Chapter 17 Concepts

Which of the following will change the pressure in a reaction involving only gases at equilibrium? Select all that apply.

Changing the volume of the container Adding an inert gas Adding or removing a reactant or product

If a system at equilibrium is disturbed by a change in concentration, the system will shift to _____ a substance that is added and to _____ a substance that has been removed.

consume; produce

When Q > K, the amount of the products must _____ and the amount of the reactants must _____ until equilibrium is established. The reaction will proceed toward the _____.

decrease; increase; reactants

Q<K

Undercomplete, proceeds to right/products

Add Reactant/ Remove Product

-Ball slides to product side

Add Product/ Remove Reactant

-Ball slides to reactant side

Pressure and K

-Increase Pressure, Decrease Volume, > least mols -Decrease Pressure, Increase Volume, > most mols

Given the following equilibrium data at a particular temperature: 2NO2Cl (g) ⇌ 2NO2 (g) + Cl2 (g) [NO2Cl] = 0.019 M, [NO2] = 0.041 M, and [Cl2] = 0.037 M calculate the value of Kc for the reaction at this temperature.

0.17

Which of the following statements correctly describe how the addition of a catalyst will affect a system at equilibrium? Select all that apply.

A catalyst does not change the value of K. Equilibrium is unaffected.

Which of the following options correctly describe how a system at equilibrium will respond to a change in temperature? Select all that apply.

A change in temperature will cause a change in Kc. If the forward reaction is exothermic, an increase in temperature causes a shift to the left.

A sample of solid C is placed in a sealed reaction vessel containing 2.0 atm H2 (g) and allowed to react according to the equation C (s) + 2H2 (g) ⇌ CH4 (g). At equilibrium the partial pressure of CH4 = 0.39 atm. Calculate Kp for the reaction.

All partial pressures used to calculate KP must be equilibrium values. 2 mol of H2 will react to form every 1 mol of CH4, so P for H2 at equilibrium=2.0-2(0.39)=0.78 Kp= 0.39/(2.0-0.78)^2 = 0.26

0.20 mol of N2 (g) and 0.15 mol of O2 (g) are placed in a 1-L container and allowed to react according to the equation N2 (g) + O2 (g) ⇌ 2NO (g), Kc = 4.10 x 10-4. Place the following steps to solve for [NO] at equilibrium in the correct order, starting with the first step at the top of the list.

Assume that the change in [N2]=x Kc= 4.10x10^-4 = 4x^2/(0.20-x)(0.15-x) Assume that x is small so that 4.10x10^-4 = 4x^2/(0.20x0.15) Solve for x, then [NO]eqm =2x Check that x<5% of both 0.20 and 0.15

If Kc = 6.2 x 10-5 = x2(0.20-x)(0.15-x), which of the following options correctly describes the correct simplifying assumption made when solving for x?

Assume that x is small so that Kc ≈ x2/(0.20)(0.15)

Match each of the following changes to the resulting effect on the pressure of an equilibrium system containing gaseous components. Increase in volume Adding a gaseous reactant adding an inert gas adding a solid

Decrease in pressure increase in pressure increase in total pressure, but no change in partial pressure of gasses no effect on pressure

0.20 mol of NO (g) is placed in a 1-L container with 0.15 mol of Br2 (g). These will react according to the balanced equation: 2NOBr (g) ⇌ 2NO (g) + Br2 (g). Given that [NOBr] = 0.18 M at equilibrium, select all the options that correctly describe the steps required to calculate Kc for the reaction.

If the change in [Br2] = -x, then the change in [NOBr] = +2x. The initial [NOBr] = 0 M. x = 0.182 = 0.09

N2O4 (g) ⇌ 2NO2 (g); Kc = 6.1 × 10-3 at 298 K If [N2O4] at the start of the reaction = 2.0 M, select ALL the options that correctly show how to calculate [NO2] at equilibrium. Assume that the change in [NO2] = +2x.

If the change in [NO2] = +2x then [N2O4]eqm = (2.0 - x). x = 2.8% of [N2O4]init so the simplifying assumption is justified. Kc = 6.1 × 10-3 ≈ 4x22.0

Which of the following statements correctly describe how an equilibrium system containing gases will respond to changes in volume and/or pressure if the temperature remains unchanged? Select all that apply.

If the total number of moles of gas in the products and reactants is the same, a change in volume will not affect the equilibrium position. If an inert gas is added, there will be no effect on the equilibrium position. If the volume is decreased, the reaction shifts in the direction that produces fewer moles of gas.

Which of the following equilibrium calculations can be simplified by taking square roots on both sides of the equation? Select all that apply.

K = 0.15 = x2/(0.1−x)(0.1−x) K = 420 = (0.1+x)(0.1+x)/(2x)2

Match each temperature change correctly to the corresponding change in K for an equilibrium system. An increase in temperature causes A decrease in temperature causes If Delta H > 0, K will decrease If Delta H < 0, K will decrease

K to increase if Delta H > 0 K to increase if Delta H < 0 When temperature decreases when temperature increases

Q>K

Overcomplete, proceeds to left/reactants

The equilibrium constant expression for a particular reaction is given by Kc = (2x)2(0.1-x)(0.2-2x)(2x)2(0.1-x)(0.2-2x). This equation cannot be simplified by taking square roots on both sides as the denominator is not a(n) ___ square. In this case, the ___ formula can be used to solve for x.

Perfect; quadratic

The Haber process is used to produce ammonia in industry. How does this process utilize Le Chatelier's principle to maximize reaction yields? The reaction is N2 (g) + 3H2 (g) ⇌ 2NH3 (g); ΔHorxn = -91.8 kJ.

