Syllabus dot points

Entropy

• S, unit = J/K

• a measure of disorder

• the universe tends over time towards disorder/chaotic (2nd Law of Thermodynamics)

• measure of how many arrangements with the same energy

• we measure entropy change, not entropy directly

Predicting entropy changes

• As temp increases, entropy increases

• States can predict (i.e. solid = least entropy)

• More particles within a system increases energy

• When a solute dissolves in a solvent, its entropy increases

Using predictions:

Determine the change in entropy of the chemical reaction:

CaO(s) + H₂O(l) --> Ca(OH)₂(s)

- there are 2 mol of reactant particles, and 1 mol of product particles = less disordered = ΔS = negative

- decrease in amount of liquids, and increase in amount of solids = less disordered = ΔS = negative

Therefore, overall the system has ΔS = negative

Spontaneity of Reactions

• spontaneous = occurs without any additional energy

• when a spontaneous process occurs, there is an increase in entropy of the universe

• we determine whether something is spontaneous by 2 factors: change in enthalpy and change in entropy

Determining entropy change from standard entropy values

• We are given standard entropy data

• Similar to heats of formation

• ΔS° = ΣS° (products) - ΣS° (reactants)

Gibbs Free Energy

• quantity that helps us determine whether a reaction will be spontaneous

• ΔG

• The change in GFE is: ΔG = ΔH-TΔS

• Combining the effects of enthalpy, entropy and temperature

Deciphering

• if ΔG is < 0 then reaction is spontaneous (exergonic)

• if ΔG is > 0 then reaction is non-spontaneous (endergonic)

Calculating with GFE for spontaneity

• State known values

• Sub into formula

• Answer the q.

Calculating temp with GFE

• GFE formula can be used to determine the minimum temperature at which reactions switch from spontaneous to not spontaneous

• sub in everything but T, and have ΔG = 0 (where reaction is neither spontaneous or NS), and will give you the point of low/high temp