Enthalpy Entropy

Intro to Entropy

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Syllabus dot points

a	analyse the differences between entropy and enthalpy
b	use modelling to illustrate entropy changes in reactions
c	predict entropy changes from balanced chemical reactions to classify as increasing or decreasing entropy
d	explain reaction spontaneity using terminology, including: (ACSCH072) – Gibbs free energy  – enthalpy – entropy
e	solve problems using standard references and 𝛥𝐺𝑜 = 𝛥𝐻 𝑜 − 𝑇𝛥𝑆𝑜 (Gibbs free energy formula) to classify reactions as spontaneous or nonspontaneous
f	predict the effect of temperature changes on spontaneity

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

Multiple Choice

Given the change of phase:

CO₂(g) —> CO₂(s)

As CO₂(g) changes to CO₂(s), the entropy of the system

• decreases

• increases

• remains the same

Multiple Choice

1. Given the following information, calculate ΔG⁰ for the reaction below at 25⁰C: (K = ⁰C + 273)

SnCl₄(l) + 2 H₂0(l) → SnO₂(S) + 4HCl(g),

ΔH⁰=133.0 kJ and ΔS⁰ =401.5 J/K

• -252.6 kJ

• -13.4 kJ

• 13.4 kJ

• 252.6 kJ

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