Question

Given the thermochemical equations:

A(g)⟶B(g)Δ?=60 kJ

B(g)⟶C(g)Δ?=−110 kJ

find the enthalpy changes for each reaction.

3A(g)⟶3B(g)Δ?=______kJ

B(g)⟶A(g)Δ?=_______kJ

A(g)⟶C(g)Δ?=_______kJ

Answer 1

1)
This reaction is 3*1st reaction
So,
ΔHo rxn = 3*given ΔH1o rxn
= 3*(60.0)
= 180.0 KJ
Answer: 180 KJ

2)
This reaction is -1*1st reaction
So,
ΔHo rxn = -1*given ΔH1o rxn
= -1*(60.0)
= -60.0 KJ
Answer: -60 KJ

3)
Lets number the given reactions as 1, 2 from top to bottom
required reaction should be written in terms of other reaction
This is Hess Law
required reaction can be written as:
Required reaction = +1 * (reaction 1) +1 * (reaction 2)

So, ΔHo rxn for required reaction will be:
ΔHo rxn = +1 * ΔHo rxn(reaction 1) +1 * ΔHo rxn(reaction 2)
= +1 * (60) +1 * (-110)
= -50 KJ
Answer: -50 KJ