1. NOTES 9 – Chemical Reactions

2. What is a chemical reaction?
Chemical reaction – a process that transforms one set of substances to another
Bonds are formed and/or broken
Atoms get rearranged
No atoms get created or destroyed (conservation of matter)
Atoms from the original substances are in the new substances

3. Reactants & Products
Reactants – substances that react to form new substances
Reactants are the “ingredients” in a chemical reaction
Products – the new substances formed in a chemical reaction
Made up of the rearranged atoms from the reactants
A chemical reaction is like baking a cake
Reactants = flour, sugar, eggs, milk, etc.
Products = cake

4. Chemical Equations Chemical reactions are written as equations
Reactants on the left
Products on the right
Arrow sign in the middle represents the change taking place
Ex. The reaction in car engines
REACTANTS  PRODUCTS
octane + oxygen  carbon dioxide + water
2 C8H O2  16 CO H2O

5. Equations Must Be Balanced
Matter is not created or destroyed in a chemical reaction
The same atoms that were in the reactants are now in the products
The mass of the reactants is equal to the mass of the products
Ex. Formation of water
Unbalanced: H2 + O2  H2O
Balanced: 2 H2 + O2  2 H2O

6. Endothermic & Exothermic Reactions
Endothermic reaction – a chemical reaction that absorbs energy
The energy in the products is greater than the energy in the reactants
Ex. photosynthesis, cold packs
Exothermic reaction – a chemical reaction that releases energy
The energy in the products is less than the energy in the reactants
Energy is usually released in the form of heat, light, or sound
Ex. burning, hot packs

7. 3 Types of Chemical Reactions
Synthesis reaction – a reaction where simple substances are changed into more complex substances
Decomposition reaction – a reaction where a complex substance is broken down into simpler substances
Combustion reaction – a reaction between oxygen and an organic compound (fuel)

8. Synthesis Reactions Synthesis reactions are endothermic
Energy is stored in the bonds of the products
Small simple substances come together to form a larger, more complex substance
Ex. Photosynthesis
Light energy is absorbed and stored as chemical energy in plants
6 CO2 + 6 H2O  C6H12O6 + 6 O2

9. Decomposition Reactions
Decomposition reactions are exothermic
More energy is released from the bonds of the reactants than is needed to form the bonds of the products
Larger, more complex substances are broken down into simpler substances
Ex. Cell Respiration
Releases energy in glucose for cell use
C6H12O6 + 6 O2  6 CO2 + 6 H2O

10. Combustion Reactions Combustion reactions are exothermic
Commonly called “burning”
Organic molecules (fuel) release heat and light in the presence of oxygen
Need some energy to start the fire – breaks open the first few molecules of fuel
Ex. Combustion of methane gas
CH4 + 2 O2  CO2 + 2 H2O

11. Combustion of Fossil Fuels
Fossil fuels (coal, oil, natural gas) contain combustible hydrocarbons
Fossil fuel provides approximately 80% of the world’s energy
Fossil fuels are non-renewable (they will eventually run out)
The products of combustion (CO2 + H2O) trap heat in the atmosphere and contribute to global warming