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If you haven't already done so, please review the section on Calorimetry.
The science of heat or energy flow in a chemical reaction is called
thermochemistry. There are three laws associated with thermochemistry.
They are as follows:
- The heat flow associated with a reaction is directly proportional
to the amount of material undergoing the reaction.
- The heat flow for a reaction is equal in magnitude but opposite
in sign for the reverse reaction
- Hess's Law: If a reaction can be expressed as the sum
of two or more independent reactions, then the heat flow for the
reaction is equal to the sum of the heat flows for the independent
reactions.
Suppose you wanted to determine the magnitude of the heat flow
for a reaction, but you are unable to perform the reaction. The
reactants are unstable, and you might start a fire in the lab. What
can you do? Well, if you remember Hess's Law, you are in luck. Find
other reactions that are easier to perform and when summed up equal
your original desired reaction. According to Hess's Law, the sum
of the heat flows for these reactions is equal to the heat flow
for your original reaction.
Let's try an example to explain this better:
Calculate the heat change for the formation of 1.00mol of strontium
carbonate from it's elements. Given the following experimental information:
- If we are going to add these equations to get our desired equation,
we must first have a desired equation. You must ask yourself:
what are we looking for?
- The answer is the heat of formation of SrCO3. So,
we must write an equation which represents the heat of formation
of SrCO3. What is heat of formation?
- It is the heat required to make 1 mole of a compound from its
elements in their standard states.
- We are trying to make SrCO3. It is formed from Sr,
C and O2. Therefore, the balanced equation is:
- Now, looking at the experimental data, can we add those equations
together to get this equation?
- The answer is yes!
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