A - Rate Equations

Question 1

What does the reaction rate tell you?

Answer 1

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

(How fast reactants are converted into products).

Question 2

How do you work out reaction rate from a graph?

Answer 2

It’s the gradient which is the change in y divided by the change in x.

Question 3

How do you work out the rate of reaction from a curved graph?

Answer 3

Draw a gradient at a particular point in the reaction and work out the gradient of the tangent. (It has the same gradient as the curve does at that point).

Question 4

What is the equation to find the rate of a reaction?

Answer 4

Change in concentration divided by time.

Question 5

The rate equation links the reaction rate to what?

Answer 5

The reactant concentrations.

Question 6

What does a rate equation tell you?

Answer 6

How the rate of reaction is affected by the concentrations of the reactants.

Question 7

What is the rate equation for the general reaction A + B —> C + D?

What are the units of rate?

Answer 7

Rate = k[A]^m[B]^n

Where m and n are the orders of reaction and k is the rate constant.

Units of rate are mol dm-3 s-1

Question 8

What is the order of reaction?

Answer 8

The power to which the concentration of a reactant is raised in the rate equation. It tells you how the concentration of the reactant in question affects the rate of reaction.

Question 9

When is the order of reaction with respect to A 0, 1 or 2?

Answer 9

If [A] changes and the rate stays the same, the order of reaction with respect to A is 0.

If [A] is proportional to rate, the order of reaction with respect to A is 1.

If the rate is proportional to [A]^2, the order of reaction with respect to A is 2. (So if [A] doubles, the rate will be 2^2=4 times faster.

Question 10

Using the general rate equation, what is the overall order of reaction?

Answer 10

m + n

Question 11

How do you work out the orders of reaction?

Answer 11

You can only find them from experiments, you can’t work them out from chemical equations.

Question 12

What is the rate constant, k?

Answer 12

A number that links the rate of reaction to the concentration of the reactants. The bigger the value of k, the faster the reaction.

Question 13

What happens to the rate constant if the temperature is increased?

Answer 13

The rate constant is always the same for a certain reaction at a particular temperature but if you increase the temperature, the rate constant will increase too.

Question 14

Explain why the rate constant must increase when temperature increases.

Answer 14

When you increase the temperature of a reaction, the rate of reaction increases. You’re increasing the number of collisions between reactant molecules, and also the energy of each collision.

But the concentrations of the reactants and the orders of reaction stay the same. So the value of k must increase for the rate equation to balance.

Question 15

What is the initial rate of reaction?

Answer 15

The rate at the start of the reaction.

Question 16

How do you find the initial rate of a reaction from a concentration-time graph?

Answer 16

By calculating the gradient of the tangent at time = 0.

Question 17

If a species has zero order, how is it included in the rate expression?

Answer 17

It’s not included.

Question 18

Briefly outline how to use the initial rates method to work out the rate equation.

Answer 18

1. Repeat an experiment several times using different initial concentrations of the reactants. But you should only change one concentration at a time, keeping the rest constant.
2. Calculate the initial rate for each experiment by finding the gradient of the tangent of the graph at time = 0.
3. See how the initial concentrations affect the initial rates of reaction to find the order for each reactant.

Question 19

You can measure the rate of reaction by continuous monitoring. What does this involve?

Answer 19

Following a reaction all the way through to its end by recording the amount of product or reactant you have at regular time intervals. The results can be used to work out how the rate changes over time.

Question 20

How can you find the order of reaction with respect to a certain reactant from a concentration-time graph?

Answer 20

1. Find the gradient at certain points on the graph to give you the rate at that particular concentration.
2. Plot each of these points on a new graph with the axes rate and concentration. Then draw a smooth line through points. The shape of the line tells you about the order of reaction with respect to that reactant.

Question 21

What will the rate-concentration graphs look like for zero, first and second orders?

Answer 21

Zero order - the line will be horizontal, meaning the changing concentration does not affect rate.

First order - a straight line through the origin, meaning the rate is proportional to the concentration of the reaction.

Second order - there will be an exponential curve, meaning the rate is proportional to the concentration squared.

Question 22

What does the Arrhenius equation show?

Answer 22

How the rate constant, k, varies with temperature.

Question 23

What is the formula for the Arrhenius equation? Give the meanings of each letter.

Answer 23

-Ea
———
RT
k = Ae

Where:
k = rate constant 
Ea = activation energy (J)
T = temperature (K)
R = gas constant (8.31 J K-1 mol-1)
A = Arrhenius constant

Question 24

What happens to the rate constant as Ea gets bigger?

Answer 24

It gets smaller.

Question 25

What does a large activation energy mean for the rate of reaction?

Answer 25

It will be slow.

Question 26

What happens to the rate constant as temperature increases?

Answer 26

K also increases.

Question 27

What do higher temperatures mean for a reaction rate?

Answer 27

The rate will increase.

Question 28

How do you get rid of the exponential in the Arrhenius equation?

Answer 28

By taking the natural log (ln) of everything in the equation.

Question 29

What are the axes of an Arrhenius plot?

What would the gradient be for this graph?

Answer 29

Y axis is ln k and the x axis is 1/T

The gradient is -Ea/R

Question 30

What is the rate-determining step?

Answer 30

The slowest step in a multi-step reaction.

Question 31

What is the overall rate of a multi-step reaction determined by?

Answer 31

The rate-determining step.

Question 32

What’s another name for the rate-determining step?

Answer 32

Rate-limiting step.

Question 33

How can the rate equation give information about the reactants in the rate-determining step?

Answer 33

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

If a reactant doesn’t appear in the rate equation, it won’t be involved in the rate determining step (and neither will anything that’s derived from it).

Question 34

Does the rate determining step have to be the first step in a mechanism?

Answer 34

No.

Question 35

What information do orders of reaction give about the rate determining step?

Answer 35

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.