Video Lesson

Lesson Objective

At the end of this section, you will be able to:

• List factors that affect the rate of a chemical reaction
• Describe the effects of changes in temperature, concentration or pressure and surface area on the rates of a chemical reactions
• Explain the effect of catalysts on the rates of chemical reaction
• Do an experiment on how the factors affect the rate of chemical reaction.

Brainstorming question

How does temperature affect the rate of a chemical reaction, and why does this relationship exist?

In what ways does concentration impact the speed of a chemical reaction, and how does this vary with different reaction orders?

What is the role of a catalyst in a chemical reaction, and how does it alter the reaction mechanism?

• How is the burning of charcoal affected by
  • a. increasing the amount of air used
  • b. adding more charcoal
• How can you increase the rate of combustion of a given block of wood?
• Why do sugar crystals dissolve more easily in hot water than in cold water?
• Why does powdered sugar dissolved easily as compared to equivalent amounts of sugar crystals?
  

Explore the effect of temperature on the rate constant and molecular collision energy.

Consider how changes in concentration influence reaction rates for zero, first, and second-order reactions.

Discuss how catalysts lower activation energy and affect reaction pathways.

Key terms/Concepts

4.3.Factor Affecting the rate of a Chemical Reactions

The rates at which reactants are consumed and products are formed during chemical
reactions vary greatly. Even a chemical reaction involving the same reactants may
have different rates under different conditions.

Change in temperature, concentration,
nature of reactant, surface area and presence of a catalyst, result in changes in rate of
reactions

4.3.1 Nature of the Reactants

The nature of the reactants plays a crucial role in determining the rate of a chemical reaction. This refers to the inherent properties of the substances involved, such as their chemical structure, bond types, and state of matter. Different reactants can react at different rates even under the same conditions.

The chemical nature of the reactants themselves affects how quickly a reaction proceeds. Reactions involving simpler molecules, ionic compounds, or gases often occur faster than those involving complex molecules, covalent compounds, or solids.

Example: Methane (CH₄) reacts quickly with oxygen during combustion compared to larger hydrocarbons like octane (C₈H₁₈).

4.3.2. Concentration of the reactants

Increasing the concentration of reactants usually increases the rate of reaction. This is because there are more particles in a given volume, leading to more frequent collisions between reactant molecules.

Example: When you increase the concentration of hydrochloric acid (HCl) in a reaction with magnesium (Mg), the reaction produces hydrogen gas (H₂) more rapidly. This is because more acid particles are available to collide with the magnesium, increasing the reaction rate.

4.3.3 Temperature of Reactants

Raising the temperature generally increases the reaction rate. Higher temperatures provide reactant particles with more kinetic energy, resulting in more frequent and energetic collision.

Example: In cooking, higher temperatures speed up the browning of meat due to faster Maillard reactions. This occurs because heat provides more energy to the reactant molecules, increasing their collision frequency and the likelihood of a reaction.

4.3.4. Surface Area

For reactions involving solids, increasing the surface area (e.g., by grinding the solid into a powder) exposes more reactant particles to the other reactant, thereby increasing the reaction rate.

Example: Finely powdered sugar burns faster than a sugar cube when exposed to a flame. The increased surface area of the powder allows more oxygen molecules to collide with the sugar at once, accelerating the combustion reaction.

4.3.5. Pressure

In reactions involving gases, increasing the pressure increases the concentration of gas molecules, leading to more frequent collisions and a faster reaction rate.

Example: In the synthesis of ammonia (NH₃) via the Haber process, increasing the pressure of nitrogen (N₂) and hydrogen (H₂) gases increases the reaction rate. Higher pressure forces gas molecules closer together, leading to more frequent collisions.

4.3.6. Catalyst

Catalysts speed up reactions by providing an alternative pathway with a lower activation energy, allowing more collisions to result in a reaction without being consumed in the process.

Example: The decomposition of hydrogen peroxide (H₂O₂) into water and oxygen is slow on its own but speeds up significantly in the presence of manganese dioxide (MnO₂) as a catalyst. The catalyst lowers the activation energy needed for the reaction.