Enthalpy and EntropyLesson 2

Which way does the reaction go?!

• At equilibrium, the reaction can go to the product side, and/or the reactant side.

• What favors one side over the other?

TWO THINGS YOU NEED TO KNOW ABOUT REACTIONS IN GENERAL

 

1. Reactions tend to go toward minimum enthalpy.

2. Reactions tend to go toward maximum entropy

1. Reactions tend to go toward minimum enthalpy

• Minimum enthalpy or minimum energy is always favoured as molecules want to stay in a state where there is minimum energy ( more stable )

Products Favoured

Remember Enthalpy (PE) on a PE graph Identify Minimum enthalpy/Minimum PE = This is where the reaction

will go.

Reactants Favoured

Min enthalpy review

• Min enthalpy favours side with heat term– If rxn is exothermic, prefers products.– If rxn is endothermic, prefers reactants.

2. Reactions tend to go toward maximum entropy

Entropy (S) = amount of randomness in a system.

Why is MAXIMUM entropy favored?

• Think about this…

If you tossed bricks off of a truck, which pile of bricks are you most likely to produce???

Maximum Entropy

• It is all to do with probability …– The possibility of a very highly organized

outcome taking place is much, much lower than a highly random outcome

– There are more random outcomes possible than highly organized ones.

How do we figure out entropy in a reaction?

Ask yourself: what is the most random phase present?

How do we figure out entropy in a reaction?

Ask yourself: what is the most random phase present?Gases aqueous solutions pure liquids Solids

most random (least organized) least random (most organized)

Entropy examples

• CaC2(s) + 2H2O(l) C2H2(g) + Ca(OH)2(aq) ↔

Entropy Examples

A(g) + B(s) 2C(g) + D(s)

The side having the most number of particles in the most random phase will have the most randomness and will be favoured.

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

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)

Entropy summary more moles of gases gases aqueous slns pure liquids Solids

Most random(max entropy) least random(min entropy)

Putting things together

Remember, reactions favour:

1- sides with minimum enthalpy

2- sides with maximum entropy.

 

Products are favoured

Are the products or Reactants favoured?

0 gases 1 gas

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

Goes to Completion

For entropy- only look at phase symbols- mainly gases

spontaneous

↔?

⇌ ProductsReactants

 

Which Way?

Right or left?Equilibrium

spontaneous

Are the Products or Reactants favoured?

1 gas 2 gases

2. N2O4(g) 2NO2(g) ΔH = +20 KJ ↔?

⇌reactants Products

 

Which Way?

Right or left?

Reactants are favoured

Does not occur Non spontaneous!

Are the Products or Reactants favoured?

3 gases 1 gas

3. 3C(s) + 3H2(g) + 45 KJ C3H6(g) ↔?

⇌reactants products

More examples: Which way will this reaction go?

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

Which way will this reaction go?

3C(s) + 3H2(g) + 45 KJ → C3H6(g)

Think…

Where would maximum entropy be for an endothermic equilibrium system?

Maximum entropyMust be on the opposite side

Minimum enthalpy

Entropy must increase

I’m Entropy

Page 47 examples

C2H2(g) + 2Cl2(g) C2H2Cl4(l) + 386 KJ

CH4(g) + O2(g) CO2(g) + 2H2O(g) + 394 KJ

4 Au(s) + 3 O2(g) + 162 KJ 2 Au2O3(s)

Summary To find out which side the reaction will go

-where is min enthalpy?–If rxn is exothermic, prefers products.–If rxn is endothermic, prefers reactants.

-where is max entropy?more moles of gases gases aqueous slns pure liquids Solids

Summary

If both min enthalpy and max entropy are at the same side, this is the side the rxn goes to.

-products (completion)

-reactants side (does not occur)

If they are at different sides, it means it is at equilibrium.

Homework

Hebden: page 48-49 questions 14-16