Unit D—Chemical Equilibrium Focusing onAcid–Base Systems

General Outcome 1: Students will explain that there is a balance of opposing reactions in chemical equilibrium systems

Knowledge 30–D1.1k Define equilibrium and state the criteria that apply to a chemical system in equilibrium; i.e., closed system, constancy of properties, equal rates of forward and reverse reactions.

Which of the following statements applies to a system at equilibrium?

A. The rate of the forward reaction is equal to the rate of the reverse reaction. B. The concentration of the reactants equals the concentration of the products. C. The rate of the forward reaction is greater than the rate of the reverse reaction. D. The concentration of the products is greater than the concentration of the reactants.

Which of the following properties would not be used to determine if the equilibrium system represented by the equation above is at equilibrium?

A. Temperature B. Pressure C. Colour D. Mass

Which of the following systems could be at equilibrium?

A. A closed bottle of carbonated water B. A block of ice in a glass of water C. Water boiling in a kettle D. A glass of pop

Knowledge 30–D1.2k Identify, write and interpret chemical equations for systems at equilibrium.

The equation that represents the equilibrium of phosphoric acid is

Knowledge 30–D1.3k Predict, qualitatively, using Le Châtelier’s principle, shifts in equilibrium caused by changes in temperature, pressure, volume, concentration or the addition of a catalyst and describe how these changes affect the equilibrium constant.

For the position of the equilibrium to shift toward the products, the pressure of the system should be __i__ by adjusting the volume of the closed system and the temperature should be __ii__ . The statement above is completed by the information in row

Row i iiA. increased increased

B. increased decreased

C. decreased increased

D. decreased decreased

When a catalyst is added to this system at equilibrium, the position of the equilibrium i and the value of ΔH° ii . The statement above is completed by the information in row

Row i iiA. shifts right increases

B. shifts right does not change

C. does not change increases

D. does not change does not change

Use the following information to answer the next two questions.

Which of the following changes, when applied to this equilibrium system, would change the value of the equilibrium constant?

A. An addition of a catalyst B. An increase in temperature C. An addition of hydrogen gas D. A decrease in the volume of the flask

The empirical evidence that could be used to determine when this system reaches equilibrium is A. colour B. density C. total mass D. total pressure

When applied to an equilibrium system, which of the following stresses would cause a change in the Kc value after the equilibrium has been re-established?

A. Addition of a catalyst B. Decrease in temperature by cooling the system C. Addition of an inert gas to increase the pressure D. Decrease in concentration by removing a product

Which of the following graphs represents the change in the equilibrium system at time x that results as the system is heated?

Which of the following stresses represents the change in the equilibrium system at time x on the graph above?

A. A catalyst is added. B. Pressure is decreased. C. Temperature is increased. D. Fluorine gas is removed from the vessel.

The stresses numbered above that would cause the equilibrium system to shift toward the products are

A. I and III only B. I, III, and IV C. II and III only D. II, III, and IV

Knowledge 30–D1.4k Define Kc to predict the extent of the reaction and write equilibrium law expressions

for given chemical equations, using lowest whole number coefficients.

Knowledge 30–D1.5k Describe Brønsted–Lowry acids as proton donors and bases as proton acceptors.

Which of the following substances can act as a Brønsted–Lowry base? A. NaCl(aq) B. CH3OH(aq) C. HCOOH(aq)

D. NaHCO3(aq)

The Brønsted–Lowry base that forms in the highest concentration in the benzoic acid solution is i , and this base will ii a proton. The statement above is completed by the information in row

Row i iiA. OH–(aq) acceptB. OH–(aq) donateC. C6H5COO–(aq) acceptD. C6H5COO–(aq) donate

Knowledge 30–D1.6k Write Brønsted–Lowry equations, including indicators, and predict whether reactants or products are favoured for acid–base equilibrium reactions for monoprotic and polyprotic acids and bases.

