Introduction to Entropy
Chemistry: IB Diploma Higher Level, topic 4.3.1
Entropy
Entropy is a measure of disorder. Entropy always increases during a reaction.
Entropy
- ΔH is not enough to determine whether or not a reaction happens.
- When NaCl dissolves, the reaction is endothermic - but it still happens!
Stability
- Substances are more stable when they have greater disorder.
- This means that everything tends to increase its disorder.
- This is why your phone cables get tangled in your pocket!
- It’s also why NaCl dissolves in water - its entropy increases.
Technical definition
- Entropy is technically defined as the number of ways molecules can be arranged, and the number of ways their energy can be arranged between them.
- Entropy is given the symbol S.
- A big and positive value of S indicates a large disorder.
Factors Affecting Entropy
Entropy is affected by many factors, including temperature, physical state and number of moles.
Temperature
- According to kinetic-molecular theory, the temperature of a substance is proportional to the average kinetic energy of its particles.
- Raising the temperature of a substance will result in more extensive vibrations of the particles in solids and more rapid translations of the particles in liquids and gases.
- At higher temperatures, the distribution of kinetic energies among the atoms or molecules of the substance is also broader than at lower temperatures.
- Thus, the entropy for any substance increases with temperature.
Physical state
- The molecules in gases are free to move. This means that they have a higher entropy than solids and liquids.
- In the same way, liquids have higher entropy than solids.
- Solids have lower entropy than liquids.
- In the solid phase, the atoms or molecules are restricted to nearly fixed positions with respect to each other and are capable of only modest oscillations about these positions.
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Phase changes
- The entropy of a substance increases (ΔS > 0) as it transforms from a relatively ordered solid, to a less-ordered liquid, and then to a still less-ordered gas.
- The entropy decreases (ΔS < 0) as the substance transforms from a gas to a liquid and then to a solid.
Number of moles
- There are more ways you can arrange ten particles than two particles. So the more moles of substance, the greater the entropy.
- Entropy also increases in a reaction that also has more moles of gaseous products than gaseous reactants.
Conditions for a reaction
- This all means that a reaction is more likely to happen if:
- The reaction increases the number of moles of substance.
- The reaction produces gas and consumes solids.
- The reaction is exothermic.
Test yourself on Introduction to Entropy
Can ΔH alone determine whether or not a chemical reaction can happen?
Why is ΔH not enough to determine whether or not a reaction happens?
Why do gas molecules have a greater entropy than solids and liquids?
Entropy of Phases