Fundamentals CHEM 14A
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This 12 page Class Notes was uploaded by Sandy Ho on Saturday October 17, 2015. The Class Notes belongs to CHEM 14A at University of California - Los Angeles taught by Lavelle in Fall 2015. Since its upload, it has received 12 views. For similar materials see Atomic and Molecular Structure, Equilibria, Acids, and Bases in Chemistry and Biochemistry at University of California - Los Angeles.
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Date Created: 10/17/15
Fundamentals Introduction to Chemistry Chemistry in society chemistry the science of matter and the changes it can undergo chemistry used since the Stone Age where skills such as turning minerals into metals making glass jewels coins and weapons demonstrated use of chemistry chemistry transformed trade stronger steel agriculture synthetic fertilizer communication transportation trains planes renewable fuels light alloys for aerial craft medicine etc The benefits came at great cost for the earth Duty to use chemistry to make earth better 3 levels of chemistry macroscopic level level dealing with the properties of large visible objects matter and its visible transformations Ex leaf changing color fuel burning microscopic level underworld of change that cannot be seen directly and is interpreted in terms of rearrangement of atoms symbolic level expression of chemical change with symbols and mathematical equations ties the 2 other levels together thinking at microscopic experimenting on macroscopic representing ndings symbolically How science is done use of the scientific method to pursue ideas 0 consists of collecting data by making observations of samples of matter finding patterns in data to form a scientific law summary of wide range of observations Ex law of constant composition a compound has the same composition regardless of source of sample developing hypotheses possible explanations of the laws requires creativity and observation Ex Dalton s atomic hypothesis that matter consists of atoms Atoms could not be seen but Dalton could imagine it conducting an experiment carefully controlled tests to verify the hypothesis forming a theory a formal explanation of a law if the repeated experiments support hypothesis 0 theories can be qualitative expressed in words and pictures and transformed to quantitative in mathematics theory can be interpreted in terms of a model a simplified version of the object of study that can be used to make predictions Branches of chemistry organic chemistry the study of compounds of carbon inorganic chemistry the study of all the other elements and their compounds physical chemistry the study of the principles of chemistry biochemistry the study of the chemical compounds reactions and other processes in living systems analytical chemistry the study of techniques for identifying substances and measuring their amounts theoretical chemistry the study of molecular structure and properties in terms of mathematical models computational chemistry the computation of molecular properties chemical engineering the study and design of industrial chemical processes including the fabrication of manufacturing plants and their operation medicinal chemistry the application of chemical principles to the development of pharmaceuticals biological chemistry the application of chemical principles to biological structures and processes Branches with roots in chemistry include molecular biology the study of the chemical and physical basis of biological function and diversity especially in relation to genes and proteins materials sciencethe study of the chemical structure and composition of materials and nanotechnologythe study of matter on the scale of nanometers at which structures consisting of small number of atoms can be manipulated green chemistry uses sustainable development the economical utilization and renewal of resources coupled with hazardous waste reduction and concern for the environment A Matter and Energy matter anything that has mass and takes up space substance a single pure form of matter gold and water not air states of matter solid a form of matter that retains its shape and does not ow atoms packed together rigid cannot move liquid a uid form of matter that has a defined surface but takes the shape of the container it occupies atoms packed together but can move past each other gas a form of matter that lls any vessel containing it atoms ee to move to where they desire O vapor the gaseous form of a substance that is normally a solid or liquid Physical Properties 0 physical property characteristic that can be observed or measured without changing the identity of the substance Ex mass temperature hardness color state of matter 0 chemical property the ability of a substance to be changed into another substance Ex a chemical property of hydrogen is that it reacts with oxygen to produce water a chemical property of zinc is that it reacts with acids to produce hydrogen gas 0 physical change identity of substance does not change only its physical properties are different Ex water eezes but it is still water 0 chemical change substance transformed into a different substance 0 physical quantity is represented by an italic or sloping letter Ex m for mass 0 result of a measurement reported as a multiple of a unit units denoted with roman letters m for meter 0 Units may be combined together into derived units to express a property that is more complicated than mass length or time Ex volume is the product of three lengths therefore the 0 derived unit of volume is meter3 denoted m3 Converting between units