Hess's Law and Thermochemical Equations - SS3 Chemistry Lesson Note

Hess's Law and thermochemical equations are important concepts in thermodynamics and are often used to calculate enthalpy changes in chemical reactions. Let's explore these concepts in more detail:

Thermochemical Equations:

Thermochemical equations are balanced chemical equations that include the enthalpy change (ΔH) associated with the reaction. The enthalpy change is typically written as a reactant or product in parentheses following the chemical formula. For example:

2H2(g) + O2(g) ⟶ 2H2O(g) ΔH = -572 kJ/mol

In this example, the thermochemical equation shows the combustion of hydrogen gas (H2) with oxygen gas (O2) to produce water vapour (H2O), and it indicates that the reaction releases 572 kJ of energy per mole of water formed.

Thermochemical equations provide crucial information about the energy changes occurring during a chemical reaction. The enthalpy change (ΔH) can be positive (endothermic, absorbing heat) or negative (exothermic, releasing heat), and it gives valuable insights into the heat flow associated with the reaction.

Hess's Law:

Hess's Law is a fundamental principle in thermodynamics that states that the enthalpy change of a chemical reaction is independent of the reaction pathway taken, as long as the initial and final conditions remain the same. In other words, the overall enthalpy change in a reaction depends only on the initial and final states of the system and not on the specific steps or intermediates involved in the reaction.

This principle is particularly useful when it is challenging to measure the enthalpy change directly. Instead of measuring it in one step, Hess's Law allows us to calculate the enthalpy change indirectly by combining multiple reactions with known enthalpies. To do this, we use the following two key principles:

a.    Enthalpy is a state function: Enthalpy is a state function, meaning its value depends only on the current state of the system, not on the path taken to reach that state. Therefore, the enthalpy change between the two states is the same, regardless of the route taken.

b.    Conservation of energy: Energy is conserved, so the total enthalpy change for a series of reactions should be the same as the enthalpy change for the desired reaction.

By manipulating and algebraically combining multiple thermochemical equations, we can cancel out intermediates and arrive at the desired overall reaction while summing the corresponding enthalpy changes. Hess's Law provides a powerful tool for calculating enthalpy changes that are not easily measured directly.

In summary, thermochemical equations and Hess's Law are essential tools in thermodynamics for calculating and understanding enthalpy changes in chemical reactions. They allow chemists to determine the energy changes associated with reactions and provide valuable insights into the behaviour of chemical systems under different conditions.