Q.Which of the following statements is incorrect about the collision theory of chemical reaction?

The key idea is that collision theory requires both sufficient energy (≥ activation energy) and proper orientation for an effective collision — not just energy alone.

Step 1: Recall the two essential conditions for an effective collision:

1. Minimum kinetic energy equal to or greater than the threshold energy.
2. Correct spatial orientation at the moment of impact. Step 2: Evaluate each statement:

• (i) is correct — the theory treats particles as hard spheres. …

The collision theory of chemical reactions is a beautifully simple model. It imagines reactant molecules as hard, billiard-ball-like spheres that bounce off each other. Not every bounce leads to a reaction — only those that meet two strict conditions. The key insight is that both energy and geometry matter. If you only check for energy, you miss half the story.

Let’s examine each statement carefully.

1. Statement (i): “It considers reacting molecules or atoms to be hard spheres and ignores their structural features.”

   This is exactly what collision theory does. It treats molecules as structureless spheres, ignoring bond angles, electron clouds, and internal vibrations. This is a simplification — and it’s correct as a description of the theory. So (i) is a true statement.

2. Statement (ii): “Number of effective collisions determines the rate of reaction.”

   The rate of a reaction is proportional to the number of collisions per second that actually lead to product formation — these are called effective collisions. More effective collisions mean a faster reaction. This is a core idea of collision theory. So (ii) is true.

3. Statement (iii): “Collision of atoms or molecules possessing sufficient threshold energy results into the product formation.”

   Here’s the trap. Threshold energy (or activation energy) is necessary — but it is not sufficient. Two molecules can smash together with enormous energy, yet if they hit each other the wrong way (e.g., a chlorine atom hitting the wrong end of a methane molecule), they simply bounce apart. The theory explicitly requires proper orientation as well. Statement (iii) says only energy is needed, which is incomplete. So (iii) is incorrect. …

Concept: Collision Theory of Chemical Reactions

Collision theory explains how chemical reactions occur and why reaction rates differ. It assumes that molecules must collide to react, but not all collisions lead to a reaction.

Method: Statement Verification by Elimination

Steps:

1. Recall the core postulates of collision theory

   • Reacting species are treated as hard spheres (structural features are ignored).
   • Reactions occur only when molecules collide with sufficient energy (≥ threshold energy) and proper orientation.
   • The rate of reaction is directly proportional to the number of effective collisions per unit time.

2. Analyze each statement against these postulates

   • (i) "It considers reacting molecules or atoms to be hard spheres and ignores their structural features."

     → Correct. This is a key assumption of collision theory.

   • (ii) "Number of effective collisions determines the rate of reaction."

     → Correct. Rate ∝ number of effective collisions.

   • (iii) "Collision of atoms or molecules possessing sufficient threshold energy results into the product formation." …

Here are the common mistakes students make with this question on collision theory, along with how to avoid each.

Mistake 1: Confusing "Threshold Energy" with "Activation Energy"

• The Error: Students often think statement (iii) is correct because they remember "molecules need activation energy to react." They miss that the statement says threshold energy, not activation energy.
• Why It’s Wrong: Threshold energy is the minimum total energy that colliding molecules must possess. Activation energy ($E_a$) is the extra energy above the average energy of reactants needed to reach the transition state. The statement in (iii) is incorrect because it says "collision of molecules possessing sufficient threshold energy results in product formation." In reality, they need activation energy, not just threshold energy. Also, even with sufficient energy, they need proper orientation — which (iii) omits.
• How to Avoid: Always distinguish:
  • Threshold energy = minimum energy for reaction.
  • Activation energy = threshold energy – average energy of reactants.
  • Remember: Sufficient energy + correct orientation = effective collision.

Mistake 2: Thinking Statement (i) is Incorrect

• The Error: Students think collision theory does consider molecular structure, so they mark (i) as the incorrect statement.
• Why It’s Wrong: Collision theory does treat molecules as hard spheres and ignores structural features (like bond angles, electron clouds). That’s a known limitation of the theory. Statement (i) is correct.
• How to Avoid: Memorise the key assumptions of collision theory:
  • Molecules are hard spheres.
  • No internal structure considered.
  • Only collisions with sufficient energy and proper orientation lead to reaction.

Mistake 3: Overlooking the "Proper Orientation" Requirement in (iv)

• The Error: Students think (iv) is incorrect because they believe only energy matters, not orientation.
• Why It’s Wrong: Statement (iv) is correct. For a collision to be effective, both sufficient threshold energy and proper orientation are required. Without correct orientation, even high-energy collisions may not break the right bonds. …