16. Chemical Equilibrium

The Reaction Quotient

Introduction to reaction quotient Qc | Chemical equilibrium | Chemistry | Khan Academy

The Reaction Quotient (Q): Predicting the Direction of Change

Increasing the temperature causes an exothermic reaction to produce more reactants.

In an exothermic reaction, energy can be considered to be on the product side of the equation and the reaction will shift to the left.

Decreasing the temperature causes an exothermic reaction to produce more products.

In an exothermic reaction, energy can be considered to be on the product side of the equation and the reaction will shift to the right.

Increasing the temperature of an endothermic reaction increases the equilibrium constant.

In an endothermic reaction, energy can be considered to be on the reactant side of the equation and the reaction will shift to the right.

All of the statements correctly describe the effect of temperature.

 Le Ch&#226;telier&#8217;s Principle states that when a system at equilibrium is disturbed, the system shifts in a direction that minimizes the disturbance. 

The reaction A21g2 + B21g2 ∆ 2 AB1g2 has an equilib- rium constant Kc = 9. The following figure represents a
reaction mixture that contains A2 molecules (red), B2 mol- ecules (blue), and AB molecules. What statement about the mixture is true? (LO 15.5) <img src="https://lightcat-files.s3.amazonaws.com/problem_images/0cd62a42627aed8e-1672267165071.jpg" alt="" /> (a) The mixture is at equilibrium, and there will be no net shift in reaction direction.
(b) The reaction will shift toward the reactants to reach equilibrium.
(c) The reaction will shift toward the products to reach equilibrium.
(d) More information is needed to answer this question.

The reaction A2 + B ∆ A + AB has an equilibrium con-stant Kc = 2. The following pictures represent reaction mix- tures that contain A atoms (red), B atoms (blue), and A2 and AB molecules. <img src="https://lightcat-files.s3.amazonaws.com/problem_images/8b6e4f94fcf5d755-1672524746435.jpg" alt="" /> (a) Which reaction mixture is at equilibrium?

(b) Will Mg(OH)2 precipitate when 4.0 g of Na2CO3 is added to 1.00 L of a solution containing 125 ppm of Mg2+?

(a) If Qc 6 Kc, in which direction will a reaction proceed
in order to reach equilibrium?

If the reaction quotient Q is equal to&#160;the equilibrium constant K then our reaction is at equilibrium.&#160;

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.&#160;

15.2 Le Chatelier's Principle | General Chemistry

Nitric oxide (NO) reacts readily with chlorine gas as
follows:
2 NO1g2 + Cl21g2 Δ 2 NOCl1g2
At 700 K, the equilibrium constant Kp for this reaction is
0.26. Predict the behavior of each of the following mixtures
at this temperature and indicate whether or not the
mixtures are at equilibrium. If not, state whether the mixture
will need to produce more products or reactants to
reach equilibrium.
(b) PNO = 0.12 atm, PCl2 = 0.10 atm, PNOCl = 0.050 atm

Consider the reaction:
NH4HS(s) ⇌ NH3(g) + H2S(g)
At a certain temperature, Kc = 8.5 * 10 - 3. A reaction mixture at this temperature containing solid NH4HS has [NH3] = 0.166 M and [H2S] = 0.166 M. Will more of the solid form or will some of the existing solid decompose as equilibrium is reached?

Halogen lamps are ordinary tungsten filament lamps in which the lamp bulb contains a small amount of a halogen (often bromine). At the high temperatures of the lamp, the halogens dissociate and exist as single atoms.
(c) When the WBr41g2 diffuses back toward the filament, it decomposes, depositing tungsten back onto the fila- ment. Show quantitatively that the pressure of WBr4 from part (a) will cause the reaction in part (a) to go in reverse direction at 2800 K. [The pressure of Br1g2 is still 0.010 atm.] Thus, tungsten is continually recycled from the walls of the bulb back to the filament, allow-ing the bulb to last longer and burn brighter.

(a) If Qc > Kc, how must the reaction proceed to reach
equilibrium?

When Q > K, the reaction will shift toward the reactant side.

If Q > K, there is too much product present.

When Q = K, the reaction has stopped.

Correct, when Q = K, the system is at equilibrium.

When Q < K, the reaction will shift toward the product side.

If Q < K, there is too much reactant present.

When Q = K, the forward reaction rate equals the reverse reaction rate.

When Q = K, the system is at equilibrium.

Q is the reaction quotient at any point in the reaction.

