The standard enthalpy of formation
"The enthalpy of formation is the energy change when 1 mole of a substance is formed from its constituent elements in their standard states"
Particular points to note:
- The elements are in their usual states under standard conditions. i.e at 25ºC and 1 atmosphere pressure (100.0 kPa).
- The enthalpy of formation of an element in its standard state is zero by this definition.
- The energy is given per mole of substance.
The standard enthalpy of formation is represented by the symbol ΔHf.
Standard enthalpy of formation equations
As the definition is according to standard states, these must be included in the equation. Once again, the key substance is the one being formed, so the elements themselves may, and often do, appear with fractional coefficients. The state symbols are important.
Example: Give the equation for the enthalpy of formation of sulfuric acid
The enthalpy of formation of sulfuric acid is represented by the following equation:
H2(g) + S(s) + 2O2(g) H2SO4(l) ΔHf = -811 kJ mol-1
Example: The enthalpy of formation of calcium carbonate is represented by the following equation:
Ca(s) + C(s) + 1½O2(g) CaCO3(s) ΔHf = -1207 kJ mol-1
Endothermic and exothermic compounds
The sign of the enthalpy of formation of a compound gives us the relative stability of the compound being formed with respect to its constituent elements.
A negative value tells us that it is an exothermic compound (compared to its elements). It DOES NOT tell us that the elements can be made to react directly together to form the compound
Similarly, a positive enthalpy of formation value tells us that the compound is relatively unstable with respect to its elements, it is an endothermic compound.
It DOES NOT mean that it is unstable, only that in comparison with its constituent elements its formation would require energy. Once again there is no implication regarding the possibility of direct formation from the elements.
Ethyne is an endothermic compound. The energy of ethyne with respect to its constituent elements in their standard states has a positive value.
i.e. the compound ethyne is at a higher energy level than the sum of its combined elements by 227kJ mol-1.
NOTE: The elements are at zero energy by definition. In reality, this does not mean that they have no chemical energy, just that we have decided to measure from this point.
Overlap between combustion enthalpy and formation enthalpy
It should be noted that the energy change 'labels' are given that best describes the specific change taking place. Usually this is unambiguous, but there are occasions where the enthalpy of formation of one substance is also the enthalpy of combustion of another.
For example, when sulfur is burned in excess oxygen under standard conditions, the enthalpy change for the process is:
S(s) + O2(g) SO2(g)
This equation represents the standard combustion enthalpy of sulfur. However, it also represents the standard enthapy of formation of sulfur(IV) oxide. Both descriptions are equally correct.
Many questions refer to the standard enthalpy of formation of water:
H2(g) + ½O2(g) H2O(l)
But this is also correctly described as the standard enthalpy of combustion of hydrogen.
Beware, though as this is not always the case. For example, the standard enthalpy of formation of sodium oxide is NOT the same as the standard combusion enthalpy of sodium.
2Na(s) + ½O2(g) Na2O(s)
In the above equation, one mole of sodium oxide is formed = ΔHf, but in the equation below
Na(s) + ¼O2(g) ½Na2O(s)
only 1 mole of sodium is combusted, therefore it is the standard combustion enthalpy of sodium, ΔHc.
The two equations are related by:
ΔHf(Na) = 2 x ΔHc(Na)