Product is removed by condensation, so that the equilibrium continually shifts to the right to generate more product. The reaction is carried out at high pressures.

Based on Le Chatelier's principle, in what ways can we optimize production of ammonia from the Haber process? Select all that apply. The reaction is N2 (g) + 3H2 (g) ⇌ 2NH3 (g); ΔHorxn = -91.8 kJ.

Remove NH3 Decrease the volume Add N2 and H2

If a reaction has a relatively ___ value of K and fairly ___ initial concentrations, equilibrium calculations can be simplified by assuming that the change in concentration (x) is ___ compared to [reactant]init. The assumption is justified as long as x is less than ___% of the initial concentration.

Small; Large; negligible

A 1-L flask containing 1.0 mol Br2, 1.0 mol Cl2, and 2.0 mol BrCl react according to the equation Br2 (g) + Cl2 (g) ⇌ 2BrCl (g); Kc = 7.0 at the reaction temperature.

The change in [Br2] will be -x. Kc = 7.0 =(2.0+2x)2/(1.0-x)(1.0-x)

Which of the following statements correctly explain why the addition of an inert gas to an equilibrium system involving gases does not cause a shift in the equilibrium position? Select all that apply.

The concentrations and the partial pressures of the gases involved in the reaction remain unchanged. The inert gas is not included in the expression for Q.

For the following reaction, what will be observed if the volume is increased at constant temperature? Select all that apply. 2NO2 (g) ⇌ 2NO (g) + O2 (g)

The equilibrium position will shift toward the products. The concentrations of all species will decrease at the instant the volume is changed.

Consider the equilibrium system C (s) + H2O (g) ⇌ CO (g) + H2 (g); ΔHorxn = 131 kJ. What will be observed if the temperature of the system is decreased? Select all that apply.

The equilibrium will shift toward the reactants. The value of Kc will decrease. The equilibrium concentration of H2O (g) will increase.

1.9 moles of HI are added to an evacuated, sealed 10.-L reaction vessel and allowed to decompose according to the following reaction: H2 (g) + I2 (g) ⇌ 2 HI (g); Kc = 1.26 × 10−3H2 (g) + I2 (g) ⇌ 2 HI (g); Kc = 1.26 × 10-3 Which of the following options correctly reflect how to calculate [H2]H2 at equilibrium? Select all that apply.

The initial [HI] = 0.19 M. If the change in [H2] = +x, then at equilibrium [I2] = x. Kc = 1.26 × 10−3 = (0.19 − 2x)/2x2

A 1-L flask contains 0.50 mol SO2Cl2, 0.30 mol SO2, and 0.16 mol Cl2, which react according to the equation SO2Cl2 (g) ⇌ SO2 (g) + Cl2 (g), Kc = 0.011 for the reaction temperature. Select all the options that correctly interpret the data provided.

The reaction will proceed toward the reactant. Kc = 0.011 =(0.30-x)(0.16-x)/(0.50+x) [SO2Cl2]eqm will be greater than 0.50 M.

If the concentration of a reactant or product is changed in a system at equilibrium, what will happen to the value of K if the temperature remains constant?

The value of K will remain unchanged because the ratio of product to reactant concentration does not change.

Which of the following options correctly describe the van't Hoff equation? Select all that apply.

The value of R used in this equation is 8.314 J/mol⋅K. The units of ΔHorxn must be consistent with the units of R when using this equation. The van't Hoff equation expresses the change in equilibrium constant as temperature changes.

Three common ways of disturbing a system at equilibrium are to change the concentrations of reactants and/or products, the temperature, or the ___ of the system.

Volume

Which of the following statements correctly reflect Le Chatelier's principle? Select all that apply.

When a system at equilibrium is disturbed, the system reacts to minimize the effect of the disturbance. When a system at equilibrium is disturbed, Q ≠ K.

An equilibrium calculation can be simplified by taking square roots on both sides of the equation when _____. Multiple choice question.

both numerator and denominator in the expression for K are perfect squares

If a system at equilibrium is disturbed by changing the concentration of a reactant or product, the equilibrium ___ will shift but the ratio of product to reactant will be ___ once the system reestablishes equilibrium. The value of K will be ___

concentrations; the same; the same

If the temperature of a system at equilibrium is increased the system will shift to use up the excess heat, favoring a(n) _____ reaction. A decrease in temperature causes more heat to be produced, favoring a(n) _____ reaction.

endothermic; exothermic

K>1

favors products (larger numerator)

K<1

favors reactants (larger denominator)

If an equilibrium system contains gaseous reactants and/or products, a change in volume will cause a change in concentration and pressure. A decrease in volume will cause the system to shift in the direction that produces _____ moles of gas in total, whereas an increase in volume causes a shift in the direction that produces _____ moles of gas in total.

fewer; more

The ______ may be used to find unknown equilibrium concentrations when the coefficients on both sides of reaction add up to two but none of the individual terms in the equilibrium expression are the same.

quadratic formula

If the temperature is increased for a system at equilibrium, an exothermic reaction (ΔH < 0) will shift toward the _____ and the value of K will _____. On the other hand, an endothermic reaction (ΔH > 0), will shift toward the _____ and the value of K will _____.

reactants; decrease; products; increase

Q=K

reaction is at equilibrium

Le Chatelier's principle states that when a chemical system at equilibrium is disturbed, the system will undergo a net reaction that ___ the effect of the disturbance and will attain a new ___ position.

reduces; equilibrium

The addition of an inert gas has no effect on the equilibrium position of a gaseous reaction because

the partial pressures of the reaction components remain the same