Hess's Law of Constant Heat Summation — Prelims Strategy

Prelims Strategy

To effectively tackle NEET questions on Hess's Law, a systematic approach is essential. Firstly, clearly identify the 'target' reaction for which the enthalpy change is to be calculated. Next, examine the given set of thermochemical equations.

For each given equation, compare its reactants and products with those of the target reaction. If a substance in a given equation is on the wrong side of the arrow compared to the target reaction, reverse the given equation and change the sign of its $\Delta H$.

If a substance's stoichiometric coefficient in a given equation does not match that in the target reaction, multiply the entire equation (and its $\Delta H$) by the necessary factor. Be meticulous with fractions.

Once all given equations are manipulated to align with the target, add them algebraically. Ensure that all intermediate species (those not present in the target reaction) cancel out from both sides of the summed equations.

Finally, sum the manipulated $\Delta H$ values to obtain the $\Delta H$ for the target reaction. Pay close attention to signs and units. For conceptual questions, remember that enthalpy is a state function, meaning $\Delta H$ is path-independent, and Hess's Law applies only to energy changes, not reaction rates.