The Reaction Quotient

The reaction quotient, Q, is used to determine if our chemical reaction is at equilibrium. 

Equilibrium Determination

Depending on if Q is greater than or less than K our reaction will shift to attain equilibrium by reaching the equilibrium constant K. The direction our reaction shifts determines whether our reactants or products are increasing or decreasing. 

Example: Q vs. K

0m

If the reaction quotient Q is equal to the equilibrium constant K then our reaction is at equilibrium. 

When the reaction quotient, Q, is smaller than the equilibrium constant, K, then the reaction shifts in the forward direction to reach equilibrium. 

When the reaction quotient, Q, is larger than the equilibrium constant, K, then the reaction shifts in the reverse direction to reach equilibrium. 

Example: Determining the Reaction Quotient

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Example: Determining Equilibrium Concentrations

3m

The Reaction Quotient Additional Practice Problems

Will a precipitate form if the metal ion concentration in a solution of M(CIO2)3 is 0.0246 M and the concentration of CIO_2 is 0.0148 M? Ksp for M(ClO2)3 = 6.25 times 10^-8 


Which of the following substances has the higher molar entropy (assume that each is in the gas phase)? 


Watch Solution

When the numerical value of Q is less than K, in which direction does the reaction proceed to reach equilibrium? Explain.

Watch Solution

The interhalogen ClF3 is prepared in a two-step fluorination of chlorine gas:

Cl2(g) + F2(g) ⇌ ClF(g)

ClF(g) + F2(g) ⇌ C1F3(g)

(a) Balance each step and write the overall equation.

(b) Show that the overall Qc equals the product of the Qc’s for the individual steps.

Watch Solution

Write Qc for each of the following:

(a) Hydrogen chloride gas reacts with oxygen gas to produce chlorine gas and water vapor.

(b) Solid diarsenic trioxide reacts with fluorine gas to produce liquid arsenic pentafluoride and oxygen gas.

(c) Gaseous sulfur tetrafluoride reacts with liquid water to produce gaseous sulfur dioxide and hydrogen fluoride gas.

(d) Solid molybdenum(VI) oxide reacts with gaseous xenon di-fluoride to form liquid molybdenum(VI) fluoride, xenon gas, and oxygen gas.

Watch Solution

Balance each reaction and write its reaction quotient,Qc:

(a) NO(g)+ O2(g) ⇌ N2O3(g)

(b) SF6(g)+ SO3(g) ⇌ SO2F2(g)

(c) SClF5(g) + H 2(g) ⇌ S2F10(g) + HCl(g)

Watch Solution

Does Q for the formation of 1 mol of NH3 from H2 and N2 differ from Q for the formation of NH3 from H2 and 1 mol of N2? Explain and give the relationship between the two Q’s.

Watch Solution

Does Q for the formation of 1 mol of NO from its elements differ from Q for the decomposition of 1 mol of NO to its elements? Explain and give the relationship between the two Q’s.

Watch Solution

Balance the equation and write the reaction quotient expression. 

U(s) + F2(g) ⇌ UF6(g)

Watch Solution

What is the value of Q when the CaSO4 solution contains 2.00×10−3M Ca+2 and 3.00×10-2M SO42-?

Express your answer numerically.

Watch Solution

Consider the reaction

Mg(s) + Fe2+(aq) → Mg2+(aq) + Fe(s) at 75°C

where [Fe2+] = 2.90M and [Mg 2+] = 0.310M

a. What is the value for the reaction quotient, Q, for the cell?

b. What is the value for the temperature, T, in kelvins?

Watch Solution

At 850 K the equilibrium constant for the following reaction is Kc = 15.

2SO2(g) + O2(g) ⇌ 2SO3(g)

If we mix the following concentrations of the three gases, predict in which direction the net reaction will proceed toward equilibrium.

[SO2] = 0.20 M
[O2] = 0.60 M
[SO3] = 0.60 M

[SO2] = 0.14 M
[O2] = 0.10 M
[SO3] = 0.40 M

[SO2] = 0.90 M
[O2] = 0.50 M
[SO3] = 0.10 M

Watch Solution

What is the value of Q when the SrCrO 4 solution contains 2.00 × 10−2 M Sr 2+ and 1.50 × 10−3M CrO42−?

