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9.E: Fundamental Equlibrium Concepts - Homework
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Chemical Equilibria
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Is a system at equilibrium if the rate constants of the forward and reverse reactions are equal?
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Answer
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yes
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If the concentrations of products and reactants are equal, is the system at equilibrium?
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Answer
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no
Equilibrium Constants
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Explain why there may be an infinite number of values for the reaction quotient of a reaction at a given temperature but there can be only one value for the equilibrium constant at that temperature.
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Answer
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the reaction quotient depends upon the concentration of the products and reactants, which can have an infinate number of values. The equilibrium constant is a constant - so it only has one value
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Explain why an equilibrium between Br
2
(
l
) and Br
2
(
g
) would not be established if the container were not a closed vessel shown in Figure 7.1.4.
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Answer
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in an open vessel the bromine gas can escape and is not able to condense back into a liquid - so the system can not go in both directions
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If you observe the following reaction at equilibrium, is it possible to tell whether the reaction started with pure NO
2
or with pure N
2
O
4
? \( \mathrm{2 NO_2} \rightleftharpoons \mathrm{N_2O_4} \)
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Answer
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no
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Write the mathematical expression for the reaction quotient, Q
c
, for the following reaction: \( \mathrm{CH_4}(g) + \mathrm{Cl_2}(g) \rightleftharpoons \mathrm{CH_3Cl}(g) + \mathrm{HCl}(g) \)
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Answer
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\[ Q=\frac{[\mathrm{CH_3Cl}][\mathrm{HCl}]}{[\mathrm{CH_4}][\mathrm{Cl_2}]} \]
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Determine if the following system is at equilibrium. If not, in which direction will the system need to shift to reach equilibrium? \( \mathrm{SO_2Cl_2}(g) \rightleftharpoons \mathrm{SO_2}(g) + \mathrm{Cl_2}(g) \) ; [SO
2
Cl
2
] = 0.12
M
, [Cl
2
] = 0.16
M
and [SO
2
] = 0.050
M
.
K
c
for the reaction is 0.078.
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Answer
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Q=0.66, Q is less than K so the reaction will go forward, increasing the concentration of products and reducing the concentration of reactants.
Shifting Equilibria: Le Châtelier’s Principle
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Methanol can be prepared from carbon monoxide and hydrogen at high temperature and pressure in the presence of a suitable catalyst. \[ \mathrm{H_2}(g) + \mathrm{CO}(g) \rightleftharpoons \mathrm{CH_3OH}(g)\]
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(a) Write the expression for the equilibrium constant (K) for the reversible reaction
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Answer
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\[ Q=\frac{[\mathrm{CH_3OH}]} {[\mathrm{H_2}][\mathrm{CO}]} \]
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(b) What will happen to the concentrations of H
2
, CO, and CH
3
OH at equilibrium if more H
2
is added?
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Answer
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H
2
and CO will decrease, CH
3
OH will increase
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(c) What will happen to the concentrations of H
2
, CO, and CH
3
OH at equilibrium if CO is removed?
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Answer
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H
2
and CO will increase, CH
3
OH will decrease
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(d) What will happen to the concentrations of H
2
, CO, and CH
3
OH at equilibrium if CH
3
OH is added?
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Answer
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H
2
and CO will increase, CH
3
OH will decrease
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(e) What will happen to the concentrations of H
2
, CO, and CH
3
OH at equilibrium if the temperature of the system is increased (Δ H = -90.2 kJ)?
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Answer
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H
2
and CO will increase, CH
3
OH will decrease
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(f) What will happen to the concentrations of H
2
, CO, and CH
3
OH at equilibrium if more catalyst is added?
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Answer
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no change
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Water gas, a mixture of H
2
and CO, is an important industrial fuel produced by the reaction of steam with red hot coke, essentially pure carbon. \[ \mathrm{C}(s) + \mathrm{H_2O}(g) \rightleftharpoons \mathrm{CO}(g) + \mathrm{H_2}(g) \]
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(a) Write the expression for the equilibrium constant for the reversible reaction
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Answer
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\[ Q=\frac{[\mathrm{CO}][\mathrm{H_2}]} {[\mathrm{H_2O}]} \]
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(b) What will happen to the concentration of each reactant and product at equilibrium if more C is added?
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Answer
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no change, since C is a solid and is not in the equlibrium constant expression
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(c) What will happen to the concentration of each reactant and product at equilibrium if H
2
O is removed?
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Answer
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amount of C will increase, concentration of H
2
O will increase, CO and H
2
will decrease
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(d) What will happen to the concentration of each reactant and product at equilibrium if CO is added?
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Answer
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amount of C will increase, concentration of H
2
O will increase, CO and H
2
will decrease
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(e) What will happen to the concentration of each reactant and product at equilibrium if the temperature of the system is increased? Δ H = 131.30 kJ
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Answer
-
amount of C will decrease, concentration of H
2
O will decrease, CO and H
2
will increase