WebLecture: Hess's Law in Practice
Kotz and Triechel, Chemistry and Chemical Reactivity Chapter 5: Sections 6-8.
5.6 Calorimetry is a method for determining the change in enthalpy during a reaction. We isolate the reaction system within a calorimetry measuring system, which itself is isolated from the environment (by insulation to prevent energy flows into or out of the calorimeter). We measure the temperature change of the calorimeter, which tells us the energy flow into or out of the reaction system, that is, the change of enthalpy of the reaction.
5.7 Hess's Law is pretty straightforward: if you want to find out what the net result of a bunch of equations is, add them up (including their heats of reaction)! You may need to multiply through some of the equations to get the numbers of moles to balance, in which case you multiply the heat of reaction by the same factor. You may need to reverse a reaction to get the proper molecules to cancel out and give you the desired result, in which case (since you are running the reaction backwards), you change the sign on the heat of reaction. Study the examples carefully so that you understand how to combine the reactions.
To determine changes, we need to know the energy involved in forming molecules from their base elements. Once we know these "energies of formation" for both the reactants and the products, we can determine the energy change between the two (because it is a state change and Hess's law holds). We can use this information to predict whether a new and untried reaction will produce or absorb a given amount of energy. Chemists have spent a lot of time ascertaining these amounts (there is a list on p. 270)....
5.8 When we write a reaction equation, the arrow direction shows us which way the reaction runs, from reactants to products. In fact, ALL reactions can potentially move either way, depending on whether energy is available from the environment or not. A reaction in temperature equilibrium with its environment will tend to run in the direction which releases energy to the environment and lowers the energy state of the system. So a reaction which is exothermic (the product state is lower than the reactant state) is product-favored. A reaction which is endothermic (the product state is higher than the reactant state) is reactant-favored; that is, the reaction will run "backwards" from the way we've written it.
ΔrH° = Σ ΔfH° (products) - Σ ΔfH° (reactants)
To find the total enthalpy change for a reaction, we add up the heats of formation of the products and subtract the sum of the heats of formation of the reactants.
Review the Videos at Thinkwell Video Lessons under THERMOCHEMISTRY.
Use your calorimeter to determine the enthalpy change associated with a specific reaction.
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