Physical Science Ch. 24

Section 1 Chemical Changes

Chemical ReactionChemical Reaction –Change in which one or more substances are converted into new substances ReactantReactant –Substances that react ProductsProducts –New substances produced

Chemical Reaction Left sideright side produce reactants  products yields

Lavoisier’s Contribution Scientist who carried out thousands of experiments to establish what we know as the Conservation of MassScientist who carried out thousands of experiments to establish what we know as the Conservation of Mass Conservation of MassConservation of Mass –Matter can neither be created nor destroyed it just changes forms

 Produces, yields, forms +Plus (s)Solid (l)Liquid (g)Gas (aq) Aqueous, substance dissolved in water

heat The reactants are heated light The reactants are exposed to light elec. An electric current is applied to the reactant

Chemical EquationChemical Equation –A way to describe a chemical reaction using chemical formulas and other symbols CoefficientsCoefficients –Tells the number of units of each substance taking place in a reaction –Only way to BALANCE a chemical equation

Section 2 Chemical Equations

Balancing Chemical Equations –Does not change the substances involved or what happens within a reaction, it only changes the way the reaction is REPRESENTED. Balanced Chemical EquationBalanced Chemical Equation –Same number of atoms on both sides of the equation

How to Balance Equations 1. Write chemical equation using symbols & formulas 2. List elements involved 3. List number of atoms for each element involved on each side 4.Choose coefficients to balance the equation 5. Recheck number of atoms for each element involved on each side

Things to remember when balancing equations Diatomic MoleculesDiatomic Molecules –Br, O, F, I, N, Cl, H Save Oxygen and Hydrogen to last to balanceSave Oxygen and Hydrogen to last to balance –Most times they balance themselves as other elements are balanced

1.Al + Br 2  AlBr 3 2.Bi + O 2  Bi 2 O 5 3.Al + C  Al 4 C 3 4.AgS + Ni  NiS 2 + Ag 5.PbO + NaCl  PbCl 2 + Na 2 O

Al + Br 2  AlBr 3 AlBr Al + 3Br 2  2AlBr 3

Bi + O 2  Bi 2 O 5 BiO Bi + 5O 2  2Bi 2 O 5

Al + C  Al 4 C 3 AlC Al + 3C  Al 4 C 3

AgS + Ni  NiS 2 + Ag AgSNi AgS + Ni  NiS 2 + 2Ag

PbO + NaCl  PbCl 2 + Na 2 O PbONaCl PbO + 2NaCl  PbCl 2 + Na 2 O

Section 3 Classifying Chemical Reactions

Types of Reactions 1.Synthesis 2.Decomposition 3.Single Displacement 4.Double Displacement

Synthesis Two or more substances reacting (combine) to form a new substanceTwo or more substances reacting (combine) to form a new substance A + B  AB 2H 2 + O 2  2H 2 O

Decomposition One substance reacting (decomposes or breaks down) to form two or more new substancesOne substance reacting (decomposes or breaks down) to form two or more new substances AB  A + B 2H 2 O  2H 2 + O 2

Single Displacement When one element replaces another element within a compoundWhen one element replaces another element within a compound AB + C  AC + B AB + C  CB + A Cu + 2AgS  CuS 2 + 2Ag

Double Displacement Positive ion of one compound replaces the positive ion of the other to form two new compoundsPositive ion of one compound replaces the positive ion of the other to form two new compounds AB + CD  AD + CB H 2 O + AgS  H 2 S + AgO

PrecipitatePrecipitate –An insoluble compound that comes out of solution during a double displacement reaction

Section 4 Chemical Reactions and Energy

More Energy Out Exergonic ReactionsExergonic Reactions –Chemical reactions that release energy Exothermic ReactionsExothermic Reactions –When the energy given off in a reaction is primarily in the form of heat

More Energy In Endergonic ReactionsEndergonic Reactions –When a chemical reactions requires more energy to break bonds than is released when new ones are formed –Energy absorbed can be in the form of light, heat, or electricity Endothermic ReactionsEndothermic Reactions –When energy needed is in the form of heat Endothermic refers not just to reactionsEndothermic refers not just to reactions –Also refers to physical changes –Epsom salt dissolving in water –Ice pack / Cold Pack

CatalystsCatalysts –A substance that speeds up a chemical reaction without being permanently changed itself When you add a catalysts the mass of the product formed remains the same, but it will form more rapidlyWhen you add a catalysts the mass of the product formed remains the same, but it will form more rapidly

InhibitorsInhibitors –Substances that are used to combine with one of the reactants to prevent certain reactions from occuring –Ex. Food preservatives They prevent chemical reactions that would cause the food to spoilThey prevent chemical reactions that would cause the food to spoil