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Equilibrium

Terms in this set (62)

equilibrium constant
the ratio of product concentrations to reactant concentrations at equilibrium, with each concentration raised to a power equal to the number of moles of that substance in the balanced chemical equation

Kc = [C]^c [D]^d / [A]^a [B]^b

ONLY GASES, NOT SOLIDS OR LIQUIDS
reaction quotient (Q)
ratio of the product of molar concentrations (or pressures) of the products to that of the reactants, each concentration (or pressure) being raised to the power equal to the coefficient in the equation
if K > Q
a reaction will proceed forward, converting reactants into products
if K < Q
the reaction will proceed in the reverse direction, converting products into reactants
if K = Q
the system is already at equilibrium.
if the volume is decreased, what happens to equilibrium?
the pressure increases, the reaction will proceed towards the side with less gas moles
if the volume is increased, what happens to equilibrium?
the pressure decreases, the reaction will proceed towards the side with more gas moles
When does changing volume not affect equilibrium?
when there is an equal amount of gas moles on both sides
How is equilibrium affected when temperature is increased?
need to determine if the reaction is endothermic (heat is a reactant) or exothermic (heat is a product)

then just use concentration rules, i.e., if it an exothermic reaction and temp is increased, then equilibrium will move to the left (toward the reactants)
How is equilibrium affected when temperature is decreased?
need to determine if the reaction is endothermic (heat is a reactant) or exothermic (heat is a product)

then just use concentration rules, i.e., if it an exothermic reaction and temp is decreased, then equilibrium will move to the right (toward the products)
Calculate the change in free energy for the following reaction at 25 °C given the following set of partial pressures:

N₂ (g) + 3H₂ (g) ↔ 2NH₃ (g)

The pressure for all 3 gases is 1 atm.

∆G° = -33 kJ
∆G = -33 kJ

∆G = ∆G° + RTlnQ

Qp = P(NH₃)² / P(N₂) * P(H₂)³

Qp = 1² / (1 * 1³) = 1

∆G = ∆G° + RTln (1)

∆G = ∆G°

∆G = -33 kJ
Calculate the change in free energy for the following reaction at 25 °C given the following set of partial pressures:

N₂ (g) + 3H₂ (g) ↔ 2NH₃ (g)

The pressure for all 3 gases is 4 atm.

∆G° = -33 kJ
∆G = -39.9 kJ

∆G = ∆G° + RTlnQ

Qp = P(NH₃)² / P(N₂) * P(H₂)³

Qp = 4² / (4 * 4³) = 0.0625

∆G = -33000 + (8.314)(298) ln (0.0625)

∆G = -39.9 kJ

∆G = -39.9 kJ
Calculate the equilibrium constant for the following reaction at 298 K.

N₂ (g) + 3H₂ (g) ↔ 2NH₃ (g)

∆G° = -33 kJ
k = 6.1 x 10⁵

∆G° = -RTlnk

-33000 = -(8.314) (298) lnk

13.32 = lnk

e^13.32 = e^lnk

k = 6.1 x 10⁵
What does it mean when k > 1?
the products are favored over the reactants (mixture contains more products than reactants)
What does it mean when k < 1?
the reactants are favored over the products (mixture contains more reactants than products)
What does it mean when k = 1?
neither the products or reactants are favored more (mixture contains an equal amount of products and reactants)
when ∆G° = 0, what is k?
k = 1
Calculate the equilibrium constant for the following reaction at 464 K.

N₂ (g) + 3H₂ (g) ↔ 2NH₃ (g)

∆G° = 0
k = 1

∆G° = -RTlnk

0 = lnk (can get rid of -RT because when you divide on both sides, it will just become 0)

e^0 = e^lnk

k = 1
when ∆G° < 0, what is k?
k > 1
when ∆G° > 0, what is k?
k < 1
Calculate the equilibrium constant for the following reaction at 1000 K.

N₂ (g) + 3H₂ (g) ↔ 2NH₃ (g)

∆G° = 106.5 kJ
k = 2.7 x 10⁻⁶

∆G° = -RTlnk

106500 = -(8.314) (1000) lnk

-12.81 = lnk

e^-12.81 = e^lnk

k = 2.7 x 10⁻⁶
Which of the following statements most accurately describes the comparison between the rate constant k and the equilibrium constant Keq?

A. k is formed from the concentration of the reactants raised to their stoichiometric coefficients, while Keq is formed from the concentration of the products over the reactants each raised to its stoichiometric coefficient.

B. The rate constant k indicates how long the reaction will take to complete, and the equilibrium constant Keq indicates the ratio of the product concentrations to the reactant concentrations at equilibrium.

