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Finding enthalpy of reaction and predicting

Hi again!

If you cannot view the question in the attachment, please follow this link.

Now my question is that, is the answer to (b) endo- or exo-thermic? The answer to the book says that it is exothermic, but an edexcel expert says that the book is wrong. Can you please clarify...

I have calculated the value of the enthalpy of the reaction.

Bonds Broken:
2H2O
Bonds Made:
2H­­2 and CO2

Therefore, the enthalpy of the reaction:

4(464)-[2(435.9)+2(805)] = -2595.2 KJmol­-1

Thus proving that the reaction is exothermic, but what I am concerned about is whether I have made a mistake anywhere.

Thank You all in advance.

kingchemist Thu, 01/13/2011 - 12:06

You are forgetting that you will also have to break up 1 mole of solid Carbon into 1 mole of gaseous C atoms (sublimation energy). This is an endothermic process. THis may be the same of atomisation energy.

However, looking at the link it appears that Kp/kPa increases with increasing temperature. To me this suggest that the number of gases is increasing as the temperature increases - and this would happen with an exothermic reaction.

The Dude 321 Thu, 01/13/2011 - 19:23

The Examiner sent me a reply, please see if it is correct.

1 The fact that Kp increase with increasing temperature is related to the fact that the reaction is endothermic in the forward direction. The relevant equation is called the van't Hoff isochore and is a well-established idea in chemical thermodynamics.

2 The number of gases is completely irrelevant. It is the pressure value in the numerator compared with the denominator that matters, not how many gases contribute to that pressure.

3 There is no relationship between the number of gases produced and the endo- or exothermic nature of a reaction.

4 Your numerical result from your equation is not correct: 4(464) - [2(435.9) + 2(805)] = - 625.8, not -2595.2 KJmol-1

5 Your calculation is in principle incorrect. The use of bond energies is based on Hess' Law, where the alternative route is the atomisation of each of the products (endothermic) and the re-assembly of the atoms into different molecules (exothermic). You have not included the atomisation energy of the carbon, which is +717 kJ mol-1. The correct calculation is therefore: {(4 x 464) + 717} - {(2 x 805) + (2 x 436)} = + 91 kJ mol-1. Endothermic, you will agree.

6 The answers in the book are not Edexcel answer schemes. They are answers written by the book's authors when they wrote the book, not by Edexcel examiners, all of whom I know.

7 The answer given in the book is wrong. A typographical error, no doubt, but wrong nonetheless.