CHEM 30
EQUILIBRIUM - In a chemical reaction, chemical equilibrium is the state in which both
reactants and products are present in concentrations which have no further tendency to change with time. Usually, this state results when the forward reaction proceeds at the same rate as the reverse reaction. The reaction rates of the forward and backward reactions are generally not zero, but equal. Thus, there are no net changes in the concentrations of the reactant(s) and product(s). Such a state is known as dynamic equilibrium.
During dynamic equilibrium there are no macroscopic changes such as pH, color , pressures, concentrations and conductivity but particles and atoms are still reacting so there are
microscope changes always occurring due to energy of the particles and environment.
There are 5 methods of expressing equilibrium 1. using words
2. using equilibrium arrows 3. using % reaction
4. using equilibrium expression and equilibrium constant 5. using graphs
1. Words :
An equilibrium is reached when the rate of forward reaction is equal to the rate of reverse reaction.
An equilibrium FAVORS the product side if there are more product concentration than the reactant concentration when the equilibrium is established
( equilibrium can be described as favoring reactant side if the opposite is true ) An equilibrium is QUANTITATIVE to product side if there are hugh amount of
product produced and a very very small amount of reactant left unreacted ( equilibrium can be quantitative to reactant side if no appreciable product is
producted )
2. Arrows
A favoring arrow will be three arrows with the third pointing to side with higher concentration
A quantitative arrow will be a single arrow pointing to side with extreme high concentration
3. % reaction
The equilibrium is said to favor product side if there are greater than 50 % of product and less than 49 % of reactant in the equilibrium mixture
( what if the equilibrium favors the reactant side ? ) or
Equilibrium
Equilibrium mixture
arrows % reaction Equilibrium constant ( K ) Reactant concentration Product concentration Favors product side
A + B
C + D
Greater than50 %
K > 1
Favors
reactant side
A + B
C + D Less than50 % K < 1
Quantitative
to product side
A + B
C
+D
→ Greater than 99 % Products/0 Quantitativeto reactant
A
B
C + D ← Less than 1 0/reactantsThe equilibrium is said to be quantitative if there are greater than 99 % of product and less than 1 % of reactant
( what about quantitative to reactant side ? )
4. equilibrium expression & constant
The equilibrium expression gives a ratio of the concentration of products compare against reactant concentration
e.g. aA + bB ⇋ cC + dD
if the product concentrations are higher than reactant, K value will be slightly higher than 1
if product concentrations are lower than reactant, K value will be slightly lower than 1
K values will be very large in a quantitative to product reaction K values will be very small in a quantitative to reactant reaction
Since equilibrium must be established at a specific temperature, the equilibrium
constant value does not change unless there is a permanent change of temperature. e.g. if an equilibrium is established at 100oC, and the K of reaction = 5.00 . then
this value will stays the same unless the equilibrium mixture is placed into a different temperature environ. Temperature change is the only method of changing the K value.
Synopsis of the above concepts [B] [A]
[D] [C]
K a b
5. equilibrium graphs
Initial concentrations are the concentrations of reactants at time 0
Equilibrium concentrations are the concentration of reactants and products after equilibrium is reached
If a time lapse picture of the reaction is plotted on graph paper, the following graph can be observed. ( assuming reaction to be : 2 A + 1 B ⇋ 2 C + 1 D )
A species concentration change on the graph is proportional to its balanced stoichiometric coefficient in a balanced chemical formula.
A C D B
concentration
time
Time at which equilibrium is established
Coefficient of B= 1 Coefficient of A = 2
PRACTICE
Write an equation to describe the following situations ( use A & B for reactants and C & D for products )
1. The equilibrium favors the products _____________________________________________
2. The equilibrium favors the reactants _____________________________________________
3. The equilibrium is quantitative to products ________________________________________
4. There is no appreciable product produced _________________________________________
5. The equilibrium is quantitative to reactant side _____________________________________
Describe the following in a sentence
1. A(aq) + B(aq) C(aq) + D(aq) ______________________________________________
2. A(l) + B(l) C(l) + D(l) _________________________________________________
3. A(g) + B(g) C(g) + D(g) __________________________________________________
4. A(aq) + B(g) C(aq) + D(g) _________________________________________________
Using arrows and sentence to describe the following situations ( use A,B and C,D )
1. An equilibrium is established when 65% of reactants used are reacted
2. An equilibrium is established when 25% of the reactants used are reacted
3. Greater than 99% of reactants had reacted when equilibrium is reached
Le Chatelier’s Principle
“If a dynamic equilibrium is subjected to a stress it readjusts so as to minimize the effect of that stress!”
