NESA Chemistry Factors that Affect Equilibrium

15 sample questions with marking guides and sample answers · Avg. score: 63.1%

Q38
2021
SCSA
12 marks
Q38
12 marks

Sulfuric acid is manufactured by the Contact process, the steps of which are outlined below.

Step One: Molten sulfur is burned in air at approximately 1000 °C:

S(l)+O2(g)SO2(g)+297kJS(l) + O_2(g) \rightarrow SO_2(g) + 297 kJ

Step Two: The resulting sulfur dioxide is converted to sulfur trioxide as shown in the following equilibrium reaction. It is conducted at a temperature of about 450 °C with a V2O5V_2O_5 catalyst at a pressure of between 100 and 200 kPa:

2SO2(g)+O2(g)2SO3(g)+198kJ2 SO_2(g) + O_2(g) \rightleftharpoons 2 SO_3(g) + 198 kJ

Step Three: The resulting sulfur trioxide is absorbed into sulfuric acid, producing oleum (H2S2O7H_2S_2O_7). Water is added to the oleum, producing 18 mol L1^{-1} sulfuric acid:

SO3(g)+H2SO4(l)H2S2O7(l)SO_3(g) + H_2SO_4(l) \rightarrow H_2S_2O_7(l) H2S2O7(l)+H2O(l)2H2SO4(aq)H_2S_2O_7(l) + H_2O(l) \rightarrow 2 H_2SO_4(aq)

Use your understanding of collision theory and chemical equilibrium to discuss the reaction conditions for Steps 1 and 2 of the Contact process, given that the aim is to produce the greatest yield in the shortest time. In your discussion, also address economic concerns where appropriate.

Reveal Answer

High temperature increases the average kinetic energy of the particles, which means that the particles collide more frequently. Also, more of these collisions will have energy higher than the activation energy, which means a greater proportion of collisions are successful, and the reaction rate increases. The vanadium catalyst increases the rate of the forward reaction, and also the rate of the reverse reaction to an equal extent, as it provides an alternative pathway with a lower activation energy. Therefore, a greater proportion of the particles will have sufficient energy to react when they collide. High pressure or concentration has more particles per unit volume and so there is a higher frequency of collisions, and the reaction rate increases. As Step 1 is a combustion reaction, it essentially goes to completion at the high temperature and does not require a catalyst or high pressure. For Step 2, high temperature, high pressure and a catalyst would favour a high rate.

For equilibrium, which is only considered for Step 2, high temperature favours the reverse reaction because it is endothermic, and this decreases the SO3_3(g) yield, which is not desired. A low temperature decreases the rate of reaction, which is also not desired. A high pressure favours the forward reaction because there are a greater number of moles of gas reactants, increasing the SO3_3(g) yield which is desired.

Economically, high pressures are costly and dangerous.

Therefore, for Step 2, a compromise is required between the high temperature for rate and the low temperature for yield. A compromise is also required between the cost of higher pressures and the pressure that allows a satisfactory yield and rate.

Marking Criteria

Rates

DescriptorMarks

1 mark each for any of the following, up to a maximum of 6 marks: Explains that high temperature increases the average kinetic energy of the particles, which means that the particles collide more frequently; Explains that more of these collisions will have energy higher than the activation energy, which means a greater proportion of collisions are successful, and the reaction rate increases; Describes that the vanadium catalyst increases the rate of the forward reaction (and also the rate of the reverse reaction to an equal extent) as it provides an alternative pathway with a lower activation energy; States that a greater proportion of the particles will have sufficient energy to react when they collide; Explains that high pressure (concentration) has more particles per unit volume and so there is a higher frequency of collisions, and the reaction rate increases; Identifies that as Step 1 is a combustion reaction, it essentially goes to completion at the high temperature (and does not require a catalyst or high pressure); States that for Step 2, high temperature, high pressure and catalyst would favour high rate.

6

Equilibrium

DescriptorMarks

1 mark each for any of the following (only considered for Step 2), up to a maximum of 3 marks: Explains that high temperature favours the reverse reaction because it is endothermic, and this decreases the SO3(g) yield; States that a low temperature decreases the rate of reaction; Explains that a high pressure favours the forward reaction because there are a greater number of moles of gas reactants, increasing the SO3(g) yield.

3

Economics

DescriptorMarks

States that high pressures are costly (and dangerous).

