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E2.2 · Predict and investigate equilibrium shifts from changing conditions

Learn to predict and investigate equilibrium shifts from changing conditions through clear examples and targeted practice.

Ontario Grade 12 Chemistry

Chemical Systems and Equilibrium

How changing conditions affects a reversible reaction at equilibrium

A sealed container of nitrogen dioxide can look darker or lighter as conditions change. Nitrogen dioxide is brown, while dinitrogen tetroxide is colourless. This visible change can provide evidence that the balance between two substances has shifted. In this lesson, you will connect such observations to particles, then use a balanced equation to predict the direction of change. A shift describes the system’s response as it moves toward a new equilibrium; it does not mean the reaction stops or that the two sides become equal in amount.

What you will learn

Describe dynamic equilibrium using forward and reverse reactions.
Predict how changes in concentration, pressure, volume, and temperature affect an equilibrium.
Explain why a catalyst does not change the position of equilibrium.
Plan a fair investigation that tests an equilibrium shift using observable evidence.

1. Bridge: what equilibrium means

A reversible reaction can proceed in both directions. The forward reaction changes reactants into products. The reverse reaction changes products back into reactants. In a closed system, these reactions can reach dynamic equilibrium. Dynamic means that changes continue; equilibrium means the forward and reverse reactions occur at equal rates.

At equilibrium, the amounts of reactants and products stay constant overall. They are not necessarily equal. For example, there may be much more reactant than product even though the forward and reverse rates are equal.

A change in conditions can disturb the balance of the two reaction directions. The system then shifts: one direction occurs faster than the other for a time, changing the amounts of substances. The system eventually reaches a new equilibrium if the conditions remain steady.

\text{forward rate} = \text{reverse rate}

Equilibrium is dynamic, not stopped.
Equal forward and reverse rates do not mean equal concentrations.
A shift describes a change in relative amounts as the system responds.

2. Observable change and particle model

Consider the reversible reaction between colourless dinitrogen tetroxide and brown nitrogen dioxide. In a sealed container, the mixture has a colour that reflects the amount of brown nitrogen dioxide present. If the colour becomes more intense after a condition changes, that is evidence that the mixture contains a greater amount of nitrogen dioxide than before. It does not, by itself, measure the exact amount.

At the particle level, both directions continue at equilibrium. If a condition changes, the balance between the directions is disturbed. For example, adding one substance means there are more of its particles in the container. The system responds by favouring the direction that uses up some of that added substance. This is a useful way to predict the direction of the shift.

A shift to the right means the system forms more products. A shift to the left means it forms more reactants. These phrases describe the overall response, not a claim that only one direction occurs.

Use an observable change as evidence, then connect it to changes in particle amounts.
Right means toward products; left means toward reactants.
A colour change can show a change in mixture composition, but it is not an exact concentration measurement.

3. Rules for predicting a shift

The equilibrium principle is a practical prediction rule: when a system at equilibrium is disturbed, it shifts in the direction that reduces the effect of that disturbance. Apply one change at a time when making a prediction.

Concentration: adding a reactant generally shifts the equilibrium toward products, which uses some of the added reactant. Removing a reactant generally shifts it toward reactants, which replaces some of what was removed. Apply the same reasoning to products: adding a product favours the reverse direction, while removing a product favours the forward direction.

Pressure and volume matter for equilibria involving gases. Increasing pressure by decreasing the container’s volume favours the side with fewer moles of gas. A mole is a counting unit for particles. Decreasing pressure by increasing volume favours the side with more moles of gas. Count only gaseous substances in the balanced equation. If both sides have the same number of gas moles, this pressure change does not favour either side.

Temperature changes the equilibrium differently from a concentration or pressure change. Treat heat as a reactant for an endothermic forward reaction, because that direction takes in heat. Raising the temperature then favours the forward direction. For an exothermic forward reaction, heat is a product, so raising the temperature favours the reverse direction. Lowering temperature has the opposite effect in each case.

A catalyst does not shift the equilibrium. It helps both directions reach equilibrium more quickly, but it does not change the equilibrium amounts. This distinction is useful when interpreting an investigation: a faster visible change is not evidence by itself that the final equilibrium position changed.

Ask what changed, then identify which direction reduces that change.
For gases, compare gas-mole counts on the two sides of the balanced equation.
For temperature, use whether the forward reaction absorbs or releases heat.
A catalyst changes how quickly equilibrium is reached, not the equilibrium position.

4. Investigating shifts fairly

An investigation can test a prediction by changing one condition and observing a suitable sign of composition. For the nitrogen dioxide equilibrium, colour is a possible observable sign. A darker mixture suggests more nitrogen dioxide than before; a lighter mixture suggests less. Do not claim that an observation occurred unless the investigation has actually been carried out.

Plan a fair test by starting with comparable sealed samples at equilibrium, where possible. Change only the condition being tested, such as temperature or volume, and keep other relevant conditions the same. Record the initial appearance, the change made, and the appearance after the system has had time to settle. Include a comparison sample that does not receive the change when practical.

For safety, an investigation involving nitrogen dioxide must only be planned or performed under appropriate teacher supervision and approved laboratory procedures. The prediction can be discussed without producing or handling this gas. The key inquiry skill is to separate the predicted shift from the evidence that would support or challenge it.

