Ch.14 - Chemical EquilibriumSee all chapters
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Ch.1 - Intro to General Chemistry
Ch.2 - Atoms & Elements
Ch.3 - Chemical Reactions
BONUS: Lab Techniques and Procedures
BONUS: Mathematical Operations and Functions
Ch.4 - Chemical Quantities & Aqueous Reactions
Ch.5 - Gases
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Ch.13 - Chemical Kinetics
Ch.14 - Chemical Equilibrium
Ch.15 - Acid and Base Equilibrium
Ch.16 - Aqueous Equilibrium
Ch. 17 - Chemical Thermodynamics
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Ch.22 - Chemistry of the Nonmetals
Ch.23 - Transition Metals and Coordination Compounds

The Reaction Quotient

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Sections
Chemical Equilibrium
ICE Chart
Le Chatelier's Principle
The Reaction Quotient
Additional Sections
Equilibrium Expressions
LearnAdditional PracticeSummary

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. 

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 #2: Determining the Reaction Quotient

Example #3: Determining Equilibrium Concentrations

Previous SectionLe Chatelier's Principle
Additional Problems
The reaction shown below has Kp = 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.
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? No conclusions about the system can be made without additional information. The system is at equilibrium. The reverse reaction will occur to a greater extent than the forward reaction until equilibrium is established. The forward reaction will occur to a greater extent than the reverse reaction until equilibrium is established.
Consider the following general reaction: 2 B4A4 (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.
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.
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.
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.
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
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?
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
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
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
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.  
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.
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
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
What is the difference between the reaction quotient, Q, and the equilibrium constant, K, for a given chemical reaction? 
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  
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.
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.    
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.    
For the reaction below Kc = 168C(s) + CO2(g) ⇌ 2CO(g)A mixture contains some C(s), [CO] = 0.50 M and [CO 2] = 0.75 M. Therefore the system willa) Shift to produce more C (s) and CO2 (g) because Q < K.b) Shift to produce more CO because Q < K.c) Shift to produce more C (s) and CO2 (g) because Q > K.d) Shift to produce more CO because Q > K.e) The system is already in equilibrium
Consider the reactionMg(s) + Fe2+(aq) → Mg2+(aq) + Fe(s) at 75°Cwhere [Fe2+] = 2.90M and [Mg 2+] = 0.310Ma. What is the value for the reaction quotient, Q, for the cell?b. What is the value for the temperature, T, in kelvins?
Consider this system at equilibrium.A(aq) ⇌ B(aq)        ΔH = -750 kJ/molWhat can be said about Q and K immediately after an increase in temperature?A) Q > K because Q increasedB) Q > K because K decreasedC) Q < K because Q decreasedD) Q < K because K increasedE) Q = K because neither changed How will the system respond to a temperature increase?A) shift leftB) shift rightC) no change
The diagram shows the free energy change of the reaction A(g) + B(g) ⇌ C(g).The reaction progress starts on the left with pure reactants, A and B  each at 1 atm and moves to pure product, C, also at 1 atm on the right. Place the statements in the appropriate place on the diagram.spontaneousnonspontaneousQ< KQ> KQ= K.
Consider this system at equilibrium.A(aq) ⇌ B(aq)        ΔH = +550 kJ/molWhat can be said about Q and K immediately after an increase in temperature?A) Q > K because Q increasedB) Q > K because K decreasedC) Q < K because Q decreasedD) Q < K because K increasedE) Q = K because neither changed How will the system respond to a temperature increase?A) shift leftB) shift rightC) no change
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.  
At 850 K the equilibrium constant for the following reaction is K c = 15.2SO2(g) + O2(g) ⇌ 2SO3(g) If we mix the following concentrations of the three gases, predict in which direction the reaction will proceed toward equilibrium: Left, Right, or No Reaction.a. [SO2] = 0.60 M, [O2] = 0.70 M, [SO3] = 0.16 Mb. [SO2] = 0.20 M, [O2] = 0.60 M, [SO3] = 0.60 Mc. [SO2] = 0.10 M, [O2] = 0.20 M, [SO3] = 0.40 M
Consider the reaction at 55°C, where [Fe2+] = 3.80 M and [Mg2+] = 0.310 MMg(s) + Fe2+(aq) → Mg+2(aq) + Fe(s)What is the value for the reaction quotient, Q, for the cell?
