Does equilibrium mean that reactant and product concentrations are equal?

No, equilibrium means that the forward and reverse reaction rates are equal, not that the concentrations are equal. The equilibrium concentrations remain constant because each substance is produced and consumed at matching rates. Their actual values depend on the reaction and temperature, and they may differ considerably. A large equilibrium constant usually corresponds to a mixture containing proportionally more products, while a small constant corresponds to proportionally more reactants. In an exam definition, always refer to equal rates and constant macroscopic properties rather than equal amounts.

When does pressure change the position of equilibrium?

A pressure change affects equilibrium position when gases are involved and the two sides of the equation contain different total numbers of gaseous moles. Increasing pressure by decreasing volume favours the side with fewer gaseous moles, while decreasing pressure favours the side with more. Solid, liquid, and aqueous coefficients are not counted in this comparison. If the gaseous mole totals are equal, changing pressure does not favour either side. Pressure changes do not alter the equilibrium constant when temperature remains constant.

Does adding a catalyst change Kc or the equilibrium yield?

A catalyst changes neither Kc nor the equilibrium yield. It provides an alternative pathway with lower activation energy for both the forward and reverse reactions. Both reaction rates therefore increase, allowing the system to reach equilibrium more quickly. The equilibrium composition remains the same because the catalyst does not alter the relative thermodynamic stability of reactants and products. An exam answer should mention both faster attainment of equilibrium and no change in equilibrium position.

How can I remember whether Q less than K means forward or reverse reaction?

Think of Q as the product-to-reactant ratio at the current moment and K as the ratio required at equilibrium. If Q K, there are proportionally too many products, so the reverse reaction is favoured. When Q = K, the system is already at equilibrium and there is no net change. This ratio-based reasoning is safer than memorising arrows without understanding them.

Should Kc calculations use initial or equilibrium concentrations?

Kc must be calculated using equilibrium concentrations for the reaction as written. Initial concentrations may be provided, but they must first be adjusted according to the stoichiometric changes that occur before equilibrium is reached. An initial-change-equilibrium table makes these relationships visible and reduces sign errors. The coefficients in the balanced equation control both the concentration changes and the powers in the Kc expression. Substituting initial values directly is valid only if the question explicitly establishes that those values are equilibrium values.

What is the best way to learn from an equilibrium video solution?

Attempt the full question before watching so that the video addresses a real gap in your method. During the solution, pause before each step and predict what should happen next. Compare how the equation, equilibrium expression, stoichiometric changes, and final conclusion are organised rather than checking only the numerical answer. Record the precise cause of your error, such as reversing Q and K or using the wrong coefficient for a concentration change. Finally, solve the same question again from a blank page and attempt a similar problem within two days to confirm that the correction is transferable.

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IB Chemistry Equilibrium: Common Mistakes

Daniel R.

• 5 Aug 2026•13

IB Chemistry Equilibrium: Common Mistakes

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IB Chemistry Equilibrium: Common Mistakes | RevisionDojo

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IB Chemistry Equilibrium: Common Mistakes

Daniel R.

• 5 Aug 2026•13

The most frequent IB Chemistry equilibrium common mistakes are not isolated knowledge gaps. They cluster around equilibrium expressions, dynamic equilibrium, Le Châtelier’s principle, reaction quotients, and multistep calculations. Most can be corrected by reviewing worked video solutions, because a complete walkthrough reveals the reasoning between reading the question and writing the final answer.

In the current IB Chemistry course, equilibrium appears under Reactivity 2.3: How far? The extent of chemical change. SL students study dynamic equilibrium, the equilibrium law, equilibrium constants, and Le Châtelier’s principle. Reaction quotients, quantitative equilibrium calculations, and the relationship between equilibrium and Gibbs energy are HL content.

The equilibrium knowledge IB students need

A reversible reaction reaches dynamic equilibrium in a closed system when the forward and reverse reaction rates are equal. The reactions continue, but the macroscopic composition remains constant because reactants and products are being consumed and produced at equal rates.

