IB Chemistry: Entropy

IB Chemistry: Entropy

Entropy

Introduction

Entropy (S) is considered to be the amount of disorder in a system.

It is an important concept in thermodynamics, which will be studied both in IB Chemistry and IB Physics. However, it isn’t a very difficult topic and can easily be mastered with some revision.

State and explain the factors that increase the entropy in a system

• Changes of state: When the product changes from state from solid to liquid and lastly, to gas, the state of disorder increases.
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• Mixing and Dissolving: If you mix two substances together, you will be increasing the disorder in the system.

  Therefore, the disorder of aqueous solutions will be larger than that of a liquid.

• Formation of gaseous products: In a reaction, if one of the products is a gas, then the disorder in the system will increase, as the disorder of a gas is typically larger than that of a solid or a liquid.
• Increased movement of particles: If you heat a liquid or a gas, you are providing the system with more kinetic energy, hence increasing the disorder in the system.
• Increased number of particles: This increases the disorder in the system.

Predict whether the entropy change (ΔS) for a given reaction or process is positive or negative.

The most common question you will be given in an exam can be solved by paying close attention to state symbols such as (g), (l), (aq).

Please note that entropy change is positive as you change state from solid to liquid to gas.

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The change in entropy of an aqueous solution is more than that of a liquid, as it is a mixture with water as a solvent, and as we mentioned above, mixing substances together results in a greater increase in entropy.

Example of Entropy Predicting:

1) X (g) + Y (g) –> A (g) + B (g)

2) X (g) + Y (g) –> A (s) + A (l)

3) X (s) + Y( s) –> A (s) + A (g)

4) X (s) + Y( s) –> A (s) + A (s)

Answers:

1) There is no change in state so the entropy change is zero, or very close to zero.

2) There is a change of state from gas to liquid/solid, so the change in entropy is negative, as the disorder in the system decreases.

3) Notice that although one of the substances is still a solid, substance B changed state to a gas, so the change in entropy is positive, as the disorder in the system increases

4) There is no change in state, so the change in entropy is zero.

Calculate the standard entropy change for a reaction (S°) using standard entropy values (S°).

• Enthalpy is very difficult to measure directly.
• However, the absolute entropy can be measured directly. We give the absolute entropy the letter (S).

ΔS = Sproducts – S reactants

Let’s do an example

1.Calculate entropy change in the following reaction:

• 3H2(g) +N2 (g) –> 2NH3 (g)
• However, we need to know the values of some absolute entropies

Hydrogen: 131 KJ-1mol-1

Nitrogen: 192 JK-1mol-1