Entropy | A-level Chemistry | OCR, AQA, Edexcel

Entropy in a Snap!

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The key points covered in this video include:
1. What makes a reaction go?
2. Introduction to entropy
3. Entropy change of the system
4. Entropy change of the surroundings
5. Calculating total entropy change
6. Predicting entropy changes

What Makes a Reaction “Go”?

Some reactions just happen, like after ignition, magnesium will just burn in oxygen without any further input from us: Whereas the reverse reaction will not happen, unless we force it to by changing something like the temperature: We call reactions that happen without us helping them along spontaneous. What decides whether a reaction will happen? It’s tempting to say that it’s the energy change - maybe reactions happen if it decreases the total energy as this is more stable? There must be more to it however because endothermic reactions do just happen. Enthalpy alone cannot tell us whether a reaction is feasible.

Entropy

Entropy is a quantity introduced in thermodynamics to make sense of why things in the universe happen the way they do. The simplest way to think about entropy is as a measure of disorder. A more involved way to treat entropy is as the number of ways particles and energy can be distributed. There are far more configurations the gas particles can be in and far more ways to distribute the energy so the entropy is higher. It’s a fundamental observation about the universe we live in that the total entropy will always increase, as will the entropy of a closed system. So in a spontaneous process, the total entropy must increase.

Calculating Entropy Changes

When we think about entropy changes we break the universe into two parts. The system is the chemical reaction we’re following. The surroundings is the rest of the universe (everything we’re not interested in). Only energy can be exchanged between the two.

Entropy Change of the system

All substances have an entropy associated with them. The units are J K^-1 mol^-1 - this will make more sense when we look at the surroundings. These values have been investigated and recorded so we may just look them up. If we want to know the entropy change use: Example: Using the data below find the standard entropy change for the reaction given by the equation.

Calculating Entropy Changes

When we think about entropy changes we break the universe into two parts. The system is the chemical reaction we’re following.

Entropy of the Surroundings

The only way the chemical reaction can change the entropy (disorder) of the surroundings is to transfer heat to it. The amount the entropy changes for a given amount of heat energy depends on how hot the surroundings already are. The heat absorbed is the same as energy released by the chemicals. The heat energy absorbed by the chemicals is the change in enthalpy, ΔH, so the heat absorbed by the surroundings is the negative of this. Now you can see why entropy has units of J K^-1 mol ^-1.

Total Entropy Change

Sometimes we refer to the total entropy change as the entropy change of the universe: Only reactions with a positive ΔStotal value can ever happen.

Predicting Entropy Changes

We can often predict whether the entropy change of a system will be positive or negative without calculating the exact value. Change of state. When ammonium carbonate is added to ethanoic acid. When we burn magnesium in oxygen. Change in number of moles. When hydrated barium hydroxide reacts with solid ammonium chloride. If we have an increase in moles but the products have states with lower entropy it will not always be obvious whether the systems entropy increases or decreases. Dissolving ionic solids in water. 1. Breaking down the lattice. 2. Hydration of the ions. The entropy of the system usually increases when we dissolve ionic solids. When we dissolve ammonium nitrate crystals in water. Remember that the system isn’t the whole story and we have to think about the changes to entropy of the surroundings too.