KEY POINTS:
- Entropy is defined as a measure of the degree of disorder of a substance, i.e. how many different ways the particles in the substance could be arranged.
- It should be logical that entropy increases as we go from solid to liquid to gas. Equally, it increases as we go from elements to compounds to mixtures.
- Every substance has a standard entropy value at a certain temperature, symbol S (units of kJ mol-1 K-1). As chemical reactions involve new substances being made, there is of course an entropy change:
ΔS = sum of entropy of products – sum of entropy of reactants - As our universe is expanding, and therefore disorder is increasing, reactions with a large increase in entropy tend to be favoured. For example, explosions – when solid substances become gases, one or two moles of reactant produce many moles of product.
- The equation for the explosion of T.N.T. (trinitrotoluene a.k.a. dynamite) is:
2 C7H5N3O6 (s) → 3 N2 (g) + 5 H2O (g) + 7 CO (g) + 7 C (s)
This involves a massive increase in entropy, which the universe really likes. Two moles of a solid becoming 15 moles of gases, and a little soot. What was the big bang if not a massive explosion?
- There are of course reactions that involve a decrease in entropy. However there must be an enthalpy change associated with it that the universe favours, i.e. exothermic reactions. This is explained on the next page.
THINK OUTSIDE THE BOX:
- Can you suggest three reactions that have a decrease in entropy? What do you notice about their enthalpy changes?
- What evidence do we have for the universe expanding?