For
• A + B ⇋ C + D The equilibrium will shift to the RIGHT if you: add A, add B, remove C or remove D. (K is unaffected)
• A + B ⇋ C + D The equilibrium will shift to the LEFT if you: remove A, remove B, add C or add D. (K is unaffected)
• A + heat ⇋ B (endothermic reaction) The equilibrium will shift to the RIGHT if you: add heat (increase the system temperature). (K will increase)
• A + heat ⇋ B (endothermic reaction) The equilibrium will shift to the LEFT if you: remove heat (decrease the system temperature). (K will decrease)
• A ⇋ B + heat (exothermic reaction) The equilibrium will shift to the RIGHT if you: remove heat (decrease the system temperature). (K will increase)
• A ⇋ B + heat (exothermic reaction) The equilibrium will shift to the LEFT if you: add heat (increase the system temperature). (K will decrease)
• A(g) ⇋ 2B(g) The equilibrium will shift to the RIGHT (the side with more mol of gas molecules) if you decrease the system pressure. (K is unaffected)
• A(g) ⇋ 2B(g) The equilibrium will shift to the LEFT (the side with fewer mol of
gas molecules) if you increase the system pressure bydecreasing the volume. (K is unaffected)
Increasing the system pressure on a gaseous equilibrium by adding an inert gas has absolutely no effect on that equilibrium. Although the total system pressure increases the partial pressures of the equilibrium components (the P’s that appear in the Kp expression) do not change! (K is unaffected)
For any equilibrium catalysts have no effect on the position of equilibrium they only cause the system to reach equilibrium more rapidly. (K is unaffected)
It is patently silly to remember each and every case outlined above. It is more profitable to remember how to work out the equilibrium shift in each instance.
Remember VPn for an easy trick…like so…If volume increases the pressure drops and the moles must increase to compensate. And vice versa…as everyone should have a VPN! Trust no one!
Le Chatelier’s Principle
1. State Le Chatelier’s Principle ?
2. For each of the following equilibrium reaction and stresses placed on it, a. State the direction of the equilibrium shift
b. Account for the changes of concentrations of reactants and products
A. H2S(g) + NaOH(aq) ⇋ NaHS(aq) + H2O(l)
( Stress : NaOH(aq) is added into equilibrium mixture )
B. CH3COOH(aq) + PO43-(aq) ⇋ CH3COO-(aq) + HPO42-(aq)
( Stress : Na2HPO4(aq) is added into the equilibrium mixture )
C. H2S(g) + NaOH(aq) ⇋ NaHS(aq) + H2O(l)
D. CH3COOH(aq) + PO43-(aq) ⇋ CH3COO-(aq) + HPO42-(aq)
( Stress : CH3COOH(aq) is removed from the equilibrium mixture )
E. 2 NaOH(aq) + H2SO4(aq) ⇋ 2 H2O(l) + Na2SO4(aq) + Energy
( Stress : if the reaction mixture is cooled )
F. H2CO3(aq) + energy ⇋ H2O(l) + CO2(g)
( Stress : if reaction mixture is heated )
G. HCl(g) + NH3(g) ⇋ NH4Cl(g) + energy
H. H2(g) + F2(g) ⇋ 2 HF(g)
( Stress : if pressure is decreased )
I. N2(g) + 3 H2(g) < = > 2 NH3(g)
( Stress : if volume of container is increased )
J. 4 NH3(g) + 5 O2(g) < = > 4 NO2(g) + 6 H2O(g) + energy
( Stress : if volume of container is decreased )
3. Give a generalization of Le Chatelier’s Principle for concentration changes ( increase and
4. Given the following graph, (a ). identify the reactants and the products of reaction
(b). write chemical equation to represent reaction ( include probable coefficient )
(c). identify time ( a, b, c or d ) at which equilibrium is established
5. Given the following equilibrium graph of a gaseous system, ( coefficients are given )
1. identify the reactants and products of this reaction __________________________________
2. write an equation to represent the chemical reaction _________________________________
3. what stress was imposed during time “A” _______________________________________
what stress was imposed during time “B” _______________________________________
what stress was imposed during time “C” _______________________________________ what stress was imposed during time “D” _______________________________________
Change of concentration
A B C Time
a b c d
Change of concentration
Time
3a
2B
1C
4. identify the direction of shift of equilibrium in response to the stress
A.