Conversion Factor W then use equation information required info given gtlt conversion factor 0 extensive property a property that does depend on the size extent of the sample If a system is divided into parts and it is found that the property of the complete system has a value that is the sum of the values of the property of all the parts then that property is extensive Ex Mass Volume 0 intensive property independent of the size of the sample Ex temperature density taken from 2 extensive properties density 332 doubling volume would double mass ratio would remain the same 0 Most substances contract slightly and become more dense as they freeze but water is unusual in that it expands slightly when it freezes thus ice is less dense than water at 0 C o The number of signi cant gures in a numerical value is the number of digits that can be justi ed by the data Thus the measurement 50 g has two signi cant gures 2 sf and 1050 g cm3 has four 4 sf The number of signi cant gures in the result of a calculation cannot exceed the number in the data 0 precision of a measurement is re ected in the number of signi cant gures justi ed by the procedure and depends on how close repeated measurements are to one another accuracy of a series of measurements is the closeness of their average value to the true value 2 types of measurement errors systematic error error that is present in every one of a series of repeated measurements has same sign and magnitude can be found and corrected dust random error error that varies in both sign and magnitude and can average to zero over a series of observations wind draft on scale can be minimized by taking multiple observations and taking the average of the results Force 0 force F is an in uence that changes the state of motion of an object O Newton s second law of motion when an object experiences a force it is accelerated The acceleration a of the object is the rate of change of its velocity and is proportional to the force that it experiences acceleration 0t force or a 0i F 0 force mass gtlt acceleration or F ma a stronger force is required to accelerate a heavy object O Velocitythe rate of change of position has both magnitude and direction so when a force acts it can change the magnitude alone the direction alone or both simultaneously The magnitude of the velocity of an object the rate of change of position regardless of the direction of the motion is called its speed v Energy 0 energy the capacity to do work 0 work the process of moving an object against an opposing force work done force gtlt distance 0 unit for energy is the joule J l J 1 kg m2 s392 O 3 types of energy 0 kinetic energy E energy that a body possesses due to its motion E k 35 mv2 0 potential energy E energy that an object possesses on account of its position in a field of force I gravitational potential energy for a particle in a gravitational field Ep mgh g acceleration of free fall usually 981 m s39z higher altitude greater gravitational potential energy I coulomb potential energy for a charged particle in an electrostatic field the Coulomb potential energy of a particle of charge Q 1 at a distance r from another particle of charge Q 2 is proportional to the two charges and inversely proportional to the distance between them E p Z 803 80 epsilon zero constant called the vacuum permittivity8854 X I C2 m391 charge on an electron is e with e 1602 X 103919 C Energy approaches zero as the distance between two particles approaches infinity positive charge energy rises O electromagnetic energy energy of electromagnetic fieldspace energy carried through space by radio waves light waves and xrays electromagnetic field is generated by the acceleration of charged particles and consists of an oscillating electric field and an oscillating magnetic field The crucial distinction is that an electric field affects charged particles whether they are stationary or moving whereas a magnetic eld affects only moving charged particles Total energy kinetic energy potential energy or E E kEp energy is conserved if no outside in uences law of conservation of energy the observation that energy can be neither created nor destroyed chemical energy change in energy when a chemical reaction takes place sum of the potential and kinetic energies of substances participating in reaction 0 thermal energy sum of the potential and kinetic energies arising from the thermal motion of atoms ions and molecules B Elements and Atoms Atoms o The smallest particle of an element that can exist is called an atom o Dalton 1807 found patterns in ratios of the masses of elements that combine together to form compounds 0 Atomic hypothesis All the atoms of a given element are identical no atoms have different mass The atoms of different elements have different masses A compound is a specific combination of atoms of more than one element In a chemical reaction atoms are neither created nor destroyed they exchange partners to produce new substances 0 element a substance composed of only one kind of atom The nuclear model 0 atoms consist of a positively charged nucleus containing protons 2000 times as heavy as an electron surrounded by electrons charge of atom is neutral 0 atomic number Z number of protons in an element s atomic nucleus founded by Moseley properties of xrays depended on atomic number 0 mass spectrometer a device for determining the mass of an atom Isotopes 0 found that not all atoms of an element has same mass discovered the neutron approx same mass as proton no charge neutrons and protons jointly known as nucleons 0 mass number A total number of protons and neutrons in a nucleus 0 Isotopes of an element