HW03 - Chemical Equilibria

Question 11.5 pts

When the chemical reaction

A + B ⇌ C + D

is at equilibrium, which of the following is true?

  1. all four concentrations are equal
  2. neither the forward nor the reverse reactions have stopped
  3. the sum of the concentrations of A and B equals the sum of the concentrations of C and D
  4. both the forward and reverse reactions have stopped

Question 21.5 pts

Explain why equilibrium constants are dimensionless.
  1. They are dimensionless because the pressures or concentrations we put in are all for the substances in their standard states.
  2. This is a trick question. Equilibrium constants have units that involve some multiple of atmospheres or moles per liter.

  3. They are not really dimensionless, but we must treat them as such in order to be able to take ln(K) in the expression:

    \Delta G^\circ\:=\:-RT\:\ln K

  4. Every concentration or pressure that enters into Kc or Kp is really divided by the corresponding concentration or pressure of the substance in its standard state.

Question 31.5 pts

The expression for KP for the reaction

4NH3(g) + 5O2(g) ⇌ 4NO(g) + 6H2O(g)

at equilibrium is:

  1. \(\displaystyle{P_{\rm H_2O}^6 P_{\rm NO}^4 \over P_{\rm O_2}^5 P_{\rm NH_3}^4}\)

  2. \(P_{\rm NH_3}^4 P_{\rm O_2}^5\)

  3. No answer text provided.
  4. \(\displaystyle{P_{\rm NH_3}^4 P_{\rm O_2}^5 \over P_{\rm NO}^4 P_{\rm H_2O}^6}\)

  5. \(\displaystyle{P_{\rm NO} P_{\rm H_2O} \over P_{\rm NH_3} P_{\rm O_2} }\)


Question 41.5 pts

Consider the following reactions at 25°C:

2NO(g) ⇌ N2(g) + O2(g)              KP = 1x1030

2H2O(g) ⇌ 2H2(g) + O2(g)           KP = 5x10-82

2CO(g) + O2(g) ⇌ 2CO2(g)          KP = 3x1091

Which compound is most likely to dissociate and give O2(g) at 25°C?

  1. CO2
  2. NO
  3. \(\displaystyle{P_{\rm NO} P_{\rm H_2O} \over P_{\rm NH_3} P_{\rm O_2} }\)

  4. H2O

Question 51.5 pts

At 600°C, the equilibrium constant for the reaction

2HgO (s) \longrightarrow 2Hg (ℓ) + O2 (g)

is 2.8. Calculate the equilibrium constant for the reaction

0.5O2 (g) + Hg (ℓ) \longrightarrow HgO (s).

  1. 0.60
  2. CO
  3. 1.1
  4. 1.7

Question 61.5 pts

Consider the reaction

2HgO (s) ⇌ 2Hg (ℓ) + O2 (g)

What is the form of the equilibrium constant Kp for this reaction?

  1. \(\displaystyle{P_{\rm O_2} \over P_{\rm HgO}^2}\)

  2. \(\displaystyle{P_{\rm Hg}^2 P_{\rm O_2}\over P_{\rm HgO}^2}\)

  3. \(P_{\rm O2}\)

  4. \(P_{\rm Hg}^2P_{\rm O2}\)


Question 71.5 pts

Kp = 2.6 x 108 at 825 K for the reaction

2H2 (g) + S2 (g) ⇌ 2H2S (g)

The equilibrium pressure of H2 is 0.0020 atm and S2 is 0.0010 atm. What is the equilibrium pressure of H2S?

  1. 0.10 atm
  2. 1.0 atm
  3. 0.0010 atm
  4. 0.36

Question 81.5 pts

Consider the reaction below

2SO2 (g) + O2 (g) ⇌ 2SO3 (g)

At 1000 K the equilibrium pressures of the three gases in one mixture were found to be 0.562 atm SO2, 0.101 atm O2, and 0.332 atm SO3. Calculate the value of Kp for the reaction.

  1. 2.64
  2. 0.298
  3. 0.171
  4. 3.46

Question 91.5 pts

Consider the following reaction:

2NO (g) + Br2 (g) ⇌ 2NOBr (g)                   Kp = 2.40 @ 373 K

Calculate Kc for this reaction at 100 °C.