B
C
D.
What is happening at each time…meaning what stress was applied at….
2 min?
4min?
10 min?
14 min?
A) Consider the gaseous equilibrium:
H2(g) + I2(g)<---> 2HI(g)
The sketches below show the pressures, P, of these three gases as a function of time during an experiment. The system is initially at
equilibrium. At some time, t, extra I2 is added.
Equilibrium Constant Expressions
Provide equilibrium expressions for the following
a. 3 H2(g) + N2(g) ⇋ 2 NH3(g) (The Haber-Bosch Process)
b. CH4(g) + H2O(g) ⇋ 3 H2(g) + CO(g)
c. C(s) + H2O(g) ⇋ H2(g) + CO(g) (Synthesis Gas Reaction)
d. Al(H2O)63+(aq) + H2O(l) ⇋ Al(OH)(H2O)52+(aq) + H3O+(aq)
e. 2 Cl-(aq) + 2 H2O(l) ⇋ Cl2(g) + H2(g) + 2 OH-(aq)
f. CaCl2(s) ⇋ Ca2+(aq) + 2Cl-(aq) (Solubility Product, Ksp)
g. HSO4-(aq) + H2O(l) ⇋ SO42-(aq) + H3O+(aq) (Acid Ionization)
h. HSO4-(aq) ⇋ SO42-(aq) + H+(aq) (Acid Dissociation)
i. CuSO4•5H2O(s) ⇋ CuSO4(s) + 5 H2O(g)
j. NH4HS(s) ⇋ NH3(g) + H2S(g)
k. 2 HgO(s) ⇋ 2 Hg(l) + O2(g)
m. CO(g) + H2O(g) ⇋ H2(g) + CO2(g)
n. CH3COOH (aq) ⇋ CH3COO-(aq) + H+(aq)
o. HA (aq) + H2O(l) ⇋ A-(aq) + H3O+(aq) (Acid Ionization, Ka)
p. A-(aq) + H2O(l) ⇋ HA (aq) + OH-(aq) (Base Ionization, Kb)
q. CH3COOH (l) + CH3OH(l) CH3COOCH3(l) + H2O(l)
r. HCOOH (aq) + NH3(aq) ⇋ HCONH2(aq) + H2O(aq)
What happens when you multiple the Ka from o with the Kb from p?
When going back from K expression to chemical formula the biggest mistakes are…
1. having the reactants over the products
2. adding the reactants and products instead of multiplying them
3.Not looking at the power each concentration was raised to and getting their coefficients wrong
The diploma will not ask you to take a K value and create a chemical equation where H20(l) was present but not shown in the expression
Select the correct answer for the following reaction : 2 C(s) + O2(g) ⇋ 2 CO(g)
Equilibrium exercise :
1. At a temperature of 350oC, the equilibrium concentrations for the reaction
N2(g) + 3 H2(g) ⇋ 2 NH3(g)
[ N2(g) ] = 0.885 mol/L ; [ H2(g) ] = 0.665 mol/L ; [ NH3 ] = 1.23 mol/L
calculate the value of the equilibrium constant for this reaction ?
2. 2 SO2(g) + O2(g) ⇋ 2 SO3(g)
The above reaction occurred in a 0.80 L flask at constant temperature. At equilibrium, the flask contained 3.00 mol of SO3(g), 2.25 moles of SO2(g) and 100.0 g of O2(g). Calculate the equilibrium
constant for the reaction at this constant temperature.
