Level 1 — RecognitionEquilibrium

Equilibrium

⏱ 20 minutes30 marksprintable — key stays hidden on paper

Level 1: Recognition (MCQ + Matching + True/False with Justification)

Time limit: 20 minutes
Total marks: 30

Section A — Multiple Choice (1 mark each) — 12 marks

Choose the single best answer.

Q1. At dynamic equilibrium, which statement is TRUE? (a) Forward reaction stops (b) Rate of forward reaction = rate of reverse reaction (c) Concentrations of reactants and products are equal (d) The reaction has gone to completion

Q2. For the reaction $N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)$ , the value of $\Delta n$ (gaseous) is: (a) $+2$ (b) $-2$ (c) $-1$ (d) $+1$

Q3. The relationship between $K_p$ and $K_c$ is: (a) $K_p = K_c(RT)^{\Delta n}$ (b) $K_p = K_c/(RT)^{\Delta n}$ (c) $K_p = K_c \cdot RT$ (d) $K_p = K_c^{\Delta n}$

Q4. If $Q < K_c$ for a reaction, the reaction will: (a) proceed in the reverse direction (b) be at equilibrium (c) proceed in the forward direction (d) stop

Q5. For $CaCO_3(s) \rightleftharpoons CaO(s) + CO_2(g)$ , the correct expression for $K_p$ is: (a) $\dfrac{p_{CO_2}\cdot p_{CaO}}{p_{CaCO_3}}$ (b) $p_{CO_2}$ (c) $\dfrac{1}{p_{CO_2}}$ (d) $p_{CaO}\cdot p_{CO_2}$

Q6. According to the Brønsted–Lowry definition, a base is a substance that: (a) donates a proton (b) accepts a proton (c) accepts an electron pair (d) produces $OH^-$ in water

Q7. The conjugate base of $H_2SO_4$ is: (a) $SO_4^{2-}$ (b) $H_3O^+$ (c) $HSO_4^-$ (d) $H_2SO_3$

Q8. At 25 °C, $K_w = 10^{-14}$ . For a neutral solution: (a) $[H^+] = 10^{-14}$ (b) $[H^+] = [OH^-] = 10^{-7}$ (c) $[H^+] = 10^{-7}, [OH^-] = 10^{-14}$ (d) $pH = 14$

Q9. Ostwald's dilution law for a weak acid gives the degree of dissociation as: (a) $\alpha = K_a C$ (b) $\alpha = \sqrt{K_a/C}$ (c) $\alpha = \sqrt{K_a C}$ (d) $\alpha = K_a/C$

Q10. The Henderson–Hasselbalch equation for an acidic buffer is: (a) $pH = pK_a + \log\dfrac{[\text{acid}]}{[\text{salt}]}$ (b) $pH = pK_a + \log\dfrac{[\text{salt}]}{[\text{acid}]}$ (c) $pH = pK_a - \log\dfrac{[\text{salt}]}{[\text{acid}]}$ (d) $pOH = pK_a + \log\dfrac{[\text{salt}]}{[\text{acid}]}$

Q11. A solution of $NH_4Cl$ (salt of strong acid + weak base) in water is: (a) acidic (b) basic (c) neutral (d) amphoteric

Q12. Adding a common ion to a saturated solution of a sparingly soluble salt: (a) increases its solubility (b) decreases its solubility (c) does not affect solubility (d) changes its $K_{sp}$

Section B — Matching (1 mark each) — 6 marks

Q13. Match Column I with Column II (acid–base concepts):

Column I	Column II
(i) Arrhenius acid	(P) electron-pair acceptor
(ii) Brønsted acid	(Q) gives $H^+$ in water
(iii) Lewis acid	(R) proton donor

Q14. Match the salt type with the nature of its aqueous solution:

Column I	Column II
(i) $NaCl$	(P) basic
(ii) $CH_3COONa$	(Q) neutral
(iii) $NH_4Cl$	(R) acidic

Section C — True/False WITH Justification (2 marks each: 1 for T/F, 1 for reason) — 12 marks

Q15. A catalyst increases the value of the equilibrium constant $K_c$ .

Q16. For an exothermic reaction, increasing temperature shifts the equilibrium toward the products.

Q17. Pure solids and pure liquids are excluded from equilibrium constant expressions.

