Given the following thermochemical step equations below, calculate ΔH for the target reaction: PCl₅(g) → PCl₃(g) + Cl₂(g)

Question

1. - 5880 kJ
2. + 1000 kJ
3. + 250 kJ
4. - 250 kJ
5. – 1000 kJ

Answer 1

This is an example of Hess' Law.

Target equation: PCl₅(g) ==> PCl₃(g) + Cl₂(g)

Given:

(1). P₄(s) + 6Cl₂(g) ==> 4PCl₃(g) ... ∆H = -2440 kJ

(2). 4PCl₅(g) ==> P₄(s) + 10Cl₂(g) ... ∆H = +3440 kJ

Add equations (1) and (2) to get...

P₄(s) + 6Cl₂(g) + 4PCl₅(g) ==> 4PCl₃(g) + P₄(s) + 10Cl₂(g) ... ∆H = -2440 kJ + 3440 kJ = 1000 kJ

This reduces to 4PCl₅(g) ==> 4PCl₃(g) + 4Cl₂(g) ... ∆H = 1000 kJ

Divided this by 4 to get the target equation:

PCl₅(g) ==> Cl₃(g) + Cl₂(g) ... ∆H = 1000 kJ / 4 = +250 kJ (ANSWER 3)