Sigma Bond
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Hi,

Recently had a mock exam on the F322 OCR A Chemistry, June 2013 paper. I'm really confused with these questions about enthalpy profile diagrams and was wondering if anyone could explain this to me.

The question is:

2N2 + O2 <--> 2N2O

It then gives an enthalpy profile which is endothermic. It states the activation energy is +447 kJ mol-1 and the enthalpy change is 164 kJ mol-1.

(i) calculate the enthalpy change when 240dm3 of N2O measured at RTP, is formed fro N2 and O2.

(Answer is 820)

(ii) What is the enthalpy change of formation of N2O?

(Answer is 82)

(iii) Calculate the activation energy for the reserve reaction.

(answer is +283)

I have a rough idea how to get the answers but I feel like i need to consolidate this knowledge. If anyone could explain it to me then it would be greatly appreciated
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JacobAlevels
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(i) 240dm3 is 10 moles of any gas. we've already got 2 moles of gas in the equation. to make that ten we'll have to multiply by 5 (2x5=10) so the enthalpy change is also multiplied by 5. 164x5=820.
(ii) enthalpy of formation stands for when 1 mol of a compound is made under s.t.p. the equation says two moles are made. so to get the enthalpy change when 1 mol is made, just divide by 2. 164/2=82.
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JacobAlevels
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oh and (iii) the activation energy of the reverse of any reaction is always the activation energy of the forward reaction minus the overall enthalpy change of the reaction (because the enthalpy of a reaction is actually the activation energy of the forward reaction minus the activation energy of the reverse reaction so just rearrange the equation). so 447-164=284.
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Sigma Bond
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(Original post by JacobAlevels)
oh and (iii) the activation energy of the reverse of any reaction is always the activation energy of the forward reaction minus the overall enthalpy change of the reaction (because the enthalpy of a reaction is actually the activation energy of the forward reaction minus the activation energy of the reverse reaction so just rearrange the equation). so 447-164=284.
Thanks a lot
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