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Chem 1110 Chapter 4

by: Samantha

Chem 1110 Chapter 4 Chem 1110

Marketplace > Auburn University > Chemistry > Chem 1110 > Chem 1110 Chapter 4
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General Chemistry I
David M Stanbury

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Aqueous solutions Hydrations Describing Reactions in Solution
General Chemistry I
David M Stanbury
Class Notes




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This 0 page Class Notes was uploaded by Samantha on Monday January 11, 2016. The Class Notes belongs to Chem 1110 at Auburn University taught by David M Stanbury in Winter 2016. Since its upload, it has received 7 views. For similar materials see General Chemistry I in Chemistry at Auburn University.


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Date Created: 01/11/16
Chem 1110 Chapter 4 1 Summary a Solventor dissolving medium water b Aqueous solutions are solutions that use water as its solvent 2 Water the Common Solvent a Water is one of the most important substances on earth i One of the most valuable functions of water involves its ability to dissolve many different substances To understand this process we need to consider the nature of water 1 Liquid water consists of a collection of H20 molecules An individual molecule is quotbentquot or Vshaped with an HOH angle of about 150 The OH bonds in the water molecule are covalent bonds formed by electron sharing between the oxygen and hydrogen atoms a The oxygen has a greater attraction for electrons than does hydrogen b Since they are not shared equally then the electrons spend more time closer to the oxygen than the hydrogens i Making the oxygen gain a slight excess negative charge and the hydrogen atoms become slightly positive ii Because of this unequal charge distribution water is said to be a polar molecule ii Hydrations a mixing of the native orbitals on a given atom to form special atomic orbitals for bonding 1 The hydration of its ions tends to cause a salt to quotfall apartquot in the water or to dissolve The strong forces present among the positive and negative ions of the solid are replaced by strong waterion interactions 2 When salts dissolve the separate into individual cations and anions b The solubility of ionic substances in water varies greatly i The differences in the solubilities of ionic compounds in water typically depend on the relative af nities of the atoms for each other and the af nities of the ions for the water molecules 3 The Nature of Aqueous Solutions Strong and Weak Electrolytes a A solution is a homogeneous mixture It is the same throughout but its composition can be varied by changing the amount of dissolved substance b A useful property for characterizing a solution is its electrical conductivity its ability to conduct an electric current i Some solutions conduct current very ef ciently strong electrolytes and others solutions conduct only a small current weak electrolytes ii Those that produce no current are called nonelectrolytes iii The extent of which a solution can conduct an electrical current depends directly on the number of ions present c Strong Electrolytes i A salt consists of an array of cations and anions that separate and become hydrated when the salt dissolves 1 Solubility is usually measured in terms of the mass of solution that dissolves per given volume of solvent or in terms of the number of moles of solute that dissolves in a given volume of solution ii One of Arrhenius39s most important discovers concerned the nature of acids 1 He found that when the substances HCI HNO3 and H2504 were dissolved in water they behaved as strong electrolytes He postulated that this was the result of ionization in water 2 Thus he proposed that an acid is a substance that produces H ions protons when it is dissolved in water 3 Because of the fact that when these acids are dissolved in water virtually every molecule dissociates to give ions therefor are called strong acids iii Another important class of strong electrolytes is the strong bases soluble compounds containing the hydroxide ion that completely dissociate when dissolved in water d Weak Electrolytes i Weak electrolytes are substances that produce relatively few ions dissolved in water The most common are weak acids and weak bases a Formulas for acids are often written with the acidic hydrogen atom or atoms listed rst Then if any nonacidic hydrogen is present they are written last Ex HC2H302 2 Acetic acid only has 1 of its molecules dissociate in aqueous solution a Because acetic acid is a weak electrolyte it is called a weak acid Any acid that dissociated only to a slight extent in aqueous solution is called a weak acid 3The most common weak base is ammonia NH3 The solution is basic since OH ions are produced It is called a weak base because the resulting solution is a weak electrolyte very few ions are present e Nonelectrolytes i Are substances that dissolve in water but do not produce any ions 4 The Composition of Solutions a To preform stoichiometric calculations we must know 1 the nature of the reaction which depends on the exact forms the chemicals take when dissolved and 2 the amounts of chemicals present in the solutions that is the composition of each solution i Most commonly used expression of concentration molarity M which is de