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Chemical Reactions What is a chemical reaction? • A chemical reaction is a chemical change where chemical substances (called reactants) react to give new chemical substances (called products). • Example – The combustion of hydrogen in oxygen is a chemical reaction which gives water. • Hydrogen and Oxygen are the reactants. • Water is the product. Chemical reaction and molecules • The chemical reaction of hydrogen and oxygen is done between the molecules of these two gases. • In this chemical reaction, the bonds of the hydrogen molecules H2 and oxygen molecules O2 break down. • The atoms H and O form new bonds to give water molecules H2O. The three molecules • Hydrogen molecule H2 is made up of 2 atoms • Oxygen molecule O2 is made up of 2 atoms • Water molecule H2O Chemical equation • A chemical equation is a way which represents the rearrangement of the atoms during a chemical reaction. • In a chemical equation, we write the formulas of the reactants and the formulas of the products separated by an arrow. hydrogen + oxygen H2 + + O2 water H2O Lavoisier’s Law • Antoine Lavoisier, a french chemist, stated the following law: – « the number of atoms in the reactants = the number of atoms in the products. » • This law makes it possible to balance chemical equations Balance a chemical equation H2 + O2 H2O - There are 2 atoms H and 2 atoms O in the reactants. - There are 2 atoms H and 1 atom O in the product. To balance this equation, we take 2 water molecules containing 2 atoms O and 4 atoms H, and we take 2 hydrogen molecules and 1 oxygen molecule. H2 and H2 + O2 H2O and H2O We should write it: 2 H2 + O2 2 H2O To read a chemical equation 2 H2 + O2 2 H2O Read this equation as follow: 2 hydrogen molecules react with 1 oxygen molecule to form 2 water molecules. Attention: - The number of atoms is the same in the reactants and the products. - The number of molecules is not necessarily the same. Problems 1. Methane CH4 reacts with oxygen O2 to form carbon dioxide CO2 and water H2O. a. Balance the equation of this reaction CH4 + O2 CO2 + H2O b. Determine the number of reactants and products molecules. Problems 2. Balance and read the following equations: a. b. c. d. Na Fe N2 Al + + + + O2 Cl2 H2 O2 Na2O FeCl3 NH3 Al2O3 Note : FeCl3 = Iron III chloride NH3 = Ammonia Al2O3 = Aluminum oxide Answers 1. CH4 + 2 O2 CO2 + 2 H2O 1 methane molecule react with 2 oxygen molecules to form 1 dioxide carbon molecule and 2 water molecules. answers 2. a. 4Na + O2 2 Na2O 4 sodium molecules react with 1 oxygen molecule to form 2 sodium oxide molecules. b. 2Fe + 3Cl2 2FeCl3 2 iron molecules react with 3 chlorine molecules to form 2 iron III molecules. Answers c. N2 + 3H2 2NH3 1 nitrogen molecule react with 3 hydrogen molecules to form 2 ammonia molecules. d. 4Al + 3O2 2Al2O3 4 aluminum molecules react with 3 oxygen molecules to form 2 aluminum oxide molecules.