Chemical Equilibrium: Complete Guide & Formula Sheet with Free PDF Download (JEE & NEET)
Chemical Equilibrium — Competishun
Chemical Equilibrium: Complete Guide & Formula Sheet with Free PDF Download (JEE & NEET)
Chemical Equilibrium is one of the most important and scoring chapters in physical chemistry. It carries high weightage in JEE and NEET, with 2-3 questions appearing every year.
This chapter deals with reversible reactions that reach a state where the forward and backward reactions occur at the same rate. It covers the law of mass action, equilibrium constants (Kp and Kc), the reaction quotient, Le Chatelier's principle, and the degree of dissociation.
This page gives you the complete guide to Chemical Equilibrium with all concepts explained in depth. You will find clear definitions, derivations, worked examples, and common mistakes to avoid. Download the free PDF below and keep it handy for quick revision before your JEE Main, JEE Advanced, or NEET exam.
Download the Chemical Equilibrium Complete Guide PDF
Get all Chemical Equilibrium concepts, formulas, and derivations in one clean PDF, free. Perfect for JEE and NEET revision.
Download Free PDFWhat is Chemical Equilibrium?
Chemical Equilibrium is a dynamic process, not a static one. The forward and backward reactions continue to occur, but at the same rate, so there is no net change in the concentrations of reactants and products.
Forward Reaction
Reactants are converted into products. The rate of the forward reaction decreases as the concentration of reactants decreases.
Backward Reaction
Products are converted back into reactants. The rate of the backward reaction increases as the concentration of products increases.
Law of Mass Action
For a general reversible reaction:
At equilibrium, Kc = [C]^c [D]^d / [A]^a [B]^b
Important Points
- Pure solids and liquids: Their concentrations are omitted from the equilibrium expression because they are constant.
- Gases: Their concentrations can be expressed in terms of partial pressures (Kp) or molar concentrations (Kc).
- Units of K: K is unitless when using activities. However, for Kc and Kp, units may be present depending on the reaction.
Equilibrium Constants — Kc and Kp
Relation Between Kp and Kc
Where Δn = (moles of gaseous products) - (moles of gaseous reactants).
| Quantity | Definition | Formula |
|---|---|---|
| Kc | Equilibrium constant in terms of concentrations | Kc = [Products] / [Reactants] |
| Kp | Equilibrium constant in terms of partial pressures | Kp = (P_Products) / (P_Reactants) |
| Kp = Kc(RT)Δn | Relation between Kp and Kc | Δn = gas products - gas reactants |
| If Δn = 0, then Kp = Kc. This is a common trap in JEE and NEET questions. | ||
Properties of Equilibrium Constant
- Depends only on temperature: K is independent of initial concentrations, pressure, volume, and the presence of a catalyst.
- Reverse reaction: K' = 1/K.
- Multiplying by a factor: If the reaction is multiplied by n, the new equilibrium constant is Kn.
- Adding reactions: When two or more reactions are added, the equilibrium constant of the resulting reaction is the product of their equilibrium constants.
Reaction Quotient (Q)
| Condition | Direction of Reaction |
|---|---|
| Q < K | Forward reaction proceeds (net forward) |
| Q = K | System is at equilibrium |
| Q > K | Backward reaction proceeds (net backward) |
| Comparing Q with K tells us which direction the reaction will shift to reach equilibrium. | |
Le Chatelier's Principle
Effect of Changes on Equilibrium
| Stress | Effect on Equilibrium |
|---|---|
| Change in Concentration | Shifts to consume the added substance or replace the removed substance. |
| Change in Pressure/Volume | Shifts towards the side with fewer moles of gas (if Δn ≠ 0). |
| Change in Temperature | For an exothermic reaction (ΔH < 0), increasing temperature shifts the equilibrium to the left. For an endothermic reaction (ΔH > 0), increasing temperature shifts the equilibrium to the right. |
| Catalyst | No effect on equilibrium position; only speeds up the attainment of equilibrium. |
| Inert Gas Addition | At constant volume, no effect. At constant pressure, shifts towards the side with more moles of gas. |
| Le Chatelier's principle is essential for predicting how equilibrium will respond to changes in conditions. | |
Degree of Dissociation (α)
Vapour Density Method
For a reaction where one mole of a reactant dissociates into gaseous products:
Where D = normal vapour density, d = observed vapour density, n = number of gaseous moles formed from 1 mole of reactant.
ICE Table Method
ICE (Initial, Change, Equilibrium) tables are used to organize the information in equilibrium problems.
- Initial: Initial concentrations (or moles) of reactants and products.
- Change: Change in concentrations as the reaction proceeds to equilibrium.
- Equilibrium: Concentrations at equilibrium.
Heterogeneous Equilibria
Example
Kp = P(CO₂)
The equilibrium constant depends only on the partial pressure of CO₂, which is constant at a given temperature.
Simultaneous Equilibria
For example, if two solids dissociate to produce a common gas:
D(s) ⇌ E(g) + C(g) · K₂
The common gas (C) affects both equilibria. This is called the common-ion effect or common-gas effect.
All Chemical Equilibrium Formulas at a Glance
| Formula | What It Means |
|---|---|
| Kc = [C]^c[D]^d / [A]^a[B]^b | Equilibrium constant in terms of concentrations |
| Kp = Kc(RT)Δn | Relation between Kp and Kc |
| Q = [C]^c[D]^d / [A]^a[B]^b | Reaction quotient |
| α = (D - d) / d(n - 1) | Degree of dissociation by vapour density |
| Kp = P(CO₂) for CaCO₃ decomposition | Heterogeneous equilibrium |
| Memorise these formulas for Chemical Equilibrium. They are the key to scoring full marks in this chapter. | |
Common Mistakes in Chemical Equilibrium
- Forgetting to omit pure solids and liquids: Their concentrations are constant and are not included in the equilibrium expression.
- Confusing Kp and Kc: Kp is used for gases (partial pressures), and Kc is used for solutions (concentrations).
- Misapplying Le Chatelier's principle: Remember that temperature changes affect K, while concentration and pressure changes do not.
- Using the wrong sign for Δn: Δn = (moles of gaseous products) - (moles of gaseous reactants).
- Forgetting that a catalyst does not affect equilibrium: It only speeds up the attainment of equilibrium.
- Not checking if Q is greater than or less than K: This determines the direction of the reaction.
Why Chemical Equilibrium Matters for JEE and NEET
- High weightage: Chemical Equilibrium appears in 2-3 questions in every JEE Main, JEE Advanced, and NEET chemistry paper.
- Foundation for physical chemistry: Understanding Chemical Equilibrium helps you with thermodynamics, electrochemistry, and acid-base chemistry.
- Direct scoring: Many questions are direct formula-based, especially Kp and Kc calculations, Le Chatelier's principle, and degree of dissociation.
- Conceptual clarity: This chapter rewards students who understand the concepts rather than just memorizing formulas.
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Frequently Asked Questions — Chemical Equilibrium
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