1

Compromise

DescriptorMarks

1 mark each for any of the following, up to a maximum of 2 marks: Identifies that for Step 2, a compromise is required between the high temperature for rate and the low temperature for yield; Identifies that a compromise is also required between the cost of higher pressures and the pressure that allows a satisfactory yield and rate.

2
Q8
2024
QCAA
Paper 2
2 marks
Q8
2 marks

In the aqueous solution of a chromate salt, an equilibrium exists between the yellow chromate (CrO42\text{CrO}_4^{2-}) ions and the orange dichromate (Cr2O72\text{Cr}_2\text{O}_7^{2-}) ions. This equilibrium can be represented by the equation shown.

2CrO42(aq)+2H+(aq)Cr2O72(aq)+H2O(l)2\text{CrO}_4^{2-}(\text{aq}) + 2\text{H}^+(\text{aq}) \rightleftharpoons \text{Cr}_2\text{O}_7^{2-}(\text{aq}) + \text{H}_2\text{O}(\text{l})
(yellow) \hspace{3cm} (orange)

Explain, at an atomic level, why no colour change occurs once the chromate–dichromate solution has established equilibrium.

Reveal Answer

At equilibrium, the forward reaction rate equals the reverse reaction rate. Therefore, yellow chromate ions and orange dichromate ions are re-formed at the same rate at which they are broken down, so the colour remains constant because the [CrO42][CrO_4^{2-}] and [Cr2O72][Cr_2O_7^{2-}] remain constant.

Marking Criteria
DescriptorMarks

Identifies that the forward reaction rate equals the reverse reaction rate at equilibrium

1

Explains that the colour remains constant because the [CrO42][CrO_4^{2-}] and [Cr2O72][Cr_2O_7^{2-}] remain constant

1
Q24
2021
VCAA
1 mark
Q24
1 mark

Which one of the following statements describes the effect that adding a catalyst will have on the energy profile diagram for an exothermic reaction?

A

The energy of the products will remain the same.

B

The shape of the energy profile diagram will remain the same.

C

The peak of the energy profile will move to the left as the reaction rate increases.

D

The activation energy will be lowered by the same proportion in the forward and reverse reactions.

Reveal Answer
A

The energy of the products will remain the same.

Correct Answer

A catalyst provides an alternative reaction pathway with a lower activation energy, but it does not alter the initial energy of the reactants or the final energy of the products.

B

The shape of the energy profile diagram will remain the same.

The shape of the energy profile diagram changes because the peak, which represents the activation energy, is lowered when a catalyst is added.

C

The peak of the energy profile will move to the left as the reaction rate increases.

The x-axis of an energy profile diagram represents reaction progress, not time. A catalyst lowers the peak vertically rather than shifting it horizontally.

D

The activation energy will be lowered by the same proportion in the forward and reverse reactions.

A catalyst lowers the activation energy of both the forward and reverse reactions by the same absolute amount, not the same proportion, since their initial activation energies are different.

Q4
2025
SCSA
1 mark
Q4
1 mark

The equation for a system at equilibrium is given below.

2NO(g)+O2(g)2NO2(g)+heat2 NO(g) + O_2(g) \leftrightharpoons 2 NO_2(g) + heat

At 25 `C, the value of K for this equilibrium is 2.19×10122.19 \times 10^{12}.

Which of the following statements about this system is true? Increasing the

A

partial pressure of NO(g) will increase the yield of NO2(g)NO_2(g) and will increase the rate of the forward reaction.

B

partial pressure of NO(g) will increase the yield of NO2(g)NO_2(g) but will decrease the rate of the forward reaction.

C

temperature will increase the yield of NO2(g)NO_2(g) but decrease the rate of the forward reaction.

D

temperature will increase the yield of NO2(g)NO_2(g) and increase the rate of the forward reaction.

Reveal Answer
A

partial pressure of NO(g) will increase the yield of NO2(g)NO_2(g) and will increase the rate of the forward reaction.

Correct Answer

According to Le Chatelier's principle, increasing the partial pressure of a reactant (NONO) shifts the equilibrium toward the products, increasing the yield. Additionally, a higher concentration of reactants increases the frequency of collisions, thereby increasing the forward reaction rate.

B

partial pressure of NO(g) will increase the yield of NO2(g)NO_2(g) but will decrease the rate of the forward reaction.

While increasing the partial pressure of a reactant does increase the yield, it also increases (rather than decreases) the reaction rate due to a higher frequency of molecular collisions.

