Entropy

Endothermic reactions

• Why do exothermic reactions take place?

  • Because the products are more stable than reactants

  • The products have a lower energy than the reactants

• Why do endothermic reactions take place?

  • The products have a higher energy than reactants

  • They will be less stable than reactants

• Another factor must be contributing so the reaction occurs: entropy

Entropy

• Entropy is a measure of the dispersal of energy in a system

• It is often described as the level of the disorder in a system - not strictly correct

• Consider the following examples

  • A gas diffusing through a room

  • Heat from a fire warming a room

  • Ice melting in beaker

• All of these involve energy being dispersed more or being more spread out

• Increasing the temperature of a substance is an example of an increase in entropy

  • Solids melting to form liquids, liquids boiling to form gases

• A solid dissolving in water is another example.

• Chemical reactions that produce more gaseous molecules than there were gaseous reactants

  • e.g. CaCO₃(s) → CaO(s) + CO₂(g)

    • This has 2 products compared to 1 reactant

    • Also 1 gaseous product and no gaseous reactants

Standard entropy S⁰

• The standard entropy of a substance is the entropy of one mole of the substance under standard conditions

• Entropy has units of JK^-1mol^-1

• Always positive

• The absolute entropy of a substance cannot be measured

• Any standard entropies are provided

• The more ordered the structure the lower the entropy

Entropy change of a reaction △S⁰

• The entropy change for a reaction can be calculated from the standard entropies

  • △S⁰ = ∑S⁰(products) - ∑S⁰(reactants)

• If the reaction has an increase in entropy (more disordered) then △S⁰ will be positive

• If the reaction has a decrease in entropy (more ordered) then △S⁰ will be negative