• 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.

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

public domain image from Louis E. Keiner via Wikipedia Creative Commons

© R A Paselk

Last modified 8 March 2013