8 - 1 Balancing Chemical Equations If you are starting with a word equation, be sure to write each chemical formula correctly.  You must remember that elements ending in - gen or - ine are diatomic! Count how many of each type of atom is present on each side of the equation. Begin by balancing elements that are found in one substance on each side.

8 - 2 Balance hydrogen and oxygen last as they sometimes balance out at the end. If there is an odd number of an element on one side and an even number on the other, the odd number will need to be evened out so try using a coefficient of 2 for that substance. If there are polyatomic ions that remain together as a unit during the reaction, count the polyatomic ion as one unit.

8 - 3 You are allowed to change coefficients but never change subscripts!  Subscripts come from the valence (oxidation) numbers of the elements and once a chemical formula is correctly written, the subscripts become etched in stone! After balancing, put in the correct physical states by using the solubility chart.

8 - 4 States of Elements and Compounds Ionic Compounds  NaCl(s), NH 4 Cl(s), MgCl 2 (s) Nonpolar Covalent Compounds  H 2 (g), F 2 (g), Br 2 (l), CO 2 (g), O 2 (g), I 2 (s) Polar Covalent Compounds  H 2 O(l), C 12 H 22 O 11 (s), NH 3 (g), SO 2 (g)

8 - 5 Elements  Na(s), Al(s), Cu(s) Types of Bonds (difference in electronegativities)  0.0 – 0.3nonpolar covalent  0.4 – 1.7polar covalent  1.8 – 4.0ionic

8 - 6 Synthesis Reactions A synthesis or a direct combination reaction has the general form: A + B A + B → AB 2H 2 (g) + O 2 (g) → 2H 2 O(g)  Two elements in their elemental (free) states form a compound. states form a compound.

8 - 7 Important Notes to Remember  Note that hydrogen and oxygen because they end in - gen are diatomic.  The reactant states are determined by what state hydrogen and oxygen are at what state hydrogen and oxygen are at room temperature (25°C, 298 K). room temperature (25°C, 298 K).  Note that the subscripts are not changed in attempting to balance the equation.  Water can be in either the liquid or gaseous state depending on the ambient gaseous state depending on the ambient temperature. temperature.

8 - 8 Decomposition Reactions A decomposition or an analysis reaction has the general form: AB AB → A + B 2H 2 O(l) → 2H 2 (g) + O 2 (g)  A compound is decomposed into its constituent elements. constituent elements.  See important notes for synthesis reactions. reactions.

8 - 9 Typical Decomposition Reactions When metallic carbonates are heated, metallic oxides and carbon dioxide are formed.  Metallic carbonates are ionic compounds containing the carbonate polyatomic (CO 3 2- ) ion.  Metallic oxides are ionic compounds containing the oxide (O 2- ) ion.

 BaCO 3 (s) → BaO(s) + CO 2 (g) When metallic hydroxides are heated, metallic oxides and water are formed.  Metallic hydroxides are ionic compounds containing the hydroxide (OH - ) ion.  Ba(OH) 2 (s) → BaO(s) + H 2 O(g) Δ Δ

When metallic chlorates are heated, metallic chlorides and oxygen are formed.  Metallic chlorates are ionic compounds containing the chlorate (ClO 3 - ) ion.  2KClO 3 (s) → 2KCl(s) + 3O 2 (g) Δ

When some acids are heated, nonmetallic oxides and water are formed.  Nonmetallic oxides are covalent (molecular) compounds containing oxide (O 2- ).  Examples are NO 2, SO 3, and CO 2.  H 2 SO 3 (aq) → H 2 O(l) + SO 2 (g) Δ

When some metallic oxides are heated, a metal and oxygen are formed.  2HgO(s) → 2Hg(l) + O 2 (g) Some decomposition reactions are produced by electrolysis (an electric current).  2H 2 O(l) → 2H 2 (g) + O 2 (g)  2NaCl(l) → 2Na(l) + Cl 2 (g) Δ

Single Replacement Reactions A single replacement reaction has the general form: A + BC → AC + B Zn(s) + Cu(NO 3 ) 2 (aq) → Zn(NO 3 ) 2 (aq) + Cu(s)

Important Notes to Remember  Note that elements (Zn, Cu) in their free or uncombined state have no charge.  Not all single replacement reactions occur.  The elemental metal, Zn, must be a more reactive metal to replace the metal in the ionic compound.  To determine if a single replacement occurs, check the Activity Series of the Elements.

