Reaction Rate

1. Topic 8 Reaction Rate

2. Chemical Reaction Rates • Chemical reactions can occur at different speeds. • Why does one reaction take longer than another? • Can you speed up or slow down reactions?

3. Four ways to Speed up Reactions • Increase Temperature • Increase Surface Area • Increase Concentration • Add a catalyst

4. Temperature • Increasing the temperature increases the rate of reaction • Why? At higher temperatures, molecules move faster and collide more frequently. • Therefore, new substances can be formed more quickly. • Example: Cooking • The amount of chemical reactions in the liquid batter are increased with higher temperatures. • Example: Preserving Fruit • Fresh fruit and vegetables are often kept in the fridge to slow down the chemical reactions

5. Surface Area • Crushing materials increases the amount of surface area that is available to react. • A powder of a substance will react much more quickly than a block of the same substance.

6. Concentration • Increasing the amount of substance per unit of volume increases the amount of substance that is available to react. • If you are doing a lab with acid, a more concentrated acid will react faster than a dilute concentration.

7. Catalysts • What is a catalyst? • A substance that speeds up the rate of a reaction without being changed itself. • A catalyst increases the speed of the reaction – it doesn’t interfere with the products being created • Natural catalysts are called enzymes • Saliva is called salivary amylase – it breaks apart your food to aid in digestion • An enzyme in your tears attacks the cell wall of the bacteria to prevent an infection.

8. Inhibitors • Inhibitors are substances that slow down chemical reactions. • Examples: • Plants have inhibitors to stop germination until conditions are right • Foods and medicines have inhibitors in them to prevent decomposition • Hydrogen Peroxide has inhibitors in it to prevent its molecules from decomposing too quickly

9. Corrosion • Corrosion is the oxidation of metals or rocks in the presence of air and moisture. • The most common type of corrosion: Rust

10. Preventing Corrosion • Corroded materials lose their strength. Once materials lose the top layer, oxygen from the air can reach the inner layers. • Prevention? • Apply a thin coat of paint • Coat it with a thin layer of zinc – this process is called galvanization • Examples? • Galvanized nails so that job sites aren’t ruined • Chromium covered car parts to protect the lead • What is the electrochemical process to coat a metal with another type of metal? • Electroplating

11. Combustion • The highly exothermic combination of a substance with oxygen. • Every time something burns, it combines with oxygen from the air. • Even the food you eat is converted to energy by combining with the oxygen you breathe.

12. Products of Combustion • The products of burning a hydrocarbon (hydrogen and carbon) are always the same • Products: CO2(g) and H2O(g) and heat CH4(g) + 2O2(g) CO2(g) + 2H2O(g) + heat C2H8(g) + 5O2(g) 3CO2(g) + 4H2O(g) + heat