- A formation reaction is a reaction where 1 mole of a species is formed from its elements in their standard states.
- The enthalpy of formation for an element in its standard state is 0 kJ/mol.
Imagine we wanted to build a molecule of octane (C8H18) from its constituent elements, carbon and hydrogen. To balance this reaction, we would need 8 carbon atoms and 9 hydrogen molecules (H2). The standard state for carbon is pure solid in the form of graphite, and that of molecular hydrogen (H2) is gas at a pressure of 1 bar. Putting all of this together, we can write the reaction to create one mole of liquid octane:
This type of reaction, where one mole of a species is formed from its elements in their standard states, is called a formation reaction. The enthalpy changes associated with formation reactions are called enthalpies of formation (ΔHf). Formation reactions are always at standard pressure (P = 1 bar, but some older tables will use P = 1 atm). Tabulated values will specify a temperature, usually T = 298 K. The enthalpy of formation for octane (i.e. the enthalpy change for the formation reaction above) at T = 298 K is ΔHf = –250 kJ/mol.
Consider the following pair of formation reactions, where the first reaction forms methanol in the liquid phase, and the second forms methanol in the gas phase, both at P = 1 bar and T = 298 K:
Which reaction releases the most energy?
Expand for answer:
At constant pressure, ΔH=q.
The first reaction has a more negative ΔH value, meaning that this reaction releases more energy as heat than the second reaction.
Consider the formation reaction for N2(g):
This is a null reaction (nothing changes). The final state (product) is identical to the initial state (reactant), provided the temperature and pressure of the system are constant. If the state of the system has not changed, the enthalpy change for this reaction must be zero (ΔHf = 0). Conceptually, this provides an important reference point: The enthalpy of formation for an element in its standard state is 0 kJ/mol.
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