Chemical Equilibrium is one of the common terms when it about Chemistry. This chapter holds a significant weightage of almost 4-5% and different styles of questions are asked in NEET. Like direct questions on reactions or sometimes questions required to be solved using formulas. Also Check NEET Chemistry Syllabus

Chemical Equilibrium is an important chapter in exams like NEET, etc. which can’t be ignored at all. Go through the detailed article which will provide you a detailed idea about chemical equilibrium.

What is Equilibrium?

Equilibrium is the state of a process where the properties such as temperature, pressure, and concentration among others of the system don’t show any change even with time. All the processes attaining equilibrium, two opposing processes are involved and Equilibrium is set to be achieved when the rates of the two opposing processes become the same.

• Physical Equilibrium: In case the opposing processes involve only physical changes, the equilibrium will be termed as Physical Equilibrium.
• Chemical Equilibrium: In case the opposing processes are chemical reactions, then the equilibrium will be termed as Chemical Equilibrium.

Physical Equilibrium

• Solid – liquid Equilibrium: H2O(s) H2O(l)
• Liquid – Gas Equilibrium: H2O(l) H2O(g)
• Solid – Solution Equilibrium: Salt(Solid) Salt(in solution)
• Gas –Solution equilibrium: CO2(g) CO2(in solution)

Some Important Questions

Ques: When the system A + B C + D is at equilibrium, choose the correct option?

1. The sum of the concentrations of A and B will be equal to the sum of the concentrations of C and D.
2. The forward reaction will stop.
3. Both, the forward and the reverse reactions have stopped.
4. The reverse reaction has stopped.
5. Nor the forward neither the reverse reaction has been stopped.

The correct option is E

Ques: 2SO3(g) 2SO2(g) + O2(g). The conventional equilibrium constant expression (Kc) for the system mentioned in the equation will be represented as:

[SO2]2/[SO3]

1. [SO2]2[O2]/[SO3]2
2. [SO3]2/[SO3]2[O2]
3. [SO2][O2]
4. none of these

The correct option is B

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What will be the Equilibrium in Chemical Process?

Reversible reaction: Reversible reactions are those reaction where not only the reactants react to form the products under different conditions but also the products also react to form reactants under the same conditions.

For instance: 3Fe(s) + 4H2O(g) Fe3O4(s) + 4H2(g).

Irreversible reaction: A reaction can’t take place in the reverse direction, which similar means that products formed don’t react to give back the reactants under the same condition.

Example: AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(g)

The formula representing a chemical equilibrium is A+bB cC+bD , where A, B are reactants while C and D are the products.

Quick Notes on Chemical Equilibrium

• The double arrow between the left-hand part and right-hand part display that the changes take place in both the directions.
• Based on the extent of reaction, before equilibrium is attained chemical reactions might be categorized into three categories. First those reactions where the process is almost completed. Second reactions, which proceed to almost only up to a little extent. And third reactions which proceed to such an extent, that the concentrations of reactants and products at equilibrium can be compared.
• The equilibrium state is dynamic nor static. The reaction will be attain equilibrium when the rate of the forward reaction will be sequal to the backward reaction.

Sample Questions

Ques: Consider the following reversible reaction. In a 3.00 liter container, some of the amounts that are found in equilibrium at 400 oC: 0.0420 mole N2, 0.516 mole H2 and 0.0357 mole NH3 would be. You need to calculate Kc. N2(g) + 3H2(g) 2NH3(g)

1. 0.202
2. 1.99
3. 16.0
4. 4.94
5. 0.503

The correct option is B

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Characteristics of Equilibrium State

• Equilibrium State can be achieved only in case the reversible reaction is carried out in closed vessel.
• The state can be achieved from any side of the reaction.
• A catalyst can speed up the approach of equilibrium but will not make any change in eth state of equilibrium.
• It is dynamic in nature, the reaction will not stop; however both forward and backward reactions will take place at equal rate.
• Change of pressure, concentration or temperature favours one of the reactions (forward or backward) resulting in shift of equilibrium point in one direction.

