All chemical bonds posses potential energy. In a chemical reaction this energy is changed

when old bonds are broken and new bonds are formed. Chemical changes are different from physical changes. When a physical change occurs there is no breaking and forming of

bonds. There are certain things that will help us identify if a chemical reaction has taken place. We call these evidences of chemical

reactions.

Light is given off: Potential energy in valance electrons is released in the form of light.Gas is given off: Indicates an increase in kinetic energy of the atom (compound) as it is changed to a gas.Temperature change: Exothermic is an increase in the speed of the molecules (heat). Exothermic is the taking in of energy.

Color change: the increase or decrease of potential energy stored in new bonds.Precipitate formed: increase or decrease of potential energy in new bonds formed.Electrical potential: The release of chemical bond energies to cause the movement of valence electrons.

Equations that show just the formulas of the reactants and products are called

skeleton equations- a skeleton equation is a chemical equation that does not indicate the relative amounts of the reactants and products.

The physical state of a substance in a reaction can be indicated in the equation by using the following symbols: (s) for solid, (l) for liquid, (g) for gas, and (aq) for a solution in water.

RULES OF BALANCING EQUATION 1. Write the skeleton equations, either word or formula, to

represent the facts. 2. Determine the correct formulas for all the reactants and

products. 3.Write the formulas for the reactants on the left and

formulas for the products on the right with a arrow in between. If two or more chemicals are involved, separate their formulas with "+" sign

4.Balance the elements one at a time by using coefficients. (small whole numbers that appear in front of a formula in an equation.) when no coefficient is written, it is assumed to be 1. It is best to begin with an element other than hydrogen or oxygen. These two elements often occur more than twice in an equations.

5. Finally, make sure that all the coefficients are in the lowest possible ratio.

COMPOSITION Composition is when two or more substances react

to form a single substance. The reactants of most common composition reactions are either two elements or two compounds. The product of a composition reaction must be a compound. Two nonmetals can often combine in more than one way. Thus for composition reactions involving nonmetals you will usually need to be told what the product is.

R + S à RS

Example: Ca + S CaS

DECOMPOSITION In a decomposition reaction a single compound is broken down into two

or more simpler products. These products can be any combination of elements and compounds. Most decomposition reactions require energy in the form of heat, light or electricity in order for them to occur.

Example: 2HgO 2Hg + O2

SINGLE REPLACEMENT In a single replacement reaction atoms of an element replace

the atoms of a second element in a compound. These reactions are also called displacement reactions. Whether one metal will replace another metal from a compound can be determined by their relative reactivities of the two metals. A reactive metal will replace any metal that is less active than itself. Metals will also replace hydrogen in water, and hydrogen in acids. A nonmetal can also replace another nonmetal from a compound. This replacement is usually limited to the halogens. Remember reactivities from bonded for life packet.

T + RS TS + R

Example: Cu + AgCl CuCl2 + Ag

DOUBLE REPLACEMENT Double replacement reactions involve

an exchange of positive ions between two compounds. These reactions generally take place between two ionic compounds in aqueous solution.

R+S- + T+U- T+S- + R+U-

Example: NaCl + AgNO3 NaNO3 + AgCl

COMBUSTION In a combustion reaction oxygen reacts with

another substance, often producing energy in the form of heat and light. Combustion reactions commonly involve hydrocarbons which are compounds of hydrogen and carbon. The complete combustion of a hydrocarbon produces the compounds carbon dioxide and water.

CxHy + O2 xCO2 + y/2H2O

Example: CH4 + O2 CO2 + 2H2O