The net ionic equation that represents the reaction that occurs at the second equivalence point in the experiment is A. H3PO4(aq) + 3NaHCO3(aq) → Na3PO4(aq) + 3H2CO3(aq) B. H3PO4(aq) + HCO3–(aq) → H2PO4–(aq) + H2CO3(aq) C. H2PO4–(aq) + HCO3–(aq) → HPO42–(aq) + H2CO3(aq) D. H2PO4–(aq) + 2HCO3–(aq) → HPO42–(aq) + 2H2CO3(aq)

Knowledge30–D1.7k Identify conjugate pairs and amphiprotic substances.

Sulfurous acid is a __i__ acid, and its conjugate base is__ ii_ . The statement above is completed by the information in row

Row i iiA. monoprotic amphiproticB. monoprotic polyproticC. polyprotic amphiproticD. polyprotic polyprotic

In the reaction represented by the equation above, the amphiprotic species is A. H2SO3(aq) B. HSO3–(aq) C. HF(aq) D. F–(aq)

Knowledge 30–D1.8k Define a buffer as relatively large amounts of a weak acid or base and its

conjugate in equilibrium that maintain a relatively constant pH when small amounts of acid or base are added.

If each pair of solutions listed above is mixed together in equal amounts, then the pair of solutions that would act as a buffer is A. I B. II C. III D. IV

Skill30–D1.3s Analyze data and apply mathematical and conceptual models to develop and assess

possible solutions write the equilibrium law expression for a given equation analyze, qualitatively, the changes in concentrations of reactants and products

after an equilibrium shift interpret data from a graph to determine when equilibrium is established and

to determine the cause of a stress on the system interpret, qualitatively, titration curves of monoprotic and polyprotic acids

and bases for strong acid-weak base and weak acid-strong base combinations, and identify buffering regions

Which of the following acids, when titrated with a 0.10 mol/L NaOH(aq) solution, could produce the titration curve shown above? A. HF(aq) B. HCl(aq) C. H2SO4(aq) D. H2CO3(aq)

General Outcome 2: Students will determine quantitative relationships in simple equilibrium systems

Knowledge30–D2.1k Recall the concepts of pH and hydronium ion concentration and pOH and hydroxide

ion concentration, in relation to acids and bases.

If the pH of a solution changes from 2 to 4, then the hydronium ion concentration A. is doubled B. is halved C. increases by a factor of 100 D. decreases by a factor of 100

During the reaction, the pH of the solution will __i__ and the pOH of the solution will __ii__ . The statement above is completed by the information in row

Row i iiA. increase increase

B. increase decrease

C. decrease increase

D. decrease decrease

Knowledge 30–D2.2k Define Kw, Ka, Kb and use these to determine pH, pOH, [H3O+] and [OH–] of acidic and basic solutions.

Which of the following solutions has the lowest pH? A. 1.5 mol/L H3PO4(aq) B. 0.50 mol/L HNO3(aq) C. 0.75 mol/L H2CO3(aq) D. 1.0 mol/L HCOOH(aq)

The Kb of the conjugate base of sulfurous acid, expressed in scientific notation, is a.b × 10–cd. The values of a, b, c, and d are _____, _____, _____, and _____.

A technician prepares 500 mL of a 0.3 mol/L NH3(aq) solution. The pH of the NH3(aq) solution is __________ .

The pOH of a 0.012 5 mol/L solution of sodium hypochlorite is A. 1.90 B. 4.19 C. 8.37 D. 9.81

Knowledge30–D2.3k Calculate equilibrium constants and concentrations for homogeneous systems and Brønsted–Lowry acids and bases (excluding buffers) when

concentrations at equilibrium are known initial concentrations and one equilibrium concentration are known the equilibrium constant and one equilibrium concentration are known

The value of the equilibrium constant for this system at 25 °C is A. 57 B. 31 C. 0.033 D. 0.017

At equilibrium, if the concentration of PCl3(g) is 0.165 mol/L and the concentration of PCl5(g) is 0.255 mol/L, then the equilibrium concentration of Cl2(g), expressed in scientific notation, is a.bc × 10–d mol/L. The values of a, b, c, and d are _____, _____, _____, and _____.

At 15.0 °C, the equilibrium constant is __________. (Record your three-digit answer in the numerical-response section on the answer sheet.)