Chemical Formulas & Equations 8 th Grade Science 2012

1. Chemical Formulas & Equations8th 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 21 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 7 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 = 2 + 1 = 3 carbon atoms • H = 3 x 2 = 6 + 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. Reactants Carbon = 1 Hydrogen = 4 Oxygen = 2 NOT BALANCED! Products Carbon = 1 Hydrogen = 2 Oxygen = 3 Let’s Practice:CH4 + O2 → H2O + CO2Is the equation above balanced?

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