Q.What is the equilibrium concentration of each of the substances in the equilibrium when the initial concentration of ICl was 0.78 M ? 2ICl
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Equilibrium Constant Calculation: From Intuition to Precision
Imagine you're at a party where people can move between two rooms. Some people prefer the kitchen (more snacks), others prefer the living room (better music). After a while, the number of people in each room stops changing — not because everyone froze, but because the rate of people leaving the kitchen equals the rate of people entering it. The system is in dynamic equilibrium.
Chemical reactions work the same way. A reversible reaction like
A+B⇌C+D doesn't stop when it reaches equilibrium. Instead, the forward reaction (making C and D) and the reverse reaction (making A and B) happen at the same rate. The concentrations of A, B, C, and D become constant — not equal, but constant.
The equilibrium constant K is a number that tells you where this balance lies. It answers the question: At equilibrium, which side of the reaction is favoured?
The Intuitive Idea
Think of a seesaw. If K is very large (say 106), the equilibrium sits heavily on the product side — almost all A and B have turned into C and D. If K is very small (say 10−6), the opposite is true: hardly any product forms. If K is around 1, both sides have comparable amounts.
So K is a ratio — a comparison of product concentrations to reactant concentrations at equilibrium, each raised to the power of their stoichiometric coefficients.
The Precise Statement
For a general reversible reaction at a given temperature:
aA+bB⇌cC+dD
the equilibrium constant Kc (for concentrations in mol/L) is:
Kc=[A]a[B]b[C]c[D]d
where [X] means the equilibrium concentration of species X in moles per litre.
The exponents come directly from the balanced chemical equation. If the coefficient of A is 2, you square its concentration. This is not optional — it's built into the definition.
What K Actually Depends On
K is constant at a given temperature. Change the temperature, and K changes. But K does not depend on:
- Initial concentrations
- Presence of a catalyst (catalysts speed up both directions equally)
- Pressure or volume changes (for Kc; Kp for gases has its own rules)
This is a critical exam point: if a problem gives you initial concentrations and asks for K, you must first find equilibrium concentrations — you cannot plug in initial values.
A Worked Example
Problem:
N2(g)+3H2(g)⇌2NH3(g)
At 500°C, equilibrium concentrations are:
[N2]=0.50 M, [H2]=1.50 M, [NH3]=0.20 M
Calculate Kc.
Solution:
Write the expression:
Kc=[N2][H2]3[NH3]2
Substitute:
Kc=(0.50)(1.50)3(0.20)2=0.50×3.3750.04=1.68750.04≈0.0237
The units cancel because the numerator and denominator both have units of (mol/L)2 and (mol/L)4 respectively — but by convention, Kc is reported without units. The numerical value is what matters.
Common Pitfalls
- Forgetting the exponents: A coefficient of 2 means square the concentration, not double it.
- Using initial concentrations: You must use equilibrium concentrations only. …
The key idea is to set up an ICE table (Initial, Change, Equilibrium) for the reaction and solve the resulting quadratic from the equilibrium expression.
Let the change in concentration of I2 be x M. From the stoichiometry:
- 2ICl⇌I2+Cl2
- Initial: [ICl]=0.78, [I2]=0, [Cl2]=0
- Change: [ICl] decreases by 2x, [I2] and [Cl2] each increase by x.
- Equilibrium: [ICl]=0.78−2x, [I2]=x, [Cl2]=x.
The equilibrium expression is:
Kc=[ICl]2[I2][Cl2]=(0.78−2x)2x⋅x=0.14
Taking the square root of both sides (since both numerator and denominator are perfect squares):
0.78−2xx=0.14≈0.3742
Solve for x: …
For the reaction 2ICl⇌I2+Cl2 with Kc=0.14 and initial [ICl]=0.78 M, the equilibrium concentrations are [ICl]=0.48 M, [I2]=[Cl2]=0.15 M.
The key idea here is that the equilibrium constant tells us the ratio of products to reactants at equilibrium. Since the reaction starts with only ICl, the products must form in equal amounts (one mole of I₂ and one mole of Cl₂ per two moles of ICl consumed). This symmetry simplifies the algebra.
Let’s walk through it step by step.
- Write the balanced equation and the expression for Kc. The reaction is:
2ICl(g)⇌I2(g)+Cl2(g)
The equilibrium constant expression is:
Kc=[ICl]2[I2][Cl2]=0.14
- Set up the change in concentrations using a variable. Let x be the concentration of I₂ that forms at equilibrium. Because the stoichiometry is 1:1 for I₂ and Cl₂, the same x applies to Cl₂:
[I2]=xand[Cl2]=x
For every mole of I₂ formed, 2 moles of ICl are consumed. So the change in ICl is −2x, and its equilibrium concentration is:
[ICl]=0.78−2x
- Substitute into the Kc expression.
