enthalpy and entropy lesson 2. which way does the reaction go?! at equilibrium, the reaction can go...
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Enthalpy and EntropyLesson 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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Products Favoured
Remember Enthalpy (PE) on a PE graph Identify Minimum enthalpy/Minimum PE = This is where the reaction
will go.
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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(s)
H2O(g)H2O(l)
NaCl(s)NaCl(aq)
N2O4(g) 2NO2(g)
glass broken glass
clean room messy room
H2O(l)
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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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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
↔?
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⇌ ProductsReactants
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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 ↔?
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⇌reactants Products
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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) ↔?
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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) ΔH = -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 entropyMust be on the opposite side
Minimum enthalpy
Entropy must increase
I’m Entropy
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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