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Wize University Chemistry Textbook > Equilibrium

Q (Reaction Quotient)

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Predicting Equilibria: The Reaction Quotient (Q)

When reactants and products are mixed, there are two questions we ask ourselves:
  1. Are we at equilibrium?
  2. If not, which direction will the reaction shift (right or left) to get to equilibrium?

We use the Reaction Quotient (Q) to determine how close we are to equilibrium conditions (at time, t) and which direction the reaction will shift.

During non-equilibrium conditions, we use Q instead of K:

3H2(g)+N2(g)→2NH3(g)\large3H_2(g)+N_2(g)\to2NH_3(g)

Q=[NH3]t2[H2]t3[N2]t\large Q=\cfrac{[NH_3]^2_t}{[H_2]^3_t[N_2]_t} K=[NH3]eq2[H2]eq3[N2]eq\large K=\cfrac{[NH_3]^2_{eq}}{[H_2]^3_{eq}[N_2]_{eq}}

There are three possible situations:
  1. Q = K: The system is at equilibrium.
  2. Q > K: The [products] is too (high/low)
    high
    compared to [reactants]. The equilibrium will shift
    left.
  3. Q < K: The [products] is too (hlgh/low)
    low
    compared to [reactants]. The equilibrium will shift
    right.

Wize Tip
The above 3 points are commonly seen on exams!
Knowing these can help you get the best mark possible :)

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Visual Representation


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Example: Is the Reaction at Equilibrium?

If the current concentrations of H2O, H2, and O2 were 1.2M, 0.7M, and 3.2M, respectively, is this reaction at equilibrium? If not, which direction must the reaction shift to reach equilibrium? K=1.2x10-2.
2 H2O(l)⇌2 H2(g)+O2(g)\rm 2\ H_2O_{(l)} \quad\xrightleftharpoons{\hspace{1.5cm}}\quad 2\ H_{2(g)}\quad+\quad O_{2(g)}

1) To figure out if reaction is at equilibrium we must compare Q and K:


Q= [O2][H2]2
=(3.2)(0.72)
Q =1.568
So Q>K Therefore we are not at equilibrium.

2) Which direction will the reaction shift?


Since Q>K, there are more products than reactants currently, so the equilibrium will shift left towards reactants.

Practice: Understanding Q vs K

Which of the following statements regarding the reaction quotient Qc and the equilibrium constant Kc is true?

Practice: Predicting and Interpreting Equilibrium


Silver chloride will react with acetic acid to form the complex ion silver acetate. A reaction mixture is prepared by adding 0.25 g of AgCl to a 1.25 L solution that already contains 4.6×10‐6M Ag(CH3COO)(aq), 2.3×10‐3M Cl-, 1.1×10‐8M H+, and 0.010M CH3COOH. Based on this information, which of the statements below is true?

AgCl(s)+CH3COOH(aq)⇌Ag(CH3COO)(aq)+Cl−(aq)+H+(aq)K=5.87×10−16AgCl(s)+CH_3 COOH(aq)⇌Ag(CH_3 COO)(aq)+Cl^- (aq)+H^+ (aq)\qquad K = 5.87×10^{-16}

