Chemical Energetics is a very important topic in Thermodynamics and is a highly application topic in A-Level H2 Chemistry Examinations.
In it, Hess’ Law and the use of Energy Cycle Diagrams is of utmost importance. Let’s take a look at some of the key concepts of Hess’ Law.
Hess’ Law of Constant Heat Summation:
It is defined as the enthalpy change (ΔH) of a particular reaction is determined only by the initial and final states of the system regardless of the pathway taken.
By Hess’ Law,
Enthalpy change for Pathway 1 = Enthalpy change for Pathway 2
ΔH (for A → D) = ΔH (for A → B) + ΔH (for B → C) + ΔH (for C → D)
Application of Hess’ Law is particularly useful when the enthalpy change of a reaction cannot be determined directly by experiment. In this cases, we will use Hess’ Law to calculate the enthalpy change from other data that is available (individual steps) and can be experimentally determined. A very common application of Hess’ Law is the Born-Haber Cycle which is used to determine the lattice energies of Ionic Compounds.
Energy Cycle Diagrams:
For calculations to be clear, we often draw an Energy Cycle Diagram which is made up of individual steps that link the initial and final states of the system.
General rules to calculate ΔH of a reaction using Hess’ Law are:
Step 1: Write down the equation representing the enthalpy change which you are required to calculate.
Step 2: Construct a fully labelled energy cycle according to the data provided, including state symbols.
Step 3: Apply Hess’ Law to calculate the required enthalpy change.
It takes some practice on questions before you can feel comfortable using it. In the next blog post, i will share with you some questions on how to use the Energy Cycle Diagram and the Hess’ Law to calculate enthalpy changes.