Calculate the value of Q.

Watch Solution

In which direction will the net reaction proceed

X(g) + Y(g) ⇌ Z(g)          Kp = 1.00 at 300 K

for the set of initial condition?

 [X] = [Y] = [Z] = 1.0 M 

a) net reaction goes to the left

b) net reaction goes to the right

c) reaction is at equilibrium

Watch Solution

At 500oC, the Kp for the below reaction is 1.45 x10 -5. When 75 atm N2, 35 atm H2, and 5 atm of NH3 are added to a container and heated to 500 oC, in which direction does the reaction proceed?

N2 (g) + 3 H2 (g) ⇌ 2 NH3 (g)

A. The reaction proceeds to the right towards products until  Qp=Kp

B. The reaction proceeds to the left towards reactants until  Qp=Kp

C. The reaction proceeds to the right towards products until  Qp>Kp

D. The reaction proceeds to the left towards reactants until  Qp<Kp

Watch Solution

Which of the following statements is TRUE?

A) If Q < K, it means the reverse reaction will proceed to form more reactants.

B) If Q > K, it means the forward reaction will proceed to form more products.

C) If Q = K, it means the reaction is at equilibrium.

D) All of the above are true.

E) None of the above are true.

 

Watch Solution

What is Δn for the following equation in relating Kc to Kp?

N2O4(g) ⇌ 2NO2(g)

A) 3

B) -1

C) -2

D) 2

E) 1

Watch Solution

A reaction with an equilibrium constant K c = 1.5 x 10 25 would consist of which of the following at equilibrium:

A. essentially all reactants

B. some reactants and products with reactants slightly favored

C. some reactants and products with products slightly favored

D. approximately equal reactants and products

E. essentially all products

Watch Solution

Consider the following general reaction:

2 B4A(l) + 2 C2 (g) → 4 A2C (g) + 4 B2 (g),           K c =  0.0596

If you have a reaction mixture that contains 0.725 M concentration of each of these substances, in which direction will the reaction proceed?

A. Q = K; the reaction mixture is at equilibrium

B. Q < K; the reaction will shift towards the reactants

C. Q > K; the reaction will shift towards the products

D. Q > K; the reaction will shift towards the reactants

E. Q < K; the reaction will shift towards the products.

Watch Solution

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. No conclusions about the system can be made without additional information.
  2. The system is at equilibrium.
  3. The reverse reaction will occur to a greater extent than the forward reaction until equilibrium is established.
  4. The forward reaction will occur to a greater extent than the reverse reaction until equilibrium is established.
Watch Solution

The reaction shown below has K= 81.9 at 25°C.

I2(g) + Cl2(g) ⇌ 2 ICl(g)

A flask contains Pi2 = 0.114 atm, PCl2 = 0.102 atm, and PICl = 0.355 atm. Choose the answer that best describes how this mixture will behave.

A) The reaction is at equlibrium, so the concentrations will not vary.

B) The reaction is not at equilibrium and will generate more products.

C) The reaction is not at equilibrium and will generate more reactants.

D) The reaction will not proceed without the addition of a catalyst.

E) The rate constant of the reaction is needed to solve this problem.

Watch Solution

The value of the equilibrium constant at a given temperature for the reaction:

                2HI(g) ⇌ H2(g) + I2(g)

        is 0.200.  If [HI] = 0.200 M, [H 2] = 0.200 M, and [I 2] = 0.200 M, determine if the reaction is at equilibrium. If it is not, in which direction will it proceed to reach equilibrium?

    A)    The reaction is at equilibrium.

    B)    The reaction is not at equilibrium, it will shift to the left.

    C)    The reaction is not at equilibrium, it will shift to the right.

    D)    It is not possible to tell if the reaction is at equilibrium.

    E)    To reach equilibrium, the value of Keq must change.

