thermodynamics of reactions
DESCRIPTION
Thermodynamics of Reactions. Spontaneity, Entropy, and Free Energy Chapter 16. 1 st Law of Thermodynamics. The energy of the universe is constant (conservation of energy). Spontaneous Processes. Can be fast OR slow. Occurs without outside intervention. True, or Not True?. Why??. - PowerPoint PPT PresentationTRANSCRIPT
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Thermodynamics of Reactions
Spontaneity, Entropy, and Free Energy
Chapter 16
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1st Law of Thermodynamics
• The energy of the universe is constant
• (conservation of energy)
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Spontaneous Processes
• Can be fast OR slow. • Occurs without outside
intervention
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True, or Not True?
• Ball rolls downhill spontaneously.• Ball rolls uphill spontaneously.• Wood burns spontaneously in O2.• CO2 and H2O spontaneously form
wood.• Steel rusts spontaneously.• Rust spontaneously turns into iron
and water.
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Its All About Entropy
• Entropy is…• All about the ability of energy to spread
out• Related to probability.• Over-simplified to be about disorder.
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Positional Entropy
• When a substance has more opportunities to exist, (ie, particles can have more than one arrangement) it is also said to have more positional entropy.
• Increasing moles of gas• Increasing volume• Decreasing pressure• Changing state of matter ( s l g )• Exothermic rxns **
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Try Me Conceptual
• Determine which of the following pairs has the most positional entropy:
− 1 mol H2 at STP or 1 Mol H2 at 100oC, 0.5 atm
− 1 mol N2 at STP or 1 mol N2 at 100 K 2atm
− 1 mol H2O(s) at 0oC or 1 mole H2O(l) at 20oC
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2nd Law of Thermodynamics
• Spontaneous processes increase the entropy of the universe.
Suniverse = Ssystem + Ssurroundings
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The Value of S
• Sign is determined by enthalpy:− exothermic− endothermic
• Magnitude is determined by temperature
T
HS gssurroundin
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Try Me Calculation
• The melting point of tungsten (W) is the second highest among the elements, at 3680 K. The enthalpy of fusion for this metal is 35.2 kJ/mol. What is the entropy of fusion?
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3rd Law of Thermodynamics
The entropy of a perfect crystal at 0 Kelvin is zero.
In PhaseOut of Phase
Angle BendingRotation
Translation
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Entropy for a chemical reaction
Sorxn = nSo
p - nSor
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Try Me
• Calculate the change in entropy at 25oC for the reaction:
2 NiS + 3 O2 2 SO2 + 2 NiO
Given Entropy Values:SO2 = 248 J/KmolNiO = 38 J/KmolO2 = 205 J/KmolNiS = 53 J/Kmol
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Free Energy
• Gibbs Free Energy- another method for determining spontaneity.
• Also indicates the amount of available energy that is capable of doing work.
• As an energy source is used, the energy is not destroyed, only converted to a non-usable form.
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G = H - TS
• Defines Gibbs energy in terms of enthalpy and entropy.
• All three factors will contribute to reaction spontaneity.
• When G is negative, the reaction is spontaneous.
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Rearrange that formula
• Lets start with the following 3 formulae:
G = H –TSSsurr = -H/T
Suniv = Ssurr + Ssyst
Suniverse = - G /T
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Fancy Pants Charts
HH SS GG Spontaneous?
__ + __ Always
__ __ +/- When temp is low
+ + +/- When temp is high
+ __ + Never
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Free Energy in Chemical Reactions
• Standard Free energy is used so that we can compare the relative tendency to occur.
• Go = Ho – TSo
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2SO2(g) + O2(g) 2SO3(g)
• The above rxn. Occurs at 25oC and 1 atm. Calculate Ho, So, and Go using the following data:
substance Hof So
kJ/mol J/Kmol
SO2(g) -297 248
SO3(g) -396 257
O2(g) 0 205
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It also works like Hess’s Law problems:
What is the Go for the reaction if the mechanism is:
Cdi(s) + O2(g) CO2(g) Go = -397kJ
Cgr(s) + O2(g) CO2(g) Go = -394 kJ
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Dependence on Pressure
• Enthalpy does not depend on pressure.
• Entropy does depend on temperature.
S low pressure > S high pressure
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G = Go + RT ln(P)
• This can be adapted to reflect partial pressures for the reaction it describes. When that occurs, the formula can be re-written:
Go = -RT ln(Q)
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@ Equilibrium
• K = Q• The free energy is the lowest
possible it will ever be for the system.
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Try Me Out
• The overall rxn for rusting iron by oxygen is
4Fe(s) + 3O2(g) 2 Fe2O3(s)
@25oC, find the equilibrium constant given:
Substance Ho So
Fe2O3(s) -826 90Fe(s) 0 27O2(g) 0 205
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Free Energy and Work
• Achieving the maximum amount of work from a process is highly unlikely because of transfers of energy.
• G represents the maximum possible quantity of work a system is capable of doing.