Worksheet #4 – Rates of Reaction
1. A catalyst increases the speed of a reaction by:
a) lowering the ΔH of the reaction
b) lowering the activation energy
c) increasing the kinetic energy of the particles
d) increasing the enthalpy of the reactants
2. What happens to the reaction rate if a catalyst is added?
a) the rate remains unchanged
b) more products are produced
c) more reactants are used
d) the reaction either speeds up or slows down
e) the reaction stops
3. What happens to the reaction rate if more reactants are added?
a) the rate remains unchanged
b) the rate increases
c) the rate always increases
d) more products are produced
e) the reaction stops
4. If the rate-determining step of a reaction is A  2B 
 C , the reaction rate is represented by
(assume it is third order overall and the concentration of B is a bigger rate determinant than concentration
of A):
a) k[A]2[B]
b) k[A][B]2
c) k[B]2
d) k[A][B]2[C]
5. At the beginning of a reaction, the reaction rate for the reactants is:
a) largest, then decreases
b) largest and remains constant
c) smallest then increases
d) smallest and remains constant
e) the same as the final rate
6. Catalysts may not:
a) speed up a reaction
b) be solids
c) form new substances in a reaction
d) be present in living tissues
7. Which one of the following would not increase the reaction rate?
a) Increasing the temperature of the reactants.
b) Dissolving two solids in water before mixing them together.
c) Diluting a solution of HCl with water before adding magnesium.
d) Grinding a solid into fine particles.
e) Adding an enzyme catalyst.
8. What is the effect on the energy diagram of a certain reaction if a catalyst is added?
The curve is lower, since the activation energy is lowered.
9. State what happens to the reaction rate if the following are done.
a)
a catalyst is added?
b)
more reactants are added?
c)
the temperature decreases?
A catalyst increases the reaction rate.
Adding more reactants increases the reaction rate.
Decreasing the temperature decreases the reaction rate.
10. According to the general rate law equation, when is the rate of reaction the fastest: at the beginning of
the reaction, when 50% of the reactants have been consumed or at the end of the reaction? Explain
briefly.
The reaction rate is fastest at the beginning of the reaction because the concentration of reactants
is the highest at the beginning of the reaction.
11. The rate law equation allows you to calculate the instantaneous rate of a chemical reaction. What
difference is there between the instantaneous and average rate of reaction?
The instantaneous reaction rate is the rate at that very instant, whereas the average reaction rate is
the average rate for the whole duration of the reaction.
12. For the following chemical reaction:
A  B 
 C
Three students measure the initial reaction rate, but under different conditions (initial concentration
of substances A and B).
Student
[A]
[B]
1
0.20 M
0.20 M
2
0.20 M
0.40 M
3
0.40 M
0.40 M
The rate constant for this reaction, at a certain temperature, is 2.5 M-1s-1 (assume it is first order with
respect to both reactants).
a)
Which student observed the reaction which proceeded the quickest?
b)
What is the rate of the quickest reaction?
c)
What is the rate of the slowest reaction?
d)
Does the rate of reaction remain constant for the duration of the reaction? Briefly explain
your answer.
e)
Give an explanation as to why each student would obtain different experimental results for
the initial reaction rate.
a)Student #3 observed the quickest reaction, because the reaction concentrations are highest.
b)The reaction rate of the quickest reaction at this time is:
rate = 2.5 M-1s-1 x [A] x [B]
= 2.5 x 0.4 x 0.4
= 0.4 M/s
c)The reaction rate of the slowest reaction at this time is:
rate = 2.5 M-1s-1 x [A] x [B]
= 2.5 x 0.2 x 0.2
= 0.1 M/s
d) No, the reaction rate doesn't remain constant during the duration of the reaction...because the
concentrations of reactants decreases as the reaction occurs. So, as the concentration
decreases, the rate also decreases.
e) Student 1-low concentrations of both reactants low rate
Student 2-low [A] but high [B] medium rate
Student 3- high concentrations of both reactants high rate
13. A student performed a chemical reaction which involved three reactants: A, B and C and measured the
initial reaction rate but varied initial concentrations. The results showed that doubling the concentration
of A doubled the reaction rate, doubling the concentration of B quadrupled the reaction rate and doubling
the concentration of C had no effect on the rate of reaction.
Find the overall reaction order and write the rate equation.
The reaction is third order overall and the rate equation is:
rate = k [A][B]2
14. A student performed a chemical reaction which involved each molecule of A reacting with two
molecules of B to form one molecule of product C.
This reaction is 0 order for A and first order for B. The rate of A being consumed is 1.2 M/s when both
initial concentrations are 0.2 M.
Determine k (include units)
Balanced reaction: A + 2B  C
rate = k[B]
1.2 M/s = k (0.2 M)
k = 6 s-1