have the same atomic number but different mass numbers Their nuclei have the same number of protons but different numbers of neutrons o isotopes of the same element usually have the same chemical and physical properties with the exception of hydrogen isotopes 3 isotopes 1 with no neutron 1 with 1 neutron and one with 2 neutrons The organization of the elements 0 114 known elements 88 naturally occurring and in significant amounts 0 The periodic table is an arrangement of the elements by atomic number that re ects their family relationships members of the same group vertical columns of the periodic table identifying the principal families of elements typically show a smooth trend in properties Columns 1 2 13 14 15 16 17 and 18 are the main groups of the table horizontal rows are called periods and are numbered from the top down four rectangular regions of the periodic table are called blocks and for reasons related to atomic structure are labeled s p d and f 0 transition metals d block except for Group 12 elements transitional in character between the reactive metals in the s block and the less reactive one in the p block 0 inner transition metals f block upper row are the lanthanoids and the lower row are the actinoids alkali metals elements in Group 1 soft metals that melt at low temp produces hydrogen violently when in contact with water alkaline earth metals elements in Group 2 similar properties to Group 1 but reactions less vigorous noble gases Group 18 combine with few elements once called inert gases colorless odorless halogens Group 17 gas properties vary with elements in elemental forms the halogens occur as diatomic molecules molecules consisting of 2 atoms hydrogen special element no group 2 elements liquid at normal temp mercury bromine ll gaseous elements elements divided into metal conducts electricity has a luster and is malleable and ductile nonmetal does not conduct electricity and is neither malleable nor ductile and metalloid has the appearance of a metal but can behave chemically like a metal or a nonmetal malleable can be hammered into thin sheets ductile can be drawn out into wires C Compounds What are compounds compound an electrically neutral substance that consists of two or more different elements with their atoms present in a definite ratio A binary compound consists of only two elements 0 Organic compounds contain the element carbon and usually hydrogen too EX propane methane sugar 0 Inorganic compounds are all the other compounds they include water calcium sulfate ammonia silica hydrochloric acid and more In addition some very simple carbon compounds particularly carbon dioxide 0 lntermetallic compounds are inorganic compounds that are formed when the atoms of two metals bond in specific proportions EX nitinol NiTi orthodontic wires FeCo magnets elements in compound bonded due to chemical change 0 atoms can bond together to form molecules a discrete group of atoms bonded together in a specific arrangement or can be present in compounds as ions a positively or negatively charged atom or molecule positively charged ion is called a cation and a negatively charged ion is called an anion An ionic compound consists of ions in a ratio that results in overall electrical neutrality a molecular compound consists of electrically neutral molecules 0 binary compounds of 2 nonmetals are molecular compounds formed by a metal and nonmetal are ionic N aCl Molecules and Molecular Compounds chemical formula of a compound represents its composition in terms of chemical symbols Subscripts show the of atoms of each element present for molecular compounds the molecular formula a chemical formula that shows how many atoms of each type of element are present in a single molecule of the compound is used structural formula shows how the atoms are linked together but not their actual threedimensional arrangement in space A group of atoms attached to another atom in the molecule is set off with parentheses CH3CHCH3CH3 or HCCH33 carbon atoms can have up to four bonds Drawing compleX molecular structures without showing the C and H atoms involves using a line structure which represents a chain of carbon atoms by a zigzag line where each short line indicates a bond and the end of each line represents a carbon atom Atoms other than C and H are shown by their symbols Double bonds are represented by a double line and triple bonds by a triple line shapesstructures of molecular compounds pictorial representations such as the space lling model atoms shown with colored spheres ballandstick model ball represents atom and the sticks represent the bonds tube structure no balls only sticks ending with a different color representing the link bw atoms density isosurface indicates the spread of the electrons and gives a sense of the shape of the molecule electrostatic potential surface calculates the net electric potential at different points of density isosurface and represents it with different colors bluepositive rednegative Ions and Ionic Compounds monatomic ions single atom ions when an atom loses an electron the neutral atom becomes positively charged usually the elements in the 1st 2 groups when an atom gains an electron the atom becomes negatively charged usually the elements near the gases who want to reach noble gas con guration for atoms toward the left or right of the periodic table atoms lose or gain electrons until they have the same number of electrons as the nearest noblegas atom charge of an ion related to its group in the periodic table many ions are diatomic meaning that they consist of two atoms bonded together or polyatomic meaning that they consist