C. A large k means that the reaction will go to equilibrium, and a large Keq means that a reaction will go to completion quickly.

D. Both constants are dependent on temperature, but only Keq is independent of concentration.
D. Both constants are dependent on temperature, but only Keq is independent of concentration.
Which expression represents the standard Gibb's free energy (∆G°) in kilojoules per mole at equilibrium for the dephosphorylation of ATP to ADP?

ATP + glucose ↔ ADP + glucose-6-phosphate;
Keq1 = 8.5 x 10²

glucose-6-phosphate + H₂O ↔ glucose + Pi;
Keq2 = 2.5 x 10²

A. -(8.314) (298) ln(2.1 x 10⁵) / 1000

B. (0.0821) (1000) (298) ln(1.1 x 10³)

C. (8.314) (298) (1000) / ln(1.1 x 10³)

D. -(0.0821) (298) ln(2.1 x 10⁵) / 1000
A. -(8.314) (298) ln(2.1 x 10⁵) / 1000
Which of the following is a true statement about the role of catalysts in a reaction?

I. Catalysts more effectively lowers the activation energy in the forward direction.
II. Catalysts generally react with one or more reactants to form intermediates that subsequently give the final reaction product..
III. If a catalyst affects the equilibrium of the reaction, it must be consumed as the reaction proceeds.
IV. Catalysts can may increase the reaction rate or selectivity or enable the reaction at a lower temperature.

A. II and IV

B. I and III

C. II, III, and IV

D. II and III
A. II and IV

III is wrong and A is the only choice without III
The Haber process involves the production of ammonia from hydrogen and nitrogen gas. In the laboratory, it was determined that the equilibrium concentrations of NH₃, H₂, and N₂ are 0.0030 M, 0.10 M, and 0.090 M, respectively. Which of the following statements most accurately describes the reaction progress when all three concentrations are at 0.3 and 3.0 M?

N₂ (g) + 3H₂ (g) ↔ 2NH₃ (g)

A. For both concentrations, the reaction will shift to the right.

B. For 3.0 M, the reaction will shift to the right, and for 0.3 M, the reaction will shift to the left.

C. For both concentrations, the reaction will shift to the left.

D. For 0.3 M, the reaction will shift to the right, and for 3.0 M, the reaction will shift to the left.
C. For both concentrations, the reaction will shift to the left.

0.3 M:

Keq = 0.003² / (0.1³) * (0.09) = 0.1

Q = (0.3²) / (0.3³) * (0.3) = 11

Keq < Q, so shift left

3 M:

Keq = 0.003² / (0.1³) * (0.09) = 0.1

Q = 3² / 3 * 3³ = 0.11

Keq < Q, so shift left
According to Le Chatêlier's Principle, addition of which of the following compounds will the equilibrium of the following endothermic reaction NOT shift to the right?

CaCO₃ (s) + CO₂ (g) + H₂O (l) ↔ Ca²⁺ (aq) + 2HCO⁻₃ (aq)

A. Na₃PO₄

B. CO₂

C. CaCO₃

D. LiOH
C. CaCO₃

C is right because CaCO₃ is a solid and does not appear in the equilibrium equation

A is wrong because adding sodium phosphate will shift the equilibrium to the right since calcium phosphate is insoluble. Calcium ion will be removed from the solution as a precipitate.

D is wrong because adding lithium hydroxide will shift the equilibrium to the right since hydroxide is a stronger base and will react with bicarbonate to make the carbonate ion.
The equilibrium constant (Keq) is usually expressed in terms of molar concentrations, which can be expressed as Kc. To differentiate between the two, for gases, the equilibrium constant can be expressed as Kp and be substituted with partial pressures. Given the following reaction below, which is the correct expression for Kp?

N₂ (g) + 3H₂ (g) ↔ 2NH₃ (g)

A. [NH₃]² / [H₂]³[N₂] (RT)⁻²

B. [H₂]³[N₂] / [NH₃]² (RT)²

C. [NH₃]² / [H₂]³[N₂] (V / RT)⁻²

D. [H₂]³[N₂] / [NH₃]² (VRT)²
A. [NH₃]² / [H₂]³[N₂] (RT)⁻²
A covalent bond has a standard Gibbs free energy of 400 kJ/mol, while the hydrogen bond and van der Waals forces have substantially lower values, respectively 25 and 5 kJ/mol. Given such ∆G° values, which of the following is equivalent to the ratio of the Keq value of a covalent bond to that of a hydrogen bond? (RT = 2500 J/mol)

A. -e¹⁵⁵

B. 10³²

C. e³²

D. e¹⁵⁵
D. e¹⁵⁵


Covalent bond:
∆G° = -RTlnKeq

400000 = -2500lnKeq

lnKeq = e¹⁶⁰

Hydrogen bond:

∆G° = -RTlnKeq

25000 = -2500lnKeq

lnKeq = e¹⁰

ratio:
e¹⁶⁰ / e¹⁰ = e¹⁵⁰ ~ e¹⁵⁵
Which of the following is considered necessary as part of standard state conditions?