0.14=(0.78−2x)2(x)(x)=(0.78−2x)2x2
- Solve for x by taking the square root of both sides. This is a neat shortcut — because both numerator and denominator are perfect squares, we avoid solving a quadratic:
0.14=0.78−2xx
Calculate 0.14:
0.14≈0.3742
So:
0.3742=0.78−2xx
- Solve the linear equation. Multiply both sides by (0.78−2x):
0.3742(0.78−2x)=x
0.2919−0.7484x=x
0.2919=x+0.7484x=1.7484x
x=1.74840.2919≈0.167 M
A common mistake is to forget that the denominator is squared. If you mistakenly write Kc=0.78−2xx2, you’ll get a different (wrong) answer. Always check the exponent in the Kc expression.
-
Check if the approximation is valid.
The change 2x=0.334 M is less than the initial 0.78 M, so the equilibrium concentration of ICl is positive: 0.78−0.334=0.446 M. This is fine — no negative concentrations.
-
Compute the equilibrium concentrations precisely.
Let’s use more exact arithmetic to avoid rounding errors.
x=1+20.140.78⋅0.14
With 0.14=10014=1014≈103.7417=0.37417: …
Showing the 12 most recent of 69 on this concept.
- AP EAPCET 2026Set eng-2026-05-12-FN1 markMCQQ.The equilibrium constants KP1 and KP2 for the gaseous reactions, X⇌2Y and Z⇌P+Q respectively are in the ratio of 1:9. If the degree of dissociation of X and Z are equal, then the ratio of their total pressures at these equilibria is (A) 1:36 (B) 1:3 (C) 9:1 (D) 4:1
›Reveal solutionSolution
Both reactions dissociate 1 mole into 2 moles of gas, but one produces 2 identical molecules (so the mole-fraction factor is 4α2) and the other produces 2 different molecules (factor α2) — that factor-of-4 difference is what converts a 1:9 ratio of KP into a 1:36 ratio of total pressures.
Concept and Intuition
For a dissociation of the type 1 mol→2 mol starting from 1 mole of pure reactant with degree of dissociation α and total equilibrium pressure P, the partial pressures scale with mole fractions times P, and total moles at equilibrium are 1+α. Whether the two product molecules are identical (as in X→2Y) or different (as in Z→P+Q) changes the numerical prefactor in KP even though the mole-count bookkeeping is otherwise identical.
Step-by-Step Solution
- Reaction 1: X⇌2Y. Start 1 mol X; at equilibrium: (1−α) mol X, 2α mol Y, total =1+α.
pX=1+α(1−α)P1,pY=1+α2αP1
KP1=pXpY2=(1−α)P1/(1+α)4α2P12/(1+α)2=(1−α)(1+α)4α2P1=1−α24α2P1
- Reaction 2: Z⇌P+Q. Start 1 mol Z; at equilibrium: (1−α) mol Z, α mol P, α mol Q, total =1+α.
pZ=1+α1−αP2,pP=pQ=1+ααP2
KP2=pZpPpQ=(1−α)P2/(1+α)α2P22/(1+α)2=1−α2α2P2 …
- AP EAPCET 2026Set eng-2026-05-12-AN1 markMCQQ.At 540 K, 0.1 mol of PCl5 is heated in a 8.0 L flask. The total pressure of the equilibrium mixture is 1 atm. The Kp (in atm) for the reaction is (R=0.082 LatmK−1mol−1) (A) 1.77 (B) 0.77 (C) 2.77 (D) 3.77
›Reveal solutionSolution
This finds the degree of dissociation of PCl5 from the observed total pressure (via total moles), then uses it to compute Kp; the answer is ≈1.77 atm.
Concept and Intuition
When PCl5 partially dissociates into PCl3 and Cl2 (1 mole of gas becoming 2 moles), the total moles — and hence the total pressure at fixed T,V — increase above what pure undissociated PCl5 would give. Comparing the observed total pressure to the pressure expected with zero dissociation lets us extract the degree of dissociation α, and from there the mole fractions (and Kp) follow directly.
Step-by-Step Solution
- Reaction: PCl5(g)⇌PCl3(g)+Cl2(g). Starting with n0=0.1 mol PCl5, at degree of dissociation α: moles are PCl5=n0(1−α), PCl3=n0α, Cl2=n0α; total =n0(1+α).
- Total moles at equilibrium from the ideal gas law: neq=RTPtotalV=(0.082)(540)(1)(8.0)=44.288.0≈0.181 mol.
