Chemical Thermodynamics - SS2 Chemistry Past Questions and Answers - page 4

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.

Thermochemical equations are essential in representing the enthalpy changes (ΔH) associated with chemical reactions. They provide a quantitative description of the heat transfer during a reaction and allow us to calculate and compare energy changes under different conditions.

Thermochemical equations are balanced similarly to regular chemical equations, but with the addition of the enthalpy change as a coefficient. The coefficient in front of the thermochemical equation represents the molar quantity of the reactants and products involved, while the ΔH value indicates the corresponding enthalpy change.

For example, consider the reaction of hydrogen gas (H2) with oxygen gas (O2) to form water (H2O):

H2(g) + 1/2O2(g) → H2O(g) ΔH = -285.8 kJ/mol

In this balanced thermochemical equation, the coefficients 2 and 1/2 represent the stoichiometric ratios of the reactants and products, respectively. The enthalpy change of -285.8 kJ/mol indicates the heat released during the reaction.

Balancing thermochemical equations is crucial to ensure the conservation of energy and atoms. The coefficients must be adjusted to achieve an equal number of atoms on both sides of the equation, while the enthalpy change remains accurate for the reaction.

Thermochemical equations and Hess's Law provide valuable tools for understanding and calculating enthalpy changes in chemical reactions, enabling the prediction and analysis of energy transformations in various systems.