1. Chemical Formulas & Equations 8th Grade Science 2012
Flipped Notes
TAKE GOOD, THOROUGH NOTES

2. 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.

3. 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

4. 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.

5. 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 atoms
How many elements are there? How many total atoms?
3 elements total atoms

6. H2SO4 How many atoms of each element are in this compound?
H = Hydrogen – 2 atoms
S = Sulfur – 1 atom
O = Oxygen – 4 atoms
How many elements are there? How many total atoms?
3 elements atoms

7. 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

8. 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’)

9. Let’s Practice: 2Fe2O3 2 Molecules of Fe2O3 =
2 X 2 = 4 Fe (iron) atoms
2 X 3 = 6 O (oxygen) atoms

10. 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

11. Let’s Practice: (CH3)2CHOH
C = 2 x 1 = = 3 carbon atoms
H = 3 x 2 = = 8 Hydrogen atoms
O = only 1 Oxygen atom

12. 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

13. Chemical Equations
Uses chemical symbols and formulas to represent a chemical reaction.
Example:
2H2 + O2 → 2H2O
Reactants Yield Products
Sign
Elements are rearranged to form new substances

14. Chemical Equations
Reactants – starting materials in a chemical reaction (left side)
Products – substances formed in a chemical reaction (right side)

15. 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.

16. 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.

17. 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.

18. 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

19. YES! NaCl + AgNO3 → NaNO3 + AgCl Reactants → Products Na = 1 Na = 1
Cl = 1 Cl = 1
Ag = 1 Ag = 1
N = 1 N = 1
O = 3 O = 3
Is this equation balanced?
YES!