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Session 8. Chemical reactions and why they happen

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This graph shows a simplified version of the change that takes place in the internal energy of the molecules as nitric oxide and carbon monoxide change into nitrogen and carbon dioxide. The x axis is labelled 'progress of reaction', and the y axis is labelled 'internal energy'. The higher (red) curve shows the change in the absence of a catalyst; the lower (blue) curve, the change when a catalyst is present.