Questions
5 questions per paper
Difficulty
Medium-Hard
Importance
Core — never skip
Overview
Chemical and ionic equilibrium describes the state where the rate of forward and backward reactions are equal, resulting in constant concentrations of reactants and products. Mastering this topic is essential for scoring in physical chemistry, as it forms the bedrock for understanding spontaneity, solubility products, and pH calculations in competitive exams.
Le Chatelier's Principle
This principle predicts how a system at equilibrium responds to changes in pressure, temperature, or concentration. It states that if a constraint is applied to a system at equilibrium, the system will shift to counteract the effect of the constraint.
- Increase in concentration of reactants shifts equilibrium to the right
- Increase in temperature favors endothermic reactions
- Increase in pressure favors the side with fewer gaseous moles
- Inert gas addition at constant volume has no effect
- Catalysts do not shift equilibrium; they only reach it faster
Ionic Equilibrium
Ionic equilibrium focuses on the dissociation of weak electrolytes in aqueous solutions. Students must be comfortable with the degree of dissociation and the application of the Law of Mass Action to electrolytes.
- Ostwald Dilution Law: alpha = square root(K/C)
- Dissociation constant K_a = (C * alpha^2) / (1 - alpha)
- Solubility Product (K_sp) defines the solubility of sparingly soluble salts
- Common Ion Effect reduces the degree of dissociation
- Degree of dissociation increases with dilution
pH and Buffer Solutions
This section covers the measurement of hydrogen ion concentration and the resistance of solutions to pH changes. Understanding the Henderson-Hasselbalch equation is mandatory for solving numerical problems regarding acidic and basic buffers.
- pH = -log[H+]
- pH + pOH = 14 at 25 degrees Celsius
- Henderson-Hasselbalch (Acidic Buffer): pH = pKa + log([salt]/[acid])
- Henderson-Hasselbalch (Basic Buffer): pOH = pKb + log([salt]/[base])
- Buffer capacity is maximum when [salt] = [acid]
Formula Sheet
Kc = [Products]/[Reactants]
Kp = Kc(RT)^delta(n)
pH = -log[H+]
pKa = -log(Ka)
Ksp = [A+]^x * [B-]^y
Exam Tip
Always verify the temperature before assuming the ionic product of water (Kw) is exactly 10^-14, as it is highly temperature-dependent.
Common Mistakes
- Applying Le Chatelier's principle to solids or liquids when changing pressure.
- Forgetting to convert molarity to hydrogen ion concentration before applying the log function.
- Neglecting the common ion effect when calculating solubility in presence of another salt.
More Revision Notes
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