1.9 Rate equations

Question 1

Define reaction rate.

Answer 1

Reaction rate is the change in amount of reactant or product per unit time.

(in solution)
Change in concentration per unit time. Units will be mol dm-3s-1

Question 2

How can you work out reaction rate from the gradient of a graph?

Answer 2

(see page 66 in the revision guide)

Question 3

What is the rate equation for this reaction?

A+B -> C+D

Answer 3

(see page 67 in the revision guide)

Question 4

What different methods can you use to measure the rate of reaction by continuous monitoring?

Answer 4

• Gas volume
• Loss of mass
• Colour change
• Change in pH

(see page 70 in the revision guide to see how)

Question 5

How does the shape of a rate-concentration graph tell you the order of the reaction?

Answer 5

(see page 71 in the revision guide)

Question 6

What is the rate determining step?

Answer 6

The slowest step in a multi-step reaction.

see page 70 for more explanation

Question 7

What information does the order of a reaction provide about the rate determining step?

Answer 7

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

(see page 70 in the revision guide)

Question 8

What is the generalised rate equation for this reaction:

aA + bB -> products

Answer 8

(see page 1 in the chemrevise revision guide for the rate equation)

Question 9

What is the symbol for rate?

Answer 9

r

Question 10

What is the usual unit of rate (r)?

Answer 10

The unit of r is usually mol dm-3 s-1

Question 11

If r = k[A]0 = k what effect does the concentration of A have on the rate of reaction?

Answer 11

For zero order: The concentration of A has no effect on the rate of reaction.

Question 12

If r = k[A]1 what effect does the concentration of A have on the rate of reaction?

Answer 12

For first order: The rate of reaction is directly proportional to the concentration of A.

Question 13

If r = k[A]2 what effect does the concentration of A have on the rate of reaction?

Answer 13

For second order: The rate of reaction is proportional to the concentration of A squared.

Question 14

Draw a general graph for r = k[A]0

Answer 14

(see page 1 in the chemrevise revision guide)

Question 15

Draw a general graph for r = k[A]1

Answer 15

(see page 1 in the chemrevise revision guide)

Question 16

Draw a general graph for r = k[A]2

Answer 16

(see page 1 in the chemrevise revision guide)

Question 17

For a first order overall reaction what is the unit of k?

Answer 17

s-1

The units depend on the overall order of the reaction. It must be worked out from the rate equation.

Question 18

For a second order overall reaction what is the unit of k?

Answer 18

mol-1dm3s-1

Question 19

For a third order overall reaction what is the unit of k?

Answer 19

mol-2dm6s-1

Question 20

What happens to the value of k if we increase the temperature?

Answer 20

The value of k refers to a specific temperature and it increases if we increase temperature.

The value of k is independent of concentration and time. It is constant at a fixed temperature.

Question 21

What is k in a rate equation?

Answer 21

k is called the rate constant.

Question 22

Answer the example question labelled 1 on page 2 in the chemrevise revision guide.

Answer 22

correct?

Question 23

What is the method called if we follow one experiment over time recording the change in concentration?

Answer 23

When we follow one experiment over time recording the change in concentration we call it a continuous rate method.

The gradient represents the rate of reaction. The reaction is fastest at the start where the gradient is steeped. The rate drops as the reactants start to get used up and their concentration drops. The graph will eventually become horizontal and the gradient becomes zero which represents the reaction having stopped.

(see page 2 in the chemrevise revision guide for an example graph)

Question 24

What are the positives and negatives of using a gas syringe for measuring rate of reaction?

Answer 24

Measuring the change in volume of a gas works if there is a change in the number of oles of gas in the reaction. Using a gas syringe is a common way of following this. It works quite well for measuring continuous rate but a typical gas syringe only measures 100ml of gas so you don’t want a reaction to produce more than this volume. Quantities of reactants need to be calculated carefully.

(see page 2 in the chemrevise revision guide for a diagram of this apparatus)

Question 25

What is the initial rate of reaction? Where is it on a graph?