of three or more atoms bonded together Most common polyatomic anions are the oxoanions polyatomic anions that contain oxygen precise of ions in compounds varies but the ratio is the same Ex NaCl there is one Na for every Cl39 ions combine in such a way that the positive and negative charges cancel all compounds are electrically neutral overall formula unit word expression of the ratio of ions inside a compound A formula unit of an ionic compound is a group of ions with the same number of atoms of each element as appears in its formula D The Nomenclature of Compounds many compounds given informal common names such as sugar salt water systematic names reveals which elements are present and the arrangement of atoms chemical nomenclature systematic naming of compounds Names of Cations name of a monatomic cation is the same as the name of the element forming it with the addition of the word ion more than I kind of cation use of oxidation number Charge of cation written in roman numerals included Cu2 is a copperII ion some cations were once denoted by the endings 0us lower charges and z39c higher charges Names of Anions Names of monatomic anions end in ide F39 uoride Oxoanions are anions that contain oxygen The suf x ate indicates a greater number of oxygen atoms than the suf x ite within the same series of oxoanions In series of three or more related oxoanions the pre x per indicates the maximum number of oxygen atoms ClO439 perchlorate the pre x hypo indicates the least number of oxygen atoms ClO39 hypochlorite anion containing hydrogen sometimes named starting with pre x bi bicarbonate Names of Ionic Compounds ionic compounds are named by starting with the name of the cation with its oxidation number if more than one charge is possible followed by the name of the anion Ex CoCl3 cobaltII chloride hydrates form crystals that incorporate a proportion of molecules of water as well as the ions of the compound itself when writing the formula a raised dot is used to separate the waters of hydration om the rest of the formula hydrates are named by giving the name of the compound then adding the suf x hydrale with a Greek pre x indicating the number of water molecules in the formula unit Ex CuSO4 SHZO copper II sulfate pentahydrate when the compound has lost its water of hydration it is called anhydrous Names of Inorganic Molecular Compounds molecular compounds are named by using Greek pre xes to indicate the number of atoms of each element present the element named second has its ending changed to ide Binary acids are named by adding the pre x hydro and changing the ending of the name of the second element to z39c acid as in hydrochloric acid for HCl in water An aqueous solutiona solution in water is indicated by aq after the formula 0 An oxoacid is an acidic molecular compound that contains oxygen z39c oxoacids are the parents of az e oxoanions and 0us oxoacids are the parents of z39z e oxoanions Oxoacids are the parents of the oxoanions in the sense that an oxoanion is formed by removing one or more hydrogen ions from an oxoacid molecule Names of Some Common Organic Compounds hydrocarbons compounds of hydrogen and carbon Ex methane CH4 ethane CZH6 benzene C6H6 Hydrocarbons that have no carbon carbon multiple bonds are called alkanes Ex methane ethane Hydrocarbons with double bonds are called alkenes Ex ethene CH2 CH2 benzene is a parent hydrocarbon of a group of compounds called aromatic compounds some atoms such as methyl are named by replacing the ending of the parent hydrocarbon s name by yl hydrocarbons are the basic framework for all organic compounds Different classes of organic compounds have one or more of the hydrogen atoms replaced by other atoms or groups of atoms such as alcohols a type of organic compound that contains an OH group Ex ethanol carboxylic acids a compound that contains the carboxyl group COOH Ex acetic acid and haloalkanes an alkane in which one or more H atoms have been replaced by halogen atoms Ex chloromethane Moles and Molar Masses The mole o The amounts of atoms ions or molecules in a sample are expressed in moles and Avogadro s constant NA is used to convert between numbers of these particles and the numbers of moles 1 mole of objects contains the same number of objects as there are atoms in exactly 12 g of carbon12 1 mole of objects means 60221 X 1023 of those objects The mole is the unit for the physical property formally called the amount of substance n number of objects per mole 60221 X 1023 is called the Avogadro s constant NA Number of objects amount in moles x number of objects per mole Avogadro or N nNA Molar mass 0 The molar mass of a compound the mass per mole of its molecules or formula units is used to convert between the mass of a sample and the amount of molecules or formula units that it contains The molar mass of an element is the mass per mole of its atoms The molar mass of a molecular compound is the mass per mole of its molecules The molar mass of an ionic compound is the mass per mole of its formula units units of molar mass in each case are grams per mole g mol39l Mass of sample amount in moles x mass per molemolar mass or m nM molar mass mass of one atom multiplied by Avogadro s constant or M m N or M m N A atom A atom average Ex nding the amount and actual number of atoms in a compound when molar mass and sample mass is given find amount in moles first by using n mM then use the number of moles to multiply by the Avogadro s constant to find the number of atoms 0 the molar mass of a compound is the sum of the molar masses of the elements that make up the molecule or the formula unit molar mass amu The