A needed step is only available in reverse, so a chemist reverses it and multiplies it by 2. If the original step had ΔH = -45 kJ, what is ΔH for the manipulated step?

Core Concept

Each manipulation acts independently on ΔH. Reversing an equation flips the sign, taking -45 kJ to +45 kJ. Multiplying the equation by 2 scales the enthalpy by the same factor because ΔH is extensive, giving +45 × 2 = +90 kJ. Both operations commute, so their order does not matter, which is what makes Hess's-law bookkeeping so dependable.