General Chemistry Lecture
General Chemistry Lecture CHE 106
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Date Created: 10/21/15
Chapter 4 Agueous Reactions and Solution Stoichiometry aqueous solution a solution in which water is the dissolving medium seawater is different from what we call freshwater because it has a much higher total concentration ofdissolved ionic substances solution a homogeneous mixture of two or more substances 0 solvent the substance present in the greatest quantity 0 solutes other substances in the solution to be dissolved in the solvent a salt solution is a good conductor ofelectricity 0 both solutions may be clear but they could differ in the electrical conductivity electrolyte a substance whose aqueous solutions contain ions nonelectrolyte a substance that does not form ions in solution one way to differentiate two aqueous solutions is to employ a device that measures their electrical conductivities o the ability ofa solution to conduct electricity depends on the number of ions it contains 0 an electrolyte solution contains ions that serve as charge carriers causing the bulb to light water is a very effective solvent for ionic compounds because of its polarity solvation a process that helps stabilize the ions in solution and prevent cations and anions from recombining when a molecular compound dissolves in water the solution usually consists of intact molecules dispersed throughout the solution 0 most are nonelectrolytes o a few molecular substances have aqueous solutions that contain ions 0 acids are the most important of these solutions eg HCI ionizes to dissociate in H and Cl strong electrolytes are those solutes that exist in solution completely or nearly completely as ions 0 essentially all soluble ionic compounds and a few molecular compounds are strong weak electrolytes are those solutes that exist in solution mostly in the form of molecules with only a small fraction in the form of ions be careful not to confuse the extent to which an electrolyte dissolve with whether it is strong or weak chemists use halfarrows in both directions to represent the ionization of weak electrolytes and a single arrow to represent the ionization of strong electrolytes chemical equilibrium a state of dynamic balance in which the rate of formation of the products ofa reaction from the reactants equals the rate of formation ofthe reactants from the products at equilibrium the concentrations ofthe reactants and products remain constant soluble ionic compounds are strong electrolytes o ionic compounds those made up of metals and nonmetals or compounds containing the ammonium ion Precipitation reactions reactions that result in the formation of an insoluble product occur when certain pairs of oppositely charged ions attract each other so strongly that they form an insoluble ionic solid 0 Precipitate an insoluble solid formed by a reaction in solution Solubility of substance at a given temperature is the amount of the substance that can be dissolved in a given quantity of solvent at the given temperature Any substance with a solubility less than 10 molL will be referred to as insoluble All common ionic compounds ofthe alkali metal ions group 1A ofthe periodic table and of the ammonium ion are soluble in water Compounds containing 8239 To predict whether a precipitate forms when we mix aqueous solutions oftwo strong electrolytes o 1 note the ions present in the reactants o 2 consider the possible combinations of the cations and anions o 3 use the Table to determine if any of these combinations are insoluble exchange reactions metathesis reactions reactions in which positive ions and negative ions appear to exchange partners a reaction between compounds that when written as a molecular equation appears to involve the exchange of ions between the two reactants o to complete and balance an exchange reaction 0 1 Use the chemical formulas of the reactants to determine the ions that are present 0 2 Write the chemical formulas of the products by combining the cation from one reactant with the anion ofthe other 0 3 Finally balance the equation molecular equation shows the complete chemical formulas ofthe reactants and products complete ionic equation a chemical equation in which dissolved strong electrolytes such as dissolved ionic compounds are written as separate ions spectator ions ions that appear in identical forms among both the reactants and products ofa complete ionic equation present but play no direct role in the reaction net ionic equation a complete ionic equation when the spectator ions are omitted from the equation includes only the ions and molecules directly involved in the reaction 0 charges must be the same on both sides of the balanced equation 0 net ionic equations are widely used to illustrate the similarities between large numbers of reaction