Example: 1.40 g of vegetable oil is placed in a bomb calorimeter with excess oxygen and ignited with a spark. If the calorimeter temperature changes from 20.000 °C to 21.195 °C, find the energy released per gram of oil . The calorimeter contains 2.50 kg of water. The calorimeter without water has a heat capacity of 1.00 kJ°C^{1}.
q = nC_{p}T, where C_{p} is the molar heat capacity at constant pressure (= 75.3 J C^{1}mol^{1} for water).
q= q_{water} + q_{calorimeter}
Hess's Law
some properties of enthalpy

When a reaction is written in reverse, the sign of H is reversed.

The magnitude of H is directly proportional to the amount of reactants. Thus if the coefficients of a reaction are multiplied, then H is multiplied by the same amount.
Standard States:

The form of a pure substance stable at one atm and 25°C. (Actually other temperatures are tabulated, so have to check when looking at tabulated values.)

For a substance in solution the standard state is defined for a concentration of exactly one molar.

For an element the standard state is the form stable at one atm and 25°C. Note that many elements have allotropes: different forms of the pure element. For example carbon has 3 allotropes, the most common of which are graphite and diamond. Graphite is the stable form (a diamond is not forever at 1 atm and 25°C!).

The enthalpy of formation of a pure element in its standard state is defined to be 0.
public domain image from Louis E. Keiner via Wikipedia Creative Commons
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© R A Paselk
Last modified 4 March 2011