1. Chemical Kinetics Chapter 14

2. Chemical Kinetics Thermodynamics – does a reaction take place? Kinetics – how fast does a reaction proceed?

3. Factors That Affect Reaction Rates  Physical State of the Reactants  Concentration of Reactants  Temperature  Presence of a Catalyst Fig 14.2

4. Reaction rate - the change in the concentration of a reactant or a product with time (M/s) A B rate = −  [A] tttt rate =  [B] tttt  [A] = change in concentration of A over time period  t time period  t  [B] = change in concentration of B over time period  t time period  t Because [A] decreases with time,  [A] is negative

5. rate = −  [A] tt rate = [B][B] tt Fig 14.3 Progress of a hypothetical reaction A → B

6. Average rate decreases as reaction proceeds As the reaction goes forward, there are fewer collisions between reactant molecules Change of Rate with Time C 4 H 9 Cl (aq) + H 2 O (l) → C 4 H 9 OH (aq) + HCl (aq)

7. Fig 14.4 Concentration of butylchloride as a function of time Instantaneous rate ≡ slope of line tangent to the curve at any point Initial rate ≡ rate at t = 0 rate = − d[CH 4 ] dtdt

8. Reaction Rates and Stoichiometry In this reaction, the ratio of C 4 H 9 Cl to C 4 H 9 OH is 1:1 Rate of consumption of C 4 H 9 Cl = rate of formation of C 4 H 9 OH C 4 H 9 Cl (aq) + H 2 O (l) → C 4 H 9 OH (aq) + HCl (aq) Rate = −  [C 4 H 9 Cl]  t =  [C 4 H 9 OH]  t

9. Reaction Rates and Stoichiometry What if the ratio is not 1:1? 2 HI (g) → H 2 (g) In such a case: Rate = − 1212  [HI]  t =  [I 2 ]  t

10. aA + bB cC + dD rate = − Δ[A] ΔtΔt 1 a = − Δ[B] ΔtΔt 1 b = Δ[C] ΔtΔt 1 c = Δ[D] ΔtΔt 1 d Reaction Rates and Stoichiometry Eqn [14.4]

11. Fig 14.5 Basic components of a spectrophotometer Br 2 (aq) + HCOOH (aq) 2Br − (aq) + 2H + (aq) + CO 2 (g) time Br 2 (aq) Br − (aq)

12. Br 2 (aq) + HCOOH (aq) 2Br − (aq) + 2H + (aq) + CO 2 (g) time  [Br 2 ]   Absorption 393 nm Br 2 (aq) Br − (aq) Beer’s Law: A = abc

13. The Rate Law Rate law - expresses the relationship of the rate of a reaction to the rate constant and the concentrations of the reactants raised to some powers aA + bB cC + dD Rate = k [A] x [B] y reaction is xth order in A reaction is yth order in B reaction is (x +y)th order overall

14. F 2 (g) + 2ClO 2 (g) 2FClO 2 (g) rate = k [F 2 ][ClO 2 ] Rate Laws Rate laws always determined experimentally Reaction order always defined in terms of reactant (not product) concentrations Order of a reactant is not related to the stoichiometric coefficient of the reactant in the balanced chemical equation 1

15. rate  [Br 2 ] rate = k [Br 2 ] k = rate [Br 2 ] = rate constant = 3.50 x 10 -3 s -1 y = mx + b Plot of rate vs [Br 2 ] Br 2 (aq) + HCOOH (aq) 2Br − (aq) + 2H + (aq) + CO 2 (g)