Enthalpy and Entropy

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Enthalpy and Entropy Lesson 2
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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?

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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
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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 )

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Remember Enthalpy (PE) on a PE graph Identify
Minimum enthalpy/Minimum PE This is where the
reaction will go.
Products Favoured
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Reactants Favoured
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Min enthalpy review

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

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2. Reactions tend to go toward maximum entropy

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

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Why is MAXIMUM entropy favored?

• Think about this

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

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How do we figure out entropy in a reaction?

• Ask yourself what is the most random phase
  present?

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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)

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Entropy examples

• CaC2(s) 2H2O(l) C2H2(g) Ca(OH)2(aq)

?
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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.

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Entropy is randomness or a measure of how spread
out, or broken up the system is.
Less Entropy More Entropy
H2O(l)
H2O(g)
H2O(s)
H2O(l)
NaCl(aq)
NaCl(s)
N2O4(g)
2NO2(g)
glass
broken glass
clean room
messy room
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Entropy summary

• more moles of gases? gases ? aqueous slns ? pure
  liquids ? Solids
• Most random(max entropy) ? least random(min
  entropy)

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Putting things together

• Remember, reactions favour
• 1- sides with minimum enthalpy
• 2- sides with maximum entropy.

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Are the products or Reactants favoured?
0 gases 1 gas
1. Zn(s) 2HCl(aq) H2(g) ZnCl2(aq)
?H -252 KJ
For entropy- only look at phase symbols- mainly
gases
Products are favoured
Goes to Completion
spontaneous
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Products
?
Reactants
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Are the Products or Reactants favoured?
2. N2O4(g) 2NO2(g) ?H
20 KJ
1 gas 2 gases

Equilibrium spontaneous
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reactants
Products
?
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Are the Products or Reactants favoured?
3. 3C(s) 3H2(g) 45 KJ
C3H6(g)
3 gases 1 gas
Reactants are favoured Does not occur Non
spontaneous!
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reactants
?
products
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More examples Which way will this reaction
go? Zn(s) 2HCl(aq) ? H2(g) ZnCl2(aq)
?H -252 KJ
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Which way will this reaction go?3C(s) 3H2(g)
45 KJ ? C3H6(g)
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• Think
• Where would maximum entropy be for an endothermic
  equilibrium system?

Maximum entropy Must be on the opposite side
Minimum enthalpy
Entropy must increase
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Page 47 examples

• C2H2(g) 2Cl2(g)? C2H2Cl4(l) 386 KJ

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• CH4(g) O2(g) ? CO2(g) 2H2O(g) 394 KJ

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• 4 Au(s) 3 O2(g) 162 KJ ? 2 Au2O3(s)

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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

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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.

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Homework Hebden page 48-49 questions 14-16