atomic weight of an element is the numerical value of its molar mass The molecular weightof a molecular compound or the formula weight of an o ionic compound is the numerical value of its molar mass Determination of Chemical Formulas Mass Percentage Composition 0 mass percentage composition the mass of each element expressed as a percentage of the total mass mass of element in the sample 0 total mass of sample X A 0 mass percentage of element Determining Empirical Formulas o The empirical formula shows the relative numbers of atoms of each element present in the compound Ex CHZO o The empirical formula of a compound is determined from the mass percentage composition and the molar masses of the elements present G To convert the mass percentages into an empirical formula convert the mass percentage of each type of atom into the relative number of atoms of that element Imagine that we have a sample with a mass of exactly 100 g In this case the mass percentage composition gives the mass in grams of each element Then divide by the molar mass of each element to convert these masses into amounts in moles and go on to find the relative numbers of moles of each type of atom by dividing each mole by the smallest mole Determining Molecular Formulas The molecular formula shows the actual numbers of atoms of each element in a molecule Ex C5HIZO6 calculate how many empirical formula units are needed to account for the molar mass determine the molar mass of an organic compound by mass spectrometry molar mass of compound molar mass of empirical formula unit used factor to multiply the empirical formula by Mixtures and Solutions Classifying mixtures mixtures with one substance mingled with another components of a mixture are merely mingled with one another they retain their own chemical properties in the mixture a compound has chemical properties that differ from those of its component elements a mixture is a physical change whereas the formation of a compound requires a chemical change Mixture components can be separated by using physical techniques composition is variable properties are related to those of it components Compound components cannot be separated by using physical techniques composition is fixed properties are unlike those of its of its components heterogeneous mixture different substances component particles can be seen separately Ex milk fat oating on water homogeneous mixture molecules or ions of the components are so well mixed that the composition is the same throughout cannot see separate regions Ex sugar is a homogeneous mixture of sugar and water homogeneous mixture is also called a solution a solution is a physical change not a chemical change the components that form it retain their identities the component of the solution present in the larger amount what is dissolved in Ex water is called the solvent and any dissolved substance is a solute Solvent determines physical state of solution Crystallization happens when the solute slowly comes out of solution as crystals as the solvent evaporates Ex salt crystals beside a lake when water evaporates precipitation a solute comes out of solution so rapidly that a single crystal does not have time to form Instead the solute forms a divided powder called a precipitate instantaneous aqueous solutions solutions in which the solvent is water Ex saltwater Nonaqueous solutions solutions in which the solvent is not water Ex used in dry cleaning solid solutions solvent is a solid Ex brass a solution of zinc in copper gaseous solutions referred as gaseous mixtures Ex atmosphere mix of nitrogen and other gases Separation techniques techniques to analyze the composition of a sample by separating the components physically Decanting use of differences in density One liquid oats on another liquid or lies above a solid and can be poured off Filtration used to separate substances when there is a difference in solubility the ability to dissolve in a particular solvent The sample is stirred with a liquid and then poured through a filter Components of the mixture that have dissolved in the liquid pass through the filter whereas solid components that do not dissolve are captured by the filter used as first stage for water supplies chromatography relies on the different abilities of substances to adsorbor stick to surfaces Distillation uses differences in boiling points to separate mixtures When a solution is distilled the components of the mixture boil away at different temperatures and condense in a cooled tube called a condenser Ex removing water which boils at 100 C om table salt sodium chloride which does not even melt until 801 C the solid salt is left behind Concentration o The molar concentration c of a solute in a solution which is called the molarity of the solute is the amount of solute molecules or formula units in moles present in a given volume of the solution in liters amounto solute molarzly or c un1tsmol L1 Dilution O chemists store a solution in a concentrated form called a stock solution and then to dilute itor reduce its concentration to whatever concentration is needed 0 To dilute a stock solution a pipet is used to transfer the appropriate volume of stock solution to a volumetric ask Then enough solvent is added to increase the volume of the solution to its final value 0 The solute remains the same in quantity Only concentration is changed n Cfmalmeal Vinitial Chemical Equations 0 chemical reactions processes by which one or more substances are converted into other substances 0 starting materials are called the reactants and the substances formed are called the products 0 The chemicals available in a laboratory