# Rate Laws.

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Rate Laws

Determine the rate law and order of a chemical reaction from experimental data.
Include: various reaction orders, rate versus concentration graphs. Additional KEY Terms Ratio

Rate is proportional to changes in A and B
A + B C Rate is proportional to changes in A and B Rate Law helps us calculate the rate of a REACTION not a reactant.

It is an expression that shows the quantitative effect of concentration changes on reaction rate.
Rate = k[A]x where: k rate constant [A] concentration of A x reaction order **The rate constant (k) and the order (x) can only be determined experimentally**

Temperature is the only factor to affect
The rate constant (k) is specific for each reaction at a specific temperature. Temperature is the only factor to affect the rate constant. Rate = k[A]x

Reaction Order

Each reactant can affect the rate differently
A + B Products Rate = k[A]x[B]y Each reactant can affect the rate differently The orders of a reaction (x and y) indicate how much each [reactant] affects the rate of a reaction.

Rate = k[A] 1 x First order reaction ( x = 1)
The reaction rate is directly proportional to changes in reactant concentration. [A] is doubled - rate doubles 2 = 21 [A] is tripled - rate triples 3 = 31 x Rate = k[A] 1

Rate = k[A]2 Second order reaction (x = 2)
The reaction rate is proportional to changes in reactant concentration squared. Rate = k[A]2 Doubling [A] - increase rate 4x 4 = 22 Tripling [A] - rate increase 9x = 32

**NOT included in the rate law if determined
Third order reaction (x = 3) The reaction rate is proportional to changes in reactant concentration cubed. Rate = k[A]3 Zero order reaction (x = 0) The rate does not depend on the [A]. Changing [A] does NOT change the rate. Rate = k **NOT included in the rate law if determined to be zero order**

Rate = k[A][B]2 1st order + 2nd order = 3rd order overall
Overall order of reaction is the sum of the orders: x + y = overall reaction order 1st order + 2nd order = 3rd order overall

Calculating Rate Law

Several ways to determine the rate law:
differential rate law - uses calculus integrated rate law – uses graphing software initial rates method – uses data tables Determining rate law: Measure the effect of changes in concentration of one reactant on rate, while keeping the other reactant constant.

Ratio Approach 8 = 2  4 2 =

= Using ratios: A + B → products same Rate = k[A]x[B] y
Trial [A] (mol/L) [B] (mol/L) Initial Rate (mol/Ls) 1 0.10 0.20 2.0 2 0.30 18.0 3 0.40 16.0 same Rate = k[A]x[B] y Rate = k[A]x[B] y Rate2 k [A]2x  [B]2y Rate1 k [A]1x  [B]1y = α 9.0 α [3]x  1 9.0 α [3]x 18.0 α x  0.2 y  0.2 x = second order

= Using ratios: A + B → products 2 Rate = k[A]2[B] y 1
Trial [A] (mol/L) [B] (mol/L) Initial Rate (mol/Ls) 1 0.10 0.20 2.0 2 0.30 18.0 3 0.40 16.0 Rate = k[A]2[B] y 1 Rate = k[A]x[B] y Rate3 k [A]3x  [B]3y Rate1 k [A]1x  [B]1y = α 8.0 α [2]2  [2]y 8.0 α  [2]y 2.0 α [2]y 16.0 α  0.4 y  0.2 y = first order

Can’t have multiple answers…pick new trials.
Using ratios: A + B → products 2 Trial [A] (mol/L) [B] (mol/L) Initial Rate (mol/Ls) 1 0.10 0.20 2.0 2 0.30 18.0 3 0.40 16.0 Rate = k[A]2[B] y You might pick bad trials Can’t have multiple answers…pick new trials. Rate2 α [A]2x  [B]2y 9.0 α [3]  [1]y 2 Rate [A]1x  [B]1y 9.0 α 9  [1]y 1 α [1]y 18.0 α x  0.2 y  0.2 y = 0 or 1 or 2…..

Common Sense Approach

x y 1 1 rate = k[H2O2] [HI] rate = k[H2O2] [HI] = 2x 2 same same
Rate2 k [H2O2]2x  [HI]2y Rate1 k [H2O2]1x  [HI]1y 2.0 α 1 ּ [2]y = x y 1 1 rate = k[H2O2] [HI] 2.0 α [2]y α x  0.2 y  0.1 rate = k[H2O2] [HI] x = first order

x 1 y Rate = k[A] [B] 8.0 α [2]x  [4]1 Rate α [A]x  [B]y
Tl [A]i mol/L [B]i Initial Rate (mol/Ls) 1 0.0100 0.0240 1.45 x 10−4 2 0.0120 7.25 x 10−5 3 0.0200 0.0480 5.80 x 10−4 x y Rate = k[A] [B] 1 8.0 α [2]x  [4]1 Rate α [A]x  [B]y 8.0 α [2]x  4 Rate3 α [A]3x  [B]3y Rate [A]2x  [B]2y 2.0 α [2]x x = first order α x   0.012

3 A (g) + B (g) + 2 C (g) 2 D (g) + 3 E (g)
a. Write the rate law for this reaction. b. Calculate the value of the rate constant (k). c. Calculate the rate for Trial #5. d. Calculate the concentration of A in Trial #6.

rate = k[A][B]2 = = 1 2 same same same same same same
same same same same same same a. Write the rate law for this reaction. Rate2 k [A]2x  [B]2y  [C]2z Rate1 k [A]1x  [B]2y  [C]2z = Rate2 [A]2x Rate1 [A]1x = rate = k[A][B]2

rate = k[A][B]2 b. Calculate the value of the rate constant (k).
To find the value of k, we use that data from ANY trial. Don’t include units for k.

rate = (200)(0.50 mol/L)(0.40 mol/L)2 rate = 16 mol/Ls
c. Calculate the rate for Trial #5. rate = k[A][B]2 rate = (200)(0.50 mol/L)(0.40 mol/L)2 rate = 16 mol/Ls

d. Calculate the concentration of A in Trial #6.
rate = k[A][B]2

Conclusion: Everything in the Rate Law must be determined experimentally:
1. Write a basic rate law with all reactants 2. Determine the order for each reactant (1, 2, 0) 3. Re-write the rate law with the determined order of reaction for each 4. Solve any problems

CAN YOU / HAVE YOU? Determine the rate law and order of a chemical reaction from experimental data. Include: various reaction orders, rate versus concentration graphs. Additional KEY Terms Ratio