Chemical Equilibrium · Reaction Shift
Watch a reversible A ⇌ B reaction approach dynamic equilibrium and examine temperature shifts, reaction quotient and forward/reverse rates.
NCERT alignment: Class 11; chapter Equilibrium. Exam practice: JEE Main, JEE Advanced, NEET.
Learning objectives
Students should use the live simulator to test a quantitative prediction, read measurements and explain the limits of the model. Do not rely on the animation alone: calculate the expected result before adjusting controls.
Theory, equations and interpretations
Dynamic chemical equilibrium
For the elementary reversible A ⇌ B process, forward rate kf[A] equals reverse rate kr[B] at equilibrium. Individual molecules continue interconverting; equilibrium does not mean reactions stop.
Important relation: A ⇌ B; Kc = [B]eq/[A]eq = kf/kr
Reaction quotient Q
At any instant Q=[B]/[A]. If Q<Kc, net conversion A→B occurs; if Q>Kc, the net reaction goes backward. For this closed one-to-one model [A]+[B]=constant.
Important relation: Q = [B]/[A]; Q < Kc → forward
Temperature and enthalpy
The van’t Hoff relationship gives an idealized effect of temperature on K. Endothermic ΔH>0 reactions favour products at higher temperature; exothermic ΔH<0 reactions favour reactants. Kc is not changed merely by adding A or B at fixed T.
Important relation: ln(K₂/K₁) = (ΔH/R)(1/T₁ − 1/T₂)
Analytical time evolution
For first-order isomerization A ⇌ B, the concentrations exponentially approach the equilibrium distribution with relaxation rate kf+kr. This is an illustrative kinetic model, not a universal equilibrium-rate law.
Important relation: [A](t) = [A]eq + ([A]₀−[A]eq)e⁻⁽ᵏᶠ⁺ᵏʳ⁾ᵗ
Practical investigations
- Detect direction from Q: Start with 86% A; calculate Q initially, compare with Kc and predict whether A or B grows.
- Heat an endothermic reaction: Use ΔH=+25 kJ/mol, compare Kc at 298 K and 365 K. Do not confuse a shift in K with a change in initial concentration.
- Reverse ΔH sign: Set ΔH negative and raise temperature; explain why product fraction decreases at equilibrium.
- Compare rate constants: Increase kf at 298 K while keeping Kref fixed. Observe approach speed and distinguish rate of attaining equilibrium from equilibrium position.
Worked exam-style questions and answers
Q1. At chemical equilibrium for A ⇌ B
Options: A. Forward reaction stops; B. Reverse reaction stops; C. Both rates are equal; D. Concentrations must be equal
Answer: C. Dynamic equilibrium means vforward=vreverse, not equal concentrations.
Q2. For A ⇌ B, Kc=4. If [B]/[A]=1, net reaction moves
Options: A. Backward; B. Forward; C. Neither; D. Undefined
Answer: B. Q<Kc so the net reaction proceeds toward B.
Q3. For an endothermic forward reaction, increasing temperature usually
Options: A. Decreases K; B. Increases K; C. Leaves K unchanged; D. Makes Q undefined
Answer: B. Van’t Hoff gives positive d lnK / dT for ΔH>0.
Q4. Which changes Kc for a fixed chemical reaction in the idealized model?
Options: A. Initial concentration; B. Catalyst; C. Temperature; D. Adding reactant
Answer: C. At fixed standard state, equilibrium constant depends primarily on temperature.
Q5. In a closed A ⇌ B first-order model, total [A]+[B]
Options: A. Is constant; B. Must be zero; C. Always doubles; D. Must equal Kc
Answer: A. One A becomes one B, conserving total concentration.
Q6. Adding a catalyst to an equilibrium mixture at fixed T
Options: A. Changes K; B. Moves equilibrium to reactants; C. Speeds both directions without shifting K; D. Stops reaction
Answer: C. Catalyst affects kinetic rates, not the thermodynamic equilibrium constant.
Common misconceptions and limitations
A ⇌ B is a simplified elementary first-order isomerization model in a closed, fixed-volume vessel. It does not represent arbitrary equilibria, gas-pressure effects, activity corrections or real experimental kinetics.
All learning content is available free of charge. The simulation is a simplified educational model. Check assumptions and units before applying a result. NANDAX SimStudio is not affiliated with NCERT or examination boards.
Teacher classroom use
Ask students to form a hypothesis, vary one parameter at a time, keep a record of graph readings, explain any change and compare results with the analytic relation. The on-screen worksheet offers questions with an answer key.