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IB Chemistry Equilibrium constant, Kc and Reaction quotient, Qc.
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Transcript of IB Chemistry Equilibrium constant, Kc and Reaction quotient, Qc.
Dynamic Equilibrium
Closed system
Reversible
Forward Rate, Kf
Reverse Rate, Kr
2NO2(g) N2O4(g)
Chemical system
Forward rate rxn Rate Combining
Backward rate rxn Rate dissociation
Reversible rxn happening, same time with same rate
Rate of forward = Rate of backward
Conc of reactant and product remain UNCHANGED/CONSTANT not EQUAL
combining dissociation
Conc vs time Rate vs time
Conc
Time
Conc NO2
Conc N2O4
With time • Conc NO2 decrease - Forward rate decrease
• Conc N2O4 increase - Backward rate increase
2NO2(g) N2O4(g)
Forward rate
Backward rate
Forward Rate = Backward Rate
Conc NO2 and N2O4 remain UNCHANGED/CONSTANT
brown colourless
How dynamic equilibrium is achieved in closed system?
Conc of NO2 decrease ↓over time
Forward rate, Kf decrease ↓ over time
Forward Rate = Reverse Rate
NO2
2NO2(g) N2O4(g)
Conc of N2O4 increase ↑ over time
N2O4
Reverse rate, Kr increase ↑ over time
NO2
N2O4
1
2
Conc of reactant/product remain constant
Rate
3
Time
Conc
NO2
N2O4
At dynamic equilibrium
As reaction proceeds Concentration
As reaction proceeds Rate
Time
Click here to view simulation
Conc vs Time
How dynamic equilibrium is achieved in a closed system?
40 0
Rate forward = ½ breakdown = ½ x 40 = 20
Rate reverse = ¼ form = ¼ x 0 = 0
20 20
Rate forward = ½ breakdown = ½ x 20 = 10
Rate reverse = ¼ form = ¼ x 20 = 5
15 25
Rate forward = ½ breakdown = ½ x 15 = 8
Rate reverse = ¼ form = ¼ x 25 = 6
13 27
Rate forward = ½ breakdown = ½ x 13 = 7
Rate reverse = ¼ form = ¼ x 27 = 7
13 27
At dynamic Equilibrium Rate forward = Rate reverse Breakdown (7) = Formation (7)
At dynamic Equilibrium Conc reactant 13 /Product 27 constant
Rate vs Time
4/1
2/1
..tan..
..tan..
1
1 reversetconsrate
forwardtconsrate
K
K
213
27
tan
treac
productK c
24/1
2/1
1
1 K
KK c
or
Dynamic Equilibrium
Reversible (closed system)
Forward Rate, K1 Reverse Rate, K-1
Kc = ratio of molar conc of product (raised to power of their respective stoichiometry coefficient) to molar conc of reactant (raised to power of their respective stoichiometry coefficient)
Conc of product and reactant at equilibrium
At Equilibrium
Forward rate = Backward rate Conc reactants and products remain CONSTANT/UNCHANGE
Equilibrium Constant Kc
aA(aq) + bB(aq) cC(aq) + dD(aq)
coefficient
Solid/liq not included in Kc
Conc represented by [ ]
K1
K-1
ba
dc
cBA
DCK
1
1
K
KKc
Equilibrium Constant Kc
express in
Conc vs time Rate vs time
A + B
C + D
Conc
Time
Click here notes on dynamic equilibrium
Excellent Notes
reversetconsrate
forwardtconsrate
K
K
..tan..
..tan..
1
1
Large Kc
• Position equilibrium shift to right • More products > reactants
Magnitude of Kc
ba
dc
cBA
DCK
Extend of reaction
How far rxn shift to right or left?