Which of the following is a false statement about the relationship between Q and K?

The reaction A2 + B2 Δ 2 AB has an equilibrium
constant Kc = 1.5. The following diagrams represent
reaction mixtures containing A2 molecules (red), B2 molecules
(blue), and AB molecules. (a) Which reaction mixture
is at equilibrium? [Sections 15.5 and 15.6]
<img src="https://lightcat-files.s3.amazonaws.com/problem_images/55341fe647122c74-1666359608563.jpg" alt="" />

Q vs. K

The following reaction, which has Kc = 0.145 at 298 K, takes place in carbon tetrachloride solution:
2 BrCl1soln2 ∆ Br21soln2 + Cl21soln2 A measurement of the concentrations shows 3BrCl4 =
0.050 M, 3Br24 = 0.035 M, and 3Cl24 = 0.030 M. (b) Determine the equilibrium concentrations of BrCl, Br1, and Cl2.

When the reaction quotient, Q, is larger than the equilibrium constant, K, then the reaction shifts in the reverse direction to reach equilibrium.&#160;

When the reaction quotient, Q, is smaller than the equilibrium constant, K, then the reaction shifts in the forward direction to reach equilibrium.&#160;

Determining the Reaction Quotient

Calculate the equilibrium concentrations at 25 °C for the reaction in Problem 15.84 if the initial concentrations are 
3N2O44 = 0.0200 M and 3NO24 = 0.0300 M.

Determining Equilibrium Concentrations

Consider the following equilibrium:
Ag + 1aq2 + Cl-1aq2 ∆ AgCl1s2
Use Le Châtelier’s principle to predict how the amount of solid silver chloride will change when the equilibrium is disturbed by: (d) Removing Cl-; also account for the change using the reaction quotient Qc

Silver sulfate dissolves in water according to the reaction: Ag2SO4(s) ⇌ 2Ag+(aq) + SO42-(aq)
Kc = 1.1 * 10-5 at298K A 1.5-L solution contains 6.55 g of dissolved silver sulfate. If addi-
tional solid silver sulfate is added to the solution, will it dissolve?

Equilibrium Constants

At 100 C, the equilibrium constant for the reaction
COCl21g2 Δ CO1g2 + Cl21g2 has the value Kc =
2.19 * 10-10. Are the following mixtures of COCl2, CO,
and Cl2 at 100 C at equilibrium? If not, indicate the direction
that the reaction must proceed to achieve equilibrium.
(a) 3COCl24 = 2.00 * 10-3 M, 3CO4 = 3.3 * 10-6 M,
3Cl24 = 6.62 * 10-6 M

Consider a reaction A21g2 + B21g2Δ2 AB1g2, atoms of A shown in red in the diagram and atoms of B shown in blue. (b) If Kc = 1, which box represents the system at Q < Kc?<img src="https://lightcat-files.s3.amazonaws.com/problem_images/90bad2993589d07c-1667324628302.jpg" alt="" />

Consider the reaction:
H2(g) + I2(g) ⇌ 2 HI(g)
A reaction mixture at equilibrium at 175 K contains PH2 = 0.958 atm, PI2 = 0.877 atm, and PHI = 0.020 atm. A second reaction mixture, also at 175 K, contains PH2 = PI2 = 0.621 atm and PHI = 0.101 atm. Is the second reac- tion at equilibrium? If not, what will be the partial pressure of HI when the reaction reaches equilibrium at 175 K?

(b) At the start of a certain reaction, only
reactants are present; no products have been formed.
What is the value of Qc at this point in the reaction?

Chemical Equilibria and Reaction Quotients

At 900 C, Kc = 0.0108 for the reaction
CaCO31s2 Δ CaO1s2 + CO21g2
A mixture of CaCO3, CaO, and CO2 is placed in a 10.0-L
vessel at 900 C. For the following mixtures, will the
amount of CaCO3 increase, decrease, or remain the same
as the system approaches equilibrium?
(c) 30.5 g CaCO3, 25.5 g CaO, and 6.48 g CO2

(b) What condition must be satisfied so that Qc = Kc?

K (Equilibrium Constant) vs Q (Reaction Quotient)

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[BOOK CHEM] Clutch Version

Le Chatelier's Principle

Chemical Equilibrium

Most chemical reactions never go to completion where all the reactants are converted into products. Instead these reactions reach a&nbsp;chemical equilibrium.&nbsp;

The reaction quotient, Q, is used to determine if our chemical reaction is at equilibrium.&#160;

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