- So n0(1+α)=0.181⇒(1+α)=0.10.181≈1.8⇒α≈0.8.
- Mole fractions at equilibrium: xPCl5=1+α1−α, xPCl3=xCl2=1+αα. …
- AP EAPCET 2026Set eng-2026-05-13-FN1 markMCQQ.For the reaction 2HI(g)⇌H2(g)+I2(g), the correct expression relating the degree of dissociation (α) of HI and equilibrium constant Kp is (A) α=21+2Kp (B) α=1+22Kp (C) α=1+2Kp2Kp (D) α=1+2Kp2Kp
›Reveal solutionSolution
Setting up the ICE table for 2HI⇌H2+I2 in terms of degree of dissociation α, using the fact that total moles don't change, and simplifying Kp in terms of mole fractions gives α=1+2Kp2Kp.
Concept and Intuition
Because 2 moles of HI decompose into exactly 1+1=2 moles of products, the total number of moles (and hence total pressure, at fixed volume/temperature) never changes during this reaction — a special simplification that lets Kp be written purely in terms of mole fractions (the total-pressure factors cancel since the numerator and denominator have the same overall power of 2).
Step-by-Step Solution
- Start with a mol of HI, degree of dissociation α. Since 2 mol HI → 1 mol H2 + 1 mol I2:
- HI remaining: a(1−α)
- H2 formed: aα/2
- I2 formed: aα/2
- Total moles =a(1−α)+aα/2+aα/2=a (unchanged).
- Mole fractions: xHI=(1−α), xH2=xI2=α/2.
- Since total moles (and hence total pressure at fixed T,V) is unchanged, Kp reduces to a pure mole-fraction expression (the Ptotal factors cancel, as numerator and denominator both carry overall power 2): Kp=xHI2xH2xI2=(1−α)2(α/2)(α/2)=4(1−α)2α2 …
- Start with a mol of HI, degree of dissociation α. Since 2 mol HI → 1 mol H2 + 1 mol I2:
- AP EAPCET 2026Set eng-2026-05-14-FN1 markMCQQ.Observe the following reaction A(g)+B(g)⇌T(K)C(g)+D(g) In a 1L closed flask, 2 moles of A(g) and 1 mole of B(g) were taken and heated to T(K). At equilibrium, the concentration of C(g) is thrice the concentration of B(g). What is the value of Kc? (A) 7.2 (B) 3.6 (C) 0.9 (D) 1.8
›Reveal solutionSolution
A standard ICE-table equilibrium problem: use the given ratio between two equilibrium concentrations to pin down the extent of reaction x, then plug into the Kc expression. Answer: Kc=1.8.
Concept and Intuition
For a reaction run in a fixed 1 L vessel, moles and molar concentrations are numerically identical, which simplifies the ICE (Initial–Change–Equilibrium) table. The extra piece of information — that [C] at equilibrium is exactly three times [B] at equilibrium — is what lets us solve for the single unknown x, the amount of A and B consumed (equal to the amount of C and D formed, since all stoichiometric coefficients are 1).
Step-by-Step Solution
- Write the ICE table (1 L flask, so [X] = moles of X):
A(g)+B(g)⇌C(g)+D(g)
Initial: A=2, B=1, C=0, D=0
Change: −x, −x, +x, +x
Equilibrium: A=2−x, B=1−x, C=x, D=x
2. Apply the given condition [C]=3[B]:
x=3(1−x)
x=3−3x⇒4x=3⇒x=0.75
- So at equilibrium: [A]=2−0.75=1.25, [B]=1−0.75=0.25, [C]=[D]=0.75.
- Compute Kc: …
- AP EAPCET 2026Set eng-2026-05-14-AN1 markMCQQ.At T(K), 15 moles of H2 reacts with 5.2 moles of I2 and forms 10 moles of HI. The equilibrium constant for the reaction 2HI(g)⇌H2(g)+I2(g) is (A) 50 (B) 100 (C) 2×10−2 (D) 1×10−2
›Reveal solutionSolution
Compute equilibrium moles from the stoichiometry of H2+I2→2HI, get K for the forward reaction, then invert it since the question asks for K of the reverse reaction 2HI⇌H2+I2.
Concept and Intuition
When a reaction is written in reverse, its equilibrium constant is the reciprocal of the forward one: Kreverse=1/Kforward. Here the physical process described (15 mol H2 + 5.2 mol I2 forming 10 mol HI) is the FORWARD synthesis, but the constant asked for is for the reverse decomposition.
Step-by-Step Solution
- Reaction (as it physically occurred): H2+I2→2HI.
