C HEMICAL R EACTIONS How do chemicals react with each other? Chapter 23

W HAT 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.

W HY 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

C HEMICAL R EACTIONS 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

C HEMICAL R EACTIONS (R XNS ) 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)

R ATE OF C HEMICAL R EACTIONS 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

E XAMPLE Isooctane, C 8 H 18 and oxygen, O 2 react to form CO 2, carbon dioxide and H 2 O, water C 8 H 18 + O 2  CO 2 + H 2 O + Energy KEY WORDS React with, combine with, and + To form, yields, forms, produces, creates 

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

E NERGY C LASSIFICATIONS Energy is absorbed from surroundings Energy is given off from rxn EndothermicExothermic

A CTIVATION E NERGY The energy required to start a reaction

C HEMICAL E NERGY Energy stored in chemical bonds Released with a reaction Ex) bonds in coal store energy that is released

C HEMICAL R EACTION 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 + Cl 2  NaCl

Decomposition - Complex compound breaks down into simpler compounds - Ex) DeFG  DeG + F - Ex) 2H 2 O  2H 2 + O 2 (electrolysis of water)

Combustion - Uses oxygen as a reactant and products will contain oxygen somewhere, usually as CO 2 - A reaction where a compound and oxygen will burn - Ex) CH 4 + O 2  CO 2 + H 2 O - Methane combines with oxygen and activation energy to burn and produce carbon dioxide and water with light/heat

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) 3CuCl 2 + 2Al  2AlCl 3 + 3Cu - Copper chloride combines with aluminum to produce aluminum chloride and copper ions - The aluminum replaced the copper -

Double Displacement - Two compounds exchange ions (they swap partners) - Ex) Pb(NO 3 ) 2 + K 2 CrO 4  PbCrO 4 + 2KNO 3 - Lead nitrate combines with potassium chromate to form lead chromate and potassium nitrate.

R EDUCTION /O XIDATION REACTION (R EDOX RXN ) 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 : O xidation I s L oss; R eduction I s G ain OR LEO says GER : L oss of E lectrons is O xidation and G ain of E lectrons is R eduction

S UMMARY OF R XN T YPES 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