involving electrolytes if every ion in a complete ionic equation is a spectator then no reaction occurs procedure for writing net ionic equations 0 1 Write a balanced molecular equation for the reaction 0 2 Rewrite the equation to show the ions that form in solution when each soluble strong electrolyte dissociates into its component ions only strong electrolytes dissolved in aqueous solution are written in ionic form 0 3 Identify and cancel spectator ions acids and bases are common electrolytes acids substances that ionize in aqueous solutions to form hydrogen ions increasing the concentration of H proton donor 0 three common acids HCI HNO3 CH3COOH o monoprotic acids yield one H per molecule of acid diprotic acid yields two occurs in two steps bases substances that accept H ions and produce OH39 ions hydroxide when they dissolve in water 0 three common bases NaOH KOH CaOH2 0 compounds that do not contain OH39 can also be bases 0 eg NH3 weak electrolyte Strong acids and strong bases are also strong electrolytes fully ionized o More reactive than weak acids when the reactivity depends only on the concentration of H o The reactivity of an acid can depend on the anion as well HCI Weak acids and weak bases are also weak electrolytes partially ionized Table 43 summarize our observations about electrolytes Strong Electrolyte Weak Electrolyte Nonelectrolyte lonic All None None Molecular Strong Acids see Weak Acids All other Table 42 Weak Bases compounds The dye known as litmus for example is changed from blue to red by an acid and from red to blue by a base Neutralization reaction when a solution ofan acid and a solution ofa base are mixed 0 The reaction have no properties of the acidic solution orthe basic solution 0 a neutralization reaction between an acid and a metal hydroxide produces water an a salt Salt any ionic compound whose cation comes from a base and whose anion comes from an acid H aq 0H39 aq 9 H20 I summarization ofthe neutralization reaction between any strong acid and any strong base Because the ions exchange partners neutralization reactions between acids and metal hydroxides are also metathesis reactions Carbonates and bicarbonates react with acids to form CO2 gas Reaction of C03239 or HC0339 with an acid rst give carbonic acid H2C03 o Carbonic acid is unstable and then it decomposes to form water and carbon dioxide gas 0 Haq HC0339 aq 9 H20 l C02 g Antacids substances that remove the excess acid Acid inhibitors substances that decrease the production of acid Oxidationreduction reactions redox reactions where the oxidation numbers of certain atoms changes 0 Electrons are transferred between reactants Corrosion the conversion of a metal into a metal compound by a reaction between the metal and some substance in its environment 0 Eg rusting is between a metal and water Oxidation loss of electrons by a substance 0 When an atom ion or molecule has become positively charged it has become oxidized When an atom ion or molecule has become more negatively charged it is reduced Reduction the gain of electrons by a substance The oxidation ofone substance is always accompanied by the reduction of another as electrons are transferred between them Oxidation number the actual charge for a monatomic ion the hypothetical charge assigned to the atom assuming that the electrons are completely held by one atom or the other positive or negative whole number assigned to an element in a molecule or ion on the basis of a set of formal rules to some degree it re ects the positive or negative character of that atom o 1 For an atom in its elemental form the oxidation numberis always zero 0 2 For any monatomic ion the oxidation number equals the charge on the ion 0 3 Nonmetals usually have negative oxidation numbers although they sometimes can be positive 0 A the oxidation number of oxygen is usually 2 Exception is peroxides OH22399 oxidation number is 1 o B the oxidation of hydrogen is usually 1 when bonded to nonmetals and 1 when bonded to metals 0 C the oxidation of uorine is 1 in all compounds other halogens have an oxidation number of1 in most binary compounds n however they have positive oxidation states when they are combined with oxygen 0 4 The sum of the oxidation numbers of all atoms in a neutral compound is zero The sum ofthe oxidation numbers in a polyatomic ion equals the charge of the ion displacement reactions a reaction in which an element reacts with a compound displacing an element form it o the ion in the solution is displaced or replaced through oxidation of an element 0 A BX 9 AX B Whenever one substance is oxidized some other substance must be reduced Activity series a list of metals arranged in order of decreasing ease of oxidation Table 45 pg 141 gives the activity series for most common metals 0 Used to predict the outcome of reactions between metals and either metal salts