Questions
5–8 MCQs per paper
Difficulty
Medium-Hard
Importance
Foundation for Electrochemistry and Ionic Equilibrium
Overview
Chemical Equilibrium represents the state where forward and backward reaction rates are equal, resulting in constant macroscopic properties. It is a cornerstone of physical chemistry that integrates kinetics, thermodynamics, and ionic behavior, making it a high-yield topic for JEE and NEET exams.
Law of Mass Action and Kp/Kc
The Law of Mass Action dictates that the rate of a reaction is proportional to the product of active masses of reactants. Understanding the relationship between equilibrium constants (Kc, Kp, Kx) and their temperature dependence via the van't Hoff equation is critical for solving multi-stage reaction problems.
- Kc = [Products]/[Reactants]
- Kp = Kc(RT)^delta_n
- Keq changes only with temperature
- Qc vs Kc comparison determines reaction direction
- Units of Kp depend on delta_n
Le Chatelier's Principle
Le Chatelier's Principle predicts how a system at equilibrium responds to external stressors like pressure, volume, concentration, or temperature changes. In entrance exams, questions often test your ability to predict shift direction for complex gas-phase equilibria under varying constraints.
- Increase in pressure shifts to side with fewer moles
- Inert gas addition at constant volume has no effect
- Temperature increase favors endothermic direction
- Catalysts do not change the position of equilibrium
- Concentration increase shifts equilibrium to consume added species
Acid-Base Equilibria
This sub-topic involves calculating pH, pOH, and degree of dissociation for weak electrolytes. Mastering the behavior of buffer solutions, salt hydrolysis, and common ion effect is essential for solving complex titrimetric and pH-based numericals.
- pH = -log[H+]
- Kw = [H+][OH-] = 10^-14 at 25C
- Ka * Kb = Kw
- Henderson-Hasselbalch equation for buffers
- Degree of dissociation alpha = sqrt(Ka/C)
Solubility Product (Ksp)
Solubility product relates to the maximum concentration of ions in a saturated solution of a sparingly soluble salt. Exam questions frequently combine Ksp with the Common Ion Effect to determine precipitation conditions or solubility reduction.
- Ksp = [A]^x[B]^y for A_xB_y salt
- Precipitation occurs if Qsp > Ksp
- Solubility decreases significantly with common ion
- Relationship between solubility 's' and Ksp for various salt types
Formula Sheet
Kp = Kc(RT)^delta_n
delta_G = -RTlnK
pH = pKa + log([Salt]/[Acid])
Ksp = (x^x)(y^y)(s^(x+y))
Exam Tip
Always verify if the reaction temperature changes before applying Le Chatelier's Principle, as pressure and concentration shifts are irrelevant if the Keq value itself is altered by temperature.
Common Mistakes
- Including pure solids or liquids in the expression for Kc/Kp
- Forgetting to adjust the equilibrium constant when reversing or multiplying the stoichiometric coefficients of a reaction
- Confusing the effect of inert gas addition at constant pressure versus constant volume
More Revision Notes
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