After allowing this solution to rest for some time:
Extra Practice
Practice: Calculating Equilibrium Concentrations
Iodine chloride (ICl) can decompose, forming iodine and chlorine gases. If a reaction vessel is prepared by filling a 5.00 L container with 0.500 mol each of ICl, I2, and Cl2.
2ICl(g)⇌I2(g)+Cl2(g)KC=0.110 (at 25°C)2ICl(g)⇌I_2 (g)+Cl_2 (g)\qquad K_C = 0.110\ (at \ 25°C)
a) In which direction will the reaction proceed (towards products or reactants)? Show how you determined your answer.
Equilibrium constant: Reaction quotient
Consider the reaction:
A+2B⇌C+DA + 2B ⇌ C + D
At a particular temperature this reaction is reactant-favoured at equilibrium. If for the current concentrations the forward reaction is favoured, which of the following statements is true?
The system: H2(g)+X2(g)⇌2HX(g)H_2(g) + X_2(g) \rightleftharpoons 2 HX(g) has a value of 24.4 for K. A system being studied in a 3.00 liter reactor was charged with 0.150 moles of H2, 0.150 moles of X2, and 0.600 moles of HX. A catalyst was introduced using a remote unit. Which statement below describes the situation?
Express the reaction quotient for the following equilibria in terms of partial pressures and concentration.
2 H2O(l)⇌2 H2(g)+O2(g)\rm 2\ H_2O_{(l)} \quad\xrightleftharpoons{\hspace{1.5cm}}\quad 2\ H_{2(g)}\quad+\quad O_{2(g)}
Express the reaction quotient for the following equilibria in terms of partial pressures and concentration.
 CH4(g)+2 O2(g)⇌CO2(g)+2 H2O(g)\rm \ {CH_{4(g)}} \quad+\quad 2\ {O_{2(g)}}\quad \xrightleftharpoons{\hspace{1.5cm}}\quad {CO_{2(g)}}\quad+\quad 2\ H_2O_{(g)}
At 425 °C, Kp = 4.18 x 10-9 for the reaction:
2HBr(g)↔H2(g)+Br2(g)2HBr_{(g)}\leftrightarrow H_{2(g)}+Br_{2(g)}
In one experiment 0.20 atm of HBr, 0.100 atm of H2 and 0.010 atm of Br2 are placed in a container. Is the reaction at equilibrium? If not, in which direction will it proceed?
At 425 °C, Kp = 4.18 x 10-9 for the reaction:
2HBr(g)↔H2(g)+Br2(g)2HBr_{(g)}\leftrightarrow H_{2(g)}+Br_{2(g)}
In one experiment 0.20 atm of HBr, 0.100 atm of H2 and 0.010 atm of Br2 are placed in a container. Is the reaction at equilibrium? If not, in which direction will it proceed?
Which of the following statements regarding the reaction quotient Qc and the equilibrium constant Kc is true?
Consider the reaction of nitrogen and oxygen gases to produce nitric oxide:
N2(g)+O2(g)⇌2NO(g)                   Kc=4.0×10−4  at  1750 oCN_2(g)+O_2(g) \rightleftharpoons 2NO(g)\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ K_c=4.0\times 10^{-4}\ \ at\ \ 1750 \ ^oC
What is the value of Kp for this reaction at 1750 °C?
At 425 °C, Kp = 4.18 x 10-9 for the reaction:
2HBr(g)↔H2(g)+Br2(g)2HBr_{(g)}\leftrightarrow H_{2(g)}+Br_{2(g)}
In one experiment 0.20 atm of HBr, 0.100 atm of H2 and 0.010 atm of Br2 are placed in a container. Is the reaction at equilibrium? If not, in which direction will it proceed?
Equilibrium: Reaction quotient
The system: H2(g) + X2(g) : 2 HX(g) has a value of 24.4 for Kc. A system being studied in a 3.00 liter reactor was charged with 0.150 moles of H2, 0.150 moles of X2, and 0.600 moles of HX. A catalyst was introduced using a remote unit. Which statement below describes the situation
Consider the following reaction to answer the next two questions:
4Cr(s)+3CCl4(g)⇄4CrCl3(g)+3C(s)        ΔH=26.9kJ4Cr(s)+3CCl_4(g) \rightleftarrows 4CrCl_3(g) +3C(s)\ \ \ \ \ \ \ \ \Delta H=26.9kJ
