1. Chemical Reactions February 21, 2002

2. Evidence for Chemical Reactions • Color change • Precipitation • Gas production • Changes in temperature • Changes in properties

3. Chemical Equations • A shorter, easier way to showing chemical reactions using symbols instead of words • Chemical Formula – shows the ratio of elements in a compound • H2O • NaCl • CO2

4. Structure of an Equation • Reactants – materials you start with • Products – materials you end with • Reactant + Reactant  Product • H2 + O2  H2O2

5. Conservation of Mass • The amount of matter in a chemical reaction does not change, so the total mass of the reactants must equal the total mass of the products

6. Classifying Chemical Reactions • Synthesis • Decomposition • Single Replacement • Double Replacement

7. Synthesis • Two or more elements combine to form a more complex compound • A + B  AB • Hand + kerchief  handkerchief • Example: C + Cl2 CCl4

8. Decomposition • Breaks down compounds into simpler substances • AB  A + B • Schoolbook  school + book • Example: Al2O3  Al + O2

9. Replacement • A reaction in which one element replaces another in a compound, or two elements in different compounds trade places • AB + C  AC + B (Single) • AB + CD  AC + BD (Double) • T.V. screen + door knob  T.V. knob + screen door • HCl + NaOH  H2O + NaCl