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Equilibrium questions in JEE and NEET chemistry reward careful bookkeeping more than clever ideas. This guide uses NCERT's Class 11 Chemistry unit on equilibrium as the reference and walks through the errors that cost the most marks. The K values in the examples are illustrative, chosen to make the arithmetic clear.
What goes into the K expression?
Pure solids and liquids stay out. NCERT explains that "the molar concentration of a pure solid or liquid is constant (i.e., independent of the amount present)", so they do not appear in the expression. For CaCO₃(s) ⇌ CaO(s) + CO₂(g), Kc = [CO₂] and Kp = p(CO₂). Gases and dissolved species stay in.
K belongs to the equation as written. NCERT notes that the equilibrium constant for the reverse reaction is the inverse of the forward one, and that multiplying an equation by a factor changes the expression for K to match; multiplying by n raises K to the power n.
Worked example. Suppose H₂(g) + I₂(g) ⇌ 2HI(g) has Kc = 50 at some temperature.
Reverse, 2HI ⇌ H₂ + I₂: Kc = 1/50 = 0.02.
Halved, HI ⇌ ½H₂ + ½I₂: Kc = (0.02)^½ ≈ 0.141.
An answer that quotes K without the equation it belongs to is incomplete.
How do you convert between Kp and Kc?
NCERT gives Kp = Kc(RT)^Δn, where Δn is "(number of moles of gaseous products) – (number of moles of gaseous reactants)". With concentrations in mol/L and pressures in bar, NCERT uses R = 0.0831 bar L mol⁻¹ K⁻¹.
Worked example. For N₂(g) + 3H₂(g) ⇌ 2NH₃(g), Δn = 2 − 4 = −2. Suppose Kc = 0.5 at 500 K. Then RT = 0.0831 × 500 ≈ 41.6, and Kp = 0.5 × (41.6)⁻² ≈ 2.9 × 10⁻⁴.
The usual slips:
Counting solids or liquids in Δn.
Using R = 8.314 J mol⁻¹ K⁻¹ when concentrations are in mol/L and pressures in bar.
Forgetting that Δn can be negative, or zero, in which case Kp = Kc.
How do you predict which way a reaction goes?
Use the reaction quotient Q, which NCERT defines "in the same way as the equilibrium constant Kc except that the concentrations in Qc are not necessarily equilibrium values". If Qc > Kc, the reaction goes backwards; if Qc < Kc, it goes forwards.
Worked example. With Kc = 50 for H₂ + I₂ ⇌ 2HI, a mixture has [H₂] = [I₂] = 0.1 M and [HI] = 0.5 M. Qc = (0.5)²/(0.1 × 0.1) = 25. Since 25 < 50, more HI forms.
What does Le Chatelier's principle change, and what does it not?
NCERT states the principle: a change in any factor that determines the equilibrium conditions "will cause the system to change in such a manner so as to reduce or to counteract the effect of the change". The common error is mixing up a shift in composition with a change in K.
Concentration, pressure, volume: these change the composition because Qc no longer equals Kc. The value of K stays the same.
Temperature: "when a change in temperature occurs, the value of equilibrium constant, Kc is changed". For an exothermic reaction K decreases as temperature rises; for an endothermic one it increases. NCERT's example is ammonia synthesis, with ΔH = −92.38 kJ mol⁻¹, where raising the temperature shifts the equilibrium to the left.
Catalyst: NCERT says a catalyst lowers the activation energy of forward and reverse reactions by exactly the same amount and "does not affect the equilibrium composition". It only gets you to equilibrium faster.
Inert gas at constant volume: NCERT says the equilibrium "remains undisturbed", because the partial pressures and concentrations of the reacting species do not change.
A checking routine for equilibrium answers
Write the balanced equation with states, and the K expression without solids or pure liquids.
If you change the equation, change K: inverse for reverse, power n for multiples.
For Kp and Kc, count gaseous moles only, and use R in bar L mol⁻¹ K⁻¹.
Before predicting a shift, compute Q and compare with K.
Ask: did the question change temperature? If not, K has not changed.
Equilibrium calculations sit on top of mole arithmetic; if those steps wobble, start with mole concept and stoichiometry errors. The rules above also make good cards; see how to make flashcards that work for physics and chemistry.
If the underlying idea of dynamic equilibrium is not yet clear, the Fermi Concept Library covers every concept in physics, chemistry, biology, mathematics and computing, each with its own lesson. For checking a full calculation, the Fermi Problem Library has worked problems with a step-by-step solver that explains the reasoning rather than printing an answer.
