5.2 Factors Affecting Rate of Reactions

Reaction Rate

• The change in concentration of a reactant or a product of a chemical reaction per unit time

What are the five factors that affect the rate of the reaction?

• Chemical Nature of Reactants

• Concentration of Reactants

• Surface Area

• Temperature of the Reaction System

• Presence of a Catalyst

Chemical Nature of Reactants

• The chemical nature of a pure substance determines its chemical properties.

• A chemical property describes how a pure substance behaves during a chemical change or reaction.

• Examples of chemical properties include a substance's tendency to undergo combustion or oxidation reactions.

• Chemical properties can strongly influence the reaction rate.

• Oxidation reactions demonstrate differences among metals:

  • Sodium and potassium are highly reactive with oxygen and other substances, reacting so quickly they are never found naturally in their pure elemental form.

  • Platinum, gold, and silver are generally unreactive, and oxidation occurs very slowly.

• Unreactive metals like platinum, gold, and silver are ideal for jewellery and electronics due to their resistance to oxidation.

Concentration of Reactants

• Hospitals warn against smoking or flames near oxygen because:

  • Normal air is about 20% oxygen.

  • Medical oxygen tanks can be 50% oxygen or higher.

  • Higher oxygen concentrations cause combustion to be rapid, violent, or explosive.

• Reaction rates often increase with higher reactant concentrations.

• Example: Zinc reacts with hydrochloric acid to form zinc chloride and hydrogen gas:

  • Equation: Zn(s) + 2 HCl(aq) → ZnCl₂(aq) + H₂(g)

• If you place zinc in increasingly concentrated hydrochloric acid:

  • Reaction is slow in dilute acid.

  • Reaction rate increases as acid concentration increases.

Surface Area

• Reaction rate can be increased by increasing the surface area of a solid reactant.

  • Example: Cutting zinc into smaller pieces speeds up its reaction with hydrochloric acid.

• Similar to how powdered sugar dissolves faster in water than sugar cubes.

• In reactions involving solids and liquids, more surface area leads to a faster reaction.

  • Only atoms or ions on the surface of a solid can react.

  • Increasing surface area is like increasing concentration for solids.

• Kindling burns faster than a log due to its higher surface area.

• Fine flour dust can combust so quickly that it may cause explosions.

• Workplaces handling fine particles (e.g., flour mills) must control dust to prevent fires and protect workers.

Temperature

• Reaction rates increase with temperature.

  • Many reactions double in rate for every 10 °C increase, and halve for every 10 °C decrease.

• Example: Cake batter changes very little at room temperature but undergoes reactions and bakes at 175 °C.

• Refrigeration slows down chemical reactions that cause food spoilage.

• Medications often include instructions to store in cool places to slow unwanted reactions.

• Cold-blooded animals (e.g. snakes, reptiles):

  • Take on the temperature of their surroundings.

  • Are less active in cooler conditions due to slower metabolic rates.

• Infrared images show cold-blooded animals as cooler colors (blue/black) and warm-blooded animals as warmer colors (yellow/orange).

Catalysts

A catalyst speeds up a chemical reaction but remains unchanged after the reaction.

• Can be reused many times.

• Only small amounts are typically needed.

  Biological Catalysts (Enzymes):

  • Enzymes are protein molecules that act as biocatalysts.

    • Enable reactions in cells to occur at moderate temperatures.

  • Example: Lactase helps digest lactose in milk.

    • Adults often produce less lactase → lactose intolerance.

  • Used in the production of:

    • Beer, cheese, yogurt, medicines, and enzyme-based detergents.

  • Most enzymes are made via fermentation using bacteria, yeast, or moulds.

  • Enzymes are sensitive to temperature and pH, so research focuses on making them more stable for industrial use.

  • Benefits of catalysts:

    • Speed up reactions.

    • Lower required temperatures → less energy used, less environmental harm.