C

temperature will increase the yield of NO2(g)NO_2(g) but decrease the rate of the forward reaction.

Increasing temperature increases the kinetic energy of the molecules, which always leads to an increase in the reaction rate, not a decrease.

D

temperature will increase the yield of NO2(g)NO_2(g) and increase the rate of the forward reaction.

The reaction forming NO2NO_2 is exothermic; therefore, increasing the temperature shifts the equilibrium toward the reactants (left), decreasing the yield of NO2NO_2.

Q21
2023
QCAA
Paper 1
4 marks
Q21
4 marks

CO(g)\text{CO(g)} reacts with O2(g)\text{O}_2\text{(g)} in a sealed container producing CO2(g)\text{CO}_2\text{(g)} to reach equilibrium.

2CO(g)+O2(g)2CO2(g)2\text{CO(g)} + \text{O}_2\text{(g)} \rightleftharpoons 2\text{CO}_2\text{(g)}

Apply collision theory to explain how increasing the concentration of O2\text{O}_2 at equilibrium will affect the concentration of CO2\text{CO}_2 if the temperature and volume are held constant.

Reveal Answer

Increasing the concentration of O2O_2 increases the number of O2O_2 molecules.
This increases the frequency of collisions between O2O_2 and COCO.
Therefore, the rate of the forward reaction increases and equilibrium shifts to the right (products) to increase the concentration of CO2CO_2.

Marking Criteria
DescriptorMarks

identifies that increasing concentration of O2O_2 increases the number of O2O_2 molecules

1

explains that an increase in [O2][O_2] increases collisions between O2O_2 and COCO

1

explains that the rate of forward reaction increases

1

identifies that equilibrium shifts to the right and concentration of CO2CO_2 increases

1
Q12
2023
VCAA
1 mark
Q12
1 mark

Consider the following features of a chemical reaction.

I. activation energy
II. ΔH\Delta H of the reaction
III. enthalpy of the reactants

Compared with the uncatalysed reaction pathway, the presence of a catalyst changes

A

I only.

B

II only.

C

II and III only.

D

I, II and III.

Reveal Answer
A

I only.

Correct Answer

A catalyst provides an alternative reaction pathway with a lower activation energy, but it does not affect the enthalpy of the reactants or the overall ΔH\Delta H of the reaction.

B

II only.

A catalyst does not change the overall enthalpy change (ΔH\Delta H) of the reaction, as the initial and final states of the system remain the same.

C

II and III only.

Neither the ΔH\Delta H of the reaction nor the enthalpy of the reactants are changed by a catalyst; only the activation energy is affected.

D

I, II and III.

While a catalyst does change the activation energy, it does not alter the enthalpy of the reactants or the overall ΔH\Delta H of the reaction.

Q25
2021
QCAA
Paper 1
5 marks
Q25

An equilibrium is formed between two differently coloured cobalt species, Co(H2O)62+\text{Co(H}_2\text{O})_6^{2+}(aq), which is pink, and CoCl42\text{CoCl}_4^{2-}(aq), which is blue. The equation for this equilibrium is shown.

Co(H2O)62+(aq)+4Cl(aq)CoCl42(aq)+6H2O(l)\text{Co(H}_2\text{O})_6^{2+}(\text{aq}) + 4\text{Cl}^-(\text{aq}) \rightleftharpoons \text{CoCl}_4^{2-}(\text{aq}) + 6\text{H}_2\text{O}(\text{l})

Q25a
3 marks

Apply Le Châtelier’s principle to predict the visible effect of adding AgNO3\text{AgNO}_3 to an aqueous blue-coloured solution containing Co(H2O)62+\text{Co(H}_2\text{O})_6^{2+} and CoCl42\text{CoCl}_4^{2-} ions.
Explain your reasoning.

Reveal Answer

Adding AgNO3AgNO_3 produces Ag+Ag^+ ions, which react with ClCl^- ions to form insoluble AgCl, therefore decreasing the concentration of ClCl^- ions.
Equilibrium shifts to reactants (left) to counteract the decrease by increasing the concentration of ClCl^- ions.
The blue solution will become lighter (pinker).

