Chemical Reactions

Balancing Chemical Equations

Types of Reactions

Unit 7

Remember – A chemical change occurs when substances react together and form NEW products.Examples of this include:

rustingCombustionTarnishingFermentation oxidationAcid rain reacting with calcium in rocks

Chemical Reactions

Evolution of heat and lightProduction of a gasFormation of a precipitate

A precipitate is a solid produced as a result of a chemical reaction in solution and that separates from the solution

Unexpected Color changeChange in energy (absorption or evolution)Formation of waterProduction of an odor

Indicators

Chemical reaction can be written in words or as formulas. When they are written using formulas and symbol this is called a chemical equation.

Chemical reactions must contain correct reactants (the beginning substances) and products (the ending substances)

Chemical equations must be BALANCED in order to follow the LAW of Conservation of Mass, which states that mass can neither be created or destroyed.Remember… what you start with has to equal

with what you end with!

Law of Conservation of Mass

To balance a chemical equation you must make sure there is the same number of each type of atom on both sides of the arrow. Only change the coefficients (the number in

front of the compounds or element), never change subscripts!

Diatomic molecules (di=2, atomic=atoms)(always exist as 2 when they are by themselves)

N2

O2

F2

Cl2

Br2

I2

H2

Balancing EquationsRemember that when balancing an equation each

side has to equal. (Law of Conservation of Mass).First:

Write out the equation: ___N2 + ___H2 ___NH3

Count how many atoms of each element are on each side.Reactants ProductsN=2 N=1H=2 H=3This equation is NOT balanced.

You will now have to add coefficients to make each side balanced.

____N2 + ___H2 _____ NH3

Copy this table in your notes

Symbol Explanation

“Yields”; indicates result of reactions

Indicates a reversible reaction

(s) A reactant or product in the solid state, also indicates a precipitate

(l) A reactant or product in the liquid state

(aq) A reactant or product in an aqueous solution (dissolved in water)

(g) A reactant or product in the gaseous state

or Reactants are heated

Pressure at which a reaction is carried out (in this case 2 atm)

Temperature at which reaction is carried out (in this case 0oC)

Formula of catalyst, in this case manganese dioxide, used to alter the rate of reaction

Synthesis2 elements or simple compounds combine to

form ONE product.Examples:

Generic equation: A + X AX8 Fe + S8 8 FeSCaO + H2O Ca(OH)2

The 5 major types of Chemical Reactions

DecompositionONE compound is broken down into simpler

partsExamples:

Generic equations: AX A + XAXY AX +Y

2 H2O 2 H2 + O2

Single ReplacementAn element and a compound combine to form a

new element and compound.Metals can replace other metals and hydrogen

in an acid. Halogens can replace halogens. Must look at activity series to determine if the

reaction will occur (it is located on your reference sheet)Generic equations:

AX + B BX + AAX + Y AY + X

2 Na + MgSO4 Mg + Na2SO4

Cl2 + 2NaBr Br2 + 2 NaCl

Double Replacement2 compounds produce 2 NEW compounds. These compounds are in aqueous solution so

the ions switch places with each other.Must consider solubility rules to determine

which product is the precipitate, if any.Generic Equation:

AX + BY AY + BX

Pb(NO3)2 + 2 KI PbI2 + 2 KNO3

CombustionOxygen combines with a compound to produce

carbon dioxide and water.This releases large amounts of energy (heat).

Generic equation:CxHy + O2 CO2 + H2O