3. The equilibrium constant for the reaction below is 10 A(aq) + B(aq) ⇋ C(aq) + D(aq)
What would be the equilibrium constant for the following reaction ? D(aq) + C(aq) ⇋ A(aq) + B(aq)
4. The equilibrium constant for the unbalanced reaction : H2(g) + F2(g) ⇋ HF(g) is 2.1 x 103
at a particular temperature. When the equilibrium system is analyzed at this temperature, the concentrations of H2(g) and F2(g) are both found to be 0.0021 mol/L. What is the concentration of
5. What is the only change we can impose on an equilibrium system that will result in a different K value ?
6. Complete the following table : H2(g) + I2(g) ⇋ 2 HI(g)
7. Write a balanced equation for each of the following gaseous equilibrium system that would give the following expressions for the equilibrium constant
8. Consider the following system at equilibrium : PCl5(g) ⇋ PCl3(g) + Cl2(g) Keq = 4.2 x 10-2. If the
equilibrium concentration of PCl5(g) is 0.012 kmol/L and that of Cl2(g) is 0.049 kmol/L, what is the
equilibrium concentration of PCl3(g) ?
Equilibrium conditions for HI system at 448oC
system [ H2(g)] ( mol/L ) [ I2(g)] ( mol/L ) [ HI(g)] ( mol/L )
A 0.40 0.50 1.00
B 0.16 8.5
C 0.333 3.75
D 3.56 22.4
E 4.0 9.0
9. Which of the following changes would affect the value of a system’s equilibrium constant ? a. addition of a reactant or product
b. increase in the total pressure c. addition of an inert gas d. decrease in the temperature
10.The following reaction occurs : CO(g) + 2 H2(g) ⇋ CH3OH(g)
a. write the equilibrium expression
b. state four ways to increase the yield of methanol
c. a quantity of methanol was thrown into the reaction vessel and when equilibrium was established, the concentration of CO(g) was 1.40 x 10-1 mol/L. Calculate the equilibrium
concentration of the methanol if the equilibrium constant for the reaction is
3.21 x 10-2
11.Rank the following equilibrium constants in order from the one most favoring the products to the one least favoring the products
K1 = 6.4 x 10-3
K2 = 3.9 x 10-2
K3 = 9.2 x 10-4 K
4 = 3.0 x 10-1 K
5 = 4.2 x 10-2
12.The equilibrium constant for the following reaction is 82 at 450oC. :
N2(g) + 3 H2(g) ⇋ 2 NH3(g)
ICE questions
13.Bromine monchloride, BrCl(g), decomposes to form chlorine and bromine.
2 BrCl(g) ⇋ Cl2(g) + Br2(g)
an amount of bromine monochloride is thrown into a 1.00 L reaction vessel. At a certain temperature, the equilibrium constant for the reaction is 11.1 and the equilibrium mixture contains 4.00 mol Cl2. How
many moles of Br2 and BrCl are present in the equilibrium mixture ?
14. At a constant temperature, 0.750 mol of HBr(g) was placed in a 1.50 L container. The HBr(g) decomposed
according to the following equation : 2 HBr(g) ⇋ H2(g) + Br2(g)
At equilibrium the concentration of H2(g) was found to be 0.130 mol/L. Calculate the value of the
equilibrium constant K.
15. 2 CO2(g) ⇋ 2 CO(g) + O2(g)
4.00mol of CO2(g) 3.00 mol of CO(g) and 3.00 mol of O2(g) are placed in a 1.00 L reaction vessel at a
certain temperature. When equilibrium was reached, the vessel contained 4.50 mol of O2(g).
16. At a certain temperature, a mixture of H2(g) and I2(g) was prepared by placing 0.100 mol of H2(g) and 0.100
mol of I2(g) into a 1.00 L flask. After a period of time, the equilibrium
H2(g) + I2(g) ⇋ 2 HI(g)
was established. When the system reached equilibrium, it was determined that the I2(g)
concentration had dropped to 0.020 mol/L. What is the value of Keq for this reaction at
this temperature ?
