TL - Rates of Reaction, Rate equations and Reaction Mechanisms

Question 1

Give 5 ways in which the rate of reaction can be followed

Answer 1

1) pH
2) Gas Volume
3) Loss of Mass
4) Colour change
5) Titration

Question 2

Define reaction rate

Answer 2

The reaction rate is the change in the amount of reactants or products per unit time
If the reactants are in solution, the rate is the change in concentration per second (moldm⁻³s⁻¹)

Question 3

Describe when you can measure pH to monitor a reaction and how you would do it

Answer 3

If one of the reactants or products is an acid or base, you could follow the reaction by monitoring the pH of the reaction mixture
The easiest way is to use a pH meter or a pH probe connected to a datalogger

Question 4

Describe 2 ways in which you could monitor a reaction by the gas given off and give an example

Answer 4

1) Collecting it in a gas syringe and record the amount you have at regular time intervals
2) Recording the mass of a solution at regular intervals, if the reaction gives off gas (because it will lose mass)

Example: Reaction between an acid and a carbonate (CO₂ given off)

Question 5

Describe how you could monitor a reaction by its colour change and give an example

Answer 5

Using a colorimeter measures the strength of a colour of a solution by measuring the light absorbance

Example: - Iodine and Propanone
- Bromine Clock

Question 6

Describe how you could monitor a reaction by using titration

Answer 6

You can monitor the concentration of a reactant or product in a solution by taking small samples of the reaction mixture at regular time intervals and titrating them

Question 7

Explain why you need to slow down the rate of reaction of a sample when using titration to monitor a reaction and describe a method to achieve this

Answer 7

So that the sample does react further whilst you are trying to measure the concentration
By adding the sample to a large known volume of distilled water so that the solution becomes very dilute

Question 8

Give the equation to calculate the rate of a reaction from the loss of mass

Answer 8

Rate = loss of mass / time

Question 9

Give the equation to calculate the rate of a reaction from the volume of gas produced

Answer 9

Rate = volume of gas produced / time

Question 10

Give the equation to calculate the rate of a reaction from the colour of a solution

Answer 10

Rate = Change in absorbance / time

Question 11

Give the equation to calculate the rate of a reaction from the pH of a solution

Answer 11

[H⁺] = 10⁻ᵖᴴ
Rate = change in [H⁺] / time

Question 12

Give the equation to calculate the rate of a reaction from the concentration of a product or reactant

Answer 12

Rate = change in concentration / time

Question 13

Describe how you would find the rate of reaction at a point for a graph of mass/volume/absorbance/concentration against time

Answer 13

The gradient of the line (or tangent if the graph’s a curve) is proportional to the rate at that point in the reaction

Question 14

Describe the relevance of the sign before the calculated rate for the change in concentration of a substance

Answer 14

The sign simply shows whether you are measuring the reactants or the products:

• If the sign is negative, you’re measuring the reactant concentration (because reactants decrease over time)
• If the sign is positive, you’re measuring the product concentration (because the products increase over time)

Question 15

Define orders of reaction and how can they be found?

Answer 15

• Orders are the index to which its concentration term in the rate equation is raised (i.e. they tell you how a reactant’s concentration affects the rate)
• Orders can only be found experimentally

Question 16

Give the rate equation and units for the reaction

A + B → C + D

Answer 16

Rate = k[A]ᵃ[B]ᵇ

Units: moldm⁻³s⁻¹

Question 17

What is the overall order for the rate equation:
Rate = k[A]ᵃ[B]ᵇ
and what does k stand for?

Answer 17

• The overall order of reaction = m + n
• k is the rate constant (the bigger it is, the faster the reaction) which is always the same for a certain reaction at a specific temperature

Question 18

Explain the increase in temperature on the rate of reaction in terms of the rate constant

Answer 18

1) Increasing the temp. means particles have me KE so
- move faster and collide more often
- a greater proportion of them have the activation energy to react
2) This means the rate of reaction increases
3) According to the rate equation, the rate depends on the concentration of reactants and the rate constant. Since the concentrations don’t change, the rate constant must be changing
4) The rate constant is higher at higher temperatures

Question 19

How would you find the rate constant from the orders and rate of reaction?

Answer 19

1) Write out the rate equation
2) Insert the concentration and the rate. Rearrange the equation and calculate the value of k
3) Find the units for k by putting the other units into the rate equation

Question 20

Describe the Initial Rates Method to work out the Rate Equations

Answer 20

The initial rate of a reaction is the rate at the very start of the reaction.