Consider the hypothetical reaction below3 A (g) + B (g) ↔ 2 C (g)           Kc = 1.5 x 10-3Which of the following statements is correct if the initial concentrations are [A] = 0.85 M, [B] = 0.36 M, and [C] = 0.005 M.A. At equilibrium, the amount of C will increase.B. At equilibrium, the amount of A will increase.C. At equilibrium, the amount of B will increase and C will increase.D. At equilibrium, the amount of A will increase and B will decrease.E. The reaction is at equilibrium.
Balance the equation and write the reaction-quotient expression, Qc.U (s) + F2 (g) ⇌ UF6 (g)Qc=    ?/?  
Write the reaction quotient for
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. [Cl2 ] = 0.30 M at equilibrium. 3. No apparent reaction takes place.  4. [NOCl] = [NO] = [Cl2 ] at equilibrium. 5. NOCl(g) is produced until equilibrium is reached.
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
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
Balance the equation and write the reaction quotient expression. U(s) + F2(g) ⇌ UF6(g)
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) K c = Q                       C) K c > Q         D) More information is needed to make a statement about Kc.
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.
The value of Kc for the reaction: 2NO2 (g) ⇌ N2O4(g) is 6.9 at 447 K. Determine the reaction quotient for a mixture of the two gases in which [NO  2] = 0.0311 M and [N2O4] = 0.00931 M.
Hydrogen lodide gas is formed when hydrogen and iodine gases are combined. H2 (g) + I2 (g) ⇌ 2HI (g)     Keq = 7.1 x 102 at 25°CA student is studying this reaction to observe how the equilibrium changes. He mixes the proper amounts of the gases so that the initial concentration of hydrogen gas is 0.90 M, the initial concentration of the lodine gas is 0.45 M and the initial concentration of hydrogen lodide gas is 0.75 M. Is the system at equilibrium, or will it shift toward the product? Set up the correct expression that will help you answer this question.Choose one: (i) Q = (0.75 M)2/(0.45M)(0.90M) where the reaction progresses toward the product (ii) Q = 1/(0.45M)(0.90M) 8 1090 where the reaction progresses toward the product (iii) Q = (0.75 M)2/(0.45M)(0.90M) where the reaction is at equillbrium (iv) KC = (0.75 M)2/(0.45M)(0.90M) where the reaction progresses toward the product 
Is a mixture of 0.0205 mol NO2(g) and 0.750 mol N2O4(g) and 0.750 mol N2O4(g) in a 5.25 - L flask at 25 ° C at equilibrium ? If not, in which direction will the reaction proceed - toward products or reactants?N2O4(g)⇌ 2NO2(g) KC = 4.61x10-3 at 25 °Ca. The reaction will proceed to the right, toward productsb. The reaction will proceed to the left, toward reactants.c. The mixture is at equilibrium.
Predict and calculate the effect of concentration changes on an equilibrium system. Some SbCl5 is allowed to dissociate into SbCl3, and Cl2 at 521 K. At equilibrium, [SbCl5] = 0.163 M, and [SbCl3] = [Cl2] = 6.39 x 10-2 M. Additional SbCl5 is added so that [SbCl5]new = 0.287 M and the system is allowed to once again reach equilibrium. SbCl5 (g) ⇌ SbCl3 (g) + Cl2 (g) K = 2.50 x 10-2 at 521 K (a) In which direction will the reaction proceed to reach equilibrium? (b) What are the new concentrations of reactants and products after the system reaches equilibrium? 
Consider this equilibrium reaction at 400 K. Br2 (g) + Cl2 (g) ⇌ 2BrCl (g)        Kc = 7.0 If the composition of the reaction mixture at 400 K is [BrCl] = 0.00415 M, [Br2] = 0.00366 M, and [Cl2] = 0.000672 M, what is the reaction quotient, Q? How is the reaction quotient related to the equilibrium constant, Kc, for this reaction?            (i) Q < K            (ii) Q > K           (iii) Q = K
The equilibrium constant for the following reaction is 0.16 M 0.5 at 3000 K: CO2 (g) <-> CO (g) + 0.5O2(g) Determine Qc if the following amounts (in moles) of each component is placed in a 3.00 L container. CO2 0.790 CO 0.410 O2 0.580 Give answer to 3 decimal places.
For the reaction 2A(g) ⇌ B(g), the equilibrium constant is Kp = 0.76. A reaction mixture initially contains 4.0 atm of gas (PA = 2.0 atm and PB = 2.0 atm). You may want to reference. Section 15.7 while completing this problem. Which statement is true of the reaction mixture? (a) The reaction mixture will proceed toward products. (b) The reaction mixture is at equilibrium. (c) The reaction mixture will proceed toward reactants (d) It is not possible to determine from the information given the future direction of the reaction mixture.
What is the name of Qc?a. Reversibility expressionb. Reaction expressionc. Mass actiond. Equilibrium expressione. Reaction quotient 
Will a precipitate form if the metal ion concentration in a solution of M(CIO2)3 is 0.0246 M and the concentration of CIO2 is 0.0148 M? Ksp for M(ClO2)3 = 6.25x10-8 Which of the following substances has the higher molar entropy (assume that each is in the gas phase)? 
2NO (g) + 2H2 (g) → N2 (g) + 2H2O (g) At the temperature the engineer picks, the equilibrium constant Kp for this reaction is 0.052. The engineer charges ("fills") four reaction vessels with nitrogen monoxide and hydrogen, and lets the reaction begin. He then measures the composition of the mixture inside each vessel from time to time. His first set of measurements are shown in the table below. Predict the changes in the compositions the engineer should expect next time he measures the compositions.