Please help, this topic is about Hess's Law

Question

Calculate the standard enthalphy of formation for methane gas , CH4, produced from graphite and hydrogen gas according to the following reaction: C(graphite) + 2H2(g)  CH4(g)

Data given:

a. C(graphite) + O2(g)  CO2(g) ΔHo= -393.5 kJ

b. H2(g) + ½ O2(g)  H2O(l) ΔHo= - 285.8 kJ

c. CH4(g) + 2O2(g)  CO2(g) + 2H2O(l) ΔHo = -890.4kJ

Answer 1

Solution:

Data given:

a) C(s) + O₂ (g)→ ΔCO₂ (g); ΔH^o=-393.5 kJ....... (i)

b) H₂(g) + ½ O₂(g)→H₂O(l); ΔH^o=-285.8 kJ……… (ii)

c) CH₄(g) + 2O₂(g)→CO₂(g) + 2H₂O(l); ΔH^o = -890.4kJ ... ... (iii)

(i) + 2 x (ii) – (iii) gives

C(s) + 2 H₂(g) →CH₄(g)

After such rearrangement

The value of ΔH^o is = [(-393.5) + 2 x (-285.8) + 890.4] kJ = -74.7 kJ