or acids 0 Any metal on the list can be oxidized by ions of elements below it Active metals are most easily oxidized and react most readily to form compounds such as alkali metals and alkaline earth metals Noble metals are metals with low reactivity very stable and form compounds less readily such as the transition metals from groups 8B to 1B 45 Concentrations of Solutions concentration the amount of solute dissolved in a given quantity of solvent or quantity of solution molarity M the concentration ofa solution as the number of moles of solute in a liter of solution 0 molarity moles solute volume of solution in liters o a conversion factor between volume of solution and moles of solute molesliter the concentration of an electrolyte solution can be speci ed either in terms ofthe compound used to make the solution or in terms of the ions that the solution contains stock solutions solutions that are used routinely in the laboratory are often purchased or prepared in concentrated form solutions of lower concentrations can then be obtained by adding water a process called dilution 0 moles solute before dilution moles solute after dilution 0 moles solute in conc son moles solute in dil soln O Mconc X VConcMdil X Vdil 46 Solution Stoichiometry and Chemical Analysis to determine the concentration of a particular solute in a solution use titration which involves combining a sample ofthe solution with a reagent solution of known concentration called a standard solution 0 equivalent point of titration is when the stoichiometrically equivalent quantities are brought together 0 to titrate an unknown acidbase indicators are used 0 the color change signals the end point of titration Chapter 5 Thermochemistry 51 The Nature of Energy energy is necessary for all life thermodynamics the study of energy and its transformations 0 thermochemistry the relationships between chemical reactions and energy changes involving heat energy the capacity to do work or to transfer heat work the energy used to cause an object with mass to move against a force o W F X d mgh heat the energy used to cause the temperature ofan object to increase 0 energy transferred from a hotter object to a colder one kinetic energy the energy of motion Ek 12 mv2 potential energy the energy related by virtue of its position relative to other objects Epmgh force any kind of push or pull exerted on an object o eg pull ofgravity one of the most important forms of potential energy in chemistry is electrostatic potential energy which arises from the interactions between charged particles 0 Eel 0 Like charges are positive opposite charges is a negative number 0 K 899 x 109JmC2 The chemical energy of these substances is due to the potential energy stored in the arrangements of their atoms 0 Thermal energy is associated with kinetic energy SI Unit for energy is joule J o 1 J 1 kgm2s2 calorie amount ofenergy required to raise the temperature OH 9 of water from 145 C to 155 C o 1 cal4184J o 1 Cal 1000 cal 4184 J the portion we single out for study is called the system everything else is called the surroundings 0 systems may be open closed exchange energy but not matter or isolated neither can be exchanged energy is transferred between systems and surroundings in two general ways as work or heat 52 The First Law of Thermodynamics first law of thermodynamics energy is conserved it is neither created nor destroyed internal energy E the sum of all the kinetic and potential energies of all its components 0 Dena EEfinal39Einitial c Any change in the energy ofthe system is accompanied by an opposite change in the energy ofthe surroundings 0 Delta E q w 0 When heat is added to a system or work is done on a system its internal energy increases Endothermic when a process occurs in which the system absorbs heat Exothermic a process in which the system loses heat lnternal energy is an example of a state s function a property of a system that is determined by specifying the system s condition or state 0 The value of a state function depends only on the present state of the system not on the path the system took to reach that state 53 Enthalpy pressurevolume work the work involved in the expansion or compression of gases 0 wP x Delta V O Dena V V nal39vinitial Enthalpy accounts for heat flow in process occurring at constant pressure when no forms of work are performed other than PV work o H E PV 0 Since these are all state functions so is Enthalpy 0 Delta H Delta E P x Delta V 0 Delta H qp P emphasizes the changes at constant pressure The change in enthalpy equals the heat gained orlost at constant pressure 0 When Delta H is positive it is an endothermic process when Delta H is negative it is an exothermic process 54 Enthalpies of Reaction enthalpy of reaction the enthalpy change that accompanies a reaction Delta ern balanced chemical equations that