F325: Module 1: Rates

Question 1

I) What are the units for ‘Rate’?

Answer 1

mol dm^-3 s^-1

Question 2

I) If an equation is: A + B –> C + D

what is the rate equation and explain what each of the parts mean?

Answer 2

rate = k [A]^m [B]^n
where:
A and B are the reactants.

[A] and [B] are the concentrations of A and B at any given time.

‘k’ is the rate constant (the larger the value of k, the faster the reaction goes).

m and n are the order of the reaction with respect to A and B.

Question 3

I) How would you calculate the total order of the reaction?

Answer 3

The sum of the orders for each reactant.

Question 4

I) How would you calculate the rate constant ‘k’?

Answer 4

k = rate/[A]^m[B]^n

Question 5

I) What happens to the rate constant if the temperature increases?

Answer 5

k increases meaning that the reaction goes faster.

Question 6

I) What happens to the rate constant if the temperature decreases?

Answer 6

k decreases meaning that the reaction slows down.

Question 7

I) What is the rate determining step?

Answer 7

The slowest step in a reaction mechanism which determines how fast the reaction goes.

Question 8

I) How could you use the rate equation of a reaction to fin the rate determining step?

Answer 8

The rate equation tells you how many molecules react with each other in the rate determining step. The order of the reactant tells you the number of molecules of that reactant that are in the rate determining step.

Question 9

I) On a concentration-time graph, what are the traits of zero, first and second order reactions?

Answer 9

Zero order - The gradient is constant which shows that the rate is unaffected by the concentration of the reactant.

First order - The gradient of the graph changes which shows that the concentration does affect the rate. Also, the half-life is a constant.

Second order - The gradient of the graph changes substantially so the concentration affects the rate. However, the half-life is not constant.

Question 10

I) What does the half-life of a concentration-time graph represent?

Answer 10

The time taken for the concentration to halve.

Question 11

I) What does the gradient represent on a concentration-time graph?

Answer 11

The rate of the reaction.

Question 12

I) What type of curve does a first order reaction plot on a concentration-time graph?

Answer 12

An exponential curve since exponential curves always have a constant half-life.

Question 13

I) What are rate-concentration graphs useful for?

Answer 13

Differentiating between first and second order reactions.

Question 14

I) What is the initial rates method?

Answer 14

The initial rates method is used for obtaining rate-concentraion graphs. This mean that the initial rate is measured in several experiments in which the reactants have different concentrations. The concentration of one reactant at a time is changed while the others remain the same.

Question 15

I) How would you find the initial rate of reaction from a concentration-time graph?

Answer 15

Measure the gradient at t = 0

Question 16

I) On a rate-concentration graph, what are the traits of zero, first and second order reactions?

Answer 16

Zero order - Flat line as changing the concentration has no effect on the rate.

First order - Straight line with gradient ‘k’ (the rate constant) since the rate is proportional to the concentration.

Second order - Resembles the top right quadrant of a quadratic graph since there is a quadratic relationship between the rate and the concentration.

Question 17

(2 marks)
5I- + IO3- + 6H+ –> 3I2 + 3H2O
Rate equation:
Rate = k [I-(aq)]^2 [IO3-(aq)]^2 [H+(aq)]^2

The proposed mechanism takes place in several steps. Suggest two reasons why it is unlikely that this reaction could take place in one step

Answer 17

• Stoichiometry in rate equation does not match stoichiometry in overall equation
• Collision unlikely with more than two ions