are called reagentsOnly when a reagent is being used in a particular reaction is it called a reactant Symbolizing Chemical Reactions 0 A chemical reaction is symbolized by an arrow 0 Reactants gt products 0 skeletal equation is a qualitative summary of a chemical reaction 0 To summarize reactions quantitativelyremember that atoms are neither created nor destroyed in a chemical reaction they simply change their partners The evidence is that there is no overall change in mass when a reaction takes place in a sealed container The observation that the total mass is constant during a chemical reaction is called the lawof conservation of mass Because atoms are neither created nor destroyed each elemental symbol represents one atom of the element Subscripts in a formula tell us how many atoms of that element are present in one molecule or formula unit The formulas are then multiplied by factors that result in the same numbers of atoms of each element on both sides of the arrow bringing a balance called a chemical equation 0 The number multiplying an entire chemical formula in a chemical equation is called the stoichiometric coefficient of the substance A coef cient of l is not written 0 chemical equations usually show the physical state of each reactant and product by using a state symbol 0 Ex s solid 1 liquid g gas aq aqueous solution 0 when a reaction requires a high temperature the Greek letter A delta is placed over the arrow 0 a catalyst a substance that increases the rate of a reaction but is not itself consumed in the reaction is sometimes used and can be written above the reaction arrow in a chemical equation 0 The stoichiometric coef cients tell us the relative numbers of moles of reactants and products taking part in the reaction Balancing Chemical Equations 0 An equation must never be balanced by changing the subscripts in the chemical formulas o The coefficients in a balanced chemical equation are the smallest possible whole numbers 0 A chemical equation expresses a chemical reaction in terms of chemical formulas the stoichiometric coefficients are chosen to show that atoms are neither created nor destroyed in the reaction Aqueous Solutions and Precipitation 0 When two solutions are mixed the result may be simply a new solution that contains both solutes In some cases however the solutes can react with each other 0 three main types of chemical reactions precipitation reactions acid base reactions and redox reactions Electrolytes O soluble substance dissolves to a significant extent in a specified solvent 0 insoluble substance does not dissolve significantly in a specified solvent substances are often regarded as insoluble o if they do not dissolve to more than about 01 mol L391 solute may be present as ions or as molecules ions the solution conducts an electric current only solutions that contain ions conduct electricity Ex little ions in water does not conduct electricity electrolyte substance that conducts electricity by the migration of ions Solutions of ionic solids are electrolytes because the ions become free to move when they dissolve electrolyte solution IONS electrolyte medium is a solution Some compounds like acids form ions when they dissolve and produce an electrolyte solution even though no ions are present before they dissolve Ex HCl A nonelectrolyte is a substance that does not conduct electricity even in solution MOLECULES A nonelectrolyte solution does not conduct electricity Ex acetone and sugar ribose Except for acids and bases most organic compounds that dissolve in water form nonelectrolyte solutions A strong electrolyte is a substance that is present almost entirely as ions in solution Three types of solutes are strong electrolytes strong acids and strong bases and soluble ionic compounds Ex NaOH HCl NaCl A weak electrolyte is a substance that is incompletely ionized in solution in other words most of the molecules remain intact Ex acetic acid distinguishing between strong and weak electrolytes measure the abilities of their solutions to conduct electricity with the same molar concentration of solute a solution of strong electrolyte is a better conductor than a weak electrolyte Precipitation reactions precipitate a cloudy finely divided solid deposit A precipitation reaction takes place when solutions of two electrolytes are mixed and react to form an insoluble solid When an insoluble substance is formed in water it immediately precipitates Ionic and net ionic equations The complete ionic equation for a precipitation reaction shows all the species as they actually exist in solution dissolved ionic compounds exist as separate aqueous ions so they are shown separately spectator ions ions that are present while the reaction takes place but do not participate in it ions that did not produce precipitate Canceling the spectator ions leaves the net ionic equation for the reaction the chemical equation that displays only the net change taking place in the reaction Putting precipitation to work precipitation reactions used to make a compound by preparing two solutions that when mixed give a precipitate of the desired insoluble compound the insoluble compound can be separated from the reaction mixture by filtration Precipitation reactions are also used in chemical analysis In qualitative analysis the identification of the substances present in a sample the formation of a precipitate is used to confirm the identity of certain ions In quantitative analysis the amount of each substance or element present is determined In gravimetric analysisthe amount of substance present is determined by measurements of mass J Acids and Bases Aqueous