Wize High School Grade 12 Chemistry Textbook > Equilibrium

The Reaction Quotient (Q)

Predicting Equilibria: The Reaction Quotient (Q)

When reactants and products are mixed, there are two questions we ask ourselves:

Are we at equilibrium?
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:

$\large3H_2(g)+N_2(g)\to2NH_3(g)$

$\large Q=\cfrac{[NH_3]^2_t}{[H_2]^3_t[N_2]_t}$ $\large K=\cfrac{[NH_3]^2_{eq}}{[H_2]^3_{eq}[N_2]_{eq}}$

There are three possible situations:

Q = K: The system is at equilibrium.
Q > K: The [products] is too (high/low) ________ compared to [reactants]. The equilibrium will shift ________
Q < K: The [products] is too (hlgh/low) ________ compared to [reactants]. The equilibrium will shift ________

Visual Representation

Predicting Equilibria: The Reaction Quotient (Q)

When reactants and products are mixed, there are two questions we ask ourselves:

Are we at equilibrium?
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:

$\large3H_2(g)+N_2(g)\to2NH_3(g)$

$\large Q=\cfrac{[NH_3]^2_t}{[H_2]^3_t[N_2]_t}$ $\large K=\cfrac{[NH_3]^2_{eq}}{[H_2]^3_{eq}[N_2]_{eq}}$

There are three possible situations:

Q = K: The system is at equilibrium.
Q > K: The [products] is too (high/low) ________ compared to [reactants]. The equilibrium will shift ________
Q < K: The [products] is too (hlgh/low) ________ compared to [reactants]. The equilibrium will shift ________

Visual Representation

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.

\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.

Example: Is the Reaction at Equilibrium?

\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?

Qc is always larger than Kc.

Qc is always less than Kc.

When Qc > Kc the reaction proceeds toward the products to achieve equilibrium.

When Qc < Kc the reaction proceeds toward the products to achieve equilibrium

When Qc = Kc the reaction proceeds toward the products to achieve equilibrium.

I don't know

Practice: Understanding Q vs K

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

Qc is always larger than Kc.

Qc is always less than Kc.

When Qc > Kc the reaction proceeds toward the products to achieve equilibrium.

When Qc < Kc the reaction proceeds toward the products to achieve equilibrium

When Qc = Kc the reaction proceeds toward the products to achieve equilibrium.

I don't know

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?

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

After allowing this solution to rest for some time:

Nearly all of the AgCl added will be consumed by the reaction.

More products will be present than initially because a reactant was added.

The concentration of Cl– will be lower than the initial value.

There will be no change in the concentrations, since solids do not affect equilibrium.

I don't know

Practice: Predicting and Interpreting Equilibrium

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

After allowing this solution to rest for some time:

Nearly all of the AgCl added will be consumed by the reaction.

More products will be present than initially because a reactant was added.

The concentration of Cl– will be lower than the initial value.

There will be no change in the concentrations, since solids do not affect equilibrium.

I don't know

Extra Practice

Equilibrium: Reaction Quotient

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

After allowing this solution to rest for some time:

Equilibrium: Reaction Quotient

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

Practice: Q and K

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:

$4Cr(s)+3CCl_4(g) \rightleftarrows 4CrCl_3(g) +3C(s)\ \ \ \ \ \ \ \ \Delta H=26.9kJ$

Equilibrium: Reaction quotient

The reaction of

$H_{2(g)}+CO_{2(g)}\leftrightarrow H_2O_{(g)}+CO_{(g)}\ at\ 2000K\ K_P=4.40$

When PH2 = 0.35atm; PCO2 =0.85atm; PH2O = 0.71 atm and PCO = 1.35 atm calculate Q.

Equilibrium: Reaction quotient

$H_{2(g)}+CO_{2(g)}\leftrightarrow H_2O_{(g)}+CO_{(g)}\ at\ 2000K\ K_P=4.40$

Will this reaction spontaneously shift towards products or reactants?

Equilibrium constant: Reaction quotient

Consider the reaction:

$A + 2B ⇌ C + D$

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?

Equilibrium: Reaction quotient

If the reaction $A⇌B$ has an equilibrium constant of K = 10–4, which one of the following statements is always true?

Equilibrium: Reaction Quotient

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

After allowing this solution to rest for some time:

Equilibrium: Reaction Quotient

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

After allowing this solution to rest for some time:

The Reaction Quotient (Q)

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Predicting Equilibria: The Reaction Quotient (Q)

Visual Representation

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Predicting Equilibria: The Reaction Quotient (Q)

Visual Representation

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Example: Is the Reaction at Equilibrium?

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

2) Which direction will the reaction shift?

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Example: Is the Reaction at Equilibrium?

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

2) Which direction will the reaction shift?

Practice: Understanding Q vs K

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Practice: Understanding Q vs K

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Practice: Predicting and Interpreting Equilibrium

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Practice: Predicting and Interpreting Equilibrium

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Extra Practice