3.5 Chemical Kinetics

Question 1

What is meant by ‘rate of reaction’?

Answer 1

The change in concentration of reactants or products over time.

Question 2

What is a rate equation?

Answer 2

For a reaction between A and B:
Rate = k[A]^m [B]^n
where m and n are integers representing the orders with respect to each reactant, and k is the rate constant.

Question 3

What is a rate constant?

Answer 3

The constant of proportionality linking the rate of reaction and the concentrations of the reactants raised to the power of their orders in the rate equation.

Question 4

How do you calculate the units of a rate constant?

Answer 4

• Rearrange the rate equation to make k the subject.
• Substitute units into the equation.
• Cancel the common units to find the units for k.

Question 5

What is meant by the term ‘order of reaction’?

Answer 5

• The order with respect to a reactant is the power to which the concentration of the reactant is raised in the rate equation.
• Overall order of a reaction = the sum of all the individual orders of the reactants.

Question 6

What do the different orders of reaction mean?

Answer 6

• Zero order: if the rate is proportional to [A]^0 then the rate of reaction is unaffected by changing [A].
• First order: if the rate is proportional to [A]^1 then rate of reaction increases at the same rate as [A] increases.
• Second order: if the rate is proportional to [A]^2 then rate will increase by the square of the factor [A] increases by.

Question 7

What is activation energy?

Answer 7

The minimum energy required for a particular reaction to occur.

Question 8

What are some different experimental techniques that allow you to obtain rate data?

Answer 8

• Titration.
• Measuring the volume of gas released from a reaction over time.
• Measuring the change in mass of a reactant over time.
• Colorimetry.
• Measuring the time taken for a colour change.

Question 9

What are two different method types that can be used to investigate reaction rates?

Answer 9

• Initial-rate method: Experiments are carried out where different initial concentrations of one reagent are used.
• Continuous monitoring method: The concentration/ volume is continuously monitored so that a concentration or volume time graph can be produced from the data.

Question 10

How do you calculate the rate from a first order concentration-time graph?

Answer 10

-Draw a tangent at the time you want to calculate the rate of reaction for.
- The gradient of this tangent will equal the rate of reaction.

Question 11

How often do you calculate k from a first order rate-concentration graph?

Answer 11

• Calculate the gradient of the line of best fit.
• The gradient will equal the rate constant, k.

Question 12

How do you calculate the gradient of a line/ tangent?

Answer 12

Gradient = change in y / change in x

Question 13

What is a rate-determining step?

Answer 13

• The slowest step of the reaction.
• Only the species that take part in the rate determining step (or steps that take place before it) affect the rate.

Question 14

What is the relationship between the rate-determining step and the orders with respect to the reactants?

Answer 14

• The species present in the rate equation are those that take part in the rate-determining step.
• For any reactant in the rate equation, the order related to it tells you how many molecules of it are involved in the rate-determining step.

Question 15

How does iodine react with propanone in acid?

Answer 15

CH3COCH3(aq) + I2(aq) —> CH3COCH2I(aq) + HI(aq)

• Solution turns yellow to colourless.
• The order with respect to iodine is zero and the order with respect to propane and acid is first.
• Rate = k[CH3COCH3][H+]

Question 16

What equation can be used to calculate activation energy?

Answer 16

The Arrhenius equation