Not how fast
ba
dc
cBA
DCK
Small Kc
• Position equilibrium shift to left • More reactants > products
cKcK
Position of equilibrium
2CO2(g) ↔ 2CO(g) + O2(g)
92103 cK
2H2(g) + O2(g) ↔ 2H2O(g)
81103cK
H2(g) + I2(g) ↔ 2HI(g)
2107.8 cK1
Kc
• Position equilibrium lies slightly right • Reactants and products equal amount
Reaction completion
Product favoured Reactant favoured Reactant/Product equal
cK
Temp dependent
Extend of rxn
Not how fast
Equilibrium Constant Kc
ba
dc
cBA
DCK
aA(aq) + bB(aq) cC(aq) + dD(aq)
Conc of product and reactant at equilibrium
Equilibrium expression HOMOGENEOUS gaseous rxn
4NH3(g) + 5O2(g) ↔ 4NO(g) + 6H2O(g) N2(g) + 3H2(g) ↔ 2NH3(g)
NH4CI(s) ↔ NH3(g) + HCI(g)
2SO2(g) + O2(g) ↔ 2SO3(g)
52
4
3
6
2
4
ONH
OHNOKc
32
1
2
2
3
HN
NHKc
11
3 HCINHKc
04
11
3
CINH
HCINHKc
12
2
2
2
3
OSO
SOKc
Equilibrium expression HETEROGENOUS rxn
CaCO3(s) ↔ CaO(g) + CO2(g)
03
1
2
1
CaCO
COCaOK c
12
1COCaOK c
CH3COOH(l) + C2H5OH(l) ↔ CH3COOC2H5(l) + H2O(l)
152
1
3
1
2
1
523
OHHCCOOHCH
OHHCOOCCHK c
Equilibrium expression HOMOGENEOUS liquid rxn
Cu2+(aq) + 4NH3(aq) ↔ [Cu(NH3)4]
2+
43
12
2
43 )(
NHCu
NHCuK c
Reactant/product same phase
Reactant/product diff phase
aA bB
2aA 2bB
bB aA
aA bB
aA bB
a
b
cA
BK
aA bB
Equilibrium Constant Kc Equilibrium Constant Kc
b
a
cB
AK
'
c
cK
K1'
inverse
X2 coefficient
2'
cc KK
coefficient
2
1
a
b
c
A
BK
21
21
'
ccc KKK 21'
a
b
cA
BK
a
b
cA
BK
a
b
cA
BK
2
2'
2
1
aA bB bB cC
a
b
ciA
BK
b
c
ciiB
CK
+ 2 reactions + aA cC
a
c
a
b
b
c
cA
C
A
B
B
CK
'
ciciic KKK '
Effect on Kc
Inverse Kc
Square Kc
Square root cK
Multiply both Kc
2
1
ciiK ciK
N2(g) + O2(g) ↔ 2NO(g)
2NO(g) + O2(g) ↔ 2NO2(g)
19103.2 ciK
6103ciiK
2NO2(g) ↔ N2(g) + 2O2(g)
13
619
107
103103.2
c
c
ciicic
K
K
KKKN2(g) + 2O2(g) ↔ 2NO2(g)
13107 cK
12'
13
'
1042.1
107
11
c
c
c
K
KK
HF(ag) ↔ H+(aq) + F -(aq)
H2C2O4(ag) ↔ 2H+(aq) + C2O4
2 -(aq)
4108.6 ciK
6108.3 ciiK
2HF(ag) + C2O42- ↔ 2F -
(aq) + H2C2O4(aq)
2HF(ag) ↔ 2H+(aq) + 2F -(aq)
2H+(ag) + C2O4
2- ↔ H2C2O4(aq)
7242'106.4108.6 cic KK
5
6
''106.2
108.3
11
cii
cK
K
12.0106.2106.4 57
'''
c
ccc
K
KKK
Kc for diff rxn Adding 2 rxns
+
Inverse rxn
Adding 2 rxns
2HF(ag) + C2O42- ↔ 2F -
(aq) + H2C2O4(aq)
+
HF(ag) ↔ H+(aq) + F -(aq)
4108.6 ciK
x2 coefficient
H2C2O4(ag) ↔ 2H+(aq) + C2O4
2 -
Inverse rxn
6108.3 ciiK
2HF(ag) ↔ 2H+(aq) + 2F -
(aq)
2H+(ag) + C2O4
2- ↔ H2C2O4(aq)
Add 2 rxn
7'106.4 cK
5''106.2 cK+
Effect on Kc Effect on Kc
Inverse rxn Inverts expression
Doubling rxn coefficient Squares expression
Tripling rxn coefficient Cubes expression
Halving rxn coefficient Square root expression
Adding 2 reactions Multiplies 2 expression
cK
1
2
cK
3
cK
cK
ii
c
i
c KK
Square Kc
Invert Kc
Multiply Kc
1
2
3
