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E2.1 · Use reversible-reaction, equilibrium-constant, solubility, and buffer terminology

Learn to use reversible-reaction, equilibrium-constant, solubility, and buffer terminology through clear examples and targeted practice.

Ontario Grade 12 Chemistry

Chemical Systems and Equilibrium

Core terminology for Ontario SCH4U expectation E2.1

A sealed container can hold a liquid and vapour of the same substance. Even when the visible amounts stop changing, particles may still move between the liquid and gas. This gives a useful starting point for learning the terms reversible reaction and dynamic equilibrium. Other chemical situations call for different terms: solubility describes how much solute dissolves, an equilibrium constant describes the composition of a system at equilibrium, and a buffer describes a solution that resists a large pH change after a small addition of acid or base. First recall that a solution contains a solute dissolved in a solvent. A chemical equation represents a change in substances; state symbols such as (s), (l), (g), and (aq) show whether a substance is solid, liquid, gas, or dissolved in water.

What you will learn

Use reversible-reaction and dynamic-equilibrium terminology accurately.
Describe what an equilibrium constant represents without confusing equilibrium with equal concentrations.
Use solubility and saturated-solution terminology under stated conditions.
Define a buffer and describe its general role.

1. Reversible reactions and dynamic equilibrium

A reversible reaction can proceed in two directions. The forward reaction changes reactants into products. The reverse reaction changes products back into reactants. A double arrow is used to show that both directions are possible. For example, a reversible change between liquid water and water vapour can be represented as

\mathrm{H_2O(l) \rightleftharpoons H_2O(g)}

A closed system is one in which matter does not enter or leave. In a closed system, a reversible reaction can reach dynamic equilibrium. Dynamic means that particles continue to change. At equilibrium, the forward and reverse reactions occur at equal rates. A rate describes how quickly a change occurs.

At dynamic equilibrium, the amounts of reactants and products remain constant overall. They do not have to be equal. The forward and reverse reactions have not stopped; their changes balance over time. The liquid-and-vapour example is a useful model: particles continue to move between the two states even if the visible amounts remain steady.

When using these terms, keep the conditions clear. Dynamic equilibrium refers to a reversible change in a closed system, with equal forward and reverse rates. It does not mean that the system contains equal amounts of each substance.

A reversible reaction can proceed in the forward and reverse directions.
At dynamic equilibrium in a closed system, the forward and reverse rates are equal.
Amounts remain constant overall at equilibrium, but they are not necessarily equal.

2. Equilibrium-constant terminology

An equilibrium constant is a number used to describe the relative amounts of reactants and products in a system at equilibrium at a stated temperature. In SCH4U, the symbol

K_c

refers to an equilibrium constant associated with concentration. Concentration is the amount of a substance in a given volume; it is commonly expressed in

\mathrm{mol/L}

The value of an equilibrium constant helps describe the equilibrium composition. A value that favours products indicates that products are present in a greater proportion relative to reactants than in a system with a value that favours reactants. This comparison concerns the equilibrium composition of the reaction as represented. It does not say that every product concentration must be greater than every reactant concentration.

An equilibrium constant is not a measure of whether forward and reverse reactions have stopped. Nor does it mean that concentrations are equal. The term applies to the composition at equilibrium, while dynamic equilibrium describes the ongoing changes and equal rates.

Use the temperature as part of the context when referring to an equilibrium constant. If a question supplies a value of

K_c

, identify it as a description of equilibrium composition. For this expectation, the key skill is to use the term correctly, not to construct or calculate an equilibrium-constant expression.

$K_c$ is an equilibrium-constant symbol associated with concentration.
An equilibrium constant describes relative reactant and product amounts at equilibrium for a stated temperature.
Equilibrium does not mean equal concentrations or stopped reactions.

3. Solubility and saturated solutions

Solubility describes how much solute can dissolve in a specified amount of solvent under stated conditions. Temperature is an important condition to include when comparing solubilities. The terms do not describe how quickly a solute dissolves; they describe the amount that can dissolve under those conditions.

An unsaturated solution can dissolve more solute under the stated conditions. A saturated solution contains as much dissolved solute as it can hold under those conditions. If extra solute is added to a saturated solution, some may remain as a solid.

For a sparingly soluble ionic solid, some particles may dissolve into ions while dissolved ions may also join to form solid. The term sparingly soluble means that only a small amount dissolves under the stated conditions; it does not mean that none dissolves. For example, the dissolution and return to solid of silver chloride can be shown as

\mathrm{AgCl(s) \rightleftharpoons Ag^+(aq) + Cl^-(aq)}

. The equation conserves both atoms and net charge.

If solid and dissolved particles are present together in a closed system at equilibrium, dissolving and solid formation continue at equal rates. This is a solubility equilibrium. The system remains dynamic even when the amounts of dissolved material and solid remain constant overall.

Solubility is the amount of solute that can dissolve under stated conditions.
A saturated solution holds as much dissolved solute as it can under those conditions.
Sparingly soluble means a small amount dissolves, not that the substance is completely insoluble.

4. Buffers: use the term precisely

A buffer is a solution that resists a large change in pH when a small amount of acid or base is added. The phrase resists a change does not mean prevents all change. A buffer has limited capacity, so a sufficiently large addition can produce a substantial pH change.

pH is a measure used to describe how acidic or basic a solution is. In this expectation, the important point is the buffer's general role: it helps limit pH change after a small addition of acid or base. The term describes a solution's behaviour; it does not name every acidic or basic solution.

