1501. 1502 where q is the reaction quotient. 1503
TRANSCRIPT
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0B
0A
0D
0C GbGaGdGcΔG
badc [B]lnRT[A]lnRT[D]lnRT[C]lnRT
0ΔG
ba
dc
[B][A]
[D][C]lnRT
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0B
0A
0D
0C GbGaGdGcΔG
badc [B]lnRT[A]lnRT[D]lnRT[C]lnRT
0ΔG
ba
dc
[B][A]
[D][C]lnRT
QlnRTΔGΔG 0
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where Q is the reaction quotient.
0B
0A
0D
0C GbGaGdGcΔG
badc [B]lnRT[A]lnRT[D]lnRT[C]lnRT
0ΔG
ba
dc
[B][A]
[D][C]lnRT
QlnRTΔGΔG 0
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where Q is the reaction quotient. For a reaction system at equilibrium and Q = Kc
0B
0A
0D
0C GbGaGdGcΔG
badc [B]lnRT[A]lnRT[D]lnRT[C]lnRT
0ΔG
ba
dc
[B][A]
[D][C]lnRT
0ΔG
QlnRTΔGΔG 0
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where Q is the reaction quotient. For a reaction system at equilibrium and Q = Kc
and hence (a very key result)
0B
0A
0D
0C GbGaGdGcΔG
badc [B]lnRT[A]lnRT[D]lnRT[C]lnRT
0ΔG
ba
dc
[B][A]
[D][C]lnRT
0ΔG
QlnRTΔGΔG 0
c0 KlnRTΔG
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For a reaction in which all the species are in the gas phase,
p0 KlnRTΔG
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It follows directly from the result
c0 KlnRTΔG
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It follows directly from the result
for Kc > 1
c0 KlnRTΔG
0ΔG0
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It follows directly from the result
for Kc > 1
for Kc < 1
c0 KlnRTΔG
0ΔG0
0ΔG0
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It follows directly from the result
for Kc > 1
for Kc < 1
for Kc = 1 In this case the position of the
equilibrium does not favor either products or reactants forming.
c0 KlnRTΔG
0ΔG0
0ΔG0 0ΔG0
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Example: The standard Gibbs energy for the reaction ½ N2(g) + 3/2 H2(g) NH3(g)
is -16.45 kJmol-1. Calculate the equilibrium constant for the reaction at 25.00 oC. In a certain experiment the initial concentrations are [H2] = 0.00345 M, [N2] = 0.00956 M, and [NH3] = 0.00678 M. Predict the direction of the reaction.
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Example: The standard Gibbs energy for the reaction ½ N2(g) + 3/2 H2(g) NH3(g)
is -16.45 kJmol-1. Calculate the equilibrium constant for the reaction at 25.00 oC. In a certain experiment the initial concentrations are [H2] = 0.00345 M, [N2] = 0.00956 M, and [NH3] = 0.00678 M. Predict the direction of the reaction.
From we have
c0 KlnRTΔG
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Example: The standard Gibbs energy for the reaction ½ N2(g) + 3/2 H2(g) NH3(g)
is -16.45 kJmol-1. Calculate the equilibrium constant for the reaction at 25.00 oC. In a certain experiment the initial concentrations are [H2] = 0.00345 M, [N2] = 0.00956 M, and [NH3] = 0.00678 M. Predict the direction of the reaction.
From we have
c0 KlnRTΔG
RTΔGKln
0c
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Example: The standard Gibbs energy for the reaction ½ N2(g) + 3/2 H2(g) NH3(g)
is -16.45 kJmol-1. Calculate the equilibrium constant for the reaction at 25.00 oC. In a certain experiment the initial concentrations are [H2] = 0.00345 M, [N2] = 0.00956 M, and [NH3] = 0.00678 M. Predict the direction of the reaction.