Marking Criteria
DescriptorMarks

Correctly identifies that ClCl^- decreases

1

Identifies that equilibrium shifts to left (reactants) to counteract the change

1

Identifies that the blue solution becomes lighter

1
Q25b
2 marks

When a sample of the equilibrium mixture is put into hot water, the mixture turns more blue. Determine whether the forward reaction of the equation is exothermic or endothermic. Explain your reasoning.

Reveal Answer

Adding heat shifts equilibrium towards the endothermic direction and produces CoCl42CoCl_4^{2-} ions, which are blue.
Therefore, the forward reaction is endothermic.

Marking Criteria
DescriptorMarks

Identifies that the forward reaction has been favoured

1

Determines that the forward reaction is endothermic

1
Q23
2024
NESA
3 marks
Q23
3 marks

Consider the following equilibrium system.

[Co(H2O)6]2+(aq)+4Cl(aq)[CoCl4]2(aq)+6H2O(l)[\text{Co(H}_2\text{O)}_6]^{2+}(aq) + 4\text{Cl}^-(aq) \rightleftharpoons [\text{CoCl}_4]^{2-}(aq) + 6\text{H}_2\text{O}(l)

[Co(H2O)6]2+(aq)[\text{Co(H}_2\text{O)}_6]^{2+}(aq) is pink and [CoCl4]2(aq)[\text{CoCl}_4]^{2-}(aq) is blue. When a solution of these ions and chloride ions is heated, the mixture becomes more blue.

Relate the observed colour change to the change in KeqK_{eq}.

Reveal Answer

The mixture becomes more blue as temperature is increased, which suggests that the concentration of [CoCl4]2[\text{CoCl}_4]^{2-} is increasing. This means that an increase in temperature favours the forward reaction. This response will result in a larger value of KeqK_{eq} as

Keq=[[CoCl4]2][[Co(H2O)6]2+][Cl]4K_{eq} = \frac{[[\text{CoCl}_4]^{2-}]}{[[\text{Co}(\text{H}_2\text{O})_6]^{2+}][\text{Cl}^-]^4}
Marking Criteria
DescriptorMarks

Provides a sound relationship between the observed colour change and KeqK_{eq}

3

Provides some information about changes in concentration of reactants and/or products

2

Provides some relevant information

1

None of the above

0
Q11
2025
SCSA
1 mark
Q11
1 mark

Refer to the following equation for the dissolution of gaseous carbon dioxide into the oceans.

CO2(g)CO2(aq)+heatCO_2(g) \leftrightharpoons CO_2(aq) + heat

Which statement best uses the above equation as part of a justification for why increasing temperatures at the surface of the ocean would lead to a decreased concentration of carbon dioxide in the oceans? Increasing the temperature

A

increases the rate of the forward reaction and decreases the rate of the reverse reaction leading to an equilibrium shift to the right.

B

increases the rate of the reverse reaction and decreases the rate of the forward reaction leading to an equilibrium shift to the left.

C

increases the rate of the endothermic reaction more than the exothermic reaction shifting the equilibrium to the left.

D

increases the rate of the exothermic reaction more than the endothermic reaction shifting the equilibrium to the left.

Reveal Answer
A

increases the rate of the forward reaction and decreases the rate of the reverse reaction leading to an equilibrium shift to the right.

Increasing temperature increases the rates of both the forward and reverse reactions due to higher kinetic energy, rather than decreasing one. Additionally, adding heat to an exothermic reaction shifts the equilibrium to the left (reactants), not the right.

B

increases the rate of the reverse reaction and decreases the rate of the forward reaction leading to an equilibrium shift to the left.

While the equilibrium shifts to the left, increasing temperature increases the rates of both reactions; it does not decrease the rate of the forward reaction.

C

increases the rate of the endothermic reaction more than the exothermic reaction shifting the equilibrium to the left.

Correct Answer

The reverse reaction is endothermic (absorbs heat). Increasing temperature increases the rate of the endothermic reaction (which has a higher activation energy) more than the exothermic reaction, causing a net shift to the left.

D

increases the rate of the exothermic reaction more than the endothermic reaction shifting the equilibrium to the left.

Increasing temperature favors the endothermic reaction (the reverse reaction here), not the exothermic reaction. The system shifts to the left to absorb the excess heat.

Q29
2025
NESA
4 marks
Q29
4 marks

Consider the following reaction.

