Question

Calculate the standard enthalpy of formation of methane (CH4) given the following enthalpy data: C(graphite) + 2H2(g) → CH4(g)   ΔH° = -74.8 kJ/mol ΔH°f[C(graphite)] = 0 kJ/molΔH°f[H2(g)] = 0 kJ/mol (Hint: Use Hess's Law and the fact that the standard enthalpy of formation of an element in its most stable state is zero.)

Answer 1

We can use Hess's Law to calculate the standard enthalpy of formation of methane  CH4 . Hess's Law states that the total enthalpy change of a reaction is the sum of the enthalpy changes of its individual steps, regardless of the order in which they occur.In this case, we are given the following enthalpy data:C graphite  + 2H2 g   CH4 g    H = -74.8 kJ/molHf[C graphite ] = 0 kJ/molHf[H2 g ] = 0 kJ/molSince the standard enthalpy of formation of an element in its most stable state is zero, we can write the reaction as:Hf[CH4 g ] = Hf[C graphite ] + 2Hf[H2 g ] + HWe know that Hf[C graphite ] = 0 kJ/mol and Hf[H2 g ] = 0 kJ/mol, so the equation simplifies to:Hf[CH4 g ] = 0 + 0 +  -74.8 kJ/mol Hf[CH4 g ] = -74.8 kJ/molTherefore, the standard enthalpy of formation of methane  CH4  is -74.8 kJ/mol.