From we have
c0 KlnRTΔG
RTΔGKln
0c
K) )(298.15molJK(8.314)molJ10x16.45(Kln
1-1-
1-3c
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Hence ln Kc = 6.636
6.636cK e
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Hence ln Kc = 6.636
therefore Kc = 762
6.636cK e
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Hence ln Kc = 6.636
therefore Kc = 762
(Note almost one significant digit is lost in evaluating the exponential.)
6.636cK e
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Hence ln Kc = 6.636
therefore Kc = 762
(Note almost one significant digit is lost in evaluating the exponential.)
To predict the direction of the reaction, use
6.636cK e
QlnRTΔGΔG 0
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Now
3/22
1/22
3][H][N
][NHQ
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Now
3/22
1/22
3][H][N
][NHQ
3/21/2(0.00345)(0.00956)0.00678
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Now
= 342
3/22
1/22
3][H][N
][NHQ
3/21/2(0.00345)(0.00956)0.00678
.000203)(0.0978)(00.00678
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Now
= 342 Since Q < Kc the reaction will move in the direction
to produce more NH3.
3/22
1/22
3][H][N
][NHQ
3/21/2(0.00345)(0.00956)0.00678
.000203)(0.0978)(00.00678
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EmployingQlnRTΔGΔG 0
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EmployingQlnRTΔGΔG 0
(342)lnK) )(298.15molKJ(8.314molkJ16.45ΔG 1-1-1-
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Employing
= – 16.45 kJ mol-1 + 14.46 kJ mol-1 = – 1.99 kJ mol-1
QlnRTΔGΔG 0
(342)lnK) )(298.15molKJ(8.314molkJ16.45ΔG 1-1-1-
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Employing
= – 16.45 kJ mol-1 + 14.46 kJ mol-1 = – 1.99 kJ mol-1
Since < 0, the reaction takes place in the forward direction.
QlnRTΔGΔG 0
(342)lnK) )(298.15molKJ(8.314molkJ16.45ΔG 1-1-1-
ΔG
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Are diamonds forever?
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Are diamonds forever?
In chemical jargon, is the transition Cdiamond Cgraphite
spontaneous at room temperature?
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Are diamonds forever?
In chemical jargon, is the transition Cdiamond Cgraphite
spontaneous at room temperature? Tabulated data: S0(diamond) = 2.44 J K-1 mol-1
S0(graphite) = 5.69 J K-1 mol-1
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Are diamonds forever?
In chemical jargon, is the transition Cdiamond Cgraphite
spontaneous at room temperature? Tabulated data: S0(diamond) = 2.44 J K-1 mol-1
S0(graphite) = 5.69 J K-1 mol-1
for the reaction Cgraphite Cdiamond = 1.90 kJ
0ΔH
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For the reaction Cdiamond Cgraphite
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For the reaction Cdiamond Cgraphite
= 1 mol x 5.69 J K-1 mol-1 – 1 mol x 2.44 J K-1 mol-1
= 3.25 J K-1
0ΔS
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For the reaction Cdiamond Cgraphite
= 1 mol x 5.69 J K-1 mol-1 – 1 mol x 2.44 J K-1 mol-1
= 3.25 J K-1 Using = - 1.90 x 103J - 298 K (3.25 J K-1)
0ΔS
000 ΔST-ΔHΔG
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For the reaction Cdiamond Cgraphite
= 1 mol x 5.69 J K-1 mol-1 – 1 mol x 2.44 J K-1 mol-1
= 3.25 J K-1 Using = - 1.90 x 103J - 298 K (3.25 J K-1) = - 1.90 x 103J - 969 J = -2.87 kJ
0ΔS
000 ΔST-ΔHΔG
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For the reaction Cdiamond Cgraphite
= 1 mol x 5.69 J K-1 mol-1 – 1 mol x 2.44 J K-1 mol-1
= 3.25 J K-1 Using = - 1.90 x 103J - 298 K (3.25 J K-1) = - 1.90 x 103J - 969 J = -2.87 kJ Conclusion: The reaction Cdiamond Cgraphite
is
spontaneous!
