NOTES 9 – Chemical Reactions

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

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

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 C8H18 + 25 O2  16 CO2 + 18 H2O

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

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

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)

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

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

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

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