Chemical Energetics (also known as Thermochemistry) 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)
Uses:
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 solid compounds.
If you have not read my previous post on the definitions of Standard Enthalpy Changes of Reactions, you should do so before moving on.
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.
3 Steps to calculate Enthalpy Changes (ΔH) of a reaction using Hess’ Law:
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.
The two most common applications are:
A) Calculations involving Enthalpy Changes of Formation
B) Calculations involving Enthalpy Changes of Combustion
Let us look at two exam-based questions and see how we apply it to solve the questions.
A) Calculations involving Enthalpy Changes of Formation
Question 1:
Calculate the enthalpy change for the reaction CaF2 + H2SO4 → 2HF + CaSO4, given that the enthalpies change of formation of CaF2, H2SO4, HF and CaSO4 are -1220, -814, -271 and -1434 kJmol-1 respectively.
Solution:
By Hess’ Law, ΔHrθ = 2(-271) + (1434) – (-1220) – (-814) = +58 kJmol-1
B) Calculations involving Enthalpy Changes of Combustion
Question 2:
Calculate the standard enthalpy change of formation of CH4 given than the standard enthalpies change of combustion of methane, graphite and hydrogen are -890.2, -393.4 and -285.7 kJmol-1 respectively.
Solution:
By Hess’ Law, ΔHfθ = -393.4 + 2(-285.7) – (-890.2) = -74.6 kJmol-1
I highly encouraged you to work on more H2 Chemistry Exam questions which are related to Chemical Energetics. Practice does make perfect, particularly for this topic. Our students are given a lot of Energetics’ questions to work on.
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