0ΔS
000 ΔST-ΔHΔG
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Electrochemistry
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Electrochemistry
Two Key Ideas in this section:
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Electrochemistry
Two Key Ideas in this section: (1) Use spontaneous redox chemistry to generate
electricity.
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Electrochemistry
Two Key Ideas in this section: (1) Use spontaneous redox chemistry to generate
electricity.(2) Use electric current to drive non-spontaneous
redox reactions.
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Oxidation-reduction reactions
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Oxidation-reduction reactions
Oxidation number: A number equal to the charge an atom would have if its shared electrons were held completely by the atom that attracts them more strongly.
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Oxidation-reduction reactions
Oxidation number: A number equal to the charge an atom would have if its shared electrons were held completely by the atom that attracts them more strongly.
Need to be able to distinguish reactions where there is a change in oxidation number (these are redox reactions) from reactions where there is no change in oxidation number.
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Example:
Na2SO4(aq) + BaCl2(aq) 2 NaCl(aq) + BaSO4(s)
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Example:
Na2SO4(aq) + BaCl2(aq) 2 NaCl(aq) + BaSO4(s) + 2- 2+ -+ 2-2+-
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Example:
Na2SO4(aq) + BaCl2(aq) 2 NaCl(aq) + BaSO4(s)
There is no change in oxidation number, there is no electron transfer taking place in this reaction.
Example: 2 Na(s) + Cl2(g) 2 NaCl(s)
+ 2- 2+ -+ 2-2+-
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Example:
Na2SO4(aq) + BaCl2(aq) 2 NaCl(aq) + BaSO4(s)
There is no change in oxidation number, there is no electron transfer taking place in this reaction.
Example: 2 Na(s) + Cl2(g) 2 NaCl(s)
+ 2- 2+ -+ 2-2+-
0 0 -+
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Example:
Na2SO4(aq) + BaCl2(aq) 2 NaCl(aq) + BaSO4(s)
There is no change in oxidation number, there is no electron transfer taking place in this reaction.
Example: 2 Na(s) + Cl2(g) 2 NaCl(s)
In this example, there is a change in oxidation number, so electron transfer is taking place.
+ 2- 2+ -+ 2-2+-
0 0 -+
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Half-Equations:
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Half-Equations:
Na Na+ + e- (1)
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Half-Equations:
Na Na+ + e- (1)
2e- + Cl2 2 Cl- (2)
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Half-Equations:
Na Na+ + e- (1)
2e- + Cl2 2 Cl- (2) -
+0
0
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Half-Equations:
Na Na+ + e- (1)
2e- + Cl2 2 Cl- (2)
These are called the half-equations.
-
+0
0
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Half-Equations:
Na Na+ + e- (1)
2e- + Cl2 2 Cl- (2)
These are called the half-equations. Multiply equation (1) by 2 and add equation (2) leads to the overall balanced equation.
2 Na + Cl2 2 NaCl
-
+0
0
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Oxidation: A process in which electrons are lost. Na Na+ + e-
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Oxidation: A process in which electrons are lost. Na Na+ + e-
Reduction: A process in which electrons are gained. 2e- + Cl2 2 Cl-
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Oxidation: A process in which electrons are lost. Na Na+ + e-
Reduction: A process in which electrons are gained. 2e- + Cl2 2 Cl-
Oxidizing Agent: A substance that gains electrons (in the reduction step). It undergoes a decrease in oxidation number. Example: Cl2 in the above eq.
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Oxidation: A process in which electrons are lost. Na Na+ + e-
Reduction: A process in which electrons are gained. 2e- + Cl2 2 Cl-
Oxidizing Agent: A substance that gains electrons (in the reduction step). It undergoes a decrease in oxidation number. Example: Cl2 in the above eq.
Reducing Agent: A substance that donates electrons in a redox reaction (specifically in the oxidation step). It undergoes an increase in oxidation number. Example: Na in the above oxidation.
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Redox reaction: A reaction that can be split apart into an oxidation step and a reduction step.
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Redox reaction: A reaction that can be split apart into an oxidation step and a reduction step.
Example: 2 Na + Cl2 2 NaCl