Rate Eqn. & Order of Rxn

1. p.01 Rate Eqn. & Order of Rxn C. Y. Yeung (CHW, 2009)

2. order of rxn w.r.t. [A] order of rxn w.r.t. [B] p.02 Rate Equation (Rate Law) the mathematical equation relating the rate of rxn to the [reactants]. Examples: 3A + 2B  C rate [A]x[B]y

3. rate constant p.03 k[A]x[B]y rate= (varies with temp.!!) * usually, order of reaction = 0, 1 or 2. Therefore, the rate equation shows that:  rate of rxn is affected by [reactants]  rate of rxn is affected by temperature

4. 1 rate1 k’ [A]m then = = rate2 k’ (2[A])m 4 m 1 1 = 2 4 p.04 Example: rate = k [A]m[B]n If [B] and temp. are kept constant, when [A] doubles, rate of reaction increases 4 times. Find the value of m. rate = k’ [A]m  rate1 = k’ [A]m and rate2 = k’ (2[A])m  m = 2  the rxn is 2nd order w.r.t. [A].

5. How to do it? [A] [A]03 [A]02 [A]01 kept constant t p.05 If [B] and temp. are kept constant, when [A] doubles, rate of reaction increases 4 times. initial rate method Repeat expt. with constant [A] and different [B], find n!

6. rate m = 1 m = 2 m = 0 [A] Determination of k, m and n by graphical method p.06 Plot rate vs [A] : rate = k’ [A]m Only m could be found!

7. log (rate) m = 2 m = 1 m = 0 log [A] p.07 rate = k’ [A]m log (rate) = m log [A] + log k’ y = mx + c log k’ i.e. Both k (=k’/[B]) and m could be found!

8. p.08 1992 Paper II, Q.2(a)

9. p.09 1992 Paper II, Q.2(a) : Ans.

10. slope of tangent at a point on the conc. - time graph. - d[A] - d[A] dt dt = k’ - d[A] = k’dt   - d[A] = k’ dt - [A] = k’t + C p.10 Differential Rate Equation: (NOT accurate!) = k’ [A]m rate = if m = 0 when t = 0, [A] = [A]0  C = - [A]0  [A] = - k’t + [A]0 integrated rate eqn. (zeroth order) No calculation of tangent slope!

11. [A] [A]0 t p.11 Therefore …. forZeroth OrderRxn Monitor the variation of conc. along with time by Chemical / Physical Method,  [A] = - k’t + [A]0 slope = - k’

12. p.12 To conclude …. (1) Rate Equation shows that the rate of rxn is affected by conc. (order) & temp (k). Find the order of rxn (m, n) by Initial Rate Method (many combinations of [A] & [B]!) Plot log (rate) = m log[A] + log k’, both m, k’ and k could be found.

13. p.13 To conclude …. (2) Initial Rate Method gives Differential Rate Equation, but m and k may not be accurately found from the initial rates (=slope of tangent). Integrated Rate Equation can give better m and k, as no calculation of tangent is involved.

14. p.14 To conclude …. (3) To collect data for Integrated Rate Eqn., only one combination of [A] & [B] is needed. Measure [A] at different t. Integrated Rate Eqn. of Zeroth Order: [A] = -k’t + [A]0

15. p.15 Next …. Integrated Rate Eqn. of 1st and 2nd Order To study whether the decomposition of H2O2 is 1st order w.r.t. [H2O2]. (expt. 8)

16. p.16 Assignment Pre-lab [due date: 12/2(Thur)] p.47 Q.1-6 [due date: 12/2(Thur)]