Physical Science: Ch 7 Notes A. 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 Common chemical reactions: rust forming, digestion of food, bleaching hair, milk souring, apples turning brown, photosynthesis, and the combustion of anything! Chemical equation – a representation of a chemical reaction in which the reactants and products are expressed as formulas A chemical equation is written as Reactants  Products Reactants – the substances that undergo change Products – the new substances formed as a result of that change 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 Notes B. 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 form 2. Draw a vertical line down from the arrow in the equation and, on each side, write the symbols of each of the atoms present 3. Count and record the numbers of each atom present on each side of the arrow 4. 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 Notes C. 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 Notes D. 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 Endothermic reaction – reaction absorbs energy from its surroundings as a reactant Exothermic reaction – reaction releases energy to its surroundings as a product