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Whn substances react hat may be absorbed from or given up to the environment. To determine how much, we need some measurements of how much heat is needed to form each of the substances in the reaction. The net heat produced or absorbed is the sum of the heats to make the products minus the sum of the heats to make the reactants. To calculate this we need a table of standard heats of formation.
This is a simple equation. The heat of formation of the following reaction has been measured:
1/2 H2 + 1/2 Cl2 > HCl produces 22.1 kcal/mole or 92.3 kJ/mole
The equation you've listed, which should be balanced, is
H2 + Cl2 > 2 HCl
The produces twice the amount of heat when it occurs so simply double the amounts shown above. Tables often list amounts of heat liberated as negative and amounts of heat absorbed as positive.
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