2NO2(g)N2O4(g)ΔH=57.2 kJ mol12\text{NO}_2(g) \rightleftharpoons \text{N}_2\text{O}_4(g) \quad \Delta H = -57.2 \text{ kJ mol}^{-1}

A sealed reaction vessel of fixed volume contains a mixture of NO2\text{NO}_2 and N2O4\text{N}_2\text{O}_4 gases at equilibrium.

Explain the impact of the addition of argon, an inert gas, on the temperature of the system.

Reveal Answer

Addition of argon gas does not change the amount of NO2_2 or N2_2O4_4 present. As the volume of the system remains consistent, the concentrations of NO2_2 and N2_2O4_4 are unchanged. Therefore Q=KeqQ = K_{eq} and the system remains at equilibrium. Accordingly, the temperature remains constant.

Marking Criteria
DescriptorMarks

Provides a thorough explanation of the impact on the temperature of this system

4

Provides an explanation that shows a sound understanding of equilibrium systems

3

Provides a description of an impact on the system

2

Provides some relevant information

1

None of the above

0
Q4
2023
QCAA
Paper 1
1 mark
Q4
1 mark

The question refers to the decomposition of hydrogen iodide gas (HI) to produce hydrogen gas (H2\text{H}_2) and iodine gas (I2\text{I}_2) in a sealed 1-litre container.

2HI(g)H2(g)+I2(g)ΔH=+53.6 kJ mol12\text{HI(g)} \rightleftharpoons \text{H}_2\text{(g)} + \text{I}_2\text{(g)} \quad \Delta H = +53.6 \text{ kJ mol}^{-1}
Colourless Colourless Purple

Identify which change would shift the system from light purple to dark purple.

A

adding HI(g)

B

adding a catalyst

C

decreasing the temperature

D

increasing the concentration of H2_2(g)

Reveal Answer
A

adding HI(g)

Correct Answer

According to Le Chatelier's Principle, adding the reactant HI\text{HI} shifts the equilibrium to the right to consume the added substance, resulting in the production of more purple I2\text{I}_2 gas.

B

adding a catalyst

A catalyst speeds up both the forward and reverse reactions equally but does not change the position of equilibrium or the concentration of I2\text{I}_2.

C

decreasing the temperature

The reaction is endothermic (ΔH>0\Delta H > 0), so decreasing the temperature shifts the equilibrium to the left (the exothermic direction) to generate heat, consuming I2\text{I}_2 and lightening the color.

D

increasing the concentration of H2_2(g)

Increasing the concentration of the product H2\text{H}_2 shifts the equilibrium to the left to consume the added product, which decreases the concentration of I2\text{I}_2.

Q17
2023
SCSA
1 mark
Q17
1 mark

In the conversion between sulfur dioxide and sulfur trioxide in a sealed vessel, the following equilibrium is established:

2 SO3(g)2 SO2(g)+O2(g)ΔH=198 kJ mol12\text{ SO}_3\text{(g)} \rightleftharpoons 2\text{ SO}_2\text{(g)} + \text{O}_2\text{(g)} \quad \Delta\text{H} = 198\text{ kJ mol}^{-1}

For this system, which of the following statements about the equilibrium constant K is correct? K will

A

increase if the temperature of the system is decreased.

B

decrease if the partial pressure of SO2(g)\text{SO}_2\text{(g)} is increased.

C

increase if the temperature of the system is increased.

D

increase if the pressure of the system is increased.

Reveal Answer
A

increase if the temperature of the system is decreased.

The reaction is endothermic (ΔH>0\Delta\text{H} > 0). Decreasing the temperature shifts the equilibrium to the left, which decreases the equilibrium constant KK.

B

decrease if the partial pressure of SO2(g)\text{SO}_2\text{(g)} is increased.

Changing the partial pressure of a reactant or product shifts the equilibrium position, but it does not change the value of the equilibrium constant KK, which is only temperature-dependent.

C

increase if the temperature of the system is increased.

Correct Answer

Because the reaction is endothermic (ΔH=198 kJ mol1\Delta\text{H} = 198\text{ kJ mol}^{-1}), increasing the temperature shifts the equilibrium to the right to absorb the added heat, resulting in a higher equilibrium constant KK.

D

increase if the pressure of the system is increased.

Changing the total pressure of the system will shift the equilibrium position (in this case, to the left where there are fewer moles of gas), but it will not change the equilibrium constant KK.

Q7
2022
SCSA
1 mark
Q7
1 mark

Refer to the following reaction at equilibrium in a closed reaction vessel.

