review of chapter 6
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Review of Chapter 6. Thermodynamics = study of energy and its interconversions Chemists focus on HEAT energy and the CHANGE IN HEAT of a reaction Ways to measure Enthalpy of a reaction Calorimetry (q = mc D T) Hess’s Law Standard enthalpy of formation Bond Energy. - PowerPoint PPT PresentationTRANSCRIPT
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Review of Chapter 6Review of Chapter 6Thermodynamics = study of energy
and its interconversionsChemists focus on HEAT energy and
the CHANGE IN HEAT of a reactionWays to measure Enthalpy of a
reaction– Calorimetry (q = mcT)– Hess’s Law– Standard enthalpy of formation– Bond Energy
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Spontaneous ReactionsSpontaneous Reactions
A Spontaneous reaction occurs without outside assistance.
Using enthalpy to predict spontaneity– If the energy of the products is lower than the
reactants, reaction is likely to occur– When this is true, the reaction is exothermic– Most exothermic reactions are spontaneous
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Spontaneous ReactionsSpontaneous Reactions
Not all spontaneous reactions are exothermic
Nor are all exothermic reactions spontaneous
Enthalpy of a reaction is not the only factor that determines if a reaction is spontaneous
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EntropyEntropy
Entropy (S) = measurement of randomness or disorder of a system
Left alone, systems tend to become more disordered
Chemists study the change in entropy (S) that accompanies a reaction
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Entropy and SpontaneityEntropy and Spontaneity
If the products are in a greater state of disorder (increase in entropy) than the reactants, reaction is likely to be spontaneous.
Entropy’s effect on spontaneity depends on temperature
Unit of entropy = J / K mol
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Spontaneous ReactionsSpontaneous Reactions
Most spontaneous reactions are
– Exothermic– Increase in Entropy
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11stst Law Thermodynamics Law Thermodynamics
Total energy of the universe remains constant
This is synonymous with the Law of Conservation of Energy
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22ndnd Law Thermodynamics Law Thermodynamics
Entropy of the universe INCREASES for any spontaneous process.– For any spontaneous process, the
sum of the change in entropy of the system and surroundings must be positive
Suniverse (+) = Ssystem + Ssurroundings
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22ndnd Law (con’t) Law (con’t)
Reactions are only spontaneous in ONE direction.
If a reaction is spontaneous, it will NOT be spontaneous in the reverse direction.
If a reaction is NOT spontaneous as written, it WILL be spontaneous in the reverse direction.
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33rdrd Law Thermodynamics Law Thermodynamics
The entropy of a perfect crystal at 0 Kelvin is zero.
All other entropy values are positive.
Change in entropy values however, can be positive or negative.
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EntropyEntropy
Change in entropy of a reaction will be positive if– Solids are melted to form pure liquids
or dissolved to form solutions.– Liquids or solids are converted to gas.– The number of moles of gas increases.
(More moles of gas on product side)– The temperature increases.
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EntropyEntropy
Predict if the following will have a positive change in entropy or negative– 2 C (s) + O2 (g) 2CO (g)
– 2 K (s) + Br2 (l) 2KBr (s)
– 2 MnO2 (s) 2 MnO (s) + O2 (g)
– O (g) + O2 (g) O3 (g)
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EntropyEntropy
Entropy is a state functionChange in entropy can be calculated
just like enthalpySo
reaction = npSo products – nrSo
reactants
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Calculating EntropyCalculating Entropy
Predict and find So for the reaction
2 NH3 (g) + CO2 (g) NH2CONH2 (aq) + H2O(l)
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Entropy at a Phase Entropy at a Phase ChangeChange
To calculate entropy at a phase change, use the equation
S = H / T**change H to joules
You must know H and the temperature to do this!
Try the example in the notes.
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Free EnergyFree Energy
What can tell us if a reaction is spontaneous?
A property called Free energy (G)Free energy = measurement of
amount of energy available to flow out of a system
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Free EnergyFree Energy
Free energy depends on three things– Change in enthalpy– Change in entropy– Temperature
Change in free energy can be calculated using the equation
Go = Ho – TSo **This equation is
important
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GGoo = = HHoo – T – TSSoo
Ho tells us the amount of work that could potentially be done by the system
So tells us the amount of work that needs to be done on the system
Go is the maximum amount of work that can be done by the system
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Free Energy SignFree Energy SignThe sign of free energy tells us
about the reaction
G value
Type of Reaction
Negative
Spontaneous as written
Positive NOT spontaneous as written(Spontaneous in reverse
direction)
Zero System is at equilibrium
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Free EnergyFree Energy
Free energy is a state functionStandard free energy of formation can
be calculated and used to determine change in free energy of a reaction
Change in free energy can be foundGo
reaction = Gof (products) - Go
f (reactants)
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Calculating Free EnergyCalculating Free Energy
Use the Gof values in the book to
determine Go for the reactionC2H5OH (l) + 3 O2 (g) 2 CO2 (g) + 3 H2O (g)
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Non-Standard ConditionsNon-Standard Conditions
To find G under non-standard conditions, you must know– Change in free energy under standard
conditions (Go)– Temperature (T)– Ratio of products to reactants (Q)
(Reaction Quotient)
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Reaction QuotientReaction Quotient
Most reactions do not go all the way to completion
Reaction quotient tells us how far a reaction has gone
Molar concentrations or pressures for gases are used to calculate the reaction quotient
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Reaction QuotientReaction Quotient
Equation to calculate reaction quotientQ = molar concentrations products
molar concentrations reactantFor the reaction aA + bB qQ + rR
Q = [Q]q [R]r
[A]a [B]b
** Amounts of pure solids and liquids not used
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Free Energy Non-StandardFree Energy Non-Standard
Once you know the– Standard change in free energy– Temperature– Reaction quotientYou can use the equationG = Go + RT lnQR = universal gas constant (8.314 J/K
mol)
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Free Energy at EquilibriumFree Energy at Equilibrium
In fact, these reactions can occur in the forward direction and the reverse direction simultaneously
For reactions that do not go all the way to completion, there comes at point at which the rate of reaction in the forward direction equals the rate of reaction in the reverse direction
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Free Energy at EquilibriumFree Energy at Equilibrium
Ta-Da!!! This point is known as equilibrium All reactions that come to equilibrium do
so at a very specific set of conditions The ratio of products to reactants at
equilibrium is expressed by the equilibrium constant (K)
At equilibrium the reaction quotient is equal to the equilibrium constant (Q = K)
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Free Energy at EquilibriumFree Energy at Equilibrium
Additionally, a system at equilibrium has no free energy available to flow out of the system. Therefore, at equilibrium, G = 0 and Q = K.
We can find the standard change in free energy for a system at equilibrium by using the equation:
Go = - RT ln K
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Free Energy at EquilibriumFree Energy at Equilibrium The sign of Go and the value of K can
give us information about how far a reaction comes toward completion before equilibrium is reached.
Sign of Go K value Favored direction
Negative Greater than 1
Products favored (greater concentration of products than reactants)
Zero Equal to 1 Equal products and reactants
Positive Less than 1 Reactants favored (greater concentration of reactants than products)