1. Chemical Reactions How do chemicals react with each other? Chapter 23

2. What is a chemical reaction? • Atoms change identity • Mg atom becomes Mg+2 • A change of color is almost always present • Cu replaces Ag and turns solution blue • All chemical reactions give or use energy • Cold packs, light sticks • Usually gas is given off • Form bubbles • Precipitate is formed • Like a snow globe, flakes of a solid • Any of these signs indicates a reaction.

3. Why do they occur? • Chemical reactions occur between atoms in order to form a bond • Bonds form in order to complete the outermost energy shell and become less reactive, ions

4. Chemical Reactions • A chemical reaction is an expression in which symbols and formulas are use to represent a chemical reaction. • Reactants are written on the left, and products are written on the right side of the yield sign () • There are some equations that are written to express reactions that never complete, they use reversible arrows

5. Chemical Reactions (Rxns) • Begin with reactants and end with products • Reactants --- Products • A + B  AB • We aren’t creating or destroying atoms, only rearranging them (Law of Conservation of Mass)

6. Rate of Chemical Reactions • The rate of a chemical reaction is a measure of how quickly reactants turn into products • Increasing these factors will increase the rate of reaction: • A catalyst is a substance that increases the reaction rate of a reaction without itself being changed by the reaction, Ex) enzymes • Speeds up and slows down a reaction

7. Example • Isooctane, C8H18 and oxygen, O2 react to form CO2, carbon dioxide and H2O, water • C8H18 + O2 CO2 + H2O + Energy • KEY WORDS • React with, combine with, and + • To form, yields, forms, produces, creates 

8. It takes energy to break a bond (heat, electric, sound, light) – can all transfer energy • Forming bonds releases energy • Both result in either a endothermic or exothermic reaction Bond breaking: requires energy Bond formation: releases energy

9. Energy Classifications Endothermic Exothermic • Energy is absorbed from surroundings • Energy is given off from rxn

10. Activation Energy • The energy required to start a reaction

11. Chemical Energy • Energy stored in chemical bonds • Released with a reaction • Ex) bonds in coal store energy that is released

12. Chemical Reaction types • Synthesis – at least two substances form a new, more complex compound • Always combine substances to form more complex substances • Ex) A + B + C  ABC • Ex) Na + Cl2  NaCl

13. Decomposition • Complex compound breaks down into simpler compounds • Ex) DeFG DeG + F • Ex) 2H2O  2H2 + O2(electrolysis of water)

14. Combustion • Uses oxygen as a reactant and products will contain oxygen somewhere, usually as CO2 • A reaction where a compound and oxygen will burn • Ex) CH4 + O2 CO2 + H2O • Methane combines with oxygen and activation energy to burn and produce carbon dioxide and water with light/heat

15. Single Displacement • Reaction in which one element will replace atoms of another element in a compound (especially within the same family) • More reactive elements will take the place of less reactive ones • Ex) 3CuCl2 + 2Al  2AlCl3 + 3Cu • Copper chloride combines with aluminum to produce aluminum chloride and copper ions • The aluminum replaced the copper

16. Double Displacement • Two compounds exchange ions (they swap partners) • Ex) Pb(NO3)2 + K2CrO4 PbCrO4 + 2KNO3 • Lead nitrate combines with potassium chromate to form lead chromate and potassium nitrate.

17. Reduction/Oxidation reaction (Redoxrxn ) • A reaction that occurs when electrons are transferred between reactants • One or more reactants is reduced and one or more is oxidized • Reduced- substances that accept e- • Oxidized- substances that give up e- • OIL RIG: Oxidation Is Loss; Reduction Is Gain • OR • LEO says GER: Loss of Electrons is Oxidation and Gain of Electrons is Reduction

18. Summary of Rxn Types • Synthesis • A + B  AB • Decomposition • DEF  D + E + F • Combustion • AB + O2  A + BO2 • Single Displacement • XA + B  BA + X • Double Displacement • AX + BY  AY + BX