Equilibrium mind map

Equilibrium: what the chapter covers

The main idea of each topic with its first key points. Open a branch above for everything, or read the full notes.

1. 1Physical Equilibrium

   Physical equilibrium occurs when two physical states or processes coexist with no observable macroscopic change, although microscopic changes continue. In a closed system, evaporation and condensation, melting and freezing, or sublimation and deposition can occur at equal rates. This dynamic nature is central to NCERT and NEET questions. Examples include ice and water at 273 K, saturated salt solution, water vapour in a closed container, and iodine crystals with iodine vapour. Important measurable ideas are vapour pressure, saturation, phase boundaries, and the effect of temperature and pressure. Physical equilibrium helps students understand why equilibrium does not mean stopping, but equal opposing rates.

   • Physical equilibrium involves no chemical composition change.
   • Saturated solution represents solute-solid equilibrium with dissolved solute.
   • Boiling occurs when vapour pressure equals external pressure.
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2. 2Chemical Equilibrium and Law of Mass Action

   Chemical equilibrium is reached in a reversible reaction when the rate of forward reaction becomes equal to the rate of backward reaction. Reactants and products continue to interconvert, but their concentrations become constant with time. According to the law of mass action, the rate of a chemical reaction is proportional to the product of active masses of reactants raised to their stoichiometric powers. For a general reaction, this leads to the equilibrium constant expression. NEET frequently tests writing Kc correctly, excluding pure solids and liquids, interpreting equilibrium composition, and understanding that equilibrium can be reached from either direction in a closed system.

   • Equilibrium is dynamic and molecular collisions continue.
   • Law of mass action connects concentration and reaction rate.
   • Stoichiometric coefficients become powers in Kc expression.
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3. 3Equilibrium Constant Kc, Kp and Reaction Quotient

   The equilibrium constant measures how far a reversible reaction proceeds before equilibrium is established. Kc uses molar concentrations, while Kp uses partial pressures for gaseous reactions. Their relation depends on the change in moles of gaseous species, Δn. The reaction quotient Q has the same expression as K but uses concentrations or pressures at any instant, not necessarily equilibrium. Comparing Q with K predicts the direction of reaction. This topic is very important for NEET numerical problems involving ICE tables, partial pressure, degree of dissociation, and deciding whether a reaction moves forward or backward.

   • Kc and Kp are dimensionless in modern thermodynamics, though units may appear in older numerical treatment.
   • Only gaseous species enter Kp expression.
   • ICE tables help organize initial, change, and equilibrium amounts.
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4. 4Le Chatelier Principle and Factors Affecting Equilibrium

   Le Chatelier principle states that when a system at equilibrium is disturbed by changing concentration, pressure, volume, or temperature, the system shifts in a direction that reduces the disturbance. It is a prediction tool for equilibrium shifts and industrial optimization. Adding reactant usually shifts equilibrium forward; increasing pressure favours the side with fewer gaseous moles; increasing temperature favours the endothermic direction. A catalyst does not shift equilibrium; it only helps equilibrium reach faster by lowering activation energy of both directions equally. NEET commonly asks conceptual questions on Haber process, Contact process, colour changes, and pressure-volume effects.

   • Le Chatelier principle predicts direction, not exact equilibrium composition.
   • Pressure affects only gaseous equilibria with unequal gaseous moles.
   • If Δn_gas = 0, pressure or volume change has no shift effect.
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5. 5Ionic Equilibrium, Electrolytes and Ostwald Dilution Law

   Ionic equilibrium deals with reversible ionization of weak electrolytes in aqueous solution. Strong electrolytes almost completely ionize, while weak electrolytes partially ionize and establish equilibrium between unionized molecules and ions. Degree of ionization, α, represents the fraction ionized and depends on concentration, dilution, temperature, and common ions. Ostwald dilution law relates the dissociation constant of a weak electrolyte with concentration and α. Dilution increases ionization of weak electrolytes, while addition of a common ion suppresses ionization. These concepts form the base for pH, buffers, salt hydrolysis, and solubility product problems in NEET.

