Rates of Reaction – Lecture Review

A set of question-and-answer flashcards reviewing all key concepts from the Rates of Reaction lecture, covering definitions, factors affecting rate, Boltzmann distribution, collision theory, catalysts, and experimental methods.

What is the definition of rate of reaction?

The rate of reaction is the change in concentration of a reactant or product per unit time.

What is the general formula for rate of reaction?

Rate = Δ[X] / Δt, where [X] is the concentration of a substance (mol dm⁻³) and Δt is the time interval in seconds.

Which five factors mainly affect the rate of a chemical reaction?

1. Concentration (or pressure for gases) 2. Temperature 3. Surface area (for solids) 4. Catalysts 5. Light (for photochemical reactions)

Why does increasing concentration increase reaction rate?

Higher concentration means more particles per unit volume, so collisions occur more frequently, leading to more successful collisions and a faster rate.

Why does increasing temperature increase reaction rate?

Particles gain kinetic energy; a greater proportion now have energy ≥ activation energy (Ea) and collisions occur more often and with higher energy, so the rate rises.

How does increasing gas pressure affect reaction rate?

Greater pressure increases particle concentration, causing more frequent collisions and therefore a higher reaction rate.

How does increasing the surface area of a solid reactant affect reaction rate?

A larger surface exposes more particles to collide, increasing collision frequency and speeding up the reaction.

What effect does a catalyst have on a reaction?

A catalyst provides an alternative pathway with lower activation energy (Ea), increasing the proportion of particles able to react while the catalyst itself is not consumed.

What is the Boltzmann distribution?

It is a graph showing the distribution of molecular energies in a sample of gas at a specific temperature.

What are three key features of the Boltzmann distribution curve?

1. Starts at the origin (no particles have zero energy) 2. Is asymptotic to the x-axis (never touches it) 3. The peak represents the most probable energy; the total area under the curve equals the number of particles.

In a Boltzmann distribution, what does the area above Ea represent?

The fraction of particles that have enough energy to react (energy ≥ activation energy).

How does increasing temperature change the Boltzmann distribution?

The curve flattens and shifts to higher energies; the peak lowers and moves right, increasing the area above Ea and thus the reaction rate.

How does the presence of a catalyst alter the Boltzmann distribution?

The activation energy line (Ea) moves left because Ea is lower, so a larger area of the curve lies above Ea, but the curve’s shape remains unchanged.

State the collision theory of chemical reactions.

Particles must collide with the correct orientation and with energy equal to or greater than the activation energy; only such successful collisions produce a reaction.

Define activation energy (Ea).

The minimum amount of energy that reacting particles must possess to successfully collide and form products.

List at least four experimental methods for measuring reaction rate.

Gas volume collection with a syringe, mass loss measurement, colorimetry for colour change, pH monitoring, titration to track concentration, or conductivity for ionic reactions.

What is meant by the initial rate of reaction and how is it determined?

The rate at time zero; it is obtained from the gradient of the tangent to a concentration-time (or volume-time) graph at t = 0.

Give two common units for reaction rate.

mol dm⁻³ s⁻¹ (or similar derived units depending on the reaction context).

How do catalysts and temperature each increase reaction rate?

Catalysts lower Ea, so a greater fraction of particles can react; higher temperature both raises collision frequency and gives particles more energy to exceed Ea.