NCERT Line-by-Line
1. TITLE
NCERT says 'Catalyst does not affect the equilibrium composition' — Here's how NEET tests it
2. THE NCERT STATEMENT
NCERT EXACT LINE:
"Catalyst does not affect the equilibrium composition of a reaction mixture. It does not appear in the balanced chemical equation or in the equilibrium constant expression."
— NCERT Class 11, Chapter 6, Page 188
3. WHY THIS LINE MATTERS
- How many times has NEET tested this? This concept is a favourite for examiners and has been tested multiple times as direct conceptual questions or Assertion-Reason pairings.
- What makes this statement tricky? Students often confuse chemical kinetics with thermodynamics. Because a catalyst speeds up a reaction, students intuitively assume it produces more product (shifting the equilibrium right).
- What do students typically misremember? Students frequently forget that temperature is the only factor that can change the numerical value of the equilibrium constant ($K_c$ or $K_p$). They mistakenly assume a catalyst alters $K_c$.
4. MCQ
According to NCERT, a catalyst added to a chemical system at equilibrium is characterized by:
A) It does not affect the equilibrium composition and does not appear in the equilibrium constant expression.
B) It alters the equilibrium composition but does not appear in the equilibrium constant expression.
C) It increases the rate of the forward reaction exclusively, thereby changing the equilibrium position.
D) It decreases the activation energy, which changes the numerical value of the equilibrium constant.
5. ANSWER + LINE BREAKDOWN
Correct Answer: A
Break down the NCERT line:
- "Does not affect the equilibrium composition" means that the final amounts and concentrations of reactants and products remain exactly the same as they would without a catalyst.
- "Does not appear... in the equilibrium constant expression" means the numerical value of $K_c$ or $K_p$ is completely independent of the presence of a catalyst.
- The key word is "composition" because it highlights that a catalyst only reduces the time taken to reach equilibrium, without altering the actual macroscopic balance of the system.
6. HOW NEET TWISTS THIS
Show 3 ways NEET can ask the same fact:
- As direct question: "Which of the following factors alters the numerical value of the equilibrium constant ($K_c$)?" (Options might include pressure, concentration, catalyst, and temperature. The answer is temperature, NOT catalyst).
- As assertion-reason: Assertion: A catalyst does not change the equilibrium constant of a reaction. Reason: A catalyst lowers the activation energy for the forward and reverse reactions by exactly the same amount. (Both are true, and the Reason correctly explains the Assertion).
- As except question: "All of the following statements about a catalyst in a reversible reaction are true EXCEPT..." (The false statement to spot would be: "It alters the concentration of reactants at equilibrium").
7. MEMORY LOCK
MEMORIZE:
Catalysts change TIME, not COMPOSITION or $K$.
Mnemonic: A catalyst is like a TAXI (or a scooter-wallah finding the shortest route) — it gets you to your destination (equilibrium) much faster, but it doesn't change the destination itself.
8. RELATED NCERT LINES
"Also remember these from the same page:"
- "Catalyst lowers the activation energy for the forward and reverse reactions by exactly the same amount."
- "It increases the rate of forward and reverse reactions that pass through the same transition state and does not affect equilibrium."
9. NCERT REFERENCE
NCERT Class 11, Chapter 6, Page 188, Paragraph 2 (Under Section 6.8.5 Effect of a Catalyst).
10. CTA
All important NCERT lines from this chapter → Equilibrium Guide