.

2. 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.

4. 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.

5. 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.

6.  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.

7.  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.

9.  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

10.  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

11.  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

12.  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

13.  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