solutions of substances called bases or alkalis are recognized by their soapy feel acids sour acids and bases change the color of certain dyes known as indicators used to identify acidbase A pH lower than 7 pH 7 is characteristic of an acidic solution A pH higher than 7 pH 7 is characteristic of a basic solution Acids and Bases in Aqueous Solution Def of acids and bases 1884 Svante Arrhenius 0 acid a compound that contains hydrogen and reacts with water to form hydrogen ions 0 base a compound that produces hydroxide ions in water 0 Arrhenius acids and bases Ex HCl is an Arrhenius acid because it releases a hydrogen ion H O a proton when it dissolves in water CH4 is not an Arrhenius acid because it does not release hydrogen ions in water Sodium hydroxide is an Arrhenius base because OH ions go into solution when it dissolves 0 Problem too specific to one particular solvent water When chemists studied nonaqueous solvents such as liquid 0 ammonia they found that a number of substances showed the same pattern of acid base behavior Def of acidbase 1923 Bronsted Lowry definition K 0 acid a proton donor Ex When a molecule of an acid dissolves in water it donates a hydrogen ion H to one of the water molecules and forms a hydronium ion H30 0 base a proton acceptor 0 plain acids and bases widely accepted 0 Bronsted acid contains an acidic hydrogen atom a hydrogen atom that can be released as a proton 0 An acidic hydrogen atom is often written as the first element in the molecular formula of inorganic acids Ex HCl HNO3 O The formula of an organic acid is different in that the acidic hydrogen atom is given at the end as part of the carboxyl group COOH When a carboxyl group loses a proton it becomes a carboxylate anion O In the case of acetic acid the anion formed is the acetate ion 0 acetic acid is a monoprotic acid an acid that can donate only one proton from each molecule Ex HCl 0 polyprotic acid an acid that can donate more than one proton from each molecule Ex HZSO4 o Ammonia is a base because it accepts protons from water and forms NH4 ions Strong and Weak Acids and Bases O deprotonation the loss of a proton protonation the gain of a proton A strong acid completely deprotonated in solution Ex HCl nitric acid and sulfuric acid A weak acid incompletely deprotonated in solution Ex acetic acid carboxylic acids A strong base completely protonated in solution Ex oxide ions and hydroxide ions alkali metal and alkaline earth metal oxides and hydroxides o A weak base incompletely protonated in solution Ex ammonia and its organic derivatives the amines 0 Completely protonated means that each base molecule or ion has acquired a proton o lncompletely protonated means that only a tiny fraction of the base molecules or ions have acquired protons Neutralization o The reaction between an acid and a base is called a neutralization reaction and the ionic compound produced in the reaction is called a saltThe general form of a neutralization reaction of a strong acid and a metal hydroxide that provides the hydroxide ion a strong base in water is Acid metal hydroxide gt salt water the cation of the salt is provided by the metal hydroxide and the anion is provided by the acid net outcome of any neutralization reaction between a strong acid and a strong base in water is the formation of water from hydrogen ions and hydroxide ions Redox Reactions Oxidation and Reduction 0 oxidation reaction with oxygen 0 Oxidation is electron loss increase of charge in species reduction is electron gain Reduction and oxidation occur together in redox reactions 0 whenever a species is oxidized another species must be reduced 0 redox reactions reduction oxidation reactions Oxidation Numbers Keeping Track of Electrons O Oxidation increases the oxidation number NOX of an element reduction decreases the oxidation number 0 redox reaction is any reaction in which there are changes in oxidation numbers Oxidizing and Reducing Agents 0 The oxidizing agent or oxidantin a redox reaction is the species that causes oxidation and in the process gets reduced 0 when an oxidizing agent acts it accepts the electrons released by the species being oxidized decrease in oxidation number 0 The reducing agent or reductant in a redox reaction is the species that causes reduction and in the process gets oxidized o The reducing agent supplies electrons to the species being reduced loses electrons 9increase in oxidation number Balancing Simple Redox Equations 0 When balancing the chemical equation for a redox reaction involving ions the total charge on each side must be balanced L Reaction Stoichiometry 0 reaction stoichiometry quantitative aspect of chemical reactions used to nd how much product to expect or how much reactant needs to be used 0 The sign is read is chemically equivalent to and these expressions are called stoichiometric relations MoletoMole Predictions o nding the amount of moles a product can be formed 0 A B gt C A B AorB use stoichiometric relation to set up a conversion factor relating the given substance to the required substance substance required 2 substance given mole ratio moles refer to coef c1ents 0 Amount of product2 produced mol l X 36321 The balanced chemical equation for a reaction is used to set up the mole ratio a factor that is used to convert the amount of one substance into the amount of another MasstoMass Predictions 0 To nd the mass of a product that can be formed from a known mass of a reactant we rst convert the grams of reactant into moles use molar mass use the mole ratio from the balanced equation and then convert the moles of product formed into a mass in grams Volumetric Analysis 0 