1) Repeat the experiment several times using different initial concentrations of reactants, only changing one of the reactant concentrations at a time
2) Calculate the initial rate for each experiment by finding the gradient of the tangent at time t=0 for a graph of concentration vs. time
3) See how the initial concentrations affect the initial rates and find the order for each reactant

Question 21

Describe the effect on the initial rate of doubling the concentration of a:

i) 0 order reactant
ii) 1st order reactant
iii) 2nd order reactant

Answer 21

i) The rate stays the same
ii) The rate doubles
iii) The rate quadruples

Question 22

Define half-life (concentration)

Answer 22

The time taken for a reactant to half in quantity

Question 23

Describe the graphs of concentration vs. time for a:

i) 0 order reactant
ii) 1st order reactant
iii) 2nd order reactant

Answer 23

i) The rate doesn’t change as concentration falls - the graph is a straight line
ii) The graph is curved. The rate decreases as the concentration does, but the half-life (t¹′²) is constant
iii) The graph is curved. The half-life (t¹′²) increases as the reaction goes on

Question 24

What is the rate-determining step in a multi-step reaction?

Answer 24

The slowest step (AKA the rate-limiting step)

Question 25

Describe how you find which reactants from the chemical equation are involved in the rate-determining step

Answer 25

If a reactant appears in the rate equation, it must be affecting the rate. So this reactant, or something derived from it, must be in the rate-determining step

Question 26

State whether the following statements are true or false:

i) The rate-determining step is the first step in the mechanism
ii) The reaction mechanism can’t usually be predicted from just the chemical equation

Answer 26

i) False, it is not always the first step in the mechanism

ii) True

Question 27

How would you find the number of molecules of a reactant which are involved in the rate-determining step?

Answer 27

The order of a reaction with respect to a reactant shows the number of molecules of that reactant which are involved in the rate-determining step

Question 28

What can you predict from the rate equation?

Answer 28

The mechanism of the rate-determining step

Question 29

Give an example of when the rate equation has predicted a different mechanism to what was first suggested theoretically

Answer 29

The decomposition of Nitrogen (V) Oxide (N₂O₅)
It was thought that it decomposed as follows:
2N₂O₅ → 4NO₂ + O₂
But the reaction was experimentally found to be 1st order, so the rate equation is:
Rate = k[N₂O₅]
Therefore, there is only 1 molecule of N₂O₅ in the rate-determining step, so a possible mechanism is:
N₂O₅ → NO₂ + NO₃
NO₃ + N₂O₅ → 3NO₂ + O₂

Question 30

Describe how a an enzyme-catalysed reaction act differently to a chemical catalyst

Answer 30

1) In a very simple 1st order reaction, a substrate, S, might become a molecule of product P:
S → P
2) As the concentration of S increases, the reaction speeds up. Because it’s a 1st order reaction, doubling [S] should double the rate of reaction
3) However, when the above reaction is catalysed by an enzyme, the first doubling of [S] doubles the rate of reaction. Eventually though, the reaction reaches a stage where it can’t go any faster - no matter how you increase [S]

Question 31

Describe the graphs for an un-catalysed and an enzyme-catalysed reaction, and state the order

Answer 31

Enzyme-catalysed reactions change order as substrate is added

1) The un-catalysed reaction is first order so the graph is just a straight line
2) The enzyme-catalysed reaction is first order when the substrate concentration is low, but it changes as more substrate is added
3) Towards the end, the line of the graph levels out until it’s horizontal. At this point, the rate is no longer affected by the concentration of S. The reaction has become 0 order

Question 32

Explain why an enzyme-catalysed reaction changes order at different substrate concentrations

Answer 32

The order changes because the rate-determining step changes
Enzyme-catalysed reactions are split into 3 steps:
Step 1) S + E → ES
Step 2) ES → EP
Step 3) EP → E + P
1) When [S] is low, step 1 is the slowest reaction because the chance of collision is low
2) For a while, step 1 speeds up as you increase [S]
3) Eventually, you reach a stage where all the enzyme molecules have formed ES complexes (enzyme is saturated) So adding more S can’t make the reaction go any faster since there are no enzymes to attach to
4) The rate of reaction now depends on how fast ES can convert to EP so step 2 becomes the rate-determining step. The reaction is now 0 order with respect to S