Physical Science: Ch 7 NotesA. Chemical Reactions

When a substance undergoes a chemical change, a chemical reaction has taken place.

Reactions are known to produce: unexpected color changes, a precipitate, gas, or a change in temperature

A chemical equation is written as Reactants Products

Common chemical reactions: rust forming, digestion of food, bleaching hair, milk souring, apples turning brown, photosynthesis, and the combustion of anything!

Reactants – the substances that undergo change

Products – the new substances formed as a result of that change

Chemical equation – a representation of a chemical reaction in which the reactants and products are expressed as formulas

Ex: C + O2 CO2 “carbon reacts with oxygen gas to form carbon dioxide”

Ex: H2O H2 + O2 “water decomposes into hydrogen gas and oxygen gas

Physical Science: Ch 7 NotesB. Balancing Chemical Equations

This must be done to follow the Law of Conservation of Mass – states that mass is neither created nor destroyed

Therefore, the products of a chemical reaction are not created out of thin air and the atoms of the reactants are not destroyed in the chemical reaction!

To balance a chemical equation:1. Write the reaction in symbol form2. Draw a vertical line down from the arrow in the equation and, on each side, write the symbols of each of the atoms present3. Count and record the numbers of each atom present on each side of the arrow4. Use coefficients to increase the number of atoms until they are balanced (Do NOT change the subscripts already present in the chemical formulas!)

Coefficients – the numbers that appear before the chemical formulas in a balanced chemical equation

Physical Science: Ch 7 NotesC. Classifying Reactions – chemical reactions are often classified by the type

of reactant or the number of reactants and products

Synthesis reaction – a reaction in which two or more substances react to form a single substance

General form: A + B AB

Example: 2H2 + O2 2H2O

Decomposition reaction – a reaction in which a compound breaks down into two or more simpler substances

General form: AB A + B

Example: 2H2O 2H2 + O2

Combustion reaction – a reaction in which a substance reacts rapidly with oxygen often producing heat and light The products are ALWAYS carbon dioxide and water

General form: X + O2 CO2 + H2O

Example: CH4 + 2O2 CO2 + 2H2O

Physical Science: Ch 7 Notes

Single Displacement – a reaction in which one element takes the place of another element in a compound

General form: A + BC B + AC

Example: Mg + HCl MgCl2 + H2

Double Displacement – a reaction in which two different compounds exchange positive ions and form two new compounds

General form: AB + CD AD + CB

Example: HCl + PbO H2O + PbCl2

Physical Science: Ch 7 NotesD. Chemical Bonds and Energy

Chemical energy – the energy stored in the chemical bonds of a substance

Chemical reactions involve the breaking and formation of chemical bonds

The BREAKING of chemical bonds REQUIRES energy

The FORMATION of chemical bonds RELEASES energy

During a chemical reaction, energy is either a required reactant or is released as a product

Exothermic reaction – reaction releases energy to its surroundings as a product

Endothermic reaction – reaction absorbs energy from its surroundings as a reactant