Rate Law represents an equation for a chemical reaction that connects the reaction rate with the concentrations or pressures of the reactants and the rate constant.
When it comes to the rate of a reaction the Rate Law focuses on the reactant concentrations, while ignoring the product concentrations.
Concept #1: Understanding the Rate Law.
By focusing on only the reactant concentrations we only care about the forward direction of our chemical reaction.
Concept #2: The Rate Determining Step.
If the reaction says it’s a SLOW step or a ONE-STEP MECHANISM then we can simply look at the coefficients of the reactants to determine the reaction orders.
Example #1: For the following reaction, use the given rate law to determine the best answer for the reaction with respect to each reactant and the overall order.
H2O2 (aq) + 3 I – (aq) + 2 H+ (aq) ---> I3 – + 2 H2O (l)
Rate = k [H2O2]2 [I –]
a) H2O2 is 1st order, I – is 1st order, 2nd order overall.
b) H2O2 is 2nd order, I – is 1st order, 3nd order overall.
c) H2O2 is 0th order, I – is 1st order, H+ is 1st order, 3rd order overall.
d) H2O2 is 2nd order, I – is 1st order, H+ is 0th order, 3rd order overall.
Example #2: Answer each of the following question based on the following chemical reaction:
Calculate the reaction order for reactant A.
Practice: Calculate the reaction orders for Reactants B and C.
Example #3: Calculate the rate constant and the new rate for the given reaction if the initial concentration of [A] = 0.300 M, [B] = 0.150 M and [C] = 0.150 M.
The Rate Constant k can have a direct impact on the rate of the reaction. In order to determine its units just utilize the equation below, where n is the overall order of the reaction:
Example #4: A certain chemical reaction has the given rate law:
Rate = k [A]3 [B]2 [C]-1
What are the units of the rate constant for the given reaction?
a) M-2 s-1 b) M2 s-1 c) M-4 s-1 d) M-3 s-1 e) M3 s-1
Example #5: The reaction of 3 A + B ---> 2 C + D, was found to be: Rate = k [A]2 [B]3. How much would the rate increase by if A were tripled while B were increased by half?
a) 0.50 b) 30.38 c) 0.75 d) 20.25 e) 1.125
Practice: If the rate law for the following reaction is found to be the following, what are the units for the rate constant, K?
Cl2 (g) + HCCl3 (g) → HCl (g) + CCl4 (g)
a) b) c) d) e)
Concept #3: A Reaction Mechanism involves a series of elementary reactions that give the overall equation.