A. Equations are written with reactants on the left separated by an arrow or double arrow from products on the right. Reactants are separated from each other by plus (+) signs. The same is true of products.

e.g. HCl + NaOH ---> NaCl + H₂O
             reactants ---> products
 

B. Conditions necessary for a particular reaction ( delta for heat, P for pressure, h nu or lambda for light etc.) are often written above or below the arrow.

e.g. ^{,delta, P, lambda}    >
      catalysts

C. An upward pointing arrow  is often used to indicate a gaseous product while a downward point arrow is used to indicate a precipitating product (solid settling out of solution).

e.g. H₂CO₃ ------> H₂O + CO₂

Zn + 2AgCl -------> ZnCl₂ + 2Ag
 

D. Before an equation can be balanced all formulas of compounds must be properly written in accordance with known oxidation numbers. The following elements nearly always appear as diatomic molecules when uncombined with any other elements: N₂, O₂, H₂, F₂, Cl₂, Br₂, I₂.
 

E. Symbols such as (s), (l), (g), (aq), written after the formula of a compound, are sometimes used to denote the physical state of the compound; solid, liquid, gas, or aqueous solution respectively.
 

F. An outline of four types of simple general reactions (decomposition, synthesis, simple replacement, double replacement) follows:
 

I. Decomposition (analysis)
 

General Rule: x = a metal or less the electronegative element

y = a non-metal or negative polyatomic anion

xy -----> x + y
 

1. Metallic oxides, when decomposed, yield the metal plus oxygen;
   a non-metallic oxide shows a similar reaction.

e.g. 2MgO -------> 2Mg + O₂
       oxide ------- > metal + oxygen
 

     P₄O₁₀ -------> 4P + 5 O₂
      oxide   ------ > nonmetal + oxygen
 

2. A peroxide, when decomposed, yields the oxide plus oxygen.

e.g. 2H₂O₂ ^hv > 2H₂O + O₂
     peroxide ---> oxide + oxygen
 

3. A ternary salt (containing 3 elements, one of which is usually oxygen), when decomposed, will yield: a. the lower ternary salt, if such salt is stable, or b. the binary salt (containing 2 elements) plus oxygen.
 

e.g. 2NaNO₃ -----> 2NaNO₂ + O₂
      tertiary ------> lower tertiary salt + oxygen
 

2KClO₃ ----> 2KCl + 3 O₂
tertiary -----> binary + oxygen
  salt               salt

4. A carbohydrate when heated in the absence of oxygen yields carbon plus water on decomposition.

e.g. C₁₂H₂₂O₁₁ ------> 12C + 11 H₂O
 

5. Binary salts when decomposed yield free elements. Electrolysis is a common method to cause decomposition.

e.g. 2NaCl---- > 2Na + Cl₂
      2H₂O -----> 2H₂+ O₂
 

6. Most ammonium salts when heated will give off ammonia (NH₃). Notable exceptions are NH₄NO₃ and NH₄NO₂.

e.g.NH₄Cl(s)---- > NH₃ + HCl
Two notable exceptions are:
NH₄NO₂(s) ----> 2H₂O + N₂
NH₄NO₃(s) ----> 2H₂O + N₂O
 

7. Metallic carbonates when heated will yield metallic oxides and CO₂ gas.
e.g. CaCO_3(s) -----> CaO + CO_2(g)
 
 

II. Synthesis

General Rules: x + y -----> xy
 

1. When most elements burn in air, the main product is an oxide.

e.g. 2Mg + O₂ -------> 2MgO
      metal + oxygen ---> metallic oxide
 

  C + O₂ -----> CO₂
nonmetal + oxygen -----> non-metal oxide
 
 

2. A metallic oxide (basic anhydride) plus water yields a base. A nonmetal oxide (acid anhydride) plus water yields an acid. Note that the oxidation numbers of atoms in the reactants and products remain unchanged.
 

e.g. MgO + H₂O------> Mg(OH)₂
    metallic + water ----> base
    oxide

   SO₃ + H₂O ------> H₂SO₄
nonmetal + water ----> acid
    oxide
 

3. A non-metallic oxide plus a metallic oxide yields a salt.

e.g. Al₂O₃ + 3 CO₂ ------> Al₂(CO₃)₃
       metal + nonmetal -----> salt
       oxide     oxide
 

4. A metal plus a non-metal yields a salt.

e.g. 2Al + 3S -----> Al₂S₃
     metal + nonmetal -----> salt
 
 
 

