Energetics II: Entropy

Question 1

What is entropy?

Answer 1

Measure of how disordered or chaotic a system is
More disordered - entropy INCREASES , therefore energetically more stable

Question 2

Why can’t enthalpy changes alone control whether reactions occur?

Answer 2

Some endothermic reactions occur at room temp and chemicals end up in a less stable higher energy state in endothermic reactions

Question 3

How does changing state cause a change in entropy? Give example?

Answer 3

Reaction between ethanoic acid and ammonium carbonate

(NH4)2CO3 (s)+ 2CH3COOH (aq) → 2CH3COO- (aq) + 2NH4+ (aq) + CO2 (g) + H2O (l)

gas/solution made is more disordered than solid ammonium carbonate because its constantly moving around
- system is more disordered and has increased entropy

Slight fall in temp during reaction - ENDOTHERMIC

Question 4

How does dissolving a solid effect entropy? Give example?

Answer 4

Ammonium nitrate dissolved in water - temp of solution DECREASES so endothermic- energy taken in from surroundings

NH4NO3 (s) + aq → NH4+ (aq) + NO3- (aq)
Level of disorder increase as particles are no longer in fixed position /free to move

Question 5

Why is it difficult to predict whether process of dissolving a solid is exo/endothermic ?

Answer 5

When an ionic solid dissolves
Bonds between the particles are broken increasing the disorder and taking in energy
Bonds between the solvent and particles are made reducing the disorder and releasing energy

Question 6

Eg of endothermic reaction between 2 solids and how the entropy changes?

Answer 6

Solid Barium hydroxide /ammonium chloride mixed /temp drops - ENDOTHERMIC

2NH4Cl (s) + Ba(OH)2.8H2O (s)→ BaCl2.2H2O (s) + 2NH3 (g)+ 8H2O (l)
Liquid/gas produced so entropy increased due to INCREASED DISORDER of particles than in solids

Question 7

What is the feasibility of a reaction determined by? What is the Equn/units?

Answer 7

The enthalpy and entropy change - these 2 factors come tgt in concept called GIBBS FREE ENERGY

ΔG= ΔHreaction – TΔSsystem
KJ mol-1 = kJ mol-1 - K ( JK-1mol-1)
Convert entropy to kJ K-1mol-1

For reaction to be feasible , gibbs free energy must be equal or less than 0
Rearranging can help determine the temp at which non spontaneous reaction becomes feasible

Question 8

What does the value of ΔG tell us?

Answer 8

When ΔG is negative - reaction is SPONTANEOUS /FEASIBLE and likely to occur
When ΔG is positive - reaction is NOT SPONTANEOUS /FEASIBLE and unlikely to occur

Question 9

Total entropy change of a reaction equation?

Answer 9

ΔStotal = ΔSsys + ΔSsurr
- energy being released causes substantial increase in entropy of surroundings if exothermic

Question 10

Standard entropy change equation /units?

Answer 10

ΔSsystem = ΣΔS products - ΣΔSreactants

Question 11

Entropy change of surroundings?

Answer 11

ΔS surroundings = - ΔH / T

T= absolute temp

Question 12

Equation that link Gibbs free energy and the equilibrium constant? What is it used for ?

Answer 12

ΔG= -RT lnK
R = gas constant (8.32 J K-1 mol-1)
T = temp constant (kelvin)
K = equilibrium constant

Used to determine whether forward or backward reaction is favoured

Question 13

What does a negative ΔG value for a reaction indicate?

Answer 13

Reaction is feasible/spontaneous
Equilibrium conc of products is GREATER than equilibrium conc of reactants
Value of equilibrium constant ( K) is greater than 1

As it gets more negative :
Forward reaction is favoured more
Value of K increases

Question 14

Limitations of using ΔG?

Answer 14

• ΔG can only be used to predict feasibility of a reaction under standard conditions - under non standard conditions ΔG must be calculated
• doesn’t take into account the kinetics of reaction e.g rate of reaction
• some reactions are feasible when ΔG is negative but kinetically not feasible since it occurs too slowly - might have a large energy barrier (Ea) which reactants have to overcome before reaction can occur