Principles of Chem II
Principles of Chem II CHEM 1212
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This 10 page Class Notes was uploaded by Vesta Rippin on Monday October 12, 2015. The Class Notes belongs to CHEM 1212 at Georgia College & State University taught by Ronald Fietkau in Fall. Since its upload, it has received 15 views. For similar materials see /class/221955/chem-1212-georgia-college-state-university in Chemistry at Georgia College & State University.
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Date Created: 10/12/15
CHEM 1212 Principles of Chemistry 11 Chapter 14 Acids and Bases 141 The Nature of Acids and Bases O acids were rst recognized as a class of substance that taste sour O Vinegar tastes sour because it is a dilute solution of acetic acid 0 lemons taste sour because of citric acid 0 bases sometimes called alkalis have a bitter taste and are slippery to the touch 0 drain cleaners are usually highly basic 0 the rst person to recognize the essential nature of acids and bases was Svante Arrhenius 0 he found that acids produce hydrogen ions in aqueous solution while bases produce hydroxide ions 0 two limitations of the Arrhenius concept 1 applied only to aqueous solutions and 2 allows for only one kind of base the hydroxide ion 0 a more general de nition was suggested by Johannes Brnnsted and Thomas Lowry 0 an acid is a proton donor and O a base is aproton acceptor known as the BronstedLowry concept 0 consider the following reaction HClaq H20l gt H3Oaq Cl aq 0 general reaction HAaq H20l lt gt H3Oaq A aq Acid Base lt gt Conjugate acid Conjugate base 0 it is important to note that the above reaction really represents a competition for the proton between the two bases can have an equilibrium situation H3031 A HA H1 A HA 0 where Ka is called the acid dissociation constant 0 the BronstedLowry model is not limited to aqueous solutions it can be extended to the gas phase for example NH3g HClg lt gt NH4C1s RFGCSU Page 1 of 6 Ch 14 Zumdahl 7th Edwpd 142 Acid Strength 0 the strength of an acid is de ned by the equilibrium position of its dissociation ionization reaction HAW H200 lt gt H30aQ A aq O a strong acid is one for which this equilibrium lies far to the right 0 a strong acid yields a weak conjugate base 0 a weak acid is one for which the equilibrium lies far to the left 0 a weak acid yields a strong conjugated base see Figure 144 see Figure 145 see Table 141 common strong acids are sul iric acid hydrochloric acid nitric acid and perchloric acid 0 note that sul lric acid is a diprotic acid 0 most acids are oxyacids in which the acidic proton is attached to an oxygen atom O sul lric nitric and perchloric acids are oxyacids 0 many common weak acids such as phosphoric acid nitrous acid and hypochiorous acid are oxyacids 0 organic acids commonly contain carboxyl group O examples are acetic acid and benzoic acid 0 some important acids in which the acidic proton is attached to an atom other than oxygen 0 example the hydrohalic acids HX where X is uorine chlorine bromine or iodine 0 Table 142 contains a list of common monoprotic acids and their Ka values note that strong acids are not listed reason equilibrium lies far to the right ie strong acids are Virtually 100 ionized Water as an Acid and a Base 0 water is an amphoteric substance ie can act as an acid or a base 0 consider 2 H201lt gt H30aq 0H aq 0 this leads to KW H3OOH H OH 0 KW called the dissociation constant for water 0 experiment shows that a 25 C H OH 10 X 10 7 which means that H gtlt OH 10 X 10 710 X 10 7 10 gtlt10 1 molzL2 units are usually omitted 0 important to recognize that value of KW is constant at 25C ie the product HOH will always be 10 X 10 14 RFGCSU Page 2 of 6 Ch 14 Zumdah17th Edwpd 143 The pH Scale 0 because H is small in aqueous solutions the pH scale provides a convenient way to represent solution acidity 0 pH 10 H1 0 can also have a pOH O pOH log OH O can also have a pK O pK log K 0 example H 10 X 10 9 M 0 pH 900 note two places passed the decimal point 0 a 1 unit change in pH represents a ten fold change in concentration 0 see Figure 148 0 note that 0 log KW log