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 = nCp
T, where Cp is the molar heat capacity at constant pressure (= 75.3 J C-1mol-1 for water).
q= qwater + qcalorimeter
Hess's Law
some properties of enthalpy
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When a reaction is written in reverse, the sign of H is reversed.
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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:
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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.)
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For a substance in solution the standard state is defined for a concentration of exactly one molar.
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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!).
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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
public domain image via Wikipedia Creative Commons
public domain image via Wikipedia Creative Commons
public domain image from NASA via Wikipedia Creative Commons
© R A Paselk
Last modified 4 March 2011