 

Watch Solution

At a certain temperature, bromine and nitric oxide react to form nitrosyl bromide:

Br 2(g) + 2 NO(g) ⇌ 2 NOBr(g). When initial amounts of Br 2, NO, and NOBr are mixed, the concentration of NOBr increases. Which statement below is TRUE?

A) K c < Q                                  B) = Q                       C) > Q         D) More information is needed to make a statement about Kc.

Watch Solution

The equilibrium constant, Kp , for the reaction:    H2(g) + I2(g)  ⇋ 2HI(g)

is 55.2 at 425°C. A rigid cylinder at that temperature contains 0.127 atm of hydrogen, 0.134 atm of iodine, and 1.055 atm of hydrogen iodide. Is the system at equilibrium?

A)  Yes.

B)  No, the forward reaction must proceed to establish equilibrium.

C)  No, the reverse reaction must proceed to establish equilibrium.

D)  Need to know the volume of the container before deciding.

E)  Need to know the starting concentrations of all substances before deciding.

Watch Solution

Consider the reversible reaction: 2NO2(g) ⇋ N2O4(g)

If the concentrations of both NO2 and N2O4 are 0.016 mol L-1, what is the value of Qc?

A)  0.016              

B)  0.50          

C)  1.0            

D)  2.0            

E)  63

 

Watch Solution

Consider the following reaction at equilibrium at 24°C:

NH4HS(s) ⇌ NH3(g) + H2S(g)      Kc = 1.58 x 10–4

If 0.0205 mol of NH3(g), 0.00800 mol of H2S, and excess solid NH4HS are mixed in a one-liter container at 24°C, in what direction will the equilibrium proceed?

Watch Solution

The equilibrium constant Kc for the reaction

2 NOCl(g) → 2 NO(g) + Cl2(g)

is 0.51 at a certain temperature. A mixture of NOCl, NO, and Cl2 with concentrations 1.3,1.2, and 0.60 M, respectively, was introduced into a container at this temperature. Which of the following is true?

1. Cl2(g) is produced until equilibrium is reached.

2. [Cl] = 0.30 M at equilibrium.

3. No apparent reaction takes place. 

4. [NOCl] = [NO] = [Cl] at equilibrium.

5. NOCl(g) is produced until equilibrium is reached.

Watch Solution

Write the reaction quotient for

Watch Solution

For the following chemical reaction: Br 2(g) + Cl2(g) ⇌ 2BrCl(g)

At 400 K, the equilibrium constant is K p = 7.0. A closed vessel at 400 K is filled with 1.00 atm of Br2(g), 1.00 atm of Cl2(g), and 2.00 atm of BrCl(g). Use Q p to determine which of the statements below is true.

A) At equilibrium, the total pressure in the vessel will be less than the initial total pressure.

B) The equilibrium partial pressure of Br 2(g) will be greater than 1.00 atm.

C) The reaction will go to completion.

D) The equilibrium partial pressure of BrCl(g) will be greater than 2.00 atm.

E) The reaction shifts to the left

Watch Solution

Consider the following reaction and its equilibrium constant:

I2(g) ⇌ 2I(g)                K p = 0.209

A reaction mixture contains 0.89 atm I 2 and 1.77 atm I. Which of the following statements is TRUE concerning this system?

A) The reaction will shift in the direction of reactants.

B) The reaction quotient will increase.

C) The reaction will shift in the direction of products.

D) The equilibrium constant will decrease.

E) The system is at equilibrium.

Watch Solution

Which of the following statements is TRUE?

A) Dynamic equilibrium occurs when the rate of the forward reaction equals the rate of the reverse reaction.

B) The equilibrium constant for the forward reaction is equal to the equilibrium constant for the reverse reaction.

C) A reaction quotient (Q) larger than the equilibrium constant (K) means that the reaction will favor the production of more products.

D) Dynamic equilibrium indicates that the amount of reactants and products are equal.

E) All of the above are true.

Watch Solution

In a reaction mixture containing only reactants, what is the value of Q?

A) -1

B) 1

C) ∞

D) 0

E) It cannot be determined without concentrations.

