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For the reaction 2AB(g)2AB(g) ⇌⇌ 2A(g)+B2(g)2A(g)+B2(g), ΔΔH = 2626 kJkJ

the following graph can be plotted:


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

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?

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)

Was the temperature increased or decreased? (Use Le Chatelier’s principle)

The forward reaction is endothermic (ΔΔH 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.

Label what is occuring at each stage on the graph


s and Le Chatelier’s principle