Do not assume that any mixture at equilibrium is a buffer. Dynamic equilibrium and buffer describe different features: dynamic equilibrium describes equal forward and reverse reaction rates in a closed system, while buffer describes resistance to a large pH change after a small addition of acid or base.

When reading a question, match the term to the feature being described. Use reversible reaction for a change that can proceed in both directions; equilibrium constant for a number describing equilibrium composition; solubility for the amount that dissolves under stated conditions; and buffer for a solution that resists a large pH change after a small addition.

A buffer resists a large pH change after a small addition of acid or base.
A buffer does not prevent every pH change and has limited capacity.
Buffer and dynamic equilibrium describe different features of a chemical system.

Worked example

Choose the term that matches each description

A sealed system contains liquid water and water vapour. Their amounts remain steady, although particles continue to change between liquid and gas. A separate solution can dissolve only a small additional amount of a solute before it is saturated. A third solution resists a large pH change after a small addition of acid. Identify the relevant terminology in each case.

Describe the water system
The system is sealed, and particles continue to change between liquid and gas. Since both directions are possible, the change is reversible. If the amounts remain steady because the forward and reverse rates are equal, the system is at dynamic equilibrium. Steady amounts do not mean that particle changes have stopped.
Describe the dissolving situation
Solubility is the amount of solute that can dissolve under stated conditions. A solution that holds as much dissolved solute as it can is saturated. The description gives no numerical amount, so the appropriate response is to name the terms rather than calculate a value.
Describe the pH behaviour
The third solution is a buffer because it resists a large pH change after a small acid addition. It does not follow that the pH cannot change at all; the description is specifically about resistance to a large change after a small addition.

Answer: The water system shows a reversible change and, with equal forward and reverse rates, dynamic equilibrium. The dissolving situation uses solubility and saturated-solution terminology. The pH-resisting solution is a buffer.

Check: The descriptions give no numerical concentrations, solubility amount, or pH values. No calculation is needed to identify the terms.

Common mistakes and how to avoid them

Saying that equilibrium means reactants and products are present in equal amounts.

Correction: At dynamic equilibrium, forward and reverse rates are equal and amounts remain constant overall. The amounts do not have to be equal.

Saying that reactions stop at dynamic equilibrium.

Correction: Particles continue to change. Equal forward and reverse rates keep the overall amounts constant.

Calling a sparingly soluble substance completely insoluble.

Correction: Sparingly soluble means that a small amount dissolves under stated conditions.

Saying that a buffer prevents every pH change.

Correction: A buffer resists a large change after a small addition of acid or base. Its capacity is limited.

Lesson summary

A reversible reaction can proceed in both directions.
Dynamic equilibrium occurs in a closed system when forward and reverse rates are equal; amounts remain constant overall, not necessarily equal.
An equilibrium constant such as $K_c$ describes relative reactant and product amounts at equilibrium for a stated temperature.
Solubility is the amount of solute that can dissolve under stated conditions; a saturated solution holds as much dissolved solute as it can under those conditions.
A buffer resists a large pH change after a small addition of acid or base, but it does not prevent all change.

Check your understanding

Question 1

At dynamic equilibrium in a closed system, which statement is correct?

Forward and reverse rates are equal, and amounts remain constant overall.
The forward reaction has stopped.
Reactant and product amounts must be equal.
correctIndex的0

Show answer and explanation

Forward and reverse rates are equal, and amounts remain constant overall.

Dynamic equilibrium means that forward and reverse changes continue at equal rates. Overall amounts remain constant, but they need not be equal.

Question 2

Which description matches solubility?

The amount of solute that can dissolve under stated conditions.
The rate at which a solute dissolves.
The pH change caused by adding acid.
correctIndex的0

Show answer and explanation

The amount of solute that can dissolve under stated conditions.

Solubility describes the amount that can dissolve under stated conditions, not the speed of dissolving.

Question 3

Which description best fits a buffer?

A solution that resists a large pH change after a small addition of acid or base.
A solution whose pH can never change.
Any solution that contains undissolved solid.
correctIndex的0

Show answer and explanation

A solution that resists a large pH change after a small addition of acid or base.

A buffer resists a large change after a small addition, but its capacity is limited.

Key terms

Reversible reaction: A reaction that can proceed in both forward and reverse directions.
Dynamic equilibrium: A state in a closed system where forward and reverse reaction rates are equal, so amounts remain constant overall.
Equilibrium constant: A number that describes the relative amounts of reactants and products at equilibrium for a stated temperature.
Solubility: The amount of solute that can dissolve in a specified amount of solvent under stated conditions.
Saturated solution: A solution holding as much dissolved solute as it can under stated conditions.
Buffer: A solution that resists a large pH change when a small amount of acid or base is added.

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Published by DoAssignment. This reviewed lesson follows Ontario Grade 12 Chemistry (SCH4U), expectation E2.1. It is a study resource, not an official curriculum publication.

Before publication, content is checked for structure, mathematical or chemical notation, calculations, course boundaries, and readability. Errors can still occur, so corrections are welcomed.

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