Q18. The pH of a $0.01\,M$ solution of a strong acid HCl is 2 at 25 °C.

Q19. Increasing pressure on $N_2(g)+3H_2(g)\rightleftharpoons 2NH_3(g)$ shifts equilibrium toward $NH_3$ .

Q20. For a weak acid, the degree of dissociation $\alpha$ increases on dilution.

Answer keyMark scheme & solutions

Section A (1 mark each)

Q1 — (b). At dynamic equilibrium the opposing rates are equal; concentrations become constant (not necessarily equal), and reactions continue microscopically. (1)

Q2 — (b). $\Delta n = n_{products} - n_{reactants} = 2 - (1+3) = -2$ . (1)

Q3 — (a). Standard derived relation $K_p = K_c(RT)^{\Delta n}$ . (1)

Q4 — (c). $Q<K$ means too few products relative to equilibrium, so net forward reaction occurs. (1)

Q5 — (b). Pure solids $CaCO_3, CaO$ are excluded; only gaseous $CO_2$ appears: $K_p = p_{CO_2}$ . (1)

Q6 — (b). Brønsted base = proton acceptor. (1)

Q7 — (c). Removing one $H^+$ from $H_2SO_4$ gives $HSO_4^-$ . (1)

Q8 — (b). Neutrality: $[H^+]=[OH^-]=\sqrt{K_w}=10^{-7}$ . (1)

Q9 — (b). Ostwald: $\alpha=\sqrt{K_a/C}$ (for $\alpha\ll1$ ). (1)

Q10 — (b). $pH = pK_a + \log([\text{salt}]/[\text{acid}])$ . (1)

Q11 — (a). $NH_4^+$ hydrolyses to give $H_3O^+$ → acidic. (1)

Q12 — (b). Common ion effect suppresses dissociation/solubility; $K_{sp}$ unchanged. (1)

Section B (1 mark each match, but graded per row: award full marks only if all correct — split ½ per correct row internally; total 3 each)

Q13. (i)–(Q), (ii)–(R), (iii)–(P). (3: 1 per correct pairing)

Arrhenius acid gives $H^+$ in water; Brønsted acid is proton donor; Lewis acid accepts an electron pair.

Q14. (i)–(Q), (ii)–(P), (iii)–(R). (3: 1 per correct pairing)

$NaCl$ (SA/SB) neutral; $CH_3COONa$ (WA/SB) basic; $NH_4Cl$ (SA/WB) acidic.

Section C (2 marks each: 1 T/F + 1 justification)

Q15 — FALSE. A catalyst speeds both forward and reverse reactions equally, reaching equilibrium faster but leaving $K_c$ unchanged. (1+1)

Q16 — FALSE. For exothermic reactions heat is a product; raising temperature shifts equilibrium toward reactants (backward), decreasing $K$ . (1+1)

Q17 — TRUE. Their concentrations (activities = 1) are effectively constant, so they don't appear in $K_c$ / $K_p$ expressions. (1+1)

Q18 — TRUE. Strong acid fully dissociates: $[H^+]=0.01=10^{-2}$ , $pH=-\log(10^{-2})=2$ . (1+1)

Q19 — TRUE. 4 mol gas → 2 mol gas; increasing pressure shifts toward fewer moles (products, $NH_3$ ). (1+1)

Q20 — TRUE. From $\alpha=\sqrt{K_a/C}$ , decreasing $C$ (dilution) increases $\alpha$ . (1+1)

[
  {"claim":"Q2: Delta n for Haber = -2","code":"result = (2 - (1+3)) == -2"},
  {"claim":"Q8: neutral [H+] = 1e-7 from Kw=1e-14","code":"result = sqrt(Rational(1,10)**14) == Rational(1,10)**7"},
  {"claim":"Q18: pH of 0.01 M HCl = 2","code":"import sympy as sp; result = sp.simplify(-sp.log(sp.Rational(1,100),10)) == 2"},
  {"claim":"Q20: alpha increases as C decreases (Ka fixed)","code":"Ka=sp.Rational(1,100000); a1=sp.sqrt(Ka/sp.Rational(1,10)); a2=sp.sqrt(Ka/sp.Rational(1,100)); result = bool(a2 > a1)"}
]