Equilibrium: Reaction quotient
Consider the following reaction:
4 HCl (g) + O2 (g) ⇌ 2 Cl2 (g) + 2 H2O (g) Kc = 0.063 at 400 K
If the reaction quotient, Q, is 0.100, which of the following statements is FALSE?
Equilibrium: Reaction quotient
The equilibrium constant expressed in terms of partial pressure for the water gas reaction (shown) is Kp = 2.6 at a temperature of 1000 K. C (s) + H2O (g) ⇌ CO (g) + H2 (g)
A closed vessel initially has a sample of solid carbon (graphite), and P(H2O(g)) = 0.600 atm and
P(CO(g)) = 1.525 atm and P(H2(g)) = 0.804 atm. Which one of the following statements is TRUE?
Equilibrium: Magnitude of equilibrium constant
The value of Kc for a particular chemical equilibrium is 1.0x1013. Hence, we can conclude:
Equilibrium: Reaction quotient
The Kc for the Haber process N2 (g) + 3 H2 (g) ⇌ 2 NH3 (g) is 9.60 at 573 K. When 1.0x10-3 moles of ammonia is mixed in a 1 L flask at 573 K with 1.0x10-2 moles of hydrogen and 5.0x10-1 moles of nitrogen, one can conclude that:
Equilibrium: Reaction quotient
Which statement is CORRECT for a reaction that has Kc = 2.43x10-12?
Equilibrium: Reaction quotient
Sulfuryl dichloride (SO2Cl2) can be prepared by the following two-step reaction sequence.
H2S (g) + 1.5 O2 (g) ⇌ SO2 (g) + H2O (g)
SO2 (g) + Cl2 (g) ⇌ SO2Cl2 (g)
Equilibrium: Reaction quotient
For the equilibrium process N2O4 (g) ⇌ 2 NO2 (g), Kc = 0.212 at 100oC. If a reaction is initiated with 0.70 mol of N2O4 and 0.35 mol of NO2 in a 1.0 liter container that is heated to 100oC. At equilibrium, how will the amounts of N2O4 and NO2 compare with the initial amounts?
In a large vessel the following reaction is taking place, PCl5(g)⇌PCl3(g)+Cl2(g)PCl_5\left(g\right)\rightleftharpoons PCl_3\left(g\right)+Cl_2\left(g\right) , Kc=4.18×10−2 at 225°CK_c=4.18 \times 10^{-2}\ at\ 225\degree C . I
A lab partner fills an identical vessel with these gases such that the initial concentrations are [PCl5]=0M, [PCl3]=7.2M, and [Cl2]=2.35M Which of the following statement is TRUE?
Does any solid Cu(OH)2 (Ksp =1.6 x 10-19) form when 0.075g of KOH dissolves in 1.0 L of 1 x 10-3 M Cu(NO3)2
Equilibrium: Reaction quotient
Consider the following hypothetical gas-phase reaction: A (g) ⇌ B (g) Kp = 3.40 at 25oC
If a flask initially contains 1.00 atm of each gas, in what direction will the reaction shift in order to reach equilibrium?
Carbon monoxide and chlorine combine in an equilibrium reaction to produce the highly toxic product, phosgene (COCl2)
CO(g)+Cl2(g)⇌COCl2(g)CO_{(g)} +Cl_{2(g)} \rightleftharpoons COCl_{2(g)}
If the equilibrium constant for this reaction is Kc = 248, predict, if possible, what will happen when the reactants and product are combined with the concentrations shown below:
The graph below shows the concentration of reactants and products for an unknown chemical reaction.
Which of the following statements are correct
i. Q < K at point A
For the reaction N2H4(g) ⇌ N2(g) + 2 H2(g), Kc = 0.026 at 200 K. If we place 2.0 M of N2H4(g), 0.80 M of H2(g), and 3.5 M of N2(g) in a closed vessel, in which way will the reaction proceed to reach equilibrium?
A graduate student sets up an equilibrium demonstration by placing NO2 and N2O4 in flask. These species are in equilibrium with eachother as shown below. Initially the reaction quotient, Q is equal to 3.5, we know that for this reaction K = 1.1 We would predict that,
2 NO2(g)⇌N2O4(g)2\ NO_{2(g)}\rightleftharpoons N_2O_{4(g)}
