Lesson Notes By Weeks and Term v4 - SHS 2

Lesson Video

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Performance objectives

• State Hess’s Law and explain its significance.
• Construct energy cycle diagrams for reactions.
• Calculate enthalpy changes using Hess’s Law.
• Apply the Born-Haber cycle to determine energy values.

Lesson summary

Welcome, students! In our last lesson, we learned how to measure the heat change (enthalpy change) of some chemical reactions using a calorimeter. But what if a reaction is too slow, like the rusting of an iron gate? Or too dangerous and explosive, like the reaction of potassium with water? Or what if the reaction produces other unwanted products, making it impossible to measure the heat change for just the reaction we are interested in? This is where Hess's Law comes in. It provides a powerful, indirect way to find the enthalpy change of *any* reaction, just by using data from other, easier-to-measure reactions.

Lesson notes

2.1. What is Hess's Law?

Hess's Law states that: > The total enthalpy change for a chemical reaction is independent of the route taken, provided the initial and final conditions are the same.

This means that whether a reaction happens in one step or in a series of steps, the total energy change will be the same. Enthalpy (H) is a state function, which means it only depends on the starting state (reactants) and the final state (products), not the path taken to get from one to the other.

Analogy: Imagine travelling from Accra to Kumasi. Route 1 (Direct): Drive directly from Accra to Kumasi. Let's say this releases a certain amount of energy (fuel). Route 2 (Indirect): Drive from Accra to Cape Coast, and then from Cape Coast to Kumasi. Hess's Law tells us that the total energy change for Route 1 is equal to the sum of the energy changes for the steps in Route 2. ΔH (Accra → Kumasi) = ΔH (Accra → Cape Coast) + ΔH (Cape Coast → Kumasi) 2.2. Applying Hess's Law: The Algebraic Method

Evaluation guide

• Explain Hess's law of constant heat summation and apply it to construct energy cycle
• diagrams as well as calculate relevant enthalpy changes .
• Activity -Based Learning: State Hess’ law and apply it to construct simple energy cycle
• diagrams.