A. 1 Molar for all solutions, including water

B. ambient pressure of 100,000 Pascals

C. pH 7 for all solutions

D. ambient temperature of 273 K or 310 K
B. ambient pressure of 100,000 Pascals

Some of us may be more familiar with seeing 1 atmosphere, but IUPAC does recommend a standard pressure of 10⁵ Pascals for standard state conditions. Since 10⁵ Pascals is 0.99 atmospheres, they are essentially the same. Therefore, an ambient pressure of 100,000 Pascals is necessary as part of the standard state conditions.

A is wrong because for a substance in solution, the standard state molality is 1 mol kg⁻¹, while standard state amount concentration is 1 mol dm⁻³. It is assumed that water does not change its concentration of 55.6 M appreciably.

D is wrong because standard state does not technically specify a temperature.
Consider the following reaction:

C (s, graphite) + H₂ (g) + O₂ (g) ↔ CH₃OH (l)

∆H° = -238 kJ

Which of the following would increase the value of Keq?

A. Increase pressure

B. Increase [H₂]

C. Decrease [C]

D. Decrease temperature
D. Decrease temperature

Increasing the Keq will increase the amount of product formed at equilibrium.

Only by increasing or decreasing the temperature will change the value of Keq since

Keq = e^(∆G / RT)

A is wrong because when we increase the pressure, the equilibrium will shift to the right according to Le Chatelier's Principle. The concentrations will change, but will readjust such that Keq remains the same

B and C are wrong because increasing or decreasing the concentration will cause the equilibrium to shift to the right or left, respectively, but the value of Keq remains constant since it is concentration-independent
irreversible reaction
a chemical reaction where products cannot revert back to reactants; reaction goes to completion; the max amount of product formed is determined by the amount of limiting reagent initially present
reversible reactions
can go in either direction under different circumstances; usually do not proceed to completion because the products can react together to form the reactants
dynamic equilibrium
forward and reverse reactions are still occurring, but they are going at the same rate, so there is no net change in the concentrations of the products or the reactants
law of mass action
The rate of a chemical reaction is proportional to the concentration of products over the concentrations of the reactants
what is the equilibrium constant for a reaction that occurs in more than one step?
the product of each step's equilibrium constant
What is the expression for the equilibrium constant for the following reaction?

3H₂ + N₂ ↔ 2NH₃
Keq = [NH₃]² / [H₂]³ * [N₂]
reaction quotient, Q
a quotient obtained by applying the law of mass action to initial concentrations rather than to equilibrium concentrations
if the equilibrium constant for a reaction written in one direction is Keq, what is the equilibrium constant for the reverse reaction?
1 / Keq
a reaction that strongly favors products will have a ___________ exponent
large, positive
a reaction that strongly favors reactants will have a ___________ exponent
large, negative
Given the [product] = 0.075 M and [reactant] = 1.5 M, determine the direction of reaction and sign of the free energy change for a Keq value of 5 x 10⁻²
at equilibrium (no net reaction)


∆G = 0

Q = 0.075 / 1.5 = 5 x 10⁻²

Q = Keq so at equilibrium
Given the [product] = 0.75 M and [reactant] = 1.5 M, determine the direction of reaction and sign of the free energy change for a Keq value of 5 x 10⁻3
Q > Keq (proceeds toward reactants)

∆G = positive

Q = 0.075 / 1.5 = 5 x 10⁻²

Q > Keq
Given the [product] = 0.75 M and [reactant] = 1.5 M, determine the direction of reaction and sign of the free energy change for a Keq value of 5 x 10⁻¹
Q < Keq (proceeds toward products)

∆G = negative

Q = 0.075 / 1.5 = 5 x 10⁻²

Q < Keq
for an increase in pressure, the system will move toward the side of the reaction with
the lower total number of gas moles
when volume increases, the system will move toward the side of the reaction with
the greater number of gas moles
if a reaction is endothermic (∆H > 0), heat functions as a _____________
reactant
if a reaction is exothermic (∆H < 0), heat functions as a _____________
product
Describe what would happen if the pH has been increased in the following reaction:

