Question

Identify the equation in which change in enthalpy is equal to change in internal energy.

Options

• \[\ce{2H2O2(l) -> 2H2O (l) + O2(g)}\]

• \[\ce{C(s) + O2(g) -> CO2(g)}\]

• \[\ce{PCl5(g) -> PCl3(g) + Cl2(g)}\]

• \[\ce{N2(g) + 3 H2(g) -> 2 NH3(g)}\]

Answer 1

\[\ce{C(s) + O2(g) -> CO2(g)}\]

Explanation:

Relationship between change in enthalpy and change in internal energy is given as:

ΔH = ΔU + Δn_gRT

In the given options,

(1) \[\ce{2H2O2(l) -> 2H2O (l) + O2(g)}\]  

Δn_g = 1

∴ ΔH = ΔU + RT

(2) \[\ce{C(s) + O2(g) -> CO2(g)}\]

Δn_g = 1 - 1 = 0

∴ ΔH = ΔU

(3) \[\ce{PCl5(g) -> PCl3(g) + Cl2(g)}\]

Δn_g = 2 - 1 = 1

∴ ΔH = ΔU + RT

(4) \[\ce{N2(g) + 3 H2(g) -> 2 NH3(g)}\]

Δn_g RT = 2 - 3 = - 1

∴ ΔH = ΔU - RT

Equation given in 2nd option has enthalpy change equal to internal change.