III. Simple Replacement - Using the Electromotive Series = E.M.S. (See the last page.)
 

1. Positive ion replacement:        x^' + xy > x^'y + x
 

A more active metal will replace a less active metal ion to form a new ion and then free the less active metal. The more active metals are better reducing agents than the less active metals.

e.g. Mg^o + Zn⁺²(aq) > Mg⁺²(aq) + Zn^o
Note: The Zn⁺² is from a soluble salt. The negative ion are merely
spectatorions but are often written in the complete equation as follows:

Mg + Zn(NO₃)₂ > Mg(NO₃)₂ + Zn
 

2. Negative ion replacement:    y^' + xy > xy^' + y

A more active non-metal will replace a less active non-metal ion to form a new
ion and free the less active non-metal. The more active non-metals are better oxidizing agents than the less active non-metals.

e.g. F₂ + 2Cl^-(aq) ---> 2F^-(aq) + Cl₂
Note: The Cl^- came from any soluble chloride salt. The positive ion is merely a spectator present in solution before and after the reaction. The complete equation can be written as follows:

F₂ + 2KCl ---> 2KF + Cl₂
 
 
 

IV. Double Replacement Reactions  ( xy + x^'y^' > xy^' + x^'y )  go to completion for one of three reasons:

(1) a gas is evolved.
(2) an insoluble substance (precipitate) is formed
(3) an unionized (or slightly ionized) species is formed (H₂O, HC₂H₃O₂, NH₄OH, etc.).

If one of these three does not occur, then there is no reaction.
 
 

1. Gas evolved:
e.g. Na₂CO₃+ 2HCl ----> 2NaCl + H₂O + CO₂

Note: H₂CO₃, NH₄OH, H₂SO₃ may decompose to
          form CO₂, NH₃, SO₂ respectively and water
 
 

2. Formation of insoluble precipitate: (see Solubility Rules on last page).
 

BaCl₂(aq)+2AgNO₃(aq)= 2AgCl(s)+Ba(NO₃)₂(aq)
or understanding the reaction is taking place in a water media.
 

BaCl₂ + 2AgNO₃ ----> 2 AgCl(s) + Ba(NO₃)₂
 

3. a. Neutralization:

acid + base --------> salt + H₂O
HCl(aq) + NaOH(aq)-------> NaCl(aq) + H₂O
Cl^- and Na⁺ are spectators present in solution before and after the reaction. Therefore, the net ionic equation without the spectator ions is:
H⁺(aq) + OH^-(aq) ------> H₂O
 

b. Metal oxide + acid -----> salt + H₂O
e.g. CuO + 2 HNO₃ > Cu(NO₃)₂ + H₂O
Metal oxides are called basic anhydrides.
 

c. Nonmetal oxide + base > salt + H₂O
e.g. N₂O₅ + Ca(OH)₂ --> Ca(NO₃)₂+ H₂O
Non-metal oxides are called acid anhydrides.

E.M.S.                                   GENERAL RULES FOR SOLUBILITY IN WATER
 

Li                                                      1. All* nitrates, chlorates, and
K                                                         acetates are soluble. (Silver
Ba                                                        acetate is only slightly soluble.
Sr
Ca                                                     2. All* chlorides, bromides, and
Na                                                         iodides are soluble except those of
Mg                                                        lead, silver and mercury(I).
Al
Mn                                                    3. All* sulfates are soluble except for
Zn                                                          those of Ba⁺⁺, Ca⁺⁺, Pb⁺⁺, and Hg(I).
Cr
Cd                                                     4. All* common salts, of Na⁺, K⁺, NH₄⁺,
Fe                                                          are soluble.
Ni
Sn                                                      5. Sulfides of all* metals, except
Pb                                                          groups IA and IIA and NH₄⁺ are
H                                                            insoluble.
Sb
As                                                      6. The oxides and hydroxides of all*
Bi                                                           metals except Na⁺, K⁺, NH₄⁺, Ba⁺⁺,
Cu                                                         Ca⁺⁺, Sr⁺⁺ are insoluble.
Hg
Ag                                                     7. All* carbonates and phosphates are
                                                              insoluble except those of Na⁺, K⁺, and NH₄⁺
                                                          8. All* silver salts are insoluble except AgNO₃
                                                              and AgClO₄.

*The word "all" as used above, while appropriate to the level of this course, may not in all* cases be universally exclusive!