- To form 10 mol HI, by 1:1:2 stoichiometry, 5 mol H2 and 5 mol I2 must have reacted.
- Equilibrium amounts (in the same fixed volume, so ratios of moles work like concentrations): [H2]∝15−5=10, [I2]∝5.2−5=0.2, [HI]∝10.
- Kforward=[H2][I2][HI]2=10×0.2102=2100=50. …
- AP EAPCET 2026Set eng-2026-05-13-AN1 markMCQQ.At T (K) in a closed 2.0 L vessel, 1 mole of H2 and 2 moles of I2 are taken initially and allowed to react till equilibrium is established. At equilibrium, the number of moles of H2 is found to be 0.2. The equilibrium constant for the dissociation of HI is (A) 1.066×101 (B) 93.75×10−2 (C) 2.132×102 (D) 9.375×10−2
›Reveal solutionSolution
This tests ICE-table equilibrium computation and the reciprocal relationship between Kc of forward and reverse reactions.
Concept and Intuition
For H2(g)+I2(g)⇌2HI(g), the equilibrium constant for the reverse reaction (dissociation of HI, 2HI⇌H2+I2) is simply the reciprocal of the forward Kc, since reversing a reaction inverts its equilibrium expression.
Step-by-Step Solution
- Initial moles in 2.0 L: H2=1, I2=2.
- At equilibrium, H2=0.2 mol, so 1−0.2=0.8 mol of H2 reacted.
- By stoichiometry (1:1:2), I2 reacted =0.8 mol, remaining I2=2−0.8=1.2 mol; HI formed =2×0.8=1.6 mol.
- Concentrations (divide by 2 L): [H2]=0.1 M, [I2]=0.6 M, [HI]=0.8 M.
- Kc for formation (H2+I2→2HI): Kc=[H2][I2][HI]2=0.060.64=10.667. …
- AP EAPCET 2026Set eng-2026-05-15-AN1 markMCQQ.At T(K) in a 10.0 L flask, 2 moles of N2O4(g) was heated. At equilibrium 20% of N2O4(g) was dissociated into NO2(g). What is the Kc for this reaction? (A) 2×10−2 (B) 4×10−2 (C) 3×10−2 (D) 6×10−2
›Reveal solutionSolution
This tests setting up an ICE table for the dissociation equilibrium N2O4⇌2NO2 using a given degree of dissociation, then computing Kc. The value is 4×10−2.
Concept and Intuition
Degree of dissociation α tells us what fraction of the initial moles broke apart at equilibrium. For N2O4⇌2NO2, each mole of N2O4 that dissociates produces 2 moles of NO2, so the ICE table must track this 1:2 stoichiometry carefully before computing concentrations and Kc.
Step-by-Step Solution
- Initial concentration: [N2O4]0=10 L2 mol=0.2 M.
- Let α=0.20 (20% dissociated). At equilibrium:
- [N2O4]=0.2(1−α)=0.2×0.8=0.16 M
- [NO2]=2×0.2×α=2×0.2×0.2=0.08 M …
- AP EAPCET 2026Set eng-2026-05-15-FN1 markMCQQ.Observe the following reaction A(g)+B(g)⇌T(K)C(g) In a 1L closed flask, 2 moles of A(g) and 1 mole of B(g) were taken and heated to T(K). At equilibrium, the concentration of B(g) is equal to twice the equilibrium concentration of C(g). What is the value of Kc? (A) 0.3 (B) 0.6 (C) 1.2 (D) 1.5
›Reveal solutionSolution
Setting up an ICE table with the given constraint [B]=2[C] solves for the extent of reaction, giving Kc=0.3.
Concept and Intuition
For A(g)+B(g)⇌C(g) in a 1 L flask, all concentrations equal mole numbers. Using one algebraic constraint from the problem (here, [B]=2[C] at equilibrium) lets us solve for the single unknown extent of reaction x, then substitute into the Kc expression.
Step-by-Step Solution
- Initial: [A]0=2, [B]0=1, [C]0=0 (mol/L, since V=1L).
- Let x mol/L of A and B react to form x mol/L of C (1:1:1 stoichiometry).
- At equilibrium: [A]=2−x, [B]=1−x, [C]=x.
- Given constraint: [B]=2[C]⇒1−x=2x⇒1=3x⇒x=31.