17. In a particular experiment, it was found that when O2(g) and CO(g) were mixed and allowed to react
According to the equation : 2 CO(g) + O2(g) ⇋ 2 CO2(g)
The [ O2(g) ] had decreased by 0.030 mol/L when the reaction reached equilibrium. How did
[ CO(g) ] and [ CO2(g) ] change ?
18.In an experiment, 0.20 mol of PCl3(g) and 0.10 mol of Cl2(g) were placed in a 1.0 L flask at 250oC. The
reaction : PCl3(g) + Cl2(g) ⇋ PCl5(g) was allowed to come to equilibrium, at which
Time it was found that the flask contained 0.12 mol of PCl3(g).
19.The equilibrium contant, K for the reaction : SO3(g) + NO(g) ⇋ NO2(g) + SO2(g) , was found to be
0.5000 at a certain temperature. If 0.300 mol of SO3(g) and 0.300 mol of NO(g) were placed in a 2.00 L
container and allowed to react, what would be the equilibrium concentration of each gas ?
20.One mole of H2(g) and one mole of I2 are introduced into a 1 L box at a temperature at 490oC. What will be
the final concentrations in the box when equilibrium is established? at this temperature, the value of K is 45.9
21. At a certain temperature, the reaction : CO(g) + 2 H2(g) ⇋ CH3OH(g) has a Keq = 0.500. If a reaction
mixture at equilibrium contains 0.120 mol/L CO(g) and 0.100 mol/L H2(g) . What is the [ CH3OH ] ?
22.Describe the relative amounts of product vs reactant in the following situations : a. large Keq value
23.Why can equilibrium exist only in a closed system ?
24. 2 CO2(g) ⇋ 2 CO(g) + O2(g) H = 566.0 kJ
Initially 4.00 mol of CO2(g) were placed in a 1.00 L flask and the reaction reached equilibrium.
Measurements taken at equilibrium indicated that 1.00 mol of CO2(g) still remained. Calculate the
equilibrium constant.
25.A particular chemical equilibrium has an equilibrium constant K = 1250. A true statement about this equilibrium would be
a. the products are favored b. the reactants are favored c. the reaction is exothermic
d. the reaction takes equilibrium quickly
26. 2 HCl(g) ⇋ H2(g) + Cl2(g)
How many moles of HCl(g) are needed in a 1.00 L flask to produce 0.600 moles of Cl2(g) at equilibrium ?
( assume the equilibrium constant = 2.00 )
28. At a particular temperature K = 2.50 for the reaction : SO2(g) + NO2(g) < = > SO3(g) + NO(g)
If all four gases had initial concentration of 1.00 mol/L. Calculate the equilibrium concentrations of the gases.
29.Ethyl acetate is syntesisized in a nonreacting solvent (not water) according to the following CH3CO2H(sol) + C2H5OH(sol) ⇋ CH3CO2C2H5(sol) + H2O(sol) K = 2.2
For which of the following sets of conditions is the system at equilbrium? For those that are not at equilibrium, in which direction will the system shift?
a. [CH3CO2C2H5] = 0.22 mol/L, [H2O] = 0.10 mol/L, [CH3CO2H] = 0.010 mol/L
[C2H5OH] = 0.010 mol/L
b. [CH3CO2C2H5] = 0.22 mol/L, [H2O] = 0.0.0020 mol/L, [CH3CO2H] = 0.0020 mol/L
[C2H5OH] = 0.010 mol/L
c. [CH3CO2C2H5] = 0.88 mol/L, [H2O] = 0.12 mol/L, [CH3CO2H] = 0.044 mol/L
[C2H5OH] = 6.0 mol/L
d. [CH3CO2C2H5] = 4.4 mol/L, [H2O] = 4.4 mol/L, [CH3CO2H] = 0.10 mol/L
[C2H5OH] = 10.0 mol/L
For the previous reaction, what must the concentration of water be for a mixture with [CH3CO2C2H5] = 2.0 mol/L, [CH3CO2H] = 0.10 mol/L, and [C2H5OH = 5.0 mol/L to be at
