How fast? The rate of chemical change practice questions.

Which change would NOT increase the rate of a reaction between a solid and a solution?

• increasing the temperature
• increasing the concentration of the reactant in the solution
• adding a suitable catalyst
• using larger lumps of the solid instead of a powder

Answer: using larger lumps of the solid instead of a powder

Larger lumps have a smaller surface area, giving fewer collisions per second, so the rate decreases rather than increases.

Identify how the average rate of a reaction that produces a gas can be determined.

• The temperature of the reaction mixture measured at the very end
• The volume of gas collected divided by the time taken
• The mass of solid catalyst added
• The final concentration of the products only

Answer: The volume of gas collected divided by the time taken

Average rate = change in a measurable quantity (here, gas volume) divided by the time interval over which it changes.

According to collision theory, determine which condition is required for a successful reactive collision.

• Particles collide in absolutely any orientation at all, provided that they possess at least a little kinetic energy
• Particles collide with energy equal to or greater than the activation energy and with correct orientation
• Particles collide with maximum kinetic energy only
• Particles simply come into contact regardless of speed

Answer: Particles collide with energy equal to or greater than the activation energy and with correct orientation

A collision leads to reaction only if colliding particles have energy ≥ activation energy AND the correct geometric orientation.

A reaction has rate = k[A]²[B]. Determine the units of the rate constant k when rate is in mol dm⁻³ s⁻¹ and concentrations in mol dm⁻³.

• s⁻¹
• mol⁻¹ dm³ s⁻¹
• mol⁻² dm⁶ s⁻¹
• mol dm⁻³ s⁻¹

Answer: mol⁻² dm⁶ s⁻¹

Overall order is 3, so units of k = (mol dm⁻³ s⁻¹)/(mol dm⁻³)³ = mol⁻² dm⁶ s⁻¹.

For the rate law rate = k[X]⁰, determine how the rate changes when [X] is tripled.

• The rate triples
• The rate increases ninefold
• The rate is unchanged
• The rate falls to one third

Answer: The rate is unchanged

A reaction that is zero order in X does not depend on [X], so tripling [X] leaves the rate unchanged.

Identify the main reason why a small rise in temperature produces a large increase in the rate of a reaction.

• A much larger fraction of collisions reaches the activation energy
• The activation energy of the reaction is lowered as the temperature rises
• The particles are closer together and so collide more frequently
• Every collision becomes a successful one once the gas is heated

Answer: A much larger fraction of collisions reaches the activation energy

Heating shifts the Maxwell–Boltzmann distribution so that the area beyond Eₐ grows sharply, which matters far more than the modest increase in collision frequency. Temperature never changes Eₐ itself — only a catalyst offers a route with a lower Eₐ.