2SO2(g)+O2(g)2SO3(g) ΔH=196 kJ mol12\,\mathrm{SO_2(g)} + \mathrm{O_2(g)} \rightleftharpoons 2\,\mathrm{SO_3(g)}\ \Delta H = -196\ \mathrm{kJ\ mol^{-1}}

Which of the following changes will initially decrease the rate at which SO2(g)\mathrm{SO_2(g)} is consumed?

A

decrease the volume of the reaction vessel

B

decrease the partial pressure of O2(g)\mathrm{O_2(g)}

C

heat the reaction vessel

D

add an appropriate catalyst

Reveal Answer
A

decrease the volume of the reaction vessel

Decreasing the volume of the vessel increases the concentration of the reactant gases, which increases the rate of the forward reaction.

B

decrease the partial pressure of O2(g)\mathrm{O_2(g)}

Correct Answer

Decreasing the partial pressure of O2(g)\mathrm{O_2(g)} lowers its concentration, which directly decreases the rate of the forward reaction where SO2(g)\mathrm{SO_2(g)} is consumed.

C

heat the reaction vessel

Heating the reaction vessel increases the kinetic energy of the molecules, which increases the rate of both the forward and reverse reactions.

D

add an appropriate catalyst

Adding a catalyst lowers the activation energy of the reaction, which increases the rate of both the forward and reverse reactions.

Q10
2023
VCAA
1 mark
Q10
1 mark

At constant temperature, which one of the following always corresponds to an increase of the yield of a chemical reaction?

A

An increase in the value of the equilibrium expression.

B

An increase in the concentration of the reactants.

C

A decrease in the activation energy.

D

An increase in the pressure.

Reveal Answer
A

An increase in the value of the equilibrium expression.

Correct Answer

The equilibrium expression represents the ratio of products to reactants at equilibrium. An increase in its value means a greater proportion of products is formed, which directly corresponds to a higher yield.

B

An increase in the concentration of the reactants.

While adding reactants can shift the equilibrium to form more products according to Le Chatelier's principle, it does not change the equilibrium constant and does not always increase the percentage yield.

C

A decrease in the activation energy.

Decreasing the activation energy (such as by adding a catalyst) increases the rate at which equilibrium is reached, but it does not alter the equilibrium position or the final yield.

D

An increase in the pressure.

Increasing pressure only increases the yield for reactions that produce fewer moles of gas than they consume. For reactions that produce more moles of gas, an increase in pressure would actually decrease the yield.

Q8
2022
QCAA
Paper 1
1 mark
Q8
1 mark

Predict how the system shown will respond when a small amount of aqueous sodium hydroxide is added.

CH3COOH(aq)+H2O(l)CH3COO(aq)+H3O+(aq)\text{CH}_3\text{COOH(aq)} + \text{H}_2\text{O(l)} \rightleftharpoons \text{CH}_3\text{COO}^-\text{(aq)} + \text{H}_3\text{O}^+\text{(aq)}

A

Equilibrium shifts to the left and the pH decreases.

B

Equilibrium shifts to the right and the pH increases.

C

Equilibrium shifts to the left and the pH remains the same.

D

Equilibrium shifts to the right and the pH remains the same.

Reveal Answer
A

Equilibrium shifts to the left and the pH decreases.

Incorrect. Adding NaOH\text{NaOH} introduces hydroxide ions (OH\text{OH}^-) which react with and remove H3O+\text{H}_3\text{O}^+. According to Le Chatelier's Principle, removing a product causes the equilibrium to shift to the right, not the left.

B

Equilibrium shifts to the right and the pH increases.

Incorrect. While adding a base technically causes a slight increase in pH, the system acts as a buffer which resists significant changes in acidity. In the context of ideal buffer behavior, the pH is described as remaining effectively constant.

C

Equilibrium shifts to the left and the pH remains the same.

Incorrect. The addition of OH\text{OH}^- consumes H3O+\text{H}_3\text{O}^+, creating a stress on the system. To counteract this loss, the equilibrium must shift to the right to produce more H3O+\text{H}_3\text{O}^+.

D

Equilibrium shifts to the right and the pH remains the same.

Correct Answer

Correct. The added OH\text{OH}^- ions neutralize H3O+\text{H}_3\text{O}^+, causing the equilibrium to shift to the right to replace the lost protons. Because the system acts as a buffer, it resists significant changes in pH, keeping it relatively constant.

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