A good conclusion states the condition changed, the observation, and what that observation suggests about the equilibrium. It should avoid claiming more than the evidence shows. For example, colour can support a prediction about relative nitrogen dioxide amounts, but does not directly provide a numerical concentration.

Change one factor at a time and keep other conditions as consistent as possible.
State predictions before observations so they are not confused with results.
Describe what the evidence supports and its limits.

Quick prediction guide

Change	Predicted response
Add reactant	Shift toward products
Add product	Shift toward reactants
Increase pressure by decreasing volume	Shift toward fewer moles of gas
Increase temperature	Shift toward the endothermic direction
Add a catalyst	No shift; equilibrium is reached faster

Worked example

Predicting the effect of compression and heating

A sealed equilibrium mixture contains colourless dinitrogen tetroxide and brown nitrogen dioxide. The forward reaction is endothermic. Predict the shift when the container is compressed, then heated. State the expected direction of the colour change after both changes, assuming the system is allowed to reach a new equilibrium.

Represent the equilibrium
The balanced equation shows two moles of gas on the left and one mole of gas on the right. The positive enthalpy change indicates that the forward reaction absorbs heat, so heating favours the forward direction.
$\mathrm{N_2O_4(g)} \rightleftharpoons 2\,\mathrm{NO_2(g)} \qquad \Delta H > 0$
Predict the effect of compression
Compression decreases volume and raises pressure. The system favours the side with fewer gas moles, which is the left side. The shift therefore favours formation of dinitrogen tetroxide and reduces the relative amount of brown nitrogen dioxide.
$1\text{ mol gas} < 2\text{ mol gas}$
Predict the effect of heating
Heating favours the endothermic forward direction. This favours formation of nitrogen dioxide, so it increases the relative amount of the brown gas. The two changes favour opposite directions. Without knowing the sizes of the changes and the conditions, the net final colour cannot be predicted with certainty.
$\text{heating} \;\rightarrow\; \text{right}$
State the justified conclusion
Compression alone predicts a shift left and a lighter colour; heating alone predicts a shift right and a darker colour. Since both changes are made, the final colour depends on which effect is stronger. A careful investigation would record the observation rather than assume one effect must dominate.

Answer: Compression favours the left side; heating favours the right side. The combined shift and final colour cannot be determined from the information given.

Check: The conclusion follows from comparing gas-mole counts for pressure and using the endothermic direction for temperature. It does not assume the two effects cancel or that one is automatically stronger.

Common mistakes and how to avoid them

Assuming equilibrium means there are equal amounts of reactants and products.

Correction: Equilibrium means equal forward and reverse reaction rates. The amounts can be different.

Predicting a pressure effect by counting every substance in the equation.

Correction: For pressure and volume changes, compare only the gaseous substances on each side.

Assuming heating always shifts an equilibrium toward products.

Correction: Heating favours the endothermic direction. That direction can be forward or reverse.

Saying a catalyst shifts equilibrium because the reaction becomes faster.

Correction: A catalyst helps both directions reach equilibrium faster but does not change the equilibrium position.

Lesson summary

At dynamic equilibrium, forward and reverse reaction rates are equal, while concentrations remain constant overall.
A change in conditions can shift the equilibrium as the system responds to the disturbance.
Concentration changes favour the direction that uses up an added substance or replaces a removed one.
For gas equilibria, increased pressure favours the side with fewer gas moles.
An increase in temperature favours the endothermic direction.
A catalyst changes the time needed to reach equilibrium, not the equilibrium position.

Check your understanding

Question 1

For

\mathrm{N_2O_4(g)} \rightleftharpoons 2\,\mathrm{NO_2(g)}

, what shift is predicted when pressure is increased by decreasing volume?

Left, toward fewer moles of gas
Right, toward more moles of gas
No shift, because the two sides have equal gas moles
The shift cannot be predicted from the equation

Show answer and explanation

Left, toward fewer moles of gas

The left side has one mole of gas and the right side has two. Increased pressure favours the side with fewer gas moles.

Question 2

A reversible reaction has an exothermic forward direction. What does raising the temperature favour?

The forward direction
The reverse direction
Both directions equally, with no shift
A change in concentration but no change in equilibrium

Show answer and explanation

The reverse direction

For an exothermic forward direction, heat is released. Raising temperature favours the direction that absorbs heat, which is the reverse direction.

Question 3

What does adding a catalyst do to a system at equilibrium?

Shifts it toward products
Shifts it toward reactants
Leaves the equilibrium position unchanged but helps equilibrium be reached faster
Makes reactant and product amounts equal

Show answer and explanation

Leaves the equilibrium position unchanged but helps equilibrium be reached faster

A catalyst affects how quickly equilibrium is reached, not the equilibrium position or the relative amounts at equilibrium.

Key terms

Reversible reaction: A reaction that can proceed in both forward and reverse directions.
Dynamic equilibrium: A condition in a closed system where forward and reverse reactions continue at equal rates, so amounts remain constant overall.
Equilibrium shift: A change in the relative amounts of reactants and products as a system responds to a changed condition.
Endothermic: Describes a reaction direction that absorbs heat.
Exothermic: Describes a reaction direction that releases heat.

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Published by DoAssignment. This reviewed lesson follows Ontario Grade 12 Chemistry (SCH4U), expectation E2.2. 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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