- So [A]=2−31=35, [B]=1−31=32, [C]=31. …
- AP EAPCET 2026Set eng-2026-05-18-FN1 markMCQQ.At 450 K, for the reaction 2SO2(g)+O2(g)⇌2SO3(g), the value of Kp is 2.0×1010. What is the value of Kc for the decomposition of sulphur trioxide at the same temperature? [R=0.083 L bar mol−1K−1] (A) 7.47×1011 (B) 1.34×10−12 (C) 7.47×10−12 (D) 1.34×1012
›Reveal solutionSolution
Convert Kp to Kc for the formation reaction using Kp=Kc(RT)Δn, then invert for the reverse (decomposition) reaction to get Kc′≈1.34×10−12.
Concept and Intuition
Kp and Kc are related through the change in moles of gas, Δng, via Kp=Kc(RT)Δng. The equilibrium constant of a reverse reaction is simply the reciprocal of the forward reaction's constant, since reversing a reaction flips which side is "products."
Step-by-Step Solution
- Forward (formation) reaction: 2SO2(g)+O2(g)⇌2SO3(g), Δng=2−(2+1)=−1.
- Kp=Kc(RT)Δng=Kc(RT)−1⇒Kc=Kp×RT.
- RT=0.083×450=37.35 (L bar/mol, matching Kp's bar units).
- Kc(formation)=2.0×1010×37.35=7.47×1011. …
- AP EAPCET 2026Set ap-2026-05-19-FN1 markMCQQ.Observe the following reaction A(g)⇌T(K)B(g)+C(g) One mole of A(g) was taken in a 1L closed flask and heated to T(K). At equilibrium, the concentration of A(g) is equal to four times the equilibrium concentration of B(g). What is Kc? (A) 0.02 (B) 0.04 (C) 0.05 (D) 0.06
›Reveal solutionSolution
A standard ICE-table equilibrium problem: express everything in terms of one unknown extent of reaction, use the given ratio to pin it down, then plug into Kc.
Concept and Intuition
Since A dissociates 1:1:1 into B and C, if x is the concentration of B (equal to C, since they're produced in equal amounts) at equilibrium, then the amount of A consumed is also x, leaving [A]=1−x. The condition "[A] is four times [B]" pins down x directly.
Step-by-Step Solution
- Initial: [A]0=1 mol/L (1 mol in 1 L), [B]0=[C]0=0.
- At equilibrium: [A]=1−x, [B]=[C]=x.
- Given [A]=4[B]: 1−x=4x⇒x=0.2.
- So [A]=0.8, [B]=[C]=0.2. …
- AP EAPCET 2026Set ap-2026-05-19-AN1 markMCQQ.At 300 K, Kc for the reaction 2A(g)⇌B(g)+2C(g) is 10−3 mol L−1. What is its Kp in bar? (R=0.08 L bar mol−1K−1) (A) 10−3 (B) 2.4×10−3 (C) 10−2 (D) 2.4×10−2
›Reveal solutionSolution
Convert Kc to Kp using Kp=Kc(RT)Δng; here Δng=1, giving Kp=2.4×10−2 bar.
Concept and Intuition
Kp and Kc differ because pressure and concentration units scale differently with moles of gas; the relation Kp=Kc(RT)Δng (with Δng = moles gaseous products − moles gaseous reactants) converts between them, using R in units matching the desired pressure unit (bar here).
Step-by-Step Solution
- Reaction: 2A(g)⇌B(g)+2C(g).
- Δng=(1+2)−2=1.
- Kp=Kc(RT)Δng=10−3 mol L−1×(0.08 L bar mol−1K−1×300 K)1. …
- AP EAPCET 2026Set eng-2026-05-18-AN1 markMCQQ.At T(K), in a 10 L flask, the following equilibrium is established 2SO2(g)+O2(g)⇌2SO3(g) The value of Kc for this reaction is 100. At equilibrium, the number of moles of SO3(g) is equal to twice the number of moles of SO2(g). What is the number of moles of O2(g) at equilibrium? (A) 0.04 (B) 0.4 (C) 0.02 (D) 0.2
›Reveal solutionSolution
Express all equilibrium concentrations in terms of moles of SO2 using the given ratio, plug into Kc, and solve for moles of O2; the volume terms conveniently cancel to give Kc=40/n(O2).
Concept and Intuition
The equilibrium constant Kc relates the concentrations (mol/L) of products and reactants, so we must divide every mole quantity by the flask volume (10 L) before substituting. Using the given relationship between moles of SO3 and SO2 lets us express everything in one unknown.
Step-by-Step Solution
- Reaction: 2SO2(g)+O2(g)⇌2SO3(g), Kc=100, V=10 L.
- Let moles of SO2 at equilibrium =a. Given moles of SO3=2a. Let moles of O2=b.
- Concentrations: [SO2]=a/10, [SO3]=2a/10=a/5, [O2]=b/10. …
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