Call Us: US - +1 845 478 5244 | UK - +44 20 7193 7850 | AUS - +61 2 8005 4826

Graphs and Le Chatelier’s principle

Graphs and Le Chatelier’s principle
Graphs can be used to represent data about equilibrium reactions. The following are some points to keep in mind when presented with a graph.

Identify the type of graph by looking at the label on the y-axis. You will find either:
a rate-time graph
a mole-time or concentration-time graph
For rate-time graphs, when the rate for the forward reaction and the rate for the reverse reaction are equal, the system is in equilibrium. 7f16cebb4faf4c8acabb51f608562219.png
For concentration-time graphs or mole-time graphs equilibrium occurs where the concentration or number of moles of the reactants and products are constant. These values need not be equal to one another. 764b1c1dce0bfcba7f7ced47ae70771c.png
When calculating Kc make sure you only take values from the sections of the graph where the y-value is constant. Kc can only be calculated when the system is in equilibrium.
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.