Science 9: Unit B – Matter and Chemical Change

1. Science 9: Unit B – Matter and Chemical Change Topic 8: Reaction Rate and Common types of Reactions

2. Reaction Rate • Different reactions occur at different speeds. However, techniques can be used and chemicals added to alter the natural rate of a reaction. For example heating a chemical mixture, stirring, swirling, and using smaller particles can all speed up a reaction-rate. On the other hand, cooling a reaction or mixture, and not moving it slows the reaction rate.

3. Catalysts and Inhibitors • Catalyst – A chemical that is added to a reaction mixture, but is NOT used up in the reaction. Its purpose is to help speed up a reaction. A catalyst in your saliva helps speed up the breakdown of potatoes into starch. • Inhibitor – A chemical that is added to a reaction mixture, but is NOT used up in the reaction. Its purpose is to help slow down or delay the reaction, usually because the reaction is unwanted. An example of an inhibitor are ‘clot busters’ which are used in blood donor clinics to slow down the clotting process in the donated blood.

4. Corrosion Reaction • Corrosion occurs when a metal reacts with oxygen to form a(n) ionic compound known as a metal oxide. The most famous example of corrosion occurs as a car rusting. Cars are usually made of iron. Iron will react with oxygen when water is present. That’s why cars in Alberta will outlast cars in Ontario and Quebec. There’s more humidity in the air in the East. The chemical equation for the formation of rust is: • 4Fe(s) + 3O2(g) → 2Fe2O3 (s)

5. How to Prevent Rust • To protect against rust forming, car makers add several protective layers of paint. In similar cases, a non-corrosive metal (does not react with oxygen) is added on top of the iron, this process is called electro-plating as it’s done electrically. Chromium is often used in car bumpers. • If zinc is used in electroplating, the process is called galvanization. • The process where a metal is electroplated with another metal is called electrolysis.

6. COMBUSTION • Combustion reactions involve the same three elements reacting to form the same two products, but in different amounts. • Combustion involves a hydrocarbon reacting with oxygen. A hydrocarbon is a compound made up of hydrogen and carbon. Examples of hydrocarbons include methane (natural gas) CH4, propane C3H8, butane C4H10, and octane (gasoline) C8H18.

7. Combusting a hydrocarbon • With any hydrocarbon, the reaction works out the same. An initial source of heat provides the energy needed to let the hydrocarbon react with surrounding oxygen to produce carbon dioxide, water, and a large amount of energy. Combustion reactions are very exothermic.

8. Two Examples of Combustion Reactions • For example, with a regular butane lighter, the reaction occurs as follows: When you press down on the button, the butane is released into the surrounding air. At the same time, the spark provides the initial energy needed to ignite the butane and oxygen producing a larger amount of energy (flame) and carbon dioxide and water vapour. • 2C4H10(l) + 13O2(g) → 8CO2(g) + 10H2O(g) + Energy • Another common example involves barbecuing. Propane gas is released from the tank and comes into contact with oxygen in the bbq. A match or spark provides the initial energy) releasing a large amount of heat energy. • C3H8(l) + 5O2(g) → 3CO2(g) + 4H2O(g) + Energy

9. Combination Reactions • Two or more reactants are combined together to form a single product. Note that the products formed are usually more stable than the reactants and so energy and catalysts are needed only to start the reaction. • Eg. H2(g) + Cl2(g) → 2HCl(l)

10. Dissociation Reactions • The opposite of a combination reaction. Here a single compound is broken up into its individual elements. Because compounds tend to be more stable than elements by themselves these reactions tend to be easily reversible. • Eg. 2HCl(l) → H2(g) + Cl2(g)