ned as moles of solution per volume of solution b In the process called dilution water is then added to achieve the molarity desired for a particular solution A typical dilution calculation involves determining how much water much be added to an amount of stock solution to achieve a solution of the desired concentration Dilution with water doesn39t alter the number of moles of solute present 5 Types of Chemical Reactions 6 Precipitation Reaction a When two solutions are mixed an insoluble substance sometimes forms that is a solid forms and separates from the solution Such a reaction is called a precipitation reaction and the solid that forms is called a precipitate b Memorize table 41 page 109 Solubility Rules c When solutions containing ionic substances are mixed it will be helpful in determining the products if you think in terms of ion interchange The key to dealing with the chemistry of an aqueous solution is to rst focus on the actual components of the solution before any reaction occurs and then gure out how those components will react with each other 7 Describing Reactions in Solution a Molecular equation i K2Cr04 BaNO32 BaCrO4s 2KNO3 b Complete Ionic equation i 2K CrO4 Ba N03 BaCrO4 s 2K 2NO3 ii Better represents the actual forms of the reactants and products in solution In a complete ionic equation all substances that are strong electrolytes are represented as ions iii The K and N03 ions are present in solution both before and after the reaction lons such as these that do not participate directly in a reaction in solution are called spectator ions c Net lonic equation i Ba CrO4 BaCrO4s ii Includes only those solution components directly involved in the reaction 8 Selective Precipitation a We can use the fact that salts have different solubilities to separate mixtures of ions b We want to separate the cations by precipitating them one at a time a process called selective precipitation i Step 1 Add an aqueous solution of NaCl to the solution containing Ag Ba and Fe ions Solid AgCl will form and can be removed leaving Ba and Fe ions in solution ii Step 2 Add an aqueous solution of NaZSO4 to the solution containing the Ba and Fe ions Solid BaSO4 will form and can be removed leaving only Fe ions in the solution iii Step 3 Add an aqueous solution of NaOH to the solution containing the Fe ions Solid FeOH3 will form and can be removed c The process whereby mixtures of ions are separated and identi ed is called qualitative analysis 9 AcidBase Reactions a An acid is substance that produces H ions when dissolved in water and a base is a substance that produces OH ions i An acid is a proton donor ii A base is a proton acceptor b The hydroxide ion is such a strong base that for purposes of stoichiometry it is assumed to react completely with any weak acid dissolved in water c An acid base reaction is often called a neutralization reaction When just enough base is added to react exactly with all of the acid in a solution we can say the acid has been neutralized d Acid Base Titrations i Acidbase titrations are example of volumetric analysis a technique in which one solution is used to analyze another The solution used to carry out the analysis is called the titrant and is delivered from a device called 10 11 a b C d a b a buret which measures the volume accurately The point in the titration at which enough titrant has been added to react exactly with the substance being determined is called the equivalence point or the end point This point is often marked by the change in color of a chemical called an indicator 1 The concentration of the titrant must be known standard solution 2The exact reaction between titrant and substance being analyzed must be known 3 The equivalence point must be known An indicator that changes color at or very near the end point is often used 4 The volume of titrant required to reach the endpoint must be known as accurately as possible Oxidation Reduction Reactions Reactions in which one or more electrons are transferred are called oxidationreduction reactions or redox reactions The concept of oxidation states or oxidation numbers provides a way to keep track of electrons in oxidationreduction reactions i Memorize Table 43 page 120 Oxidized i Loses electrons ii Oxidation state increases iii Reducing agent electron donor Reduced i Gains electrons ii Oxidations state decreases iii Oxidizing agent electron acceptor Balancing OxidationReduction Equations The Oxidation States Method i Assign each atom their oxidation states ii Look at the reduced and oxidized atoms iii Compare the charges and make them equal to each other 1 In using this method to balance an OR equation we can nd the coefficients for the reactants that make the total increase in oxidation state balance the total decrease The Half Reaction Method i To separate the reactions into two halfreactions one involving oxidation and the other involving reduction The general procedure is to balance the equations for the halfreactions separately and then to add them to obtain the overall balanced equation 1 Write the two equations 2 Balance all the elements except H and O 3 Balance O using H20 and Balance H using H 4 Balance the electron charges 5 Make sure that each reaction has the same number of electrons 6 Add the halfreactions together and cancel identical species


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