show the associated enthalpy change in this way are called thermochemical equations 0 1 Enthalpy is an extensive property 0 2 The enthalpy change for a reaction is equal in magnitude but opposite in sign to Delta H for the reverse reaction 0 3 The enthalpy change for a reaction depends on the state ofthe reactants and products 55 Calorimetm The value of Delta H can be determined experimentally by measuring the heat ow accompanying a reaction at constant pressure calorimetry measurement of heat flow 0 calorimeter instrument used to measure heat ow heat capacity C the temperature change experienced by an object when it absorbs a certain amount of heat 0 amount of heat required to raise 1 K or1 C molar heat capacity Cm the heat capacity ofone mole of a substance specific heatCs the heat capacity of one gram of a substance speci c heat quantity of heat transferred q grams ofa substancem x temperature change Delta T o qm x Delta T 0 Delta T in K Delta T in C For an exothermic reaction heat is lost by the reaction and gained by the solution so the temperature of the solution rises qsoln specific heat of solution x grams of solution x Delta T qan o CsxngeltaT for dilute aqueous solutions the specific heat ofthe solution will be approximately the same as that ofwater 418 JgK one of the most important types of reactions studied using calorimetry is combustion o a bomb calorimeter is a device used with a vessel called a bomb carried out under constantvolume conditions corresponds to a change in internal energy 0 qrxn Ccal x Delta T 56 Hess s Law because enthalpy is a state function the enthalpy change Delta H associated with any chemical process depends only on the amount of matter that undergoes change and on the nature ofthe initial state of the reactants and the final state ofthe products Hess s law if a reaction is carried out in a series of steps Delta H for the overall reaction will equal the sum of the enthalpy changes for the individual steps 0 Helps calculate difficult energy changes H is a state function so for a particular set of reactants and products Delta H is the same whether the reaction takes place in one step or in a series of steps 57 Enthalpies of Formation there are enthalpies of vaporization enthalpies of fusion enthalpies of combustion enthalpy of formation Delta Hfthe formation of a compound from its constituent elements enthalpy change standard enthalpy change ofa reaction is defined as the enthalpy change when all reactants and products are in their standard state Delta H w a degree sign 0 standard state is its pure form at atmospheric pressure and the temperature is 298 K 25 C o magnitude of any change depends on temperature pressure and state standard enthalpy of formation ofa compound Delta H degree f is the change of enthalpy for the reaction that forms one mole of the compound from its elements with all substances in their standard states kJmol the standard enthalpy of formation ofthe most stable form of any element is zero because there is no formation reaction needed when the element is already in it standard state standard enthalpy change of a reaction sum of the standard enthalpies of formation of the products minus the standard enthalpies of formation ofthe reactants 61 The Wave Nature of Light electronic structure the arrangements ofelectrons in atoms refers to the number of electrons and their distribution around the nucleus and their energies electromagnetic radiation radiant energy moves through a vacuum at the speed of light 3 X 10quot8 ms a form of energy that has wave characteristics and that propagates through a vacuum at the speed of light wavelength the distance between two adjacent peaks frequency the number of complete wavelengths or cycles that pass a given point each second Hz Hertz o C A X v 62 Quantized Energy and Photons quantum the smallest quantity ofenergy that can be emitted or absorbed as electromagnetic radiation Ehxv Planck s constant h and the value is 6626 X 103934 Js Because energy can be released only in speci c amounts we say that the allowed energies are quantized their values are restricted to certain quantities Photoelectric effect the emission of electrons from a metal surface induced by light A certain amount ofenergy work function is required for an electron to overcome the attractive forces that hold it in the metal Photon the smallest increment a quantum of radiant energy a photon of light with frequency v has an energy equal to hv Energy of photon E h X v Radiant energy itself is quantized Light is both wavelike and particlelike 63 Line Spectra and the Bohr Model radiation composed ofa single wavelength is monochromatic spectrum is produced when radiation from such sources is separated into its different wavelength components 0 continuous spectrum rainbow of colors containing ight ofa wavelengths with no gaps in