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I'm having problems constructing a hess cycle in order to determine the enthalpy change for the thermal decomposition of potassium hydrogen carbonate. I have the 3 equations I need when KHCO₃ and K₂CO₃ are added to HCl. I'm having problems linking the equations. Can you help.

Ulex writes
It is difficult to do a diagram in this reply, so I'll try to explain how to construct the Hess cycle 'triangle'. Draw a downward pointing triangle on a piece of paper and fill in the parts I've described below on it.
 
The decomposition of potassium hydrogencarbonate is represented by the equation which is at the top of the Hess cycle.
 
Eqn.1: 2KHCO₃ -> K₂CO₃ + CO₂ + H₂O
 
Put the reactants at the left hand point of the Hess's Law 'triangle' and the products at the right hand point. To calculate the enthalpy change for this reaction you want to get from left to right by going around the other two sides of the triangle.
 
The two reactions involving HCl needed here are:
 
Eqn.2: 2KHCO₃ + 2HCl -> 2KCl + 2CO₂ + 2H₂O
 
Eqn.3: K₂CO₃ + 2HCl -> 2KCl + CO₂ + H₂O
 
Both these reactions end in the same products. Put 2KCl + 2CO₂ + 2H₂O below Eqn. 1, to form the bottom point of the triangle.
 
Now add 2HCl to both sides of Eqn.1. This does not change the reaction, or the enthalpy, but connects each side of it to the products below. So your Hess cycle calculation becomes, if you draw arrows in the appropriate directions:
 
Enthalpy change for Eqn.1 = Enthalpy change for Eqn.2 - Enthalpy change for Eqn.3. Don't forget that the enthalpy change for Eqn.2 is for TWO moles of KHCO₃.

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updated: 23 February 2007