Transcript Chemical Formulas - Mr. Stewart's Science Classes

How to balance chemical equations
HOW MANY WORDS CAN YOU MAKE USING THE
26 LETTERS OF THE ALPHABET?
• Many thousands?
• Letters are used to form words.
• In the same way, chemical symbols are put
together to make chemical formulas that describe
substances.
a - p - p - l - e = apple
H - H - O = H2 O
• Words are put together to form a sentence.
• In the same way, chemical formulas can placed together
to describe a chemical reaction in an equation.
I ate an apple.
2H2 + O2  2H2O
A CHEMICAL EQUATION is a symbolic
representation of a chemical reaction.
• The burning of methane gas in oxygen is:
• CH4 + 2 O2 → CO2 + 2 H2O
Review:
Element symbols
• All elements are represented
by a 1 or 2 letter symbol
• For example
• C = Carbon
• Ne = Neon
• O = Oxygen
• The symbols are shown on the
periodic table
Review:
Chemical Formulas
• Shows the elements and number of atoms of each
element in a molecule
• H2SO4
Subscript
Elements:
• Hydrogen; 2 atoms
• Sulfur: 1 atom
• Oxygen: 4 atoms
7 atoms total
A subscript is a
number written below
and to the right of a
chemical symbol in a
formula. It shows how
many atoms of the
element are present
in substance.
Coefficients
• A coefficient is a number that is placed in front of a
chemical symbol or formula.
• The coefficient represents the number of molecules of
that compound or atom needed in the reaction.
• For example:
• 2H2SO4 : 2 molecules of Sulfuric Acid
coefficient
• 2H2SO4 : 2 molecules of Sulfuric Acid
• A coefficient is distributed to ALL elements in a
compound
• 2 – H2 (for a total of 4 H atoms)
• 2 – S (for a total of 2 S atoms)
• 2 – O4 (for a total of 8 O atoms)
Tip! Multiply the subscript by the coefficient
Reading Chemical Equations
• The starting materials in a chemical reaction are the
reactants.
• The substances formed from a reaction are the products.
Reactants
Products
CH4 + 2 O2 → CO2 + 2 H2O
• A plus sign (+) separates the formulas from one another.
• The arrow,
, also called the yields sign, separates the
reactants from the products.
Balancing Equations
• The Law of Conservation of Mass states that
matter is neither created nor destroyed…
• This means that a chemical equation must show the
same numbers and kinds of atoms on both sides of the
arrow!
How To Balance…
In order to be balanced, ALL atoms must be counted.
• Only coefficients are changed when balancing a chemical
equation.
• Ready – Set – Practice!