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Law of Mass Action & Equilibrium Constant

The law states that the rate at which a substance reacts would be directly proportional to its active mass; rate of a chemical reaction will be directly proportional to the product of active masses of reactants, where each will be increased to a power equal to the corresponding stoichiometric coefficient that appears in the balanced chemical equation.

Fore reaction aA+bBcC+bD

Rate of reaction ∝ [A]a.[B]b

or rate of reaction = K[A]a[B]b

Here K is the constant rate or velocity constant of the reaction at a particular temperature.

Unit of rate constant (K)

(Here n is order of reaction.)

For the unit concentration of reactants rate of the reaction will be equal to the rate constant or specific reaction rate.

Note: Active mass is defined as the molar concentration of the reacting substances that take part in the reaction.

Thus,

Active mass will be equal to the number of moles/volume in litres

Active mass of solid is taken as unity.

Also, Active mass of reactant (a) = Conc. × activity coefficient

i.e. a = Molarity × f for dilute solution f = 1

Applying Law of mass action for general reversible reaction would be aA + bB cC + dD

Rate of forward reaction ∝ [A]a[B]b

or Rf = Kf [A]b [B]b

Similarly for backward reaction

Rb = Kb[C]c [D]d

At equilibrium Kf[A]a[B]b = Kb[C]c[D]d

The above equation is known as the equilibrium equation and Kc is known as equilibrium constant. The equilibrium constant for the reverse reaction is the inverse of the equilibrium constant for the reaction in the forward direction.

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The Le–Chatelier’s Principle

Statement: When a chemical reaction at equilibrium is subjected to any stress, the equilibrium will get shifted in that direction where the stress effect is lower.

What will be the effects of addition of Inert Gases:

• If there is an addition of inert gas at constant volume, there will be no effect on equilibrium.
• If there is an addition of inert gas at constant pressure, Equilibrium will get shifted in a direction where there is surge in total moles of gases.

Effect of Change in Temperature:

• In a system at equilibrium, both exothermic and endothermic reactions will occur at the same time.
• With a surge in temperature, the equilibrium will shift in the direction of the endothermic reaction.
• While a decrease in temperature will result in a shift in sthe equilibrium in the direction of exothermic reaction.

Effect of Change in Concentration:

• When the concentration of reactants increases, the equilibrium will shift towards the forward direction.
• While when the concentration of the products is reduced, the equilibrium will be shifted towards the backward direction.

Effect of Change in Pressure:

• The surge in pressure will shift the equilibrium in the direction of lesser number of gaseous molecules.
• The decrease in pressure will shift the equilibrium in the direction where more number of gaseous molecules are present.

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Kinds of Acids and Bases

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Common Ion effect

Along with an ion to equilibrium, in case there are same ion it will make the equilibrium `reaction move in a direction to consume that ion.

This clearly indicate that water will dissociate lesser under the presence of HCl.

Buffer Solutions: Resists change in its pH when a change happens due to the addition of a small amount of acid or base is being referred to as buffer solution.

Weak acid–Salt buffer

It is created by a combination of a weak acid and a salt of the acid with a strong base. For example, CH3COOH & CH3COONa pH = pKa + log [Conjugated base]/ [Acid]

For weak dibasic acid, pH = (pKa1 + pKa2 )/2

Here pKa1 and pKa2 are 1st and 2nd dissociation constant of the acid.

Weak base–salt buffer

It is formed by combination of a weak base and a salt of the base with a strong acid. Example, NH4OH and NH4Cl pH = pKb + log [Conjugated acid]/ [Base]

Solubility and Solubility Product

Amount (moles) of the salt that has made the solution saturated per liter of solution is referred to as the solubility of the salt. For salt AB, AB ↔A+ +B-, Ksp = [A+][B-]

Solution that don’t have the product of the concentration of the ions more than KSP of the salt in solution.

