Entropy, S, is the quantitative measure of the degree of disorder in a system.
All substances possess some degree of disorde because particles are always in constant motion. Thus S is always a positive number.

One of the ideas involved in the concept of entropy is that nature tends to move from order to disorder in isolated systems. For example, gas molecules spread out over time to fill a space, increasing their entropy over time.
Entropy helps us to explain things that happen naturally:

gas spreading through a room
heat from a fire spreading though a room
ice melting in a warm room
salt dissolving in water

In all these examples, energy is changing from being localised (concentrated) to becoming more spread out (diluted). Entropy increases whenever particles become more disordered.

The entropy of a pure substance increases with increasing temperature.
Entropy increases during the changes in state that give more randomness.
Solid --> liquid --> gas
Entropy also increases when the number of gaseous molecules increases.

Entropy changes are calculated by subtracting the entropy of the reactants from the entropy of the products.

If a change makes a system more random, entropy change is positive.
If a change makes a system more ordered, entropy change is negative.

	Created by siobhan.quirk almost 13 years ago

Undergraduate A2 Chemistry (Energy) Note on Entropy, created by siobhan.quirk on 11/07/2013.