Enthalpy and Entropy

Lesson # 10

There are two factors that determine the relative amount of reactants and products at equilibrium.  

1.Reactions tend to proceed toward minimum enthalpy.

2.Reactions tend to go toward maximum entropy

Products are Favoured

Enthalpy is PE – In an Exothermic Reaction

The low side (side with the energy) is always favoured- minimum

A + B = C + energy

Reactants are Favoured

Enthalpy is PE – In an Endothermic Reaction

The low side (side with the energy) is always favoured- minimum

A + energy = B + C

Entropy is randomness or a measure of how spread out, or broken up the system is.

Watch Simulation

Less Entropy More Entropy

H2O(s)

H2O(g)H2O(l)

NaCl(s)NaCl(aq)

N2O4(g) 2NO2(g)

glass broken glass

clean room messy room

H2O(l)

Less entropy (s) (l) (aq) (g) More Entropy

P6O9(g) 3P2O3(g)

√ √

√ √

√ max entropy √ min enthalpy

You can tell the yeild of a reaction with enthalpy and entropy

equlibrium

products favoured

⇌

⇌

If they are both on one side that side is favoured

If they are on different sides it’s an equilibrium

 

Which Way?

Right or left?

Products are favoured

Large Keq

Are the products or Reactants favoured?0 gases 1 gas

1. Zn(s) + 2HCl(aq) H2(g) + ZnCl2(aq) ΔH = -252 KJ

√ max entropy

√ min enthalpy

Goes to Completion

For entropy- only look at phase symbols- mainly gases

High yield

↔?

+ E

⇌ ProductsReactants

 

Which Way?

Right or left?Equilibrium

Keq is about 1

Are the Products or Reactants favoured?

1 gas 2 gases

2. N2O4(g) 2NO2(g) ΔH = +20 KJ

√ max entropy

√ min enthalpy

↔?+ E

⇌reactants Products

 

Reactants are favoured

Keq is small

Are the Products or Reactants favoured?

3. 3C(s) + 3H2(g) + 45 KJ C3H6(g)

√ max entropy

√ min enthalpy

Low yield

Reaction does not occur!

↔?

⇌reactants products

4. Describe the change in entropy and enthalpy for an endothermic equilibrium system.

Maximum entropyMust be on the opposite side

Minimum enthalpy

Entropy must increase

I’m Entropy

Enthalpy must increase

5. Describe the change in entropy and enthalpy for an exothermic equilibrium system.

Minimum enthalpy

Maximum entropy

Entropy is decreasing

Enthalpy is decreasing

I’m Entropy

6. Describe entropy and enthalpy as increasing or decreasing.

Zn(s) + 2HCl(aq) → H2(g) + ZnCl2(aq) ΔH = -252 KJ

√ max entropy

√ min enthalpy

Enthalpy decreases

Entropy increases

7. Describe entropy and enthalpy as increasing or decreasing.

3C(s) + 3H2(g) + 45 KJ → C3H6(g)

√ max entropy

√ min enthalpy

Enthalpy increases

Entropy decreases

Equilibrium is a compromise between the tendencies toward minimum enthalpy and maximum entropy.