Chemical Formulas & Equations

8th Grade Science 2010

H2O NaCl CO2

2H2 + O2 = 2H2O

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

– 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.

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.

• 2H2O - 2 molecules of H2O

= 4 Hydrogen atoms and 2 Oxygen atoms

2Fe2O3

• 2 Molecules of Fe2O3

=

• 2 X 2 = 4 Fe (iron) atoms

• 2 X 3 = 6 O (oxygen) atoms

Using Parentheses

• Multiply the subscript with each element in parentheses.

• Al2(SO4)3

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

(CH3)2CHOH

• 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.

• 2H2 + O2 → 2H2OReactants 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.

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.

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

CH4 + O2 → H2O + CO2

• 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!

2H2 + O2 → 2H2O

• Is this equation balanced?

Reactants → Products

H = 4 H = 4O = 2 O = 2

Is this equation balanced?

Yes!