Academia Aeternum
तमसो मा ज्योतिर्गमय
Wisdom · Knowledge · Eternity
Chapter 06
The Balance Point of Reactions
When forward and reverse reactions race at equal speeds, chemistry finds its stillness — dynamic, invisible, perfect.
12 key topics in this chapter — each linked to NCERT exercises and exam questions.
Everything you need to master this chapter — from concept notes to previous year questions.
Essential expressions — understand derivations, not just results.
Important points to remember — curated from CBSE Board and entrance exam question patterns.
CBSE always tests Le Chatelier's Principle with 3–4 marks — predict the shift in equilibrium for changes in concentration, pressure, and temperature.
The relationship Kp = Kc(RT)^Δng is a very frequent 2-mark question; note that Δng = moles of gaseous products − moles of gaseous reactants.
pH questions: "find pH of 0.01 M HCl" → pH = −log(0.01) = 2. Practice all three types: strong acid, strong base, weak acid/base.
Buffer solutions and Henderson–Hasselbalch appear in 5-mark CBSE questions. Remember: pH = pKa when [A⁻] = [HA].
Ksp and solubility interconversion: if Ksp = s² for a 1:1 salt, then s = √Ksp.
Common ion effect reduces solubility — always explain it via Le Chatelier's principle.
Targeted tips for JEE Main, JEE Advanced, NEET, and BITSAT.
JEE Main tests Kc, Kp, and their ratio extensively. Memorise that Q > K means reaction proceeds in reverse; Q < K means forward.
JEE Advanced combines equilibrium with thermodynamics: ΔG° = −RT ln K. A negative ΔG° means K > 1 (product-favoured).
NEET focuses on buffer solutions and pH calculations — Henderson–Hasselbalch and blood pH regulation (bicarbonate buffer) are high-yield.
BITSAT rapidfire: "for which reaction does Kp = Kc?" → when Δng = 0. Identify such reactions instantly.
Students consistently lose marks on these — know them before your exam.
Including pure solids or liquids in the Kc expression — only aqueous and gaseous species appear.
Confusing Kc with Kp — remember to account for (RT)^Δng when converting.
Using wrong Δng sign: count gaseous products minus gaseous reactants (not total).
Forgetting that pH + pOH = 14 is valid only at 25°C (Kw = 1×10⁻¹⁴).
The non-negotiable concepts every student must carry out of this chapter.
Chemical equilibrium is dynamic — both forward and reverse reactions occur at equal rates.
K > 1 favours products; K < 1 favours reactants; K = 1 means equal concentrations at equilibrium.
Temperature is the only factor that changes the equilibrium constant K.
Le Chatelier's Principle predicts direction of shift when a system at equilibrium is disturbed.
pH < 7 is acidic, pH > 7 is basic, pH = 7 is neutral (at 25°C).
A buffer resists pH change by containing a weak acid and its conjugate base.
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