Watch Solution

Give the direction of the reaction, if Q >> 1

A) The forward reaction is favored.

B) The reverse reaction is favored.

C) cannot be determined.

D) If the temperature is raised, then the forward reaction is favored.

E) If the temperature is raised, then the reverse reaction is favored.

Watch Solution

In which of the following equations will the value of Kp be less than, greater than or equal to the value of Kc at 37°C?

a. C (s) + O2 (g) ↔ CO2 (g)

b. 2 BrNO (g) ↔ 2 NO (g) + Br 2 (g)

c. Br2 (g) + Cl2 (g) ↔ 2 BrCl (g)

d. 2 H2 (g) + S2 (g) ↔ 2 H2S (g)

e. CH4 (g) + H2O (g) ↔ CO (g) + 3 H 2 (g)

Watch Solution

For the reaction:

PCl3 (g) + Cl3 (g) ⇌ PCl5 (g)           K c = 24.3

a system is prepared with

[PCl3] = 0.10 M; [Cl 2] = 0.15 M; [PCl 5] = 0.60 M.

Which response is correct? 

a. the reaction is at equilibrium and concentrations will not change

b. the amount of PCl 5 will increase

c. more PCl3 will form

d. the reaction will shift right

e. the amount of PCl 3 will increase while the concentration of Cl 2 will decrease.

 

 

Watch Solution

The reaction 2 A → B + C has a Kc of 0.2. The reaction is commenced with initial concentrations [A] = 0.2 M, [B] = 0.2 M and [C] = 0.2 M. In what direction does the reaction occur?

1. Cannot be determined without knowing the temperature.

2. The reaction shifts to the right to reach equilibrium

3. The reaction is already at equilibrium

4. The reaction shifts to the left to reach equilibrium.

 

 

Watch Solution

What is the difference between the reaction quotient, Q, and the equilibrium constant, K, for a given chemical reaction? 

Watch Solution

A mixture of 0.2 moles of H2S(g), 0.5 moles of S8(s), and 0.35 moles of H2(g) is placed in a 2L container where the equilibrium

8H2S(g) ⇌ 8H2(g) + S8(s)

is established. If the equilibrium constant for this reaction is Kc = 162, which of the following statements is true?

a) Q = 162 and the reaction is already at equilibrium.

b) Q = 44 and the reaction proceeds to the left

c) Q = 88 and the reaction proceeds to the right

d) Q = 44 and the reaction proceeds to the right

e) Q = 88 and the reaction proceeds to the left

Watch Solution

The reaction

2 NO(g) + Br2(g) ⟺ 2NOBr(g)

has an equilibrium constant of 116.6  at 25°C. If Mickey Mouse starts with the reaction mixture PNO = 0.300, PBr2 = 0.225, PNOBr = 0.855 at 25°C, predict what will happen as the reaction proceeds towards equilibrium. The initial partial pressures are given in atm.

a) More reactants will form

b) More products will form

c) The reaction is already at equilibrium

d) It is impossible for the process to come to equilibrium from these starting conditions

e) Not enough information is given

Watch Solution

Given the chemical reaction:

4 H3PO4 (aq) ⇌ 6 H2 (g) + 8 O2 (g) + P4 (s)              Kc = 1.35 X 10  −12

Which direction will the reaction move if there is initially 0.25 M H 3PO4, 0.0020 M H2, 0.150 M O2 and 61.94 grams of P4.

a. The reaction will move to the right because Q = 2.1 X 10  -21

b. The reaction will move to the right because Q = 4.2 X 10 −21

c. The reaction will move to the left because Q = 2.1 X 10 −21

d. The reaction will move to the left because Q = 4.2 X 10 −21

e. The reaction will move to the right because Q = 0.0012

Watch Solution

If a 1.00 L vessel contains 0.010 atm SO3, 0.010 atm O2, and 0.020 atm SO2, is the system at equilibrium with respect to the reaction below? If not, what will happen in order to establish equilibrium?

2 SO2 + O2 ⇌ 2 SO3                   Kp = 0.14

Watch Solution