Explain the concept of Hess s Law and its signi... - SS2 Chemistry Chemical Thermodynamics Question

Hess's Law states that the total enthalpy change of a reaction is independent of the pathway taken from the reactants to the products. In other words, the enthalpy change of a reaction depends only on the initial and final states and is not affected by the intermediate steps.

Hess's Law is significant in thermochemistry as it allows us to calculate the enthalpy change of a reaction indirectly by combining known enthalpy changes of other reactions. This is particularly useful when direct measurement of the enthalpy change is not feasible or practical.

To determine the enthalpy change of a reaction using Hess's Law, we can break the desired reaction into a series of intermediate steps, for which the enthalpy changes are known. By manipulating and combining these equations, we can cancel out common reactants and products to obtain the overall reaction with its corresponding enthalpy change.

For example, consider the combustion of methane (CH4) to form carbon dioxide (CO2) and water (H2O). Since measuring the enthalpy change directly is challenging, we can use Hess's Law. We break the reaction into two steps:

Step 1: CH4(g) + 2O2(g) → CO2(g) + 2H2O(g) ΔH1 = -890.3 kJ/mol

Step 2: C(s) + O2(g) → CO2(g) ΔH2 = -393.5 kJ/mol

By reversing and manipulating Step 2, we get:

Step 2': CO2(g) → C(s) + O2(g) ΔH2' = +393.5 kJ/mol

Adding Step 1 and Step 2' cancels out CO2, resulting in the desired reaction:

CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)

ΔH = ΔH1 + ΔH2' = -890.3 kJ/mol + 393.5 kJ/mol = -496.8 kJ/mol

Thus, using known enthalpy changes and manipulating equations, we can determine the enthalpy change of a reaction using Hess's Law.