 # Chemical Formulas & Equations 8 th Grade Science 2010 H 2 O NaCl CO 2 2H 2 + O 2 = 2H 2 O.

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Chemical Formulas & Equations 8 th Grade Science 2010 H 2 O NaCl CO 2 2H 2 + O 2 = 2H 2 O

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. –NaCl – Sodium chloride (salt) –Na – Sodium, Cl - Chlorine –H 2 O – Dihydrogen monoxide (water) –H – Hydrogen, O - Oxygen –CO 2 – Carbon dioxide –C – Carbon, O - Oxygen

Subscripts Numbers written below and to the right of a chemical symbol in a formula. H 2 O – 2 Hydrogen, 1 Oxygen Tells how many atoms of an element are in each molecule. Only used if more than one atom is present.

C9H8O4C9H8O4 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

H 2 SO 4 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

Coefficients # placed in front of a chemical symbol or formula Identifies the # of molecules of a substance –2H 2 O = 2 molecules of H 2 O –3FeO 3 = 3 molecules of FeO 3

Coefficients X Subscripts To determine the number of atoms using coefficients you multiply the coefficient with the subscript. 2H 2 O - 2 molecules of H 2 O = 4 Hydrogen atoms and 2 Oxygen atoms

2Fe 2 O 3 2 Molecules of Fe 2 O 3 = 2 X 2 = 4 Fe (iron) atoms 2 X 3 = 6 O (oxygen) atoms

Using Parentheses Multiply the subscript with each element in parentheses. Al 2 (SO 4 ) 3 Al = 2 atoms of Aluminum S = 3 X 1 = 3 atoms of Sulfur O = 4 X 3 = 12 atoms of Oxygen

(CH 3 ) 2 CHOH C = Carbon = 2x1 = 2+1 = 3 carbon atoms H = Hydrogen = 3x2 = 6+2 = 8 Hydrogen atoms O = Oxygen = 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. 2H 2 + O 2 → 2H 2 O Reactants Yield Products Sign

Chemical Reactions Elements are rearranged to form new substances in a chemical equation to show that a chemical reaction has occurred.

Chemical Equations Reactants –starting materials in a chemical reaction. Products –substances formed in a chemical reaction.

C 12 H 22 O 11 + H 2 O → H 2 O + C 12 H 22 O 11 Does this equation show the atoms rearranged to form NEW substances? Reactants = C 12 H 22 O 11 and H 2 O Products = C 12 H 22 O 11 and H 2 O No! No new substances have been formed.

Zn + 2HCl → ZnCl 2 + H 2 Does this equation show elements rearranged to form new substances? Reactants = Zn and 2HCl Products = ZnCl 2 and H 2 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.

Chemical Equations Must Be Balanced! Law of Conservation of Mass The # of atoms in the reactants must equal the # of atoms in the products for each element

CH 4 + O 2 → H 2 O + CO 2 Reactants Carbon = 1 Hydrogen = 4 Oxygen = 2 NOT BALANCED! Products Carbon = 1 Hydrogen = 2 Oxygen = 3

NaCl + AgNO 3 → NaNO 3 + AgCl Reactants → Products Na – 1Na - 1 Cl – 1Cl - 1 Ag – 1Ag - 1 N – 1N - 1O – 3 Is this equation balanced? YES!

2H 2 + O 2 → 2H 2 O Is this equation balanced? Reactants → Products H = 4H = 4 O =2O = 2 Is this equation balanced? Yes!

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