Answer 25

The initial rate is the rate at the start of the reaction, where it is fastest. It can be calculated from the gradient of a continuous monitoring conc vs time graph at time = zero.

Question 26

Describe the typical method used for calculation the rate of reaction for this reaction:

Mg = HCl -> MgCl2 + H2

Answer 26

Typical method:

1. Measure 5cm3 of 1.0moldm-3 of hydrochloric acid and add to a conical flask.
2. Set up the gas syringe in the stand.
3. Weigh 0.20g of magnesium.
4. Add the magnesium ribbon to the conical flask, place the bung firmly into the top of the flask and start the timer.
5. Record the volume of hydrogen gas collected every 15 seconds for 3 minutes.

Question 27

In reactions where there are several reactants, what is the rate of a reactant that is kept in a large excess in this reaction?

Answer 27

In reactions where there are several reactants, if the concentration of one of the reactant is kept in a large excess then that reactant will appear not to affect the rate and will be pseudo-zero order. This is because its concentration stays virtually constant and does not affect rate.

Question 28

Why is a measure of the initial rate of a reaction preferable?

Answer 28

A measure of initial rate is preferable as we know the concentrations at the start of the reaction.

Question 29

What does it tell us if different reactions have the same initial rate?

Answer 29

Different volumes of the same initial concentrations will have the same initial rate (if other conditions are the same) but will end at different amounts.

(see page 3 in the chemrevise revision guide for a graph example)

Question 30

What 2 different ways can be used to calculate the initial rates of reaction?

Answer 30

1. The initial rate can be calculated from taking the gradient of a continuous monitoring of conc vs time graph at time = zero.
2. Initial rate can also be calculated from clock reactions where the time taken to reach a fixed concentration is measured.

Question 31

Give an example of a common clock reaction.

Answer 31

(see page 3 in the chemrevise revision guide)

Question 32

How do you work out the orders of reactants from experimental initial rate data?

Answer 32

Normally to work out the rate equations we do a series of experiments where the initial concentrations of reactants are changed (one at a time) and measure the initial rate each time.

Question 33

Read the top of page 4 in the chemrevise revision guide.

Answer 33

Understand?

Question 34

Answer the example question labelled 1 on page 4 in the chemrevise revision guide.

Answer 34

correct?

Question 35

Answer the example question labelled 1 on page 5 in the chemrevise revision guide using the information just above it.

Answer 35

correct?

Question 36

Give the Arrhenius equation.

Rearrange it to give the equation for calculation the value of k.

Answer 36

(see page 6 in the revision guide) (too many subscripts for it to make sense on here)

Question 37

Answer the example question labelled 1 on page 6 in the chemrevise revision guide. (Information above it to help)

Answer 37

correct?

Question 38

What is the rate-determining step?

Answer 38

The slowest step in a reaction.

A mechanism is a series of steps through which the reaction progresses, often forming intermediate compounds. If all the steps are added together they will add up to the overall equation for the reaction.

Each step can have a different rate of reaction. The slowest step will control the overall rate of reaction. The slowest step is called the rate-determining step.

Question 39

How does the number of moles in each substance relate to the order of reaction?

Answer 39

The molecularity (number of moles of each substance) of the molecules in the slowest step will be the same as the order of reaction of each substance.

e.g.
0 moles of A in a slow step would mean A is zero order.
1 mole of A in the slow step would mean A is first order.

Question 40

Read example 1 on page 7 in the chemrevise revision guide.

Answer 40

understand?

Question 41

Read example 2 on page 7 in the chemrevise revision guide.

Answer 41

understand?

Question 42

Read example 3 on page 7 in the chemrevise revision guide.

Answer 42

understand?

Question 43

Read example 4 on page 7 in the chemrevise revision guide.

Answer 43

understand?

Question 44

Read example 5 on page 7 in the chemrevise revision guide.

Answer 44

understand?