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Reaction kinetics (CHEM 2 past paper q) 1 mark

it's question 1aiv) on this link: (you also need to use the enthalpy diagram from question 1a)

http://filestore.aqa.org.uk/subjects/AQA-CHEM2-W-QP-JUN10.PDF

I don't understand why the overall value for the enthalpy change is p - q. I understand that the overall enthalpy change must be negative since the reaction is exothermic, but then why can't the expression just be -p - q, as this would also give a negative value?

Thank you.

(edited 10 years ago)

Reply 1

EierVonSatan

The diagram has enthalpy on the left hand axis, the enthalpy change is the difference between the products and the reactants. It's not just any old negative value.

Reply 2

Magenta96

Original post by EierVonSatan

The diagram has enthalpy on the left hand axis, the enthalpy change is the difference between the products and the reactants. It's not just any old negative value.