Energy changes
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I'm trying to work out the enthalpy change on:
2KHCO3 (s) -> K2CO3(s) + H2O(l)
I am aware that this standard enthalpy change of reaction is impossible without the use of a Hess' law cycle, but I need a method to determine how to work out this full cycle. If possible could you give me some places of reference to find this, or explain this?
Ulex writes
The first thing to mention is that your equation is incomplete. The thermal decomposition of potassium hydrogencarbonate produces carbon dioxide as well as potassium carbonate and water. It should read:
2KHCO3(s) -> K2CO3(s) + CO2(g) + H2O(l)
The next point is that there are two approaches to this problem and I am not sure which one you are looking for. I’ll do both.
1. This is entirely theoretical. Write the equation as above. At the third apex of the Hess triangle put all the elements in their states normal at 25°C and 1 atm. pressure. Now look up the enthalpy changes of formation for all the substances in the equation and enter them on the triangle. The result should look a bit like the one at the bottom of p106 in the Nuffield Chemistry Students’ Book only it will have more items on it. Now use Hess’ Law to calculate the enthalpy change you require.
2. This is the practical way. What you have to do is to think of something which you can do to the substances on both sides of the equation which will result in the same products. It has to be something practical and something which you can do in a calorimeter so that you can measure enthalpy changes. The simplest answer in this case is to react the potassium hydrogen carbonate with dilute HCl and, in a second experiment, to react potassium carbonate with dilute HCl. In both reactions the products are potassium chloride solution, carbon dioxide and water. As you are reacting a solid with an acid solution, you can put the acid in the calorimeter, measure its temperature, add the solid and allow the reaction to finish and them measure the temperature of the resulting solution. You should be able to work out suitable quantities to use (I’ll leave that to you) and use the measurements to work out the two enthalpy changes (that’s for you too!).
The third apex of the Hess triangle is potassium chloride, carbon dioxide and water. Solve your triangle to get the enthalpy change you want.
The two answers should agree!
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updated: 29 November 2004
