Chemical Formulas & Equations

8th Grade Science 2012

Flipped NotesTAKE GOOD, THOROUGH NOTES

Chemical Compounds

• A compound is when 2 or more elements chemically combine.

• They are formed by chemical bonds between atoms.

• The properties of a compound are often very different from the elements themselves.

Chemical Formulas

• Formulas that use chemical symbols and numbers to represent what makes up a substance.

Examples:– NaCl – Sodium chloride (salt)

– Na – Sodium, Cl - Chlorine

– H2O – Dihydrogen monoxide (water)– H – Hydrogen, O - Oxygen

– CO2 – Carbon dioxide– C – Carbon, O - Oxygen

Subscripts• Numbers written below and to the right

of a chemical symbol in a formula.

H2O – 2 Hydrogen, 1 Oxygen

• Tells how many atoms of an element are in each molecule.

• Only used if more than one atom is present.

Let’s Practice:C9H8O4

How many atoms of each element are in this compound?

• C = Carbon – 9 atoms

• H = Hydrogen – 8 atoms

• O = Oxygen – 4 atomsHow many elements are there? How

many total atoms?• 3 elements 21 total atoms

H2SO4

How many atoms of each element are in this compound?

• H = Hydrogen – 2 atoms

• S = Sulfur – 1 atom

• O = Oxygen – 4 atomsHow many elements are there? How many total

atoms?• 3 elements 7 atoms

Coefficients• # placed in front of a chemical symbol

or formula• Identifies the # of molecules of a

substance

2H2O = 2 molecules of H2O

3FeO3 = 3 molecules of FeO3

Coefficients X Subscripts• To determine the number of atoms using

coefficients you multiply the coefficient with the subscript.

Example:

• 3H2O - 3 molecules of H2O

H = 3 x 2 = 6 Hydrogen atoms O = 3 x 1 = 3 Oxygen atoms (remember:

when there isn’t a subscript after the element, there is an invisible ‘1’)

Let’s Practice:2Fe2O3

• 2 Molecules of Fe2O3

=

• 2 X 2 = 4 Fe (iron) atoms

• 2 X 3 = 6 O (oxygen) atoms

Using Parentheses • Multiply the subscript outside the

parenthesis with each element inside parentheses only.

Example:

• Al2(SO4)3

• Al = 2 atoms of Aluminum• S = 3 X 1 = 3 atoms of Sulfur• O = 3 X 4 = 12 atoms of Oxygen

Let’s Practice:(CH3)2CHOH

• C = 2 x 1 = 2 + 1 = 3 carbon atoms

• H = 3 x 2 = 6 + 2 = 8 Hydrogen atoms

• O = only 1 Oxygen atom

Chemical Reactions• Occurs when one or more substances change to

produce one or more different substances.

• Atoms are rearranged to form a totally new substance with properties that are completely different

• Indicators of a chemical reaction– Gas formation – Solid formation – precipitate– Energy change – light, heat, or sound– Color change

Chemical Equations• Uses chemical symbols and formulas to

represent a chemical reaction.Example:

• 2H2 + O2 → 2H2OReactants Yield Products

Sign

• Elements are rearranged to form new substances

Chemical Equations

• Reactants – starting materials in a chemical reaction (left side)

• Products – substances formed in a chemical reaction (right side)

Let’s Practice:C12H22O11 + H2O → H2O + C12H22O11

• Does this equation show the atoms rearranged to form NEW substances?

• Reactants = C12H22O11 and H2O

• Products = C12H22O11 and H2O

• No! No new substances have been formed.

Zn + 2HCl → ZnCl2 + H2

• Does this equation show elements rearranged to form new substances?

• Reactants = Zn and 2HCl

• Products = ZnCl2 and H2

• Yes! – New substances are formed.

Law of Conservation of Mass

• The mass of atoms and molecules is neither created nor destroyed in chemical reactions.– The # of atoms for each element in the

reactants must equal the # of atoms for each element in the products in a chemical reaction.

– Chemical Equations must be balanced.

Let’s Practice:CH4 + O2 → H2O + CO2

Is the equation above balanced?• Reactants

• Carbon = 1• Hydrogen = 4• Oxygen = 2

•NOT BALANCED!

• Products

• Carbon = 1• Hydrogen = 2• Oxygen = 3

NaCl + AgNO3 → NaNO3 + AgCl

Reactants → ProductsNa = 1 Na = 1Cl = 1 Cl = 1Ag = 1 Ag = 1N = 1 N = 1O = 3 O = 3

Is this equation balanced?

YES!