N2(g) + 2O2(g) ↔ 2NO2(g)
H2 + I2 ↔ 2HI
50cK
12
1
2
2
IH
HIK c
2HI ↔ H2 + I2
2
1
2
1
2'
HI
IHK c
02.050
11'
c
cK
K
2SO2 + O2 ↔ 2SO3
12
2
2
2
3
OSO
SOKc
200cK
SO2 + O2 ↔ SO3
1.14200'
cc KK
2
1
4SO2 + 2O2 ↔ 4SO3
40000
200
,
22'
c
cc
K
KK
N2(g) + 3H2(g) ↔ 2NH3(g)
32
1
2
2
3
HN
NHKc
Kc is 170 at 500K Determine if rxn is at equilibrium when conc are at: [N2] =1.50, [H2] = 1.00, [NH3] = 8.00
00.150.1
00.8
3
2
1
2
2
3
c
c
Q
HN
NHQ
• Rxn not at equilibrium
• Shift to right, favour product
• Qc must increase, till equal to Kc
IB Questions
Determine Kc for inversing rxn
inverse
Determine Kc for halving rxn
2
1
1
2
2
2
2
3
OSO
SOK c
halving
Determine Kc for doubling rxn
2SO2 + O2 ↔ 2SO3 doubling
12
2
2
2
3
OSO
SOKc
200cK
2
1
2
2
2
2
3
OSO
SOK c
2 1
3 4
170cK7.42cQ
cc KQ
Kc and Qc
H2(g) + I2(g) ↔ 2HI(g)
cK
Constant at fixed Temp
12
1
2
2
IH
HIK c
At equilibrium
Independent of initial conc
Initial conc of H2 , I2 and HI
00.4cQ
12
1
2
2
IH
HIQc
4.461012.01014.1
1052.21212
22
cK 4.46cK
Expt Initial Conc H2
Initial Conc I2
Initial Conc HI
1 0.0500 0.0500 0.100
Initial conc of H2 , I2 and HI
Expt Initial Conc H2
Initial Conc I2
Initial Conc HI
1 2.40 x 10-2 1.38 x 10-2 0
Expt Equilibrium Conc H2
Equilibrium Conc I2
Equilibrium Conc HI
1 1.14 x 10-2 0.12 x 10-2 2.52 x 10-2
At equilibrium conc
Not at equilibrium
H2(g) + I2(g) ↔ 2HI(g)
00.4
050.0050.0
100.02
cQ
Predict the direction of rxn
Difference between
cQ
Conc of product/reactant at
equilibruim conc
Reaction quotient at particular time
Not at equilibrium conc
Varies NOT constant
Kc and Qc
H2(g) + I2(g) ↔ 2HI(g)
12
1
2
2
IH
HIK c
4.461012.01014.1
1052.21212
22
cK 4.46cK
At equilibrium conc
cc KQ cc KQ cc KQ
Reaction at equilibrium
More product > reactant
Rxn shift left more reactant
→
cc KQ
cQ
Bring Qc down
More reactant > product
Rxn shift right → more product
Bring Qc up cQ
cc KQ
cQ
Expt Initial Conc H2
Initial Conc I2
Initial Conc HI
1 0.0500 0.0500 0.100
Initial conc of H2 , I2 and HI
12
1
2
2
IH
HIQc
00.4050.0050.0
100.02
cQ
cQ
Expt Initial Conc H2
Initial Conc I2
Initial Conc HI
1 0.0250 0.0350 0.300
Initial conc of H2 , I2 and HI
12
1
2
2
IH
HIQc
1030350.00250.0
300.02
cQ
Click here to view notes
Kc from reaction stoichiometry
H2(g) + I2(g) ↔ 2HI(g)
4.46 sameK c
12
1
2
2
IH
HIK c
Kc = 46.4 ( 730K) At equilibrium 4 diff initial conc of H2 , I2 and HI
4.461012.01014.1
1052.21212
22
cKRxn 1
same
Qc = Kc - rxn at equilibrium, no side/shift occur Qc < Kc – rxn shift right, favour product Qc > Kc – rxn shift left, favour reactant
Rxn 2, 3, 4 diff initial conc
more products
H2(g) + I2(g) ↔ 2HI(g)
cQ
Rxn shift to right
Rxn shift to left more reactants
treac
productQc
tan
treac
