Hess's Law and Heat of Formation - SS1 Chemistry Lesson Note

Hess's Law is a fundamental principle in thermodynamics that relates to the enthalpy change of a chemical reaction. It states that the overall enthalpy change of a reaction is independent of the pathway taken from the initial to the final state, as long as the initial and final conditions are the same. In other words, the enthalpy change is a state function, meaning it only depends on the initial and final states and not on the specific steps involved in the reaction.

 

Hess's Law is based on the principle of conservation of energy. According to this law, if a reaction can be expressed as a series of intermediate steps, then the overall enthalpy change of the reaction is equal to the sum of the enthalpy changes of each individual step. This principle allows chemists to calculate the enthalpy change of a reaction indirectly by using known enthalpy changes of other reactions.

 

Heat of formation, also known as the standard enthalpy of formation, is the enthalpy change that occurs when one mole of a compound is formed from its constituent elements in their standard states (usually at 25 degrees Celsius and 1 atmosphere of pressure). The standard enthalpy of formation is typically denoted by the symbol ΔHf°.

 

For example, let's consider the combustion of methane (CH4) to form carbon dioxide (CO2) and water (H2O). The enthalpy change (ΔH) for this reaction can be calculated using Hess's Law as follows:

 

Reaction 1: CH4 + 2O2 -> CO2 + 2H2O         ΔH1 = -890 kJ/mol

 

Reaction 2: C + O2 -> CO2                          ΔH2 = -393 kJ/mol

 

Reaction 3: 2H2 + O2 -> 2H2O                      ΔH3 = -572 kJ/mol

 

To obtain the desired reaction of CH4 + 2O2 -> CO2 + 2H2O, we can combine reactions 1, 2, and 3 as follows:

CH4 + 2O2 -> C + 2H2O                            ΔH = ΔH1

 

C + O2 -> CO2                                          ΔH = -ΔH2

 

2H2 + O2 -> 2H2O                                     ΔH = -ΔH3

 

Summing up these equations gives us the desired reaction:

 

CH4 + 2O2 -> CO2 + 2H2O                    ΔH = ΔH1 - ΔH2 - ΔH3

 

By substituting the known values of ΔH1, ΔH2, and ΔH3, we can calculate the enthalpy change (ΔH) for the combustion of methane.

 

Hess's Law is often used to calculate the heat of formation of a compound. By using a series of reactions whose enthalpy changes are known, chemists can determine the enthalpy change of the desired reaction indirectly. This is particularly useful for compounds that cannot be directly measured or synthesised.

 

The application of Hess's Law to calculate the heat of formation involves the following steps:

 

1. Identify the desired reaction for which you want to calculate the heat of formation.

 

1. Determine a series of intermediate reactions that can lead from the elements in their standard states to the desired compound.

 

1. Write the balanced chemical equations for each intermediate reaction, including the corresponding enthalpy change for each reaction. These enthalpy changes can be obtained from experimental data or from published tables.

 

1. Manipulate the intermediate reactions algebraically, if necessary, to cancel out any compounds or elements that appear on both sides of the equations.

 

1. Sum up the enthalpy changes of all the intermediate reactions to obtain the overall enthalpy change for the desired reaction. This overall enthalpy change will be equal to the heat of formation of the compound.

 

Hess's Law and the concept of heat of formation are widely used in thermochemistry and play a crucial role in understanding and predicting the energy changes that occur during chemical reactions. They provide a valuable tool for calculating enthalpy changes and studying the thermodynamic stability of compounds.