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Unformatted text preview: 18 A few comments on calorimetry We always need to think about how heat is defined in a calorimetry experiment. We never just blithely write down Q = nC Δ T and make the problem fit the formula. Let’s consider two kinds of calorimetry problems: 1. Heat transfer as two objects equilibrate. This kind of experiment is usually done in a “coffee cup” kind of system. There is no heat transfer to the surrounding. All heat transfer occurs between the objects that equilibrate inside the calorimeter. One object gains energy while the other loses energy. Melting ice, equilibrating hot iron in cold water, are all example of this process. 2. Heat given off or absorbed a chemical reaction. In this case, Q(reaction) = - Q(calorimeter). In a constant volume calorimeter, Q(reactants) = moles of reactants × Δ E per mole of reacts Q(calorimeter) = C(calorimeter) Δ T The calorimeter is a mixture of components – steel, etc. We generally don’ t know how many moles of each component are present, so we determine an average calorimeter constant that has units of Joules per degree (no moles) The most important issue is that we need to think rather than writing down...
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This note was uploaded on 02/13/2010 for the course CHM 29275 taught by Professor Farrar during the Spring '10 term at Rochester.
- Spring '10