For a general reaction:

aA + bB ⇌ cC + dD

an equilibrium concentration expression has the form:

Kc = ([C]^c[D]^d) / ([A]^a[B]^b)

The coefficients in the balanced equation become exponents. At a fixed temperature, the value of the equilibrium constant describes the equilibrium position: a large value generally indicates a product-favoured equilibrium, while a small value indicates a reactant-favoured equilibrium. It does not tell you how quickly equilibrium is reached.

Common IB Chemistry equilibrium mistakes and fixes

Common mistake	Why it is wrong	Practical fix
Describing equilibrium as static	Both reactions continue at equilibrium	State that forward and reverse rates are equal
Using initial concentrations in Kc	Kc uses equilibrium values	Label data as initial, change, and equilibrium
Ignoring stoichiometric powers	Coefficients determine exponents	Balance the equation before writing Kc
Saying pressure always causes a shift	Pressure matters only when gaseous mole totals differ	Count gaseous coefficients on each side
Saying a catalyst increases yield	It changes rates, not equilibrium composition	Explain that both directions are accelerated
Saying concentration changes K	At constant temperature, composition changes but K does not	Separate equilibrium position from K
Guessing the direction from Q	Q must be compared with K	Write the three-case Q versus K rule
Using Celsius in ΔG° = −RT ln K	Thermodynamic temperature must be in kelvins	Convert using T/K = temperature/°C + 273.15

Mistake 1: treating equilibrium as a stopped reaction

Students often write that reactions “stop” when equilibrium is established. That contradicts the word dynamic and can lose a definition mark.

A complete answer should identify three features:

The system is closed.
Forward and reverse reactions continue.
Their rates are equal, so concentrations remain constant.

Notice that equal rates do not mean equal concentrations. For example, an equilibrium mixture can contain much more product than reactant while the two reaction rates remain equal.

Worked-solution fix: Pause a video after the question is introduced and write your own definition. Compare it word by word with the model response, especially the distinction between rate and concentration.

Mistake 2: constructing Kc from memory instead of the equation

Students commonly reverse the numerator and denominator, forget exponents, or include a coefficient as a multiplier. For N₂(g) + 3H₂(g) ⇌ 2NH₃(g), the expression is:

Kc = [NH₃]² / ([N₂][H₂]³)

The coefficient 3 produces the exponent on [H₂]; it does not produce 3[H₂]. Pure solids and pure liquids are omitted from heterogeneous equilibrium expressions because their activities are constant, while dissolved and gaseous species are included as appropriate.

Worked-solution fix: Follow the same sequence every time: balance the equation, place products over reactants, convert coefficients into powers, and check physical states. The RevisionDojo equilibrium Questionbank is useful for repeating this process across differently written equations.

Mistake 3: confusing equilibrium position with the value of K

A concentration or pressure change can shift the equilibrium position, but K remains constant if temperature is unchanged. Temperature is the condition that changes the equilibrium constant for a specified reaction.

A catalyst creates another common confusion. It lowers the activation energy for both forward and reverse pathways, so equilibrium is reached faster, but neither K nor the equilibrium composition changes.

Keep the distinction explicit:

Concentration or pressure change: may alter equilibrium composition, but not K at constant temperature.
Catalyst: alters how quickly equilibrium is reached, but not K or yield.
Temperature change: alters K and the equilibrium composition.

Mistake 4: applying Le Châtelier’s principle mechanically

Statements such as “the equilibrium moves to oppose the change” are incomplete unless the direction and chemical reason are given. If pressure increases because volume decreases, the equilibrium shifts toward the side with fewer moles of gas, provided the gaseous mole totals differ.

For N₂(g) + 3H₂(g) ⇌ 2NH₃(g), increasing pressure favours ammonia because the reaction moves from four moles of gas to two. If both sides contain equal gaseous mole totals, changing pressure does not favour either direction.

Temperature questions require you to identify the endothermic direction. For an exothermic forward reaction, heat can be treated qualitatively as a product. Increasing temperature therefore favours the endothermic reverse direction and decreases the equilibrium yield of products.

Worked-solution fix: Watch for the exact order used by a strong solution: identify the disturbance, determine which direction counteracts it, and state the resulting change in reactant or product amount. The R2.3 equilibrium practice collection provides focused practice under the current syllabus structure.

