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Help a level chemistry

Help I don’t get why they found the energy released per mile of AgNo3, my working is on the whiteboard but I don’t get the markscheme I worked backwards from the delta h equals q/n equation to finding out delta t in the q equals mc delta t equation, which makes sense to me… plz help since I feel like this is a common q, which I can’t get my head around

(edited 12 months ago)

Reply 1

lordaxil

Firstly, check the calculation on final line of your blackboard - it's not numerically correct. Secondly, remember that the heat of reaction is for every 2 moles of silver nitrate. In your calculation, you have assumed it is per mole.

Reply 2

Alevelhelp.1

Original post by lordaxil

I don’t get it 😭

Reply 3

lordaxil

No problem - let me try to explain a different way. When you are given an enthalpy for a reaction in kJ/mol, then it is specific to the amount of reactants given in the chemical equation. For example, in the example reaction below:

A + B --> C + D DeltaH = -200 kJ/mol

then DeltaH refers to the enthalpy per mole of reactant A (or equivalently B, since they are equimolar).

However, if the reaction were:

A + 2B --> C + D DeltaH = -200 kJ/mol

instead then it would make a difference whether it was per mole of A or per mole of B, since there are 2 moles of B reacting with every mole of A. If you wanted the DeltaH per mole of B, then it would be half that of the reaction above, i.e. -100 kJ/mol.

In the original question above, the silver nitrate is controlling the extent of reaction (since Mg is in excess) so any heat released must be expressed per mole of silver nitrate, whereas the original equation was given per mole of Mg. So, you have to half the DeltaH.

Does that make sense?