More Single Replacement Reactions Mg + HNO 3 → ? Mg(s) + 2HNO 3 (aq) → Mg(NO 3 ) 2 (aq) + H 2 (g) This reaction occurs because Mg is above H on the Activity Series indicating that it is a more active metal.

More Single Replacement Reactions Ag + Pb(NO 3 ) 2 → ? Ag + Pb(NO 3 ) 2 → NR This reaction does not occur because Ag is below Pb on the Activity Series indicating that it is a less active metal.

Double Replacement Reactions A double replacement reaction has the general form: AB + CD AB + CD → AD + CB Ba(OH) 2 (s) + H 2 SO 4 (aq) → CaSO 4 (s) + 2H 2 O(l)

Important Notes to Remember Not all double replacement reactions occur. For such a reaction to occur, one of the following must take place:  A precipitate may be formed (i.e. AgCl(s)).  A gas may be formed (i.e. CO 2 (g))  Water may be formed (i.e. H 2 O(l)).

Combustion Reactions Organic compounds (compounds of carbon) contain either carbon and hydrogen (hydrocarbons) or carbon, hydrogen, and oxygen. CO and CO 2 are not considered organic compounds. When such compounds burn completely in excess oxygen, combustion occurs and the products are always CO 2 (g) and H 2 O(l).

Combustion Reactions When such compounds burn without sufficient oxygen, combustion occurs and the products are always CO(g) and H 2 O(l). When balancing combustion reactions, follow these steps in order:  balance the carbons.  balance the hydrogens.  balance the oxygens.

Balancing Combustion Reactions C 6 H 12 O 6 + O 2 → C 6 H 12 O 6 + O 2 → CO 2 + H 2 O C 6 H 12 O 6 + O 2 → 6CO 2 + H 2 O C 6 H 12 O 6 + O 2 → 6CO 2 + 6H 2 O C 6 H 12 O 6 + 6O 2 → 6CO 2 + 6H 2 O C 6 H 12 O 6 (s) + 6O 2 (g) → 6CO 2 (g) + 6H 2 O(l)

Balancing Combustion Reactions C 8 H 18 + O 2 → C 8 H 18 + O 2 → CO 2 + H 2 O C 8 H 18 + O 2 → 8CO 2 + H 2 O C 8 H 18 + O 2 → 8CO 2 + 9H 2 O 2C 8 H 18 (g) + 25O 2 (g) → 16CO 2 (g) + 18H 2 O(l) 25 2

What Must Balance In A Reaction? ammonium carbonate + calcium choride → ? ammonium carbonate + calcium choride → ammonium chloride + calcium carbonate (NH 4 ) 2 CO 3 + CaCl 2 → NH 4 Cl + CaCO 3 (NH 4 ) 2 CO 3 (aq) + CaCl 2 (aq) → 2NH 4 Cl(aq) + CaCO 3 (s)

Atoms 2 N atoms → 2 N atoms 8 H atoms → 8 H atoms 1 C atom → 1 C atom 3 O atoms → 3 O atoms 1 Ca atom → 1 Ca atom 2 Cl atoms → 2 Cl atoms

Mass (NH 4 ) 2 CO 3 (aq) + CaCl 2 (aq) → 2NH 4 Cl(aq) + CaCO 3 (s) g g = g g g = g Moles 2 mol reactant ≠ 3 mol product