5.2.2—Calculations: cycles and relevant energy terms

In an energy cycle, arrows represent enthalpy changes between the same states. Choose a route to the target, then combine the signed values along that route.

Write the target reaction explicitly, align intermediate species, and apply reversal/multiplication rules before calculating. A final sign check should match whether the target is exothermic or endothermic.

If direct A→C is unknown, but A→B=−80 kJ mol⁻¹ and B→C=+25 kJ mol⁻¹, then ΔH(A→C)=−55 kJ mol⁻¹.

Do not treat every number around a cycle as positive. A value belongs to its arrow direction; reversing that arrow reverses its sign.