between 0 line spectrum a spectrum containing radiation ofonly specific wavelengths Rydberg equation the calculation of the wavelength ofa the spectral lines of hydrogen 1 A RH1n211n22 RH1096776 x1o7m391 Bohr s model explains the line spectrum ofthe hydrogen atom it cannot explain the spectra of other atoms Bohr based his model on three postulates o 1 Only orbits of certain radii corresponding to certain definite energies are permitted for the electron in a hydrogen atom o 2 An electron in a permitted orbit has a speci c energy and is in an allowed energy state An electron in an allowed energy state will not radiate energy and therefore will not spiral into the nucleus 0 3 Energy is emitted or absorbed by the electron only as the electron changes from one allowed energy state to another This energy is emitted or absorbed as a photon Ehv E 218 x 1018 J1n2 o N is the principal quantum number when it is in nity the energy is zero and this is when the electron is completely removed from the nucleus This zeroenergy state is higher in energy than the states with negative energies 0 Delta E Ef Ei Ephoton hV The existence of discrete spectrum lines can be attributed to the quantized jumps of electrons between energy levels Bohr introduced two new things 0 1 Electrons exist only in certain discrete energy levels which are described by quantum numbers 0 2 Energy is involved in moving an electron from one level to another 64 The Wave Behavior of Matter De Broglie suggested that as the electron moves about the nucleus it is associated with a particular wavelength 0 A hmv o mv momentum o matter waves used to describe the wave characteristics of material particles uncertainty principle Heisenberg s principle states that it is inherently impossible for us to know simultaneously both the exact momentum ofthe electron and its exact location in space 0 the uncertainty in the position of the electron 1 x 10399 m 65 Quantum Mechanics and Atomic Orbitals check out Table 62 wave functions a mathematical description of an allowed energy state an orbital for an electron in the quantum mechanical model of the atom probability density psi squared 0 describe the electron in an atom in the quantum mechanical model we should know ofthe probability that the electron will be in a certain region of space at a given instant LJ2 is the probability density or the electron density because it gives a good estimate of an electron s location orbitals the wave functions describes a speci c distribution of electron density in space as given by the orbital s probability density the principal quantum number n when it increases the orbital becomes larger the electron has higher energy and is farther away from the nucleus the second quantum number the angular momentum quantum number l can have integral values from O to n1 for each value of n this defines the shape of the orbital usually designated by the letters s p d fcorresponding to 0 1 2 3 the magnetic quantum number m can have values in between land including zero describes the orientation ofthe orbital in space electron shell the collection of orbitals with the same value of n subshell the set of orbitals that have the same n and values 0 1 the shell with principal quantum number n will consist of exactly n subshell o 2 Each subshell consists of a speci c number oforbitals each orbital corresponds to a different allowed value of m1 each s subshell consists of one orbital and each p subshell consists of three orbitals each d subshell consists of five orbitals and so forth 3 The total number oforbitals in a shell is n2 where n is the principal quantum number ofthe shell resulting number of orbitals for shells is 14916 when the electron occupies the lowest energy orbits 1s the hydrogen atom is said to be in its ground state 0 when the electron occupies any other orbital the atom is in an excited state 66 Representation of Orbitals the electron density for the 1s orbital is that it is spherically symmetn39o in other words the electrondensity at a given distance from the nucleus is the same regardless of the direction in which proceed from the nucleus all of the other 3 orbitals are spherically symmetric as well I quantum number forthe s orbitals is O and therefore the m1 quantum n umber must be 0 for each value of n there is only one s orbital radial probability density the probability that we will nd the electron at a specific distance from the nucleus 0 radial probability function the probability that the electron will be found at a certain distance from the nucleus 0 when an electron occupies the 1s orbital it is most likely to be found 0529 A from the nucleus Bohr s radius 0 node an intermediate point at which a probability function goes to zero the p orbitals have electron density concentration in two regions on either side ofthe nucleus separated by a node at