Characteristics of Equilibrium Constant (Kc): The 3 Golden Rules
In Chemical Equilibrium, examiners rarely ask you to calculate the Equilibrium Constant (Kc) from basic concentrations. Instead, they give you the Kc of one reaction and ask you to find the Kc of a heavily modified version of that reaction.
To solve these puzzles quickly without writing pages of algebra, you must master the three mathematical characteristics of the equilibrium constant. Let's decode them!
Video Tutorial: Manipulating Kc
Watch Abhishek Sengar sir from CHEMCA derive the three golden rules and apply them simultaneously to solve a complex multi-step equilibrium problem.
The 3 Golden Rules of Kc
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Reversing the Equation → INVERSE (1 / K):
If a chemical equation is written in the reverse direction, the new equilibrium constant becomes the reciprocal (inverse) of the original constant.
Original: A ⇌ B (Constant = K)
Reversed: B ⇌ A (New Constant = 1 / K) -
Adding Equations → MULTIPLY (K1 × K2):
If two or more chemical equations are added together to form a new net equation, the equilibrium constant for the net equation is the product of their individual constants.
Eq 1: A ⇌ B (Constant = K1)
Eq 2: C ⇌ D (Constant = K2)
Net (Eq 1 + Eq 2): A + C ⇌ B + D (New Constant = K1 × K2) -
Multiplying by a Number 'n' → POWER (Kn):
If an entire chemical equation is multiplied by a stoichiometric factor n, the new equilibrium constant is the original constant raised to the power of n.
Original: A ⇌ B (Constant = K)
Multiplied by n: nA ⇌ nB (New Constant = Kn)
Note: If you divide by 2, it's the same as multiplying by 1/2, so the new constant becomes K1/2 (which is the square root, √K).
Fig: A quick visual summary. Addition becomes Multiplication. Multiplication becomes a Power.
Practice Questions for JEE & NEET
Examiners will combine these rules into a single question. Let's see if you can apply multiple operations at once!
Question 1: Given the reaction: N2(g) + 3H2(g) ⇌ 2NH3(g) has an equilibrium constant K1 = 16.
Calculate the equilibrium constant (K2) for the reaction: NH3(g) ⇌ 1/2 N2(g) + 3/2 H2(g).
Answer: K2 = 0.25 (or 1/4).
Reasoning:
We must perform two operations on the original equation to get the new equation:
- Step 1 (Reverse): We need NH3 on the reactant side. Reversing the equation changes the constant to 1 / K1 = 1/16.
(2NH3(g) ⇌ N2(g) + 3H2(g)) - Step 2 (Multiply by 1/2): We only want 1 mole of NH3, not 2. So we multiply the entire reversed equation by 1/2.
Rule 3 says multiplying by n raises the constant to the power of n.
K2 = (1/16)1/2
K2 = √(1/16) = 1/4 = 0.25.
Question 2: If adding two chemical equations means you multiply their equilibrium constants (K1 × K2), what happens to the new equilibrium constant if you subtract Equation 2 from Equation 1?
Answer: You divide the constants (Knew = K1 / K2).
Reasoning:
Subtracting Equation 2 is mathematically identical to reversing Equation 2 and then adding it to Equation 1.
By Rule 1, reversing Eq 2 makes its constant 1 / K2.
By Rule 2, adding them together means you multiply their constants:
Knew = K1 × (1 / K2) = K1 / K2.
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