Kinetics

Question 1

What is the rate of reaction?

Answer 1

The change in the amount of reactants or products per unit time.

Question 2

Describe how you could use a colourimeter to follow the rate of a reaction with one coloured reactant.

Answer 2

• A colourimeter can measure the absorbance of the solution.
• Once the absorbance data is collected, the concentration (of the coloured substance) can be calculated…
• …using a calibration curve.

Question 3

How can a concentration-time graph be used to work out the rate of a reaction at a particular time?

Answer 3

The rate of a reaction can be worked out by the gradient at a particular time on the graph.

Question 4

The rate of the acid-catalysed reaction between bromine and methanoic acid was investigated.
Br₂ + HCOOH → 2H⁺ +2Br⁻ +CO₂
Describe one method that could be used to follow the reaction rate.
[2 marks]

Answer 4

• Measure the volume of CO₂(g) produced…
• …using a gas syringe.
  OR
• Measure how the absorbance/colour of the solution changes over time…
• …using a colourimeter.
  OR
• Measure the decrease in mass (since CO₂ is given off)…
• …using a mass balance.

Question 5

If the concentration of a certain reactant was recorded at set intervals over the course of the reaction, outline how the rate of reaction, at any particular time, could be determined.
[3 marks]

Answer 5

• Plot a graph of concentration of the reactant against time.
• Draw a tangent to the curve at a particular time.
• Calculate the gradient of the tangent.

Question 6

State two assumptions you need to make when carrying out an initial rates experiment.

Answer 6

Any of the two:

• The concentration of the other reactants isn’t changing significantly.
• The temperature stays constant.
• The reaction has not proceeded too far a measurement was taken.

Question 7

What happens at the end point of a clock reaction?

Answer 7

Clock reactions tend to have an easily observable endpoint, which is usually a colour change.

Question 8

What does the order of a reaction with respect to a particular reactant tell us?

Answer 8

The order of reaction with respect to that particular reactant tells us how the reactant’s concentration affects the rate.

Question 9

If the rate of a reaction doubles when you double the concentration of reactant X, what is the order of reaction with respect to reactant X?

Answer 9

It’s the first order with respect to reactant X.

Question 10

How do you work out the overall order of a reaction from a rate equation?

Answer 10

The sum of the orders of all the different reactants will give you the overall order.

Question 11

What’s the half-life of a reaction?

Answer 11

The half-life of a reaction is the time it takes for half of the reactant to be used up.