   • Ionic equilibrium applies to aqueous weak electrolytes.
   • For weak acid HA, Ka = Cα²/(1-α).
   • If α is very small, Ka ≈ Cα².
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6. 6Acids, Bases, pH, pKa and pKb

   Acids and bases can be understood through Arrhenius, Bronsted-Lowry, and Lewis concepts. Arrhenius acids produce H+ in water and bases produce OH-. Bronsted acids donate protons, while Bronsted bases accept protons. Lewis acids accept electron pairs and Lewis bases donate electron pairs, making this the broadest concept. pH measures hydrogen ion concentration and is crucial for NEET numericals. Strong acids and bases ionize almost completely, whereas weak acids and bases require Ka or Kb. pKa and pKb are logarithmic strength indicators: smaller pKa means stronger acid, and smaller pKb means stronger base.

   • Lewis theory includes reactions without proton transfer.
   • Water is amphoteric because it can donate or accept H+.
   • Strong acid pH is calculated directly from concentration if fully ionized.
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7. 7Hydrolysis of Salts and pH of Salt Solutions

   Salt hydrolysis occurs when ions of a salt react with water to produce H+ or OH-, making the solution acidic or basic. Salts of strong acid and strong base do not hydrolyse appreciably and give neutral solution. Salts of strong acid and weak base produce acidic solutions because the cation hydrolyses. Salts of weak acid and strong base produce basic solutions because the anion hydrolyses. Salts of weak acid and weak base require comparison of Ka and Kb. NEET often asks direct pH nature, hydrolysis constant, and relation between hydrolysis and conjugate acid-base strength.

   • Hydrolysis is reverse of neutralization for salt ions.
   • Conjugate base of weak acid is basic and hydrolyses.
   • Conjugate acid of weak base is acidic and hydrolyses.
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8. 8Buffer Solutions and Henderson Equation

   A buffer solution resists change in pH when small amounts of acid or base are added. Acidic buffers contain a weak acid and its salt with a strong base, such as CH3COOH and CH3COONa. Basic buffers contain a weak base and its salt with a strong acid, such as NH4OH and NH4Cl. Buffer action occurs due to the common ion effect: added H+ or OH- is consumed by buffer components. Henderson-Hasselbalch equations allow quick pH calculation. Buffers are important in biological systems like blood, in analytical chemistry, and in NEET problems involving pKa, pKb, and concentration ratios.

   • Buffers resist pH change but do not keep pH absolutely constant under unlimited acid or base.
   • The salt supplies conjugate ion needed to neutralize added acid or base.
   • Dilution changes buffer capacity but pH changes only slightly if ratio remains same.
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9. 9Solubility Product, Precipitation and Common Ion Effect

   Solubility product, Ksp, is the equilibrium constant for dissolution of a sparingly soluble salt in water. When a salt such as AgCl dissolves, a dynamic equilibrium exists between solid salt and its ions in solution. The ionic product is calculated using actual ion concentrations at any instant. If ionic product is less than Ksp, no precipitate forms; if equal, the solution is saturated; if greater, precipitation occurs. Common ion effect decreases solubility by shifting dissolution equilibrium backward. This topic is highly important for NEET problems on precipitation, salt solubility, group separation, and comparing Ksp values.

   • Ksp is a constant at a given temperature.
   • Molar solubility must be converted using stoichiometry before writing Ksp.
   • For AB salt, Ksp = s² if solubility is s.
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10. 10Formula Sheet, Quick Revision and NEET Mind Map

    This topic collects the most exam-oriented formulas, decision rules, and NCERT concepts from the complete Equilibrium chapter. For NEET, students must rapidly connect physical equilibrium, chemical equilibrium, Le Chatelier principle, ionic equilibrium, pH calculations, buffer equations, hydrolysis, and solubility product. Most errors occur when students use initial concentration instead of equilibrium concentration, include solids in K expressions, forget Δn is only for gases, or apply pH formulas outside their conditions. The mind map helps revise the chapter as one connected system: equilibrium constants describe position, Q predicts direction, Le Chatelier predicts shift, and ionic equilibria explain pH, buffers, and precipitation.

    • Pure solids and liquids are excluded from K, Q, and Ksp expressions.
    • K changes only with temperature, not with catalyst or concentration.
    • Strong electrolytes ionize almost completely; weak electrolytes use equilibrium constants.
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