HOH log H log OH 0 pKw pH pOH O pKW log 10 X 10 14 1400 at 25 therefore 1400 pH pOH 144 Calculating the pH of Strong Acid Solutions 0 since a strong acid dissociates totally in water H is equal to the molarity of the acid 145 Calculating the pH of Weak Acid Solution Example Calculate the pH and pOH of a 025 M solution of propanoic acid HProp gt H Prop K Ka HProp HProp 13 X 10 5 pqpq 13 X 10 5 025 X 18 X 10 3 pH log18gtlt103 274 pOH 14 2741126 0 note Ka values list in Table 142 and Appendix A51 RFGCSU Page 3 of 6 Ch 14 Zumdah17th Edwpd Percent Dissociation O dissociation amount dissociated molL initial concentration molL x 100 0 from the example above 0 18 X 10 3 M025 Mx100 072 O for a given weak acid the percent dissociation increases as the acid becomes more dilute 0 see discussion in text 146 Bases 0 Consider NaOHs gt Naaq OH aq 0 sodium hydroxide is a strong base that is it dissociates completely 0 all Group 1 hydroxides LiOH NaOH KOH RbOH and CsOH are strong 0 the Group 2 hydroxides are also strong CaOH2 BaOH2 SrOH2 O the OH is equal to the concentration of the strong base 0 many types of proton aceptors bases do not contain the hydroxide ion NH3aq HzOl lt gt NH OH aq 0 general formula Baq HzOl lt gt BH OH aq Kb BHOH B 0 see Table 143 0 Example Calculate the pH of a 010 M solution of analine a weak base RNH2aq HzOl lt gt RNH OH aq Kb RNH3OH RNH2 40 x 10 0 xx0 10 x 63 x 10 10 M pOH log 63 x 10 10 520 pH 14 520 880 147 Polyprotic Acids 0 see table 144 RFGCSU Page 4 of 6 Ch 14 Zumdah17th Edwpd 148 AcidBase Properties of Salts 0 salt is simply another name for ionic compound Salts That Produce Neutral Solutions 0 salts that consist of the cations of strong bases and the anions of strong acids have no effect on H when dissolved in water eg solutions of KCl NaCl NaNO3 KNO3 will have a pH of 7 Salts That Produce Basic Solutions 0 in an aqueous solution of sodium acetate NaC2H302 the major species are Nat C2H3O2 and H20 0 What are the acidbase properties of each component pH will be determined by C2H3O2 C2H302 31 H200 lt gt HC2H302339Q OH 3 1 0 therefore have a Kb Salts That Produce Acidic Solutions 0 when NHACI dissolves in water obtain NH and C1 NH is a weak acid and C1 has no effect on the pH NH aq lt gt NH3aq Haq 0 see Table 145 0 see Table 146 149 The Effect of Structure on AcidBase Properties 0 see Tables 147149 RFGCSU Page 5 of 6 Ch 14 Zumdah17th Edwpd CHEM 1212 Principles of Chemistry 11 Chapter 11 Properties of Solutions 0 see Table 111 111 Solution Composition 0 can use qualitative terms dilute and concentrated to describe solutions but need quantitative descriptions O recall molarity M de ned as moles of solute per liter of solution 0 mass percent also known as weight percent is the percent by mass of the solute in the solution 0 also have mole fraction symbolized by the Greek lowercase letter chi which is the ratio of the number of moles of a given component to the total number of moles of solution molality m is the number of moles of solute per kilogram of solvent 0 normality is de ned as the number of equivalents per liter of solution 0 01 M HCl 01 N HC101 M H2SOA 02 N H2SOA 112 The Energies of Solution Formation 0 what factors affect solubility axiom like dissolves like 0 polar solvent to dissolve a polar or ionic solute and a nonpolar solvent to dissolve a nonpolar solute O the formation of a liquid solution takes place in three distinct steps 0 1 Breaking up the solute into individual components expanding the solute O 2 Overcoming intermolecular forces in the solvent to make room for the solute expanding the solvent 0 3 Allowing the solute and solvent to interact to form the solution 0 1 and 2 require energy since forces must be overcome to expand the solute and solvent 0 3 usually releases energy 0 the enthalpy change associated with the formation of the solution called the enthalpy heat of solution delta H5013 is the sum of the delta H values for the steps 1 2 and 3 O in the case of NaCl have enthalpy heat of hydration which combines 2 and 3 see Table 113 113 Factors Affecting Solubility Structure Effects 0 fatsoluble hydrophobic versus watersoluble