Find the ΔH for the reaction below, given the following reactions and subsequent ΔH values: H2SO4(l) → SO3(g) + H2O(g)

Question

Find the ΔH for the reaction below, given the following reactions and subsequent ΔH values: H₂SO₄(l) → SO₃(g) + H₂O(g)

 

H2S(g) + 2O2(g) → H2SO4(l)	ΔH = -235.5 kJ/mol
H2S(g) + 2O2(g) → SO3(g) + H2O(l)	ΔH = -207 kJ/mol
H2O(l) → H2O(g)	ΔH = 44 kJ/mol

Answer 1

Hess' Law: rearrange the given equations to obtain the target equation.

TARGET EQUATION: H2SO4(l) ==> SO3(g) + H2O(g)

Given:

(1) H₂S(g) + 2O₂(g) → H₂SO₄(l) ... ∆H = -235.5 kJ/mol

(2) H₂S(g) + 2O₂(g) → SO₃(g) + H₂O(l) ... ∆H = -207 kJ/mol

(3) H₂O(l) → H₂O(g) ... ∆H = 44 kJ/mol

Procedure:

reverse (1): H2SO4(l) ==> H2S(g) + 2O2(g) ... ∆H = +235.5

Copy (2): H2S(g) + 2O2(g) ==> SO3(g) + H2O(l) ... ∆H = -207

Copy (3): H2O(l) ===> H2O(g) ... ∆H = 44

-----------------------------------------------------------------------add them up and combine and/or cancel like terms...

H2SO4(l) + H2S(g) + 2O2(g) + H2O(l) ==> H2S(g) + 2O2(g) + SO3(g) + H2O(l) + H2O(g)

H2SO4(l) ==> SO3(g) + H2O(g) = TARGET EQUATION

∆H = 235.5 + (-207) + 44

∆H = 72.5 kJ/mol