Equilibrium: Reaction quotient
The system: H2(g) + X2(g) : 2 HX(g) has a value of 24.4 for Kc. A system being studied in a 3.00 liter reactor was charged with 0.150 moles of H2, 0.150 moles of X2, and 0.600 moles of HX. A catalyst was introduced using a remote unit. Which statement below describes the situation
Consider the following reaction to answer the next two questions:
4Cr(s)+3CCl4(g)⇄4CrCl3(g)+3C(s)        ΔH=26.9kJ4Cr(s)+3CCl_4(g) \rightleftarrows 4CrCl_3(g) +3C(s)\ \ \ \ \ \ \ \ \Delta H=26.9kJ
Which of the following statements regarding the reaction quotient Qc and the equilibrium constant Kc is true?
Equilibrium: Reaction quotient
For the equilibrium process N2O4 (g) ⇌ 2 NO2 (g), Kc = 0.212 at 100oC. If a reaction is initiated with 0.70 mol of N2O4 and 0.35 mol of NO2 in a 1.0 liter container that is heated to 100oC. At equilibrium, how will the amounts of N2O4 and NO2 compare with the initial amounts?
Equilibrium: Reaction quotient
A reaction vessel is prepared by putting 0.2 mol of H2S (g) and 0.05 mol of NH3 (g) into a sealed 1.0 L vessel containing 0.05 mol of ammonium hydrosulfide. Based on the reaction given below, will there be a change in the amounts of ammonium hydrosulfide in the flask after waiting for some time?
𝑁𝐻4𝐻𝑆(𝑠)⇌𝐻2𝑆(𝑔)+𝑁𝐻3(𝑔)𝐾𝐶=0.11𝑁𝐻_4𝐻𝑆 (𝑠) ⇌𝐻_2𝑆 (𝑔)+𝑁𝐻_3 (𝑔) \qquad𝐾_𝐶=0.11
Equilibrium: Using The Reaction Quotient
Silver chloride will react with acetic acid to form the complex ion silver acetate. A reaction mixture is prepared by adding 0.25 g of AgCl to a 1.25 L solution that already contains 4.6×10‐6 M Ag(CH3COO)(aq), 2.3×10‐3 M Cl-, 1.1×10‐8 M H+, and 0.010 M CH3COOH. Based on this information, which of the statements below is true?
AgCl(s)+CH3COOH(aq)⇌Ag(CH3COO)(aq)+Cl−(aq)+H+(aq)K=5.87×10−16AgCl(s)+CH_3 COOH(aq)⇌Ag(CH_3 COO)(aq)+Cl^- (aq)+H^+ (aq)\qquad K = 5.87×10^{-16}
After allowing this solution to rest for some time:
Equilibrium: Calculating Equilibrium Concentrations
Iodine chloride (ICl) can decompose, forming iodine and chlorine gases. If a reaction vessel is prepared by filling a 5.00 L container with 0.500 mol each of ICl, I2, and Cl2.
2ICl(g)⇌I2(g)+Cl2(g)KC=0.110 (at 25°C)2ICl(g)⇌I_2 (g)+Cl_2 (g)\qquad K_C = 0.110\ (at \ 25°C)
a) In which direction will the reaction proceed (towards products or reactants)? Show how you determined your answer.
Use the following scenario for the next two questions
The reaction for the reforming of methane from steam is given by:
CH4(g)+H2O(g)⇌CO(g)+3H2(g)CH_4 (g) + H_2 O (g) ⇌ CO (g) + 3 H_2 (g)
Consider the reaction: A+2B⇌C+DA+2B\rightleftharpoons C+D
At a particular temperature this reaction is reactant-favoured at equilibrium. If for the current concentrations the forward reaction is favoured, which of the following statements is true?
Chemical Equilibria
Consider this balanced chemical reaction and the molecular representation for both of the next two questions: 2AB(g)⇌A2(g)+B2(g)2AB(g)⇌A_2(g)+B_2(g)
For the following isomerization reaction between butane and isobutane, which statement best describes the system at time 2?
Equilibrium: Reaction Quotient
At 425 °C, Kp = 4.18 x 10-9 for the reaction:
2HBr(g)↔H2(g)+Br2(g)2HBr_{(g)}\leftrightarrow H_{2(g)}+Br_{2(g)}
In one experiment 0.20 atm of HBr, 0.100 atm of H2 and 0.010 atm of Br2 are placed in a container. Is the reaction at equilibrium? If not, in which direction will it proceed?