CH 13 2 Rate of a Chemical Reaction Average rate of reaction aA bB cC dD Rate 1 a A t 1 b B t 1 c C t 1 d D t The Rate Law A products Rate k A n k rate constant n reaction order Determining the Order of a Reaction The order of a reaction can be determined only by experiment A common way to determine reaction order is by the method of initial rates When the A doubles the rate doubles This means the initial rate is directly proportional to the initial concentration therefore it is first order Example A products A M 1 2 4 Initial Rate M s 015 030 060 Second Order A M Initial Rate M s 015 060 240 1 2 4 Rate quadruples when doubling concentration Zero Order A M Initial Rate M s 015 015 015 1 2 4 Rate is unaffected by concentration change If unsure or numbers not obvious rate 2 rate 1 k A n2 k A n1 Reaction Order for Multiple Reactants aA bB cC dD Rate k A m B n m reaction order with respect to A n reaction order for B m n are experimentally determined Overall Order m n 13 4 Integrated Rate Law Dependence of Concentration on Time Relationship between the concentrations of the reactants and time First order integrated rate law ln A t kt ln A 0 ln A t A 0 kt A Products y mx b or A t is concentration at anytime A 0 is initial concentration Second order integrated rate law 1 A t kt 1 A 0 Zero order integrated rate law A t kt A 0 y mx b y mx b Half Life of a Reaction t1 2 First Order Half Life 693 k t 1 2 t1 2 is independent of the initial concentration Second Order Half life t 1 2 1 k A 0 Zero Order Half life t 1 2 A 0 2k 13 6 Reaction Mechanisms reaction mechanism is a series of individual chemical steps by which an overall chemical reaction occurs H2 g 2ICl g 2HCl g I2 g Step 1 H2 g ICl g HI g HCl g Step 2 HI g ICl g HCl g I2 g Each step is called an Elementary Step cannot be broken down into smaller steps Reaction intermediates things that are formed in one step and consumed in another 14 3 The Equilibrium Constant aA bB cC dD K C c D d A a B b K 0 neither reaction is favored large K forward reaction favored products Small K reaction favors reverse reaction reactants 14 4 Equilibrium Constant in Terms of Pressure aA bB cC dD Koverall K1 K2 K p K c RT n n c d a b 14 5 Hetergeneous Equilibria solids and liquids solids and liquids are left out of Kc expressions 14 6 Calculating Equilibrium Constant from Measured Equilibrium Concentrations ICE tables DO EXAMPLES 14 7 The Reaction Quotient Predicting the Direction of Change is like Kc but at any point in the reaction not just at equilibrium aA bB cC dD Qc C c D d A a B b Only reactants Qc 0 Only products Qc infinite Q K reaction towards products Q K reaction at equilibrium Q K reaction towards reactions 14 8 Finding Equilibrium Concentrations Finding equilibrium concentrations when given K and all but one concentration Example 14 8 on paper Examples 14 9 14 10 Finding equilibrium concentrations when given K and initial concentrations or pressures Finding equilibrium partial pressures when you are given the equilibrium constant and initial partial pressures Example 14 11 More practice Examples 14 12 14 13 14 9 Le Ch telier s Principle How a system at equilibrium responds to disturbances Changing Concentrations Increasing concentration of reactants Q K favors products Increasing concentration of products Q K favors reactants Decreasing concentration of reactants Q K favors reactants Decreasing concentration of products Q K favors products Changes in concentrations DO NOT matter in solids and liquids Changing Pressure gas Increase in pressure decrease in volume shifts to side with fewer moles of gas Decrease in pressure increase in volume shifts to side with more moles of When equal moles are present on each side no shift takes place Also when adding inert gas at a fixed volume no shift takes place Effect of Temperature Change on Equilibrium Exothermic A B C D Heat Add Heat shifts to the left K decreases Remove Heat shifts to the right K increases Endothermic A B Heat C D Add Heat shifts right K increases Remove Heat Shifts left K decreases 15 3 Definitions of Acids and Bases Arrhenius Definition Acid Base a substance that produces H ions in aqueous solutions a substance that produces OH ions in aqueous solutions HCl aq H aq Cl aq HCl is an acid under Arrhenius because it produces H ion in solution NaOH aq Na aq OH aq NaOH is a base under Arrhenius because it produces OH ion in solution Br nsted Lowry Definition Acid Base Amphoteric proton H ion donor proton H ion acceptor HCl aq H2O l H3O aq Cl aq substances that can act as acids and bases HCl is and acid under Br nsted because it donates a proton to H2O in solution NH3 aq H2O l NH4 aq OH aq NH3 is a base under Br nsted because is accepts a proton from H2O in solution 15 4 Acid Strength and the Acid Ionization Constant Ka strong acid completely ionizes in solution o HCl H2O H3O Cl weak acid only partially ionizes o Cl H2O H3O Cl double arrows Acids can be classified according to their number of protons 1 Monoprotic Acid 2 Diprotic Acid 3 Triprotic Acid H3A HA Acid strength depends on attraction between H and the anion of the acid The stronger the acid the weaker the conjugate base visa versa Acid Ionization Constant Ka The equilibrium constant for the ionization reaction of a weak acid HA aq H2O l H3O aq A aq Ka H3O A HA Smaller Ka weaker acid Larger Ka stronger acid 15 5 Autoionization of water and pH Water is amphoteric it can act as either acid or base Water acts as an acid and a base with itself in pure water this is called autoionization H2O l H2O l H3O aq OH aq The equilibrium constant is called the ion product constant Kw Kw H3O OH Kw 1 10 14 at 25 C in pure water An acidic solution contains an acid the creates additional H3O ions causing H3O to increase However Kw will always be the same 1e 14 A basic solution contains a base that creates additional OH ions causing OH to increase Kw remains 1e 14 pH scale a way to quantify acidity and basicity Sig figs in M the decimal places on the calculated pH pH log H3O 0 7 pH acidic 7 14 pH basic 7 pH neutral Calculating H3O from pH 1 pH log H3O 2 4 80 log H3O 4 80 log H3O 3 4 10 4 …
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