The table below shows some values for standard enthalpy changes of combustion.
Substance
incrementHc Ө / kJ mol−1
C(s)
−394
H2(g)
−286
CH4(g)
−890
Use these values in the table above to calculate the standard enthalpy change of the reaction
C(s) + 2H2(g) ® CH4(g)
-210 kJ
-604 kJ
-76 kJ
-318 kJ
The correct calculation is:
ΔH = ΔHf (products) - ΔHf (reactants)
ΔH = [(-890 kJ/mol) - (2*(-286 kJ/mol))] - (-394 kJ/mol)
ΔH = (-890 kJ/mol + 572 kJ/mol) - (-394 kJ/mol)
ΔH = -318 kJ/mol
Therefore, the standard enthalpy change of the reaction C(s) + 2H2(g) ® CH4(g) is -318 kJ. The option closest to this value is -318 kJ.
To calculate the standard enthalpy change of the reaction C(s) + 2H2(g) → CH4(g), we need to sum up the standard enthalpy changes of combustion for each individual substance.
The standard enthalpy change for the combustion of C(s) is -394 kJ/mol.
The standard enthalpy change for the combustion of 2H2(g) is 2 times the value for H2(g), which is -2 * -286 kJ/mol = 572 kJ/mol.
Adding the standard enthalpy changes for each substance:
-394 kJ/mol + 572 kJ/mol = 178 kJ/mol.
Therefore, the standard enthalpy change of the reaction C(s) + 2H2(g) → CH4(g) is 178 kJ/mol.