productQc
tan
cQ
cc KQ cc KQ cc KQ
12
1
2
2
IH
HIKc
Kc and Qc
H2(g) + I2(g) ↔ 2HI(g)
12
1
2
2
IH
HIK c
00.4cQ
12
1
2
2
IH
HIQc
4.461012.01014.1
1052.21212
22
cK 4.46cK
Expt Initial Conc H2
Initial Conc I2
Initial Conc HI
1 0.0500 0.0500 0.100
Initial conc of H2 , I2 and HI
At equilibrium conc
Not at equilibrium
H2(g) + I2(g) ↔ 2HI(g)
00.4
050.0050.0
100.02
cQ
cc KQ cc KQ
Reaction at equilibrium
More reactant > product
Rxn shift right → more product
Bring Qc up cQ
cQ
cc KQ
00.4cQ 4.46cK<
Kc and Qc
H2(g) + I2(g) ↔ 2HI(g)
12
1
2
2
IH
HIK c
103cQ
12
1
2
2
IH
HIQc
4.461012.01014.1
1052.21212
22
cK 4.46cK
Initial conc of H2 , I2 and HI
At equilibrium conc
Not at equilibrium
H2(g) + I2(g) ↔ 2HI(g)
cc KQ cc KQ
Reaction at equilibrium
More product > reactant
Rxn shift left more reactant
→
cc KQ
cQ
Bring Qc down cQ
Expt Initial Conc H2
Initial Conc I2
Initial Conc HI
1 0.0250 0.0350 0.300
1030350.00250.0
300.02
cQ
103cQ 4.46cK>
How dynamic equilibrium is shifted when H2 is added ?
• Add H2 , Qc decrease • Position equilibrium shift to right • Rate forward and reverse increase • New equilibrium conc achieved when Rate forward Kf = Rate reverse Kr • More product NH3 ,but Kc unchanged
N2(g) + 3H2(g) ↔ 2NH3(g) 07.4cK
Equilibrium disturbed H2 added. More reactant
At equilibrium Conc reactant/product no change
At new equilibrium Conc reactant/product no change
24.2cQ
Equilibrium Conc H2 = 0.82M Equilibrium Conc N2 = 0.20M Equilibrium Conc NH3 = 0.67M
32
1
2
2
3
HN
NHKc
31
2
82.020.0
67.0cK
New Conc H2 = 1.00M Conc N2 = 0.20M Conc NH3 = 0.67M
32
1
2
2
3
HN
NHQc
31
2
00.120.0
67.0cQ
07.4cK
New Equilibrium Conc H2 = 0.90M New Equilibrium Conc N2 = 0.19M New Equilibrium Conc NH3 = 0.75M
31
2
90.019.0
75.0cK
32
1
2
2
3
HN
NHKc
07.4cK
Shift to the right - Increase product - New Conc achieve - Qc = Kc again
cc KQ
How dynamic equilibrium is shifted when H2 is added ?
• Add H2 , Qc decrease • Position equilibrium shift to right • Rate forward and reverse increase • New equilibrium conc achieved when Rate forward Kf = Rate reverse Kr • More product NH3 ,but Kc unchanged
Rate forward Kf = Rate reverse Kr
N2(g) + 3H2(g) ↔ 2NH3(g) 07.4cK
07.4 cc KQ
Equilibrium disturbed H2 added. More reactant
cc KQ
Equilibrium shift to right Rate forward Kf > Rate reverse Kr
cQ
At equilibrium Conc reactant/product no change
At new equilibrium Conc reactant/product no change
Qc increase until Qc = Kc
cQ
Rate forward Kf = Rate reverse Kr
cc KQ
cc KQ cc KQ
Click here to view simulation Click here simulation using paper clips Click here simulation on reversible rxn
Click here on reversible rxn
Simulation on Dynamic equilibrium
Click here on equilibrium constant