Using the table below that shows some values for standard enthalpy changes of combustion, calculate the standard enthalpy change of the reaction, C(s) + 2H2(g) rightwards arrowCH4(g)

Question

Substance

ΔH°f (kJ/mol)

C(s)

−394

CH4(g)

−890

H2(g)

−286

Answer 1

The equation for the reaction is:

C(s) + 2H2(g) → CH4(g)

To calculate the standard enthalpy change of this reaction, we need to first find the enthalpy change for the formation of the products and reactants from their constituent elements.

For CH4(g), the enthalpy change of formation is given as -890 kJ/mol.

For H2(g), the enthalpy change of formation is given as -286 kJ/mol.

For C(s), the enthalpy change of formation is given as -394 kJ/mol.

Using Hess's law, we can relate these formation enthalpies to the enthalpy change of the combustion reaction:

ΔH°combustion = Σ ΔH°f(products) - Σ ΔH°f(reactants)

Substituting the formation enthalpies for the products and reactants, we get:

ΔH°combustion = [ΔH°f(CH4) + 2ΔH°f(H2)] - ΔH°f(C)

ΔH°combustion = [(-890 kJ/mol) + 2(-286 kJ/mol)] - (-394 kJ/mol)

ΔH°combustion = -890 kJ/mol + (-572 kJ/mol) + 394 kJ/mol

ΔH°combustion = -1068 kJ/mol

Therefore, the standard enthalpy change of the reaction C(s) + 2H2(g) → CH4(g) is -1068 kJ/mol.