3.6 ENTROPY AND FEASIBILITY OF REACTIONS

Entropy definition

Entropy is measure of disorder or freedom of movement of particles in a system

Measure of entropy

Solid - Low

• Atoms fixed position no freedom

Liquid - Medium

• Atoms free to some movement any direction stay close

Gas- High entropy

• Free movement all direction and position

When does entropy increase

As particles become more dispersed and have greater freedom of movement

All natural processes tend towards maximum entropy

Why do gases have greatest entropy

Particles have the greatest freedom of movement and most dispered

Order of entropy

Gas > Liquid > Solid

Standard entropy af a substance ΔS

Entropy of one mole in given physical state under standard conditions

Calculating ΔS

ΔS = S _Products - S _{All reactants}

Second law of thermodynamics

Entropy always tend to increase in any isolated system not in equilibrium

Gibbs free energy

ΔG=ΔH−TΔS

• ΔG Gibbs free energy change

• ΔH Enthalpy change

• T K

• ΔS Entropy change

Unit of S

Joules per Kelvin per mole JK ^-1 mole ^-1

Values of ΔG

ΔG < 0 Spontaneous

ΔG > 0 Not spontaneous

ΔG = 0 Equilibrium

• A reaction is feasible if ΔG is negative

Linking ΔH, ΔS, and ΔG

ΔH Negative ΔS positive Spontaneous - +

• Exothermic -disorder increase

ΔH Positive ΔS Negative Never spontaneous + -

• Endothermic -Disorder decrease

ΔH Negative ΔS negative Depend on temp - -

• Spontaneous at low K

ΔH Positive ΔS positive depend on temp + +

• Spontaneous high K

Why can endothermic reactions be spontaneous

Has a positive ΔH

If reaction has a large positive ΔS entropy change

Higher temperatures TΔS becomes larger

TΔS > ΔH then = Δ G is negative

Minimum temperature

ΔG = 0

T = ΔH / ΔS

What happens to ΔS when Solid → Gas

ΔS is positive

• Gas produced

• Particles become more dispersed greater freedom movement

What happens to ΔS when 2 mol (gas) → 2 mol Gas

ΔS = 0

• Number remains same disorder does not change

What happens to entropy when solid dissolved in water

• Particles more separated and dispersed in solution

• Ions gain greater freedom of movement

• Entropy increase

Why is gas entropy higher than liquid

• Gas particles far apart move freely

• Maximum freedom of movement

• Liquids restricted

If ΔH and ΔS negative when is reaction sponatenous

At low temperatures

Positive standard enthalpy of formation meaning

Compound is less stable than the elements