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A change in concentration of a substance

RATE-TIME GRAPHS

What is responsible for the change at t = 1010 minutesminutes in the graph below?

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.

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)

Are both rates affected equally?

Yes, both rates are increased by the same amount.

What was responsible for the change at t = 1010 minutesminutes?

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

Concentration-time and mole-time graphs

• A change in concentration of a substance would appear as a sharp increase or decrease in the concentration or number of moles of that substance and a gradual change in the other substances.
• A change in temperature would affect both the forward and reverse reactions. However, one reaction would be affected more than the other. So the reactants and products would be affected gradually, in the opposite direction (one increased, the other decreased).
• A change in pressure of the reaction would cause a sharp increase or decrease in all the reactants and products. For an increase in pressure, if the forward reaction is then favoured the reactant concentrations will decrease, and if the reverse reaction is then favoured the product concentrations will decrease.
• The addition of a catalyst would increase both the forward and reverse reaction rates, meaning the equilibrium is reached faster. So if the reaction is already at equilibrium there will be no effect on a concentration-time or moles-time graph.