titration used for determining the concentration of a solute O acid base titrations an acid reacts with a base 0 redox titrations the reaction is between a reducing agent and an oxidizing agent 0 In titrations one solution is added to another The volume of the second solution is known and the volume of the rst solution required for complete reaction is measured A known volume of the solution being analyzed which is called the analyte is transferred into a ask usually with a volumetric pipet Then a solution containing a known concentration of the other reactant is measured into the ask from a buret until all the analyte has reacted The solution in the buret is called the titrantand the difference between the initial and nal volume readings of the buret tells us the volume of titrant that has been added to the ask The determination of concentration or amount by measuring volume is called volumetric analysis 0 In an acid base titration the analyte is a solution of a base and the titrant is a solution of an acid or vice versa An indicator a watersoluble dye color change is used to detect the stoichiometric pointthe stage at which the volume of titrant added is exactly that required by the stoichiometric relation between titrant and analyte O determining the concentration of the reactant in the analyte or its mass use molarity of titrant to nd amount of reacting titrant species in vol supplied nd amount of moles of titrant species added from volume of titrant and it molarity ntl trcmt Clitram x Vtitrcmt calculate amount of analyte write chemical equation nd mole ratio bw titrant and analyte species use to convert amount of titrant to analyte I quotanalyte Illmam gtlt mole ratio I calculate initial molarity of analyte by diving amount of analyte by initial vol of analyte quotanalyte I Canalyte V analyte I for determining mass use step 1 and 2 then convert the moles to mass using molar mass 0 The stoichiometric relation between analyte and titrant species together with the molarity of the titrant is used in titrations to determine the molarity of the analyte M Limiting Reactants 0 not all the reactants react exactly as described in chemical equations 0 Some of the starting materials may be consumed in a competing reaction a reaction taking place at the same time as the one in which we are interested and using some of the same reactants 0 Another possibility is that the reaction might not be complete at the time we make our measurements 0 Or the reaction does not go to completion it stops once a certain proportion of the reactants has been consumed 0 so the actual product might be less than calculated from reaction stoichiometry Reaction Yield Theoretical yield of a reaction the max quantity amount mass or volume of product that can be obtained om a given quantity of reactant Section L percentage yield the fraction of the theoretical yield actually produced expressed as a percentage 2 actual yield 0 0 Percent Y 1610 theore calyield X 100 The Limits of Reaction limiting reactant in a reaction the reactant that governs the maximum yield of product decides how much product can be made 0 Method 1 use the mole ratio from the chemical equation to determine whether there is enough of one reactant to react with anotherConvert the mass of each reactant into an amount in moles if necessary by using the molar masses of the substances Choose one of the reactants and use the stoichiometric relation to calculate the theoretical amount of the second reactant needed for complete reaction with the first If the actual amount of the second reactant is greater than the amount needed compare mol given and mol required then the second reactant is present in excess in this case the first reactant is the limiting reactant If the actual amount of the second reactant is less than that calculated then all of it will react so it is the limiting reactant and the first reactant is in excess excess or limited 0 Method 2 Convert the mass of each reactant into an amount in moles if necessary by using the molar masses of the substances For each reactant calculate how many moles of the product it can fonnThe reactant that can produce the least product is the limiting reactant Combustion Analysis used in modern chemical laboratories to determine the empirical formulas of organic compounds sample is ignited and allowed to burn in a tube while a plentiful supply of oxygen passes through The use of excess oxygen ensures that the sample is the limiting reactant All the hydrogen in the compound is converted into water and all the carbon is converted into carbon dioxide In the modern version of the technique the product gases are separated chromatographically and their relative amounts determined by measuring the thermal conductivity the ability to conduct heat of the gas streaming from the apparatus use mass of products created from burner In a combustion analysis the amounts of C H and O atoms in a sample of a compound and thus its empirical formula are determined om the masses of carbon dioxide and water produced when the compound burns in excess oxygen In the presence of excess oxygen each carbon atom in the compound ends up in one molecule of carbon dioxide Therefore we can write 1 mol C in sample 1 mol CO2 as product in the presence of excess oxygen each hydrogen atom in a compound contributes to a water molecule when the compound burns We can infer that 2 mol H in sample 1 mol H20 as product find mass of each individual element by using mole ratio stoichiometric relations convert to grams to find missing element have moles then empirical formula
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