hydrophilic vitamins 0 see Figure 114 RFGCSU Page 1 of 5 Ch 11 Zumdah17th Edwpd O fatsoluble vitamins can build up in the fatty tissues of the body 0 positive effect the can tolerate for a time a diet de cient in vitamins A D E or K 0 negative effect if excessive amounts of these vitamins are consumed their buildup can lead to the ilhiess hypervitaminosis O in contrast watersoluble vitamins are excreted by the body and must be consumed regularly eg vitamin C Pressure Effects 0 while pressure has little effect on the solubilities of solids or liquids it does significantly increase the solubility of a gas soda 0 see Figure 115 0 the relationship between gas pressure and the concentration of dissolved gas is given by Henry s law P kC P represents the partial pressure of the gaseous solute above the solution C represent the concentration of the dissolved gas and k is a constant characteristic of a particular solution 0 Henry s law holds only when there is no chemical reaction between the solute and solvent oxygen versus hydrogen chloride Temperature Effects for Aqueous S olutions 0 see Figure 116 0 predicting the temperature dependence of solubility is very dif th O the only sure way to determine the temperature dependence of a solid s solubility is by experiment 0 the behavior of gases dissolving in water appears less complex the solubility of a gas in water typically decreases with increasing temperature 0 see Figure 117 114 The Vapor Pressures of Solutions 0 liquid solutions have physical properties significantly different from those of the pure solvent 0 example antifreeze O to explore how a nonvolatile solute affects a solvent consider experiment represented in Figure 119 0 observation of experiment is that the presence of a nonvolatile solute lowers the vapor pressure of a solvent 0 the dissolved nonvolatile solute decreases the number of solvent molecules per unit volume less opportunity to escape from the surface see Figure 1110 0 detailed studies of the vapor pressure of solution containing nonvolatile solutes were carried out by Francois M Raoult 18301901 0 have Raoult s haw Psoln Xsolva jwlvem where Psoln is the observed vapor pressure of the RFGCSU Page 2 of 5 Ch 11 Zumdah17th Edwpd solution Xsolvem is the mole fraction of solvent and P pure solvent 0 see Figure 1111 is the vapor pressure of the solvent Nonideal Solutions 0 for liquidliquid solution where both components are volatile a modi ed form of Raoult s law applies see eqn on page 532 see Figure 1112 page 533 O a liquidliquid solution that obeys Raoult s law is called an ideal solution 0 Raoult s law is to solutions what the ideal gas law is to gases 0 if the solvent has a special affinity of the solute eg hydrogen bonding fewer solvent molecules will escape and the observed pressure will be lower than the value predicted by Raoult s law negative deviation 0 in contrast if two liquids mix endothermically this indicates that the solventsolvent interactions are weaker than the interactions among the molecules in the pure liquids more energy is required to expand the liquids than is released when the liquids are mixed more molecules escape and have a positive deviation from Raoult s law 0 see Figure 1113 0 see Table 114 115 BoilingPoint Elevation and FreezingPoint Depression 0 because changes of state depend on vapor pressure the presence of a solute also affects the freezing point and boiling point of a solvent 0 colligative properties are 0 freezing point depression 0 boiling point elevation and O osmotic pressure they are grouped together because they depend only on the number and not on the identity of the solute in an ideal solution Boiling Point Elevation 0 the normal boiling point of a liquid occurs at the temperature where the vapor pressure is equal to one atmosphere 0 a nonvolatile solute lowers the vapor pressure of the solvent but the pressure must be 1 atm before boiling occurs means that a nonvolatile solute elevates the boiling point of the solvent 0 the change in boiling point can be represented by the equation AT Kbmsolute where AT is the boiling point elevation Kb is a constant that is