Collision Theory

Collision theory states that a reaction occurs only when particles collide with both sufficient energy (at least equal to the activation energy) and in the correct orientation so that reactive parts of the molecules actually meet.

A collision that satisfies both conditions is called an effective collision. If either condition is not met — even if the energy is high but orientation is wrong, or orientation is correct but energy is too low — the particles simply bounce apart unchanged.

The correct answer is 'Sufficient energy and correct orientation'. The other options mix in factors like pressure, concentration, and surface area, which affect how often collisions happen but are not the two conditions required within a single collision.

The question asks for the two conditions that must be met within a single collision for a reaction to occur — these are intrinsic to collision theory itself.

'High temperature and high pressure' are external factors that affect how frequently collisions happen. They are not conditions that describe what must happen during a collision itself, so this option is incorrect.

'Correct orientation and high concentration' is wrong because concentration affects the frequency of collisions, not whether a single collision is successful. It is not one of the two required conditions.

'Sufficient energy and high surface area' is incorrect because surface area is a factor for solid reactants that affects collision frequency, not a condition required within a single collision event.

'Sufficient energy and correct orientation' is the only option that correctly identifies both conditions intrinsic to collision theory for a successful reaction.

Replacing chips with powder changes the surface area of the solid marble. The total mass — and therefore the total amount of marble — stays the same.

Reactions between a solid and a liquid only occur at the surface of the solid. Grinding the solid into powder dramatically increases the total surface area, exposing far more CaCO₃ particles to the H⁺ ions in the acid.

With more particles exposed, collisions between H⁺ ions and CaCO₃ particles are more frequent. This increases the number of effective collisions per second, so the reaction rate increases.

The correct answer is 'The powder has a greater surface area, so more particles are exposed and collisions are more frequent.'

The question asks for the collision theory explanation for why powder reacts faster than chips — the key variable is the physical form of the solid.

'The powder dissolves in the acid, increasing the concentration' is incorrect. Marble (CaCO₃) is an insoluble solid — it does not dissolve to form a solution. The reaction occurs at its surface.

'The powder has a lower activation energy' is incorrect. Activation energy is a property of the chemical reaction itself, not the physical size or shape of the solid particles.

'The smaller pieces move faster and have more kinetic energy' is incorrect. The kinetic energy of solid particles depends on temperature, not their size. At the same temperature, chips and powder have the same average kinetic energy.

'The powder has a greater surface area, so more particles are exposed and collisions are more frequent' is the only correct collision theory explanation.