Ionic Product

1. Product of ionic concentration mainly due to ions that are already present in water or from a salt.
2. I.P. might bse or might not be equal to Ksp.
3. If ionic Product (IP) Is lesser than Ksp ; precipitation will occur till I.P. become equals to Ksp
4. If Ionic Product is more than Ksp; a precipitate will not be formed and the solution will be unsaturated.
5. If Ionic Product will be equal to Ksp; a precipitate will not be create an the solution is satuated in that salt.

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Previous Year Solved Sample Question from Chemical Equilibrium

Ques: The equilibrium is constant at 427oC for the reaction: N2(g) + 3H2(g) 2NH3(g) is Kp = 9.4 x 10-5. What will be the worth of Go in the reaction at 427o.

1. -33 kJ
2. -54 kJ
3. 54 kJ
4. 33 kJ
5. 1.3 J

The correct option is C

Ques: For a particular reaction, which of the beneath statements can be made about K, the equilibrium will be constant?

1. Remains same at different reaction conditions.
2. Concentration increases if one of the products is increased.
3. It changes with changes in the temperature.
4. It increases if the concentration of one of the reactants is improved.
5. It might be altered by the addition of a catalyst.

The correct option is C

Ques: Let us consider that the equilibrium is constant for a reaction A + 2B C + 5/2 D which has a value of 4.0. Calculate the value of the equilibrium constant for the reaction 2C + 5D 2A + 4B if the temperature remains the same?

1. 0.25
2. 0.063
3. 2.0
4. 8.0
5. 16

The correct option is B

Ques: What will be the pH of a solution when 0.01 M HCl and 0.1 M NaOH are mixed in equal volumes?

1. 12.65
2. 1.04
3. 7.0
4. 2.0

The correct option is A

Ques: From the following aqueous solutions which one is the best conductor of electricity?

1. NH3
2. CH3COOH
3. HCl
4. C6H12O6

The correct option is C

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Important Formulas for Chemical Equilibrium

What will be relations between KC and KP for different types of reactions

• When n = 0, Kp = Kc e.g. for reaction A B.

• When n = +ve, Kp > Kc e.g. for reaction A 2B.

• When n = –ve, Kp < Kc e.g. for reaction 2A B.

Synthesis of ammonia (Haber's process)

• N2 (g) + 3H2 (g) 2NH3 (g) + 22.4 kcal
• Low temperature (500°C)
• High pressure (200 – 1000 atm.)
• Excess of N2 and H2

Synthesis of NO (nitric acid birkland eyde process)

• N2 (g) + O2 (g) 2NO(g)– 43.2 kcal
• High temperature
• Excess of N2 and O2
• No effect of pressure

Formation of SO3 (sulphuric acid contact process)

• 2SO2 (g) + O2 (g) 2SO3 + 42.0 kcal
• Low temperature
• High pressure
• Excess of SO2 and O2

Formation of nitrogen dioxide

• 2NO + O2 2NO2 + 27.8 kcal
• Les temperature
• More pressure
• Excess of NO and O2

Dissociation of nitrogen tetraoxide

• N2O4 2NO2 – 14 kcal
• High temperature
• Low pressure
• Excess of N2O4

Oxidation of CO by steam (Bosch process)

• CO + H2O CO2 + H2 + x kcal
• Low temperature
• Excess of steam and CO
• No effect of pressure

Dissociation of PCl5

• PCl5 PCl3 + Cl2 – 15 kcal
• More temperature
• Low pressure
• Excess of PCl5

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Some useful tips to prepare for Chemical Equilibrium

• Prepare a proper time-table and study accordingly for the same. Check NEET Study Plan
• Develop a habit of preparing notes that can be revised later.
• Give more mock tests as they help keep track of your performance and will help make all the concepts clear.
• Make a note and understand how the concepts of chemical equilibrium operated.
• Give a thorough read on all the sub-sections and understand the significance of the chemical equilibrium. Read More NEET 2021 Preparation