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Chemical Formulas & Equations 8th Grade Science 2012
Flipped Notes TAKE 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 atoms How many elements are there? How many total atoms? 3 elements total atoms

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

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 = = 3 carbon atoms H = 3 x 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 → 2H2O Reactants 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

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!

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