IB Chemistry Topic R1.2.2 Hess’s Law Revision Notes (SL/HL)

IB Chemistry Topic R1.2.2 Hess’s Law Revision Notes (SL/HL) | RevisionDojo

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Hess's Law: The Conservation of Energy in Chemical Reactions

Analogy

Consider that you’re navigating a city’s metro system, perhaps Tokyo or London.
To get from Station A to Station B, you have multiple route options.
Some may involve transfers, others may be direct, but no matter which path you take, the total distance between A and B remains the same.
This concept parallels Hess’s Law in chemistry: the enthalpy change of a reaction depends only on the starting and ending points, not the route taken.

What is Hess's Law?

Hess’s Law is an application of the law of conservation of energy, which states that energy cannot be created or destroyed: only transferred or transformed.

Definition

Hess’s Law

The total enthalpy change for a reaction is the sum of the enthalpy changes for each step of the reaction pathway.

To illustrate, consider the oxidation of sulfur to sulfur trioxide: $$S(s) + \dfrac{3}{2} O_2(g) \to SO_3(g) \quad \Delta H = -396 \, \text{kJ mol}^{-1}$$
This reaction can occur in two steps:
1. Step 1: $$S(s) + O_2(g) \to SO_2(g) \quad \Delta H = -296 \, \text{kJ mol}^{-1}$$
2. Step 2: $$SO_2(g) + \dfrac{1}{2} O_2(g) \to SO_3(g) \quad \Delta H = -100 \, \text{kJ mol}^{-1}$$
Adding the enthalpy changes for these steps gives the same total enthalpy change as the overall reaction:
$$\Delta H = (-296) + (-100) = -396 \, \text{kJ mol}^{-1}$$

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Hess's Law: The Conservation of Energy in Chemical Reactions

Analogy

Consider that you’re navigating a city’s metro system, perhaps Tokyo or London.
To get from Station A to Station B, you have multiple route options.
Some may involve transfers, others may be direct, but no matter which path you take, the total distance between A and B remains the same.
This concept parallels Hess’s Law in chemistry: the enthalpy change of a reaction depends only on the starting and ending points, not the route taken.

What is Hess's Law?

Hess’s Law is an application of the law of conservation of energy, which states that energy cannot be created or destroyed: only transferred or transformed.

Definition

Hess’s Law

The total enthalpy change for a reaction is the sum of the enthalpy changes for each step of the reaction pathway.

To illustrate, consider the oxidation of sulfur to sulfur trioxide: $$S(s) + \dfrac{3}{2} O_2(g) \to SO_3(g) \quad \Delta H = -396 \, \text{kJ mol}^{-1}$$
This reaction can occur in two steps:
1. Step 1: $$S(s) + O_2(g) \to SO_2(g) \quad \Delta H = -296 \, \text{kJ mol}^{-1}$$
2. Step 2: $$SO_2(g) + \dfrac{1}{2} O_2(g) \to SO_3(g) \quad \Delta H = -100 \, \text{kJ mol}^{-1}$$
Adding the enthalpy changes for these steps gives the same total enthalpy change as the overall reaction:
$$\Delta H = (-296) + (-100) = -396 \, \text{kJ mol}^{-1}$$

Definition

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Note

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Tip

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Structure 1. Models of the particulate nature of matter5 subtopics

Structure 2. Models of bonding and structure4 subtopics

Structure 3. Classification of matter2 subtopics

Reactivity 1. What drives chemical reactions?4 subtopics

Reactivity 2. How much, how fast and how far?3 subtopics

Reactivity 3. What are the mechanisms of chemical change?4 subtopics

IB Chemistry R1.2.2 Hess’s Law Notes

Structure 1. Models of the particulate nature of matter5 subtopics

Structure 2. Models of bonding and structure4 subtopics

Structure 3. Classification of matter2 subtopics

Reactivity 1. What drives chemical reactions?4 subtopics

Reactivity 2. How much, how fast and how far?3 subtopics

Reactivity 3. What are the mechanisms of chemical change?4 subtopics

Hess's Law: The Conservation of Energy in Chemical Reactions

What is Hess's Law?

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Duis aute irure dolor in reprehenderit

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Hess's Law: The Conservation of Energy in Chemical Reactions

What is Hess's Law?

anim nostrud sit dolore minim proident quis fugiat velit et eiusmod nulla quis nulla mollit dolor sunt culpa aliqua

Duis aute irure dolor in reprehenderit

Excepteur sint occaecat cupidatat non proident

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