Given that H2(g) + half O2(g) giving H2O(l) has an enthalpy change of -286 kJ, what is it for 2H2(g) + O2(g) giving 2H2O(l)?

1. Enthalpy change is an extensive property, so it scales with the amounts written. 2. Every coefficient in the second equation is twice the first, so the factor is 2. 3. Multiply the enthalpy change by the same factor: 2 times minus 286 kJ. 4. That gives minus 572 kJ. 5. Halving instead would give minus 143 kJ, which belongs to the half-scale equation. 6. The sign stays negative because the direction of the reaction has not been reversed. _Source: OpenStax Chemistry (CC BY 4.0), Ch 5 "Thermochemistry", section 5.3 Enthalpy_