(Not completed!) Chapter 16: Entropy and Gibbs Free Energy

16.1 → Two basic drives

Exothermic reactions & spontaneity

Exothermic reactions agree with the general concept of:

• Chemicals move from a state of higher chemical energy to one of lower chemical energy

• And energy is then released, generally in the form of heat energy

Endothermic reactions & spontaneity

• However some endothermic reactions occur spontaneously. For example:

  • Solutions of MgCl2(aq) + Na2CO3(aq) → MgCO3( + 2NaCl(aq) ΔH = +48kJ mol^-1

• Exothermic processes absorb heat as they occur. This means the systems are moving naturally towards states of higher energy.

• This suggests that there must be some other natural tendency in addition to the drive towards lower energy

• We find that, in nature, in addition to a drive towards lower energy, there is a drive towards increased chaos or increased randomness

General rule of thumb

• In chemical reactions it is generally the drive towards minimum energy that dominates. This is why exothermic reactions are much more common than endothermic ones.

• However in some reactions the drive towards maximum randomness is stronger than the energy drive, and this means that sometimes endothermic reactions can be spontaneous.

Entropy

• The physical quantity of entropy, S, measured the amount of randomness or chaos or lack of ordered structure in a substance.

• Move from an ordered state to a less ordered one = increase in entropy, meaning ΔS is positive

• Move from a less ordered state to a more ordered one = decrease in entropy, meaning ΔS is negative.

Estimates of randomness and entropy

• Qualitative generalisations about entropy:

  • Increase in entropy = Solid → Liquid → Aqueous → Gas

  • Despite different solids having different entropy values, these differences are small compared to the differences between the entropies of solids and liquids, or liquids and gases.

  • If a solid dissolves in a solvent there is an increase in entropy

• Quantitative generalisations about entropy:

  • Consider the change in number of moles of solid, liquid or gas in the reaction or whether a solid has been converted to a solution

16.2 → Entropy

What is entropy?

• Entropy (ΔS) is the measurement of randomness or chaos in a substances

• Increased entropy = more disoriented, more chaotic

• Decreased entropy - less disoriented, less chaotic

• Unit = J K^-1 mol^-1

Entropy of states

• Gases have greatest entropy

• Solids have lowest entropy

• Melting & evaporating = increased entropy

• Freezing and condensing - decreased entropy

Standard molar entropy (S°)

• The entropy of one mole of the substance in its standard state at the specified temperature (often 298K)

• It’s often just called entropy

• Unit = J K^-1 mol^-1

• It is possible to measure absolute values for entropy, unlike enthalpy, where we can only measure changes in enthalpy.

  • This means that elements in their standard states have non-zero entropies

Standard entropy change formula

ΔS° = standard entropies of products - standard entropies of reactants

16.3 → Combining the two drives - Gibbs free energy