The standard heat of formation of carbon disulphide (l) given that the standard heat of combustion of carbon(s) , sulphur(s) and carbon disulphide(l) are -393.3, -293.72 and -1108.76 kJ mol⁻¹ respectively is

Options

(a) 128.02 kJ mol⁻¹
(b) 12.802 kJ mol⁻¹
(c) -128.02 kJ mol⁻¹
(d) -12.802 kJ mol⁻¹

Correct Answer:

128.02 kJ mol⁻¹

Explanation:

(i) C(s) + O₂(g) → CO₂(g); ΔH₁ = -393.3 kJ.
(ii) S(s) + O₂(g) → SO₂(g); ΔH₂ = -293.72 kJ.
(iii) CS₂(l) + 3O₂(g) → CO₂ + 2SO₂ ; ΔH₃ = -1108.76 kJ.
On adding (i) and (ii) and subtracting (iii), we get,
C(s) + 2S(s) → CS₂(g), ΔH = -393.3 + 2 (-293.72) + 1108.76 = +128.02 kJ mol⁻¹.