Hess's law is a fundamental principle in thermochemistry that allows us to determine the overall enthalpy change of a reaction by rearranging thermochemical equations. A thermochemical equation is a chemical equation that includes the enthalpy of reaction, denoted as ΔHrxn. The key concept is that any modification to the original equation will directly affect the enthalpy of reaction.
For example, consider the thermochemical equation:
2 Mg(s) + O2(g) → 2 MgO(s) with ΔHrxn = -1204 kJ.
There are three primary operations that can be performed on this equation, each influencing ΔHrxn:
1. **Multiplying the Equation**: If we multiply the entire equation by a factor, say 2, the coefficients change accordingly:
4 Mg(s) + 2 O2(g) → 4 MgO(s)
In this case, the enthalpy change also doubles:
ΔHrxn = 2 × (-1204 kJ) = -2408 kJ.
2. **Dividing the Equation**: Conversely, if we divide the equation by 2, the coefficients adjust to:
1 Mg(s) + 1/2 O2(g) → 1 MgO(s)
Here, the enthalpy change is halved:
ΔHrxn = -1204 kJ / 2 = -602 kJ.
3. **Reversing the Reaction**: If we reverse the reaction, the products become reactants and vice versa:
2 MgO(s) → 2 Mg(s) + O2(g)
When the reaction is reversed, the sign of ΔHrxn also changes:
ΔHrxn = +1204 kJ.
In summary, Hess's law emphasizes that the enthalpy of reaction is directly proportional to the coefficients in the thermochemical equation. Any changes made to the equation—whether multiplying, dividing, or reversing—must be reflected in the corresponding enthalpy value, allowing for the calculation of overall enthalpy changes in complex reactions.