In order to calculate a H overall (net) using Hess's idea we need to be able to manipulate the steps of a reaction so that they can add up to the overall reaction.
Manipulating Steps
We can manipulate the steps of reactions in only two ways.
a) We can reverse the equations and write it backwards from product to reactant. When we do this the value of the H for the step changes sign. WHY?
b) We can multiply the equations by any common factor.When we do this we must also multiply the heat by the same factor.
for example
C(s) + O2(g) -----> CO2 (g) + 393.5 kJ
if two moles of CO2 (g) appeared in the overall reaction we might multiply this reaction by 2 to get:
2C (s) + 2 O2 (g) -----> 2 CO2 (g) + 787 kJ
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Important procedure
 Calculating Heats of Reaction using Hess's Law
1) Write the overall equation for the reaction if not given.
2) Manipulate the given equations for the steps of the reaction so they add up to the overall equation.
3) Add up the equations canceling common substances in reactant and product.
4) Add up the heats of the steps = heat of overall reaction.
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Examples of Hess's Law
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Click on the pencil to view several examples of Hess's Law calculations using the addition of the steps in a reaction.
Questions and Activities on Hess Law
a) Attempt the following question to check your understanding of Hess's Law calculation.
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Hess's Law calculations
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b) After doing the example above complete the following activity.
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Assignment : Hess's Law Calculations