Chemistry » Chemical Equilibrium » Le Chatelier's Principle

# Rate-Time Graphs

## Rate-time graphs

• A change in concentration of a substance would favour the reaction that decreases the amount of that substance. This will appear as a sharp increase in the rate of either the forward or reverse reaction and a sharp decrease in the rate of the other reaction.

The increased rate will then gradually decrease and the decreased rate will gradually increase until they are equal again.

• A change in pressure of the reaction would cause a sharp increase or decrease in all the reactants and products. This will have the same effect as a change in concentration, although the increase or decrease would be more gradual.

• A change in temperature of a substance would affect both rates in the same direction (either both increase or both decrease). However, the effect will be unequal, with the endothermic reaction favoured by an increase in temperature, and the exothermic reaction favoured by a decrease in temperature.

• The addition of a catalyst would favour both the forward and reverse reactions by the same amount.

## Example: Rate-Time Graphs

### Question

For the reaction $$2\text{AB}(\text{g})$$ $$\rightleftharpoons$$ $$2\text{A}(\text{g}) + \text{B}_{2}(\text{g})$$, $$\Delta$$H = $$\text{26}$$ $$\text{kJ}$$

the following graph can be plotted:

What stress has occurred in this system? Label the graph with what is happening at each stage.

### Step 1: Check the axes so that you know what the variables are on this graph

The axes are labelled rate and time. Therefore this is a rate-time graph.

### Step 2: Are the rates both affected in the same way or is one rate increased and the other decreased when the stress is applied?

Both rates are affected in the same way (increased) therefore the stress must be a catalyst or a change in temperature.

(A change in pressure or concentration would favour one reaction direction only)

### Step 3: Are both rates affected equally?

No, the forward rate is increased more than the reverse rate. Therefore the stress must be a change in temperature.

(A catalyst would increase both rates equally)

### Step 4: Was the temperature increased or decreased? (Use Le Chatelier’s principle)

The forward reaction is endothermic ($$\Delta$$H is positive). The forward reaction was favoured more than the reverse reaction. An increase in temperature will favour the reaction that cools the reaction vessel (the endothermic reaction). Therefore the stress must have been an increase in temperature.

## Example: Rate-Time Graphs

### Question

What is responsible for the change at t = $$\text{10}$$ $$\text{minutes}$$ in the graph below?

### Step 1: Check the axes so that you know what the variables are on this graph

The axes are labelled rate and time. Therefore this is a rate-time graph.

### Step 2: Are the rates both affected in the same way or is one rate increased and the other decreased when the stress is applied?

Both rates are affected in the same way (increased) therefore the stress must be a catalyst or a change in temperature.

(A change in pressure or concentration would favour one reaction direction only)

### Step 3: Are both rates affected equally?

Yes, both rates are increased by the same amount.

### Step 4: What was responsible for the change at t = $$\text{10}$$ $$\text{minutes}$$?

The addition of a catalyst (a change in temperature would affect both rates, but unequally).