NEET 2025 UDAAN: Chemical Equilibrium | Physical Chemistry | Part 1 | Anushka Choudhary

Key Concepts

• Chemical Equilibrium: A state where the rate of forward and reverse reactions are equal.
• Physical Equilibrium: Equilibrium between different phases of the same substance.
• Ionic Equilibrium: Equilibrium involving ions in a solution.
• Irreversible Reactions: Reactions that proceed in only one direction and go to completion.
• Reversible Reactions: Reactions that can proceed in both forward and backward directions.
• Rate of Reaction: The speed at which a reaction occurs.
• Equilibrium Constant: A value that expresses the relationship between reactants and products at equilibrium.
• Le Chatelier's Principle: States that if a change of condition is applied to a system in equilibrium, the system will shift in a direction that relieves the stress.

Practical Applications of Equilibrium

• Respiration: The balance between oxygen intake and carbon dioxide output in the human body.
• Carbon Monoxide Poisoning: Disruption of equilibrium due to high levels of carbon monoxide.
• Cold Drinks: Equilibrium between dissolved carbon dioxide and gaseous carbon dioxide.
• Crystallization: Equilibrium between dissolved and solid phases of a substance.
• Haber Process: An industrial process for producing ammonia efficiently.

Types of Reactions: Reversible vs. Irreversible

Irreversible Reactions

• Definition: Reactions that cannot be reversed and proceed in only one direction.
• Characteristics:
  • Move in one direction only.
  • Always complete.
  • Only products are formed.
  • Take place in open containers.
  • Represented by a single arrow.
• Example: Combustion, boiling in an open container.
• Explanation for Open Container: If the product is a gas, it escapes, driving the reaction to completion.

Reversible Reactions

• Definition: Reactions that can proceed in both forward and backward directions simultaneously.
• Characteristics:
  • Proceed in both forward and backward directions.
  • Never get completed.
  • Attain equilibrium.
  • Products can form reactants again.
  • Take place in closed vessels.
  • Represented by a double arrow.

Defining Equilibrium: Rate vs. Amount

• Equilibrium Definition: At equilibrium, the rate of the forward reaction (RF) is equal to the rate of the backward reaction (RB).
  • RF = RB
• Important Clarification: The amounts (moles) of reactants and products at equilibrium are not necessarily equal, but their rates of change are equal. The amounts are constant.
• Common Misconception: Confusing equal rates with equal amounts.

Graphical Representation of Equilibrium

Rate vs. Time Graph

• Forward Rate: Starts high (dependent on reactant concentration) and decreases over time.
• Backward Rate: Starts low (dependent on product concentration) and increases over time.
• Equilibrium Point: The point where the forward and backward rates become equal and constant.

Concentration vs. Time Graph

• Reactant Concentration: Decreases over time and eventually becomes constant at equilibrium.
• Product Concentration: Increases over time and eventually becomes constant at equilibrium.
• Equilibrium Point: The point where both reactant and product concentrations become constant, not necessarily equal.
• Key Point: Equilibrium is achieved when concentrations become constant, regardless of whether they are equal.

Problem-Solving Approach for Equilibrium Problems

1. Write the Balanced Chemical Equation: Ensure the reaction is correctly balanced.
2. Initial Conditions: Identify the initial moles (or concentrations) of reactants and products.
3. Change (x): Define the change in moles (or concentration) of reactants and products in terms of 'x'.
4. Equilibrium Conditions: Express the equilibrium moles (or concentrations) in terms of initial conditions and 'x'.
5. Use Given Information: Use any given information about equilibrium concentrations to solve for 'x'.
6. Calculate Equilibrium Concentrations: Substitute the value of 'x' to find the equilibrium concentrations of all species.
7. Draw the Graph: Plot the concentration vs. time graph based on the calculated values, showing the concentrations becoming constant at equilibrium.

Example Problem and Solution

Reaction: N2 + 3H2 ⇌ 2NH3

Initial Conditions:

• N2: 4 moles
• H2: 8 moles
• Volume: 1 Liter

Equilibrium Condition: 2 moles of NH3 are present at equilibrium.

Solution:

1. ICE Table (Initial, Change, Equilibrium):

   | | N2 | 3H2 | 2NH3 | | :---------- | :---- | :---- | :---- | | Initial | 4 | 8 | 0 | | Change | -x | -3x | +2x | | Equilibrium | 4-x | 8-3x | 2x |

2. Solve for x:

   • 2x = 2 (Given)
   • x = 1

3. Equilibrium Concentrations:

   • N2: 4 - 1 = 3 moles
   • H2: 8 - 3(1) = 5 moles
   • NH3: 2 moles

4. Graph: Plot the concentrations of N2, H2, and NH3 over time, showing them reaching constant values of 3, 5, and 2 respectively.

Synthesis/Conclusion

The video provides a detailed explanation of chemical equilibrium, emphasizing the difference between rate and amount at equilibrium. It covers the practical applications of equilibrium, the types of reactions (reversible and irreversible), and the graphical representation of equilibrium. The instructor clarifies the common misconception that equilibrium implies equal amounts of reactants and products, highlighting that it is the rates of forward and backward reactions that are equal. The video also demonstrates a step-by-step approach to solving equilibrium problems, including setting up ICE tables and calculating equilibrium concentrations. The key takeaway is that understanding the concept of equilibrium is crucial for solving numerical problems and interpreting concentration vs. time graphs.