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.
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.
Example: Find the value of H for the reaction:
2 CO2 + 7 H2 C2H6 + 4 H2O(g)
From Table find H values:
Often just list compounds, since known to be from elements.
| C + O2 CO2
|| H = -393.5 kJ mol-1
| 2 C + 3 H2 C2H6
|| H = -84.6 kJ mol-1
|H2 + 1/2 O2 H2O(g)
|| H = -241.8 kJ mol-1
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© R A Paselk
Last modified 8 March 2013