  1. 10 atm
  2. 73.5
  3. 19.7
  4. 0.0784

Question 101.5 pts

Calculate the equilibrium constant at 25°C for a reaction for which ΔG° = -4.22 kcal/mol. Include the sign if needed and round to the second decimal place.
 

Question 111.5 pts

The figure below represents a reaction at 298 K.

free-energy-diag1.svg

Based on the figure, which of the following statements (if any) are FALSE?

  1. At point C, the system is at equilibrium.
  2. None of the other statements are false.
  3. For this reaction, ∆G° is negative.
  4. At point D, the reaction will move toward the reactants to get to equilibrium.
  5. At point B, Q < K.

Question 121.5 pts

Consider the reaction:

Cgraphite (s) + O2 (g) ⇌ CO2 (g)          ∆G° = –400 kJ

Which of the following is a possible value of K for this reaction?

  1. At point B, Q < K.
  2. 10-70
  3. 1070
  4. 0.56

Question 131.5 pts

The reaction

A + B ⇌ C + 2D

has an equilibrium constant of 3.7 x 10-3. Consider a reaction mixture with:

[A] = 2.0 x 10-2 M

[B] = 1.7 x 10-4 M

[C] = 2.4 x 10-6 M

[D] = 3.5 x 10-3 M

Which of the following statements is definitely true?

  1. -0.56
  2. The reverse reaction will occur to a greater extent than the forward reaction until equilibrium is established.
  3. The system is at equilibrium.
  4. The forward reaction will occur to a greater extent than the reverse reaction until equilibrium is established.

Question 141.5 pts

The reaction

N2 (g) + 3H2 (g) ⇌ 2NH3 (g)

has an equilibrium constant, Kp, of 4.0 x 108 at 25°C. What will eventually happen if 44.0 moles of NH3, 0.452 moles of N2, and 0.108 moles of H2 are put in a 10.0 L container at 25°C.

  1. More N2 and H2 will be formed.
  2. No conclusions about the system can be made without additional information.
  3. It is impossible to know what will happen unless we know what the equilibrium constant is at 298 K.
  4. Nothing. The system is at equilibrium.

Question 151.5 pts

Given the hypothetical reaction:

X(g) ⇌ Y(g)

Predict what will happen when 1.0 mol Y is placed into an evacuated container.

  1. Nothing. The products are already formed, so no reaction occurs.
  2. More NH3 will be formed.
  3. As the reaction progresses, Q will increase until Q = K.
  4. As the reaction progresses, ∆G° will decrease until ∆G° = 0.

Question 161.5 pts

What happens to the concentration of NO(g) when the total pressure on the reaction below is increased (by compression) when it is at equilibrium?

3NO2 (g) + H2O (ℓ) ⇌ 2HNO3 (aq) + NO (g)

  1. it remains the same
  2. it is impossible to tell
  3. As the reaction progresses, Q will decrease until Q = K.
  4. it increases

Question 171.5 pts

Consider the following reaction:

2SO2 (g) + O2 (g) ⇌ 2SO3 (g)

where ΔHrxn =  –198 kJ. The amount of SO2(g) at equilibrium increases when...

  1. the volume is increased.
  2. the temperature is decreased.
  3. SO3 is removed.
  4. it decreases

Question 181.5 pts

Suppose the reaction mixture

N2 (g) + 3H2 (g) ⇌ 2NH3 (g)

is at equilibrium at a given temperature and pressure. The pressure is then increased at constant temperature by compressing the reaction mixture, and the mixture is then allowed to reestablish equilibrium. At the new equilibrium...

  1. there is the same amount of ammonia present as there was originally.
  2. there is less ammonia present than there was originally.
  3. more oxygen is added.
  4. the nitrogen is used up completely.

Question 191.5 pts

Consider the system:

2N2O5 (g) ⇌ 2N2O4 (g) + O2 (g)

at equilibrium at 25°C. If this is an exothermic reaction and the temperature was raised, would the equilibrium be shifted to produce more N2O5 or more N2O4?

  1. it is impossible to tell
  2. more N2O5
  3. there would be no change
  4. more N2O4

Question 201.5 pts

The equilibrium constant K for the synthesis of ammonia is 6.8x105 at 298 K. What will K be for the reaction at 375 K?

N2(g) + 3H2(g) ⇌ 2NH3(g)        ∆H° = -92.22 kJ/mol

  1. there is more ammonia present than there was originally.
  2. 1.42 x 109
  3. 326
  4. 6.85 x 105