the nucleus 0 a dumbbellshaped orbital that has two lobes o p orbitals increase in size as they move form 2p to 3p to 4p and so forth when n is 3 or greater we encounter the d orbitals 0 four of the d orbital contour representations have a fourleaf clover shape and each lies primarily in a plane the lobes are in between the axes the dz2 looks very different from the other four It has two lobes along the z axis and a doughnut in the xy plane 0 still has the same energy as the other d orbital shapes when n is 4 or greater there are seven equivalent forbitals for which l3 0 these shapes are way more complicated than the d orbitals 67 ManyElectron Atoms in a manyelectron atom for a given value of n the energy of an orbital increases with increasing value of although the shapes ofthe orbitals for manyelectron atoms are the same as those for hydrogen the presence of more than one electron greatly changes the energies ofthe orbitals the diagram with all the boxes like Figure 625 on Page 233 is a qualitative energylevel diagram orbitals with the same energy are said to be degenerate a situation in which two or more orbitals have the same energy Uhlenbeck and Goudsmit postulated that electrons have an intrinsic property called electron spin that causes each electron to behave as if it were a tiny sphere spinning on its own axis 0 The spin is quantized o The new quantum number the spin magnetic quantum number is denoted as MS 12 or 12 3 4 r A A m set of four quantum numbers n I M and Ms For a given orbital the values except the spin one are xed 68 Electron Con guralio n5 electron con guratlonr lhe Way in which lhe eleclronS are diSlribuled among lhe VariouS orb alS of an alom energy states orbital Orhnabiagmm and the anows Iepmsem the spins an unpaired electron is one not accompanied by a partner of opposite spin V Table 63 offerS Eleclron Con guralionS of Several Lighler ElemenlSm Hund s Rule number of electrons With the same sprn rs maxrmrzed lectrons arranged in this way are said to have paraIespms This rule is based on the fact that electrons repel each other and that by anolher lhuS minimizing eleclronreleclron repulSionS The outermost electrons of the atom are the ones largely responsible for the chemical behavior ofan element The condensed electron con guration of an element is when the electron con guration of the nearest noblegas element of lower atomic number is represented by its chemical symbol in brackets The noblegas core is the electron represented by the symbol for a noble gas Core electrons the electrons that are not in the outermost shell of an atom Outershell electrons are the electrons given after the noblegas core Valence electrons are the outershell electrons including the electrons involved in chemical bonding For lighter elements with an atomic number of less than 30 all of the outer electrons are valence electrons All the members ofthe alkali metal group 1A have a single svalence electron beyond a noblegas con guration Transition elements transition metals 0 Electrons are added to the 3d orbitals singly until all ve orbitals have one electron each Add tl electrons are then placed in the 3d oritbals with spin pairing until the shell is completely lled The lanthanides and actinides have their own set of orbitals known as 4f It takes 14 electrons to ll the 4f orbitals completely The 14 elements corresponding to the lling ofthe 4f orbitals are either the lanithanide elements or the rare earth elements 0 Some electron con gurations involve 5d electrons Actinide elements the nal row ofthe periodic table begins by lling the 7s orbitals uranium and plutonium are the best known 0 They are radioactive and most are not found in nature 69 Electron Con gurations and the Periodic Table the periodic table is structured so that elements with the same pattern of outershell electron configuration are arranged in columns the periodic table is your best guide to the order in which orbitals are lled representative elements maingroup elements an element from within the s and p blocks of the periodic table Figure 629 o For representative elements we do not consider completely full d orf subshells to be among the valence electrons and for transition elements we likewise do not consider a completely full fsubshell to be among valence electrons fblock metals lathanide and actinide elements in which the 4f or 5f orbitals are partially occupied the 1s subshell is the first s subshell the 2p is the rst p subshell the 3d is the first d subshell and the 4fis the first fsubshell the valence electron configuration of a halogen is nsznps although the 3d electrons are outershell electrons they are not involved in chemical bonding and are therefore not considered valence electrons Figure 631 Valence electron configurations ofthe elements pg 242
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