characteristic of the solvent and is called the molal boiling point elevation constant and m is the molality of the solute in the solution 0 see Figure 1114 solute RFGCSU Page 3 of 5 Ch 11 Zumdah17th Edwpd see Table 115 Freezing Paint Depressitm when a solute dissolved in a solvent the freezing point of the solution is lower than that of the pure solvent 0 recall that the vapor pressures of ice and liquid water are the same at 0 C when a solute is dissolved in water the solution has low vapor pressure than that of pure ice no ice forms as the solution is cooled the vapor pressure of the ice and that of the liquid water in the solution will eventually become equal have a new freezing point which is below 0 C the freezing point has been depressed the equation for freezing point depression is analogous to that for boiling point elevation AT Kfmsohme where AT is the freezing point depression Kf is a constant that is characteristic of the solvent and is called the molal freezing point depression constant and m is the molality of the solute in the solution solvent 116 Osmotic Pressure 0 osmotic pressure another of the colligative properties can be understood from Figure 1116 0 the ow of solution into the solution through the semipermeable membrane is called osmosis O the difference in the levels of the solutions see figure 1116 indicates the is a greater hydrostatic pressure on the solution than on the pure solvent 0 the excess pressure is called the osmotic pressure 0 osmosis can be prevented by applying a pressure to the solution the minimum pressure that sops the osmosis is equal to the osmotic pressure of the solution 0 see Figure 1117 0 experiments show that the dependence of the osmotic pressure on solution concentration is represented by the equation 139 MRT where T is the osmotic pressure in atmospheres M is the molarity R is the gas constant and T is the Kelvin temperature 0 in osmosis a semipermeable membrane prevents transfer of all solute particles 0 a similar phenomenon called dialysis occurs at the walls of most plant and animal cells in this case the membrane allows transfer of both solvent molecules and small solute molecules and ions 0 an important application of dialysis is the use of artificial kidney machines to purify the bloo 0 see Figure 1119 page 542 0 solution that have identical osmotic pressures are said to be isotonic solutions 0 uids administered intravenously must be isotonic with body uids 0 red blood cells in a hypertonic solution will experience cremation shrivelling 0 red blood cells in a hypotonic solution will experience hemolysis rupturing 0 salt used to preserve foods RFGCSU Page 4 of 5 Ch 11 Zumdah17th Edwpd Reverse Osmosis if a solution in contact with pure solvent across a semipermeable membrane is subjected to an external pressure larger than its osmotic pressure reverse osmosis occurs 0 can be used to produce potable water 117 Colligative Properties of Electrolyte Solutions 0 as seen previously the colligative properties of solutions depend on the total concentration of solute particles 0 the relationship between the moles of solute dissolved and the moles of particles in solution is usually expressed using the van t Hoff factor 139 which is equal to the moles of particles in solution divided by the moles of solute dissolved O i for NaCl is 2 for K2804 3 for Fe2POA3 is 5 0 see Table 116 observed values differ quotom expected values due to ion pairing 0 see Figure 1122 0 note how equations are modified for electrolyte solutions see text 118 Colloids O mud can be suspended in water by vigorous stirring 0 when stirring stops most of the particle rapidly settle out but even after several days some of the smallest particles remain suspended O the scattering of light by these particles is called the Tyndall effect 0 a suspension of tiny particles in some medium is called a colloidal dispersion or a colloid O the destruction of a colloid called coagulation usually can be accomplished either by heating or by adding an electrolyte 0 see Table 117 RFGCSU Page 5 of 5 Ch 11 Zumdah17th Edwpd
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