Question

C₂H₂ (g) + 3H₂ (g) --> 2CH₄ (g)

ΔH_f:    227                                  -75      (kJ/mol)

S°:    201            131               186      (J/mol K)

What will be the heat absorbed or released when 2.0 mol of C₂H₂ reacts at constant pressure according to the above reaction, measured in kJ?

Answer 1

Solution:

Given

ΔHf of C2H2 = 227 kJ/mol

ΔHf of H2 = 0 kJ/mol

ΔHf of CH4 = 186 kJ/mol

ΔHrxn = nΔHf(products) - nΔHf(reactants)

Where n is number of moles.

Reaction now becomes,

2C2H2 (g) + 6H2 (g) --> 4CH4 (g)

Knowing that the standard enthalpy of formation of any element in its most stable form is zero for H2.

ΔHrxn = 4 * 75 – (2*227 + 6*0)

            = -154 kJ

This is exothermic reaction. 154 kJ heat released released when 2.0 mol of C2H2 reacts at constant pressure.