Mistake 5: misusing the reaction quotient at HL

The reaction quotient, Q, uses the same algebraic expression as K, but it is calculated from concentrations or pressures at a particular moment rather than necessarily at equilibrium. Comparing Q with K predicts the direction of net change:

Q < K: the forward reaction is favoured until equilibrium is restored.
Q > K: the reverse reaction is favoured.
Q = K: the system is at equilibrium.

Students often reverse these conclusions. A reliable check is to ask what must happen to the product-to-reactant ratio. If Q is too small, more products must form to raise Q toward K.

Mistake 6: losing stoichiometry in equilibrium calculations

HL calculations usually require a structured table rather than immediate substitution. Suppose A ⇌ 2B, with initial concentrations [A] = 1.00 mol dm⁻³ and [B] = 0. If x mol dm⁻³ of A reacts, the equilibrium concentrations are [A] = 1.00 − x and [B] = 2x, not x.

Use an initial-change-equilibrium table:

Stage	A	B
Initial	1.00	0
Change	−x	+2x
Equilibrium	1.00 − x	2x

Then substitute only the equilibrium row into Kc. Check the mathematical root against chemical constraints: concentrations cannot be negative, and x cannot exceed the available reactant concentration.

Worked-solution fix: Do not merely watch the arithmetic. Record how the presenter defines x, applies the stoichiometric ratio, selects a valid root, and reports units or significant figures. These hidden decisions are often where marks are lost.

Mistake 7: mishandling reversed or multiplied equations

The equilibrium constant belongs to the equation exactly as written. If the reaction is reversed, the new constant is the reciprocal:

Kreverse = 1 / Kforward

If every coefficient is multiplied by a factor n, the new constant is Kⁿ. For example, doubling an equation squares its equilibrium constant. Students who keep the original K after changing the equation are describing a different equilibrium expression with an incompatible numerical value.

Mistake 8: making errors in the Gibbs energy relationship at HL

For standard Gibbs energy and equilibrium, IB Chemistry uses:

ΔG° = −RT ln K

Common errors include using Celsius, using log instead of the natural logarithm ln, and leaving ΔG° in kJ mol⁻¹ while using R = 8.31 J K⁻¹ mol⁻¹. Convert units before substitution and use temperature in kelvins.

The sign also has chemical meaning. If K > 1, then ln K is positive and ΔG° is negative, corresponding to a product-favoured standard equilibrium. This does not mean the reaction is fast, because thermodynamic favourability and reaction rate are separate ideas.

How to review worked video solutions effectively

Passive viewing rarely changes exam performance. Use a deliberate review cycle with the IB Chemistry past-paper video walkthroughs:

Attempt the question without assistance.
Mark the exact line where your method became uncertain.
Watch the per-question worked solution and pause before each major step.
Compare the setup, not only the final number.
Label the error as conceptual, algebraic, stoichiometric, or exam-language related.
Reattempt the question from a blank page one or two days later.

After watching, practise a parallel problem in the equilibrium topic hub. Use Chemistry Flashcards for definitions and Q-versus-K rules, then move to mixed Chemistry predicted papers so equilibrium is tested alongside other topics. Jojo AI can help identify where your response differs from the expected marking logic, but you should still write and check every chemical step yourself.

Exam checklist for equilibrium questions

Before finalising an answer, ask:

Is the equation balanced?
Am I using equilibrium rather than initial values?
Did coefficients become exponents or stoichiometric changes correctly?
Have I separated rate, equilibrium position, and K?
For pressure, did I count only gaseous species?
For temperature, did I identify the endothermic direction?
At HL, did I compare Q with K in the correct direction?
Are temperature and energy units compatible?
Is my numerical root chemically possible?

Conclusion

Equilibrium errors usually come from skipping a reasoning step rather than from the topic being inherently unpredictable. Reliable answers distinguish equal rates from equal concentrations, construct K from the balanced equation, apply Le Châtelier’s principle conditionally, and organise HL calculations before substituting numbers.

RevisionDojo can support this correction process through worked past-paper videos, targeted Questionbank practice, Flashcards, and Jojo AI feedback. Start with the per-question video walkthroughs, record the method you missed, and then test the corrected method on a new equilibrium problem.

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