Molar enthalpy question

Question

Define each word in the term standard molar enthalpy of formation. Use the equation as written below to determine the standard molar enthalpy of formation for Cr2O3(s).

2Cr2O3(s) + 2256.8 kJ → 4Cr(s) + 3O2(g)

Answer 1

Standard refers to the standard state of each element or compound; 25°C and standard pressure (1 atmosphere).

Molar refers to the heat of formation per mole.

Enthalpy is the heat absorbed or given off in a process at constant pressure conditions.

The reaction equation as written indicates that heat is absorbed for the decomposition of the compound to its elements. Written in reverse, heat is given off.

The heat of formation equation is defined as the heat given off or absorbed when a compound in its standard state is formed from its constituent elements in their standard states. Note that the heats of formation of elements in their standard states are zero.

Writing the chemical equation in reverse then gives

4Cr(s) + 3O2(g) → 2Cr2O3(s) ΔH^o_f = - 2256.8 for 2 moles of Cr2O3 or - 1128.4 for one mole.