H₂SO₄ (aq) ↔ H⁺ (aq) + HSO₄⁻ (aq)
reaction will go towards products

if pH increases, then [H⁺] decreases, so will go towards products
Describe what would happen if the pressure of the reaction vessel is decreased in the following reaction:

2C (s) + O₂ (g) ↔ 2CO (g)
reaction will go towards products

side with more moles of gas
Describe what would happen if the reaction vessel is warmed in the following reaction:

CH₄ (g) + 2O₂ (g) ↔ CO₂ (g) + 2H₂O (l) + heat
reaction will go towards reactants

side opposite of heat
Describe what would happen if water is removed without changing temperature in the following reaction:

H₃PO₄ (aq) + H₂O (l) ↔ H₃O⁺ (aq) + H₂PO₄⁻ (aq)
reaction will go towards reactants

water decreases, so reaction shifts left
kinetic products
formed at lower temperature will smaller heat transfer; less stable; faster
thermodynamic products
formed at higher temps with larger heat transfer; more stable; slower
A reaction is found to stop just before all reactants are converted to products. Which of the following could be true about this reaction?

A. The reaction is irreversible and the forward rate is greater than the reverse rate

B. The reaction is irreversible and the reverse rate is too large for products to from.

C. The reaction is reversible, and the forward rate is equal to the reverse rate.

D. The reaction is reversible, and the reverse rate is greater than the forward rate
C. The reaction is reversible, and the forward rate is equal to the reverse rate.
Carbonated beverages are produced by dissolving carbon dioxide in water to produce carbonic acid:

CO₂ (g) + H₂O (l) ↔ H₂CO₃ (aq)

When a bottle containing carbonated water is opened, the taste of the beverage gradually changes as the carbonation is lost. Which of the following statements best explains this?

A. The change in pressure and volume causes the reaction to shift to the left, thereby decreasing the amount of aqueous carbonic acid

B. The change in pressure and volume causes the reaction to shift to the right, thereby decreasing the amount of gaseous carbon dioxide

C. Carbonic acid reacts with environmental oxygen and nitrogen

D. Carbon dioxide reacts with environmental oxygen and nitrogen
A. The change in pressure and volume causes the reaction to shift to the left, thereby decreasing the amount of aqueous carbonic acid
If Kc >> 1:

A. The equilibrium mixture will favor products over reactants

B. The equilibrium mixture will favor reactants over products.

C. The equilibrium concentrations of reactants and products are equal.

D. The reaction is essentially irreversible
A. The equilibrium mixture will favor products over reactants
Acetic acid dissociates in solution according to the following equation:

CH₃COOH ↔ CH₃COO⁻ + H⁺

If sodium acetate, is added to a solution of acetic acid in excess water, which of the following effects would be observed in the solution?

A. Decreased pH

B. Increased pH

C. Decreased pKeq (pKa)

D. Increased pKeq (pKa)
B. Increased pH
Which of the following actions does NOT affect the equilibrium position of a reaction?

A. adding or removing heat

B. adding or removing a catalyst

C. increasing or decreasing concentrations of reactants

D. increasing or decreasing volumes of reactants
B. adding or removing a catalyst
In a sealed 1 L container, 1 mole of nitrogen gas reacts with 3 moles of hydrogen gas to form 0.05 moles of NH₃ at equilibrium. Which of the following is closest to the Kc of the reaction?

A. 0.0001

B. 0.001

C. 0.01

D. 0.1
A. 0.0001
Increasing temperature can alter the Keq of a reaction. Why might increasing temperature indefinitely be unfavorable for changing reaction conditions?

A. the equilibrium constant has a definite limit that cannot be surpassed

B. the products or reactants can decompose at high temps

C. Increasing temp would decrease pressure, which may or may not alter reaction conditions

D. if a reaction is irreversible, its Keq will resist changes in temp
B. the products or reactants can decompose at high temps
Which of the following is true of equilibrium reactions?

I. An increase in k¹ results in a decrease in k⁻¹
II. As the concentration of products increases, the concentrations of reactants decreases
III. The equilibrium constant is altered by changes in temperature

A. I only

B. II and III

C. I and III

D. I, II, and III
B. II and III
Compound A has a Ka (equilibrium constant of acid dissociation) of approximately 10⁻⁴. Which of the following compounds is most likely to react with a solution of compound A?

A. HNO₃

B. NO₂

C. NH₃

D. N₂O₅
C. NH₃

NH₃ is the only base
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