CI - Orders and rate equations

Question 1

What does the rate equation link?

Answer 1

Reaction rate to reactant concentrations.

Question 2

What do rate equations tell you?

Answer 2

How the rate is affected by the concentrations of reactants.

Question 3

For a general reaction: A + B –> C + D, what is the rate equation?

Answer 3

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

Question 4

What do the m and n mean in a rate equation?

Answer 4

They are the orders of reaction with respect to reactant A and reactant B.

m tells you how the concentration of reactant A affects the rate and n tells you the same for reactant B.

Question 5

What do square brackets mean?

Answer 5

The concentration of whatever’s inside them.

Question 6

what is ‘k’ in the rate equation?

Answer 6

The rate constant.

Question 7

What does the size of the rate constant ‘k’ tell you about the rate of reaction?

Answer 7

The bigger it is, the faster the reaction.

Question 8

What is important to know about k the rate constant?

Answer 8

It is always the same for a certain reaction at a particular temperature.

The units of k vary, so you have to work them out.

Question 9

What can you do if you know the rate constant and orders?

Answer 9

Calculate the rate.

Question 10

What do orders in a rate equation tell you?

Answer 10

How a reactant’s concentration affects the rate.

Question 11

When will the order of reaction with respect to A be 0?

Answer 11

If [A] changes and the rate stays the same.

Question 12

When will the order of reaction with respect to A be 1?

Answer 12

If the rate is proportional to [A].

Question 13

When will the order of reaction with respect to A be 2?

Answer 13

If the rate if proportional to [A]^2.

Question 14

How do you work out the overall order of reaction?

Answer 14

The sum of the orders of all the different reactants.

Question 15

What is the only place you can find the orders of reaction from?

Answer 15

Experiments.

Question 16

How do you find out the orders of the reactants?

Answer 16

• Think about the general formula: A+B–>C+D
• Look at how each reactant affects the rate one by one.
• Use the experimental data by:
  1. Use the initial rates method to construct a rate-concentration graph and examine its shape.
  2. Use the initial rates method to directly compare the initial rate for different concentrations of A.
  3. Continuously monitor the change in concentration of A against time and construct a concentration-time graph. Use the graph to compare successive half-lives for the reaction.

Question 17

What needs to be present to work out the individual orders of the reactants using initial rates methods?

Answer 17

You need to have all of the reactants that you’re not investigating present in excess so that the concentration of what you are not investigating won’t change much during the reaction and will pretty much remain constant. Therefore, any change in the rate can only be due to the change in concentration of the reactant you are investigating.

Question 18

Why do you need to have all of the reactants that you’re not investigating present in excess when working out the orders of the reactants?

Answer 18

So that the concentration of what you are not investigating won’t change much during the reaction and will pretty much remain constant. Therefore, any change in the rate can only be due to the change in concentration of the reactant you are investigating.

Question 19

What does the shape of a rate-concentration graph tell you?

Answer 19

The order for a reactant.

Question 20

How do you use your concentration-time graph to construct a rate-concentration graph?

Answer 20

1) Find the gradient, which represents the rate, at various points along the concentration-time graph.
2) On a new graph, plot these rate values against concentration. Join up the points with a line or smooth curve.

Question 21

What can you use to construct a rate-concentration graph to work out the order of reaction for a reactant?

Answer 21

Your concentration-time graph.

Question 22

What does a horizontal line on a rate-concentration graph mean?

Answer 22

Means that the changing concentration doesn’t change the rate so it is order 0.

Question 23

What does a straight line through the origin on a rate-concentration graph mean?

Answer 23

The rate is proportional to [X], and it is order 1.

Question 24

What does a curved line on a rate-concentration graph mean?

Answer 24

It is second order as the rate will be proportional to [X]^2.

Question 25

What can you calculate from the order and rate of reaction?

Answer 25

Rate constant.