3.4 Equilibrium Constant (Kc) — IAL Edexcel AS Chemistry

এই chapter টা শুনতে কঠিন লাগলেও আসলে খুব logical। Equilibrium constant Kc basically বলে দেয় reaction equilibrium অবস্থায় কোন দিকটা বেশি favoured। চল casual Bangla তে clean করে বুঝি।

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What is Chemical Equilibrium?

Chemical equilibrium তখন হয় যখন:

• Forward reaction rate = backward reaction rate
• Concentration constant থাকে (zero না)

মানে reaction থামে না, just দুই দিক same speed এ চলে।

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What is Equilibrium Constant (Kc)?

Kc হলো equilibrium অবস্থায় products আর reactants এর concentration এর ratio। General reaction:

$$\mathrm{aA}+\mathrm{bB}\rightleftarrows \mathrm{cC}+\mathrm{dD}$$

Kc expression:

$$Kc=\frac{{\left[}^C}{{\left[}^D}{\left[}^A{\left[}^B$$

Simple rule: Products উপরে, reactants নিচে।

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Important Rules for Writing Kc

• Only aqueous (aq) and gaseous (g) species include হবে
• Solid (s) আর pure liquid (l) include হবে না
• Power = balancing coefficient

Exam-এ solid/liquid ঢুকালে direct mark loss।

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Meaning of Kc Value

• Kc > 1 → products favoured
• Kc < 1 → reactants favoured
• Kc ≈ 1 → noticeable amount of both

Bangla hint: বড় Kc মানে reaction ডান দিকে যেতে চায়।

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Worked Example (Calculation)

For the reaction:

$$H2+I2\rightleftarrows 2\mathrm{HI}$$

Given equilibrium concentrations:

[H₂] = 0.20 mol dm⁻³
[I₂] = 0.20 mol dm⁻³
[HI] = 1.60 mol dm⁻³

Kc expression:

$$Kc=\frac{{\left[}^{\mathrm{HI}}}{\left[H2\right]\left[I2\right]}$$

Substitute values:

Kc = (1.60²) / (0.20 × 0.20) = 64

So products strongly favoured।

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Does Kc Change?

• Change in concentration → Kc unchanged
• Change in pressure/volume → Kc unchanged
• Change in temperature → Kc changes

Exam favourite line: Kc only depends on temperature।

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MCQ Practice (AS Level)

Q1. Which change will affect the value of Kc?

A) Increasing concentration
B) Adding catalyst
C) Increasing temperature
D) Changing pressure

Answer: C

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Q2. If Kc is very large, the equilibrium lies:

A) To the left
B) To the right
C) In the middle
D) At start only

Answer: B

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Exam-Style Structured Question

Q. Consider the equilibrium:

$$N2+3H2\rightleftarrows 2\mathrm{NH}3$$

(a) Write the expression for Kc.
(b) State what a large value of Kc indicates.
(c) Explain what happens to Kc if temperature is increased.

Worked Answer

(a)

$$Kc=\frac{{\left[}^{\mathrm{NH}}}{\left[N2\right]{\left[}^H}$$

(b) Large Kc মানে products favoured

(c) Temperature change করলে equilibrium shift হয়, তাই Kc change হয়

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Common Exam Mistakes

• Solid/liquid include করা
• Power ভুল দেওয়া
• Kc direction বলে instead of value explanation

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Final Revision Tip

Kc question দেখলেই: equation → Kc expression → value meaning → temperature check। এই flow ঠিক রাখলে marks আসবেই।