Writing Chemical Reactions

    This is a very important section on in chemistry, especially for the Advanced Placement examinations.  Writing chemical reactions is usually question 1 of section II and is always present.  You can not use a calculator and should give approximately 10 minutes for this part.  There are Eight chemical reactions and you must choose FIVE of these to predict.  You must give the unbalanced net ionic formula for the written reaction. 

Example: A strip of magnesium is added to a solution of silver nitrate

                    Mg + Ag⁺ ---> Mg²⁺ + Ag

    Each of the FIVE reactions are worth 3 points each, one point for correct reactants, two for products.  If the equation is correct but the charge on the ions is incorrect, 1 point is deducted.  This part is worth 15% of section II.   We will show various methods and tricks to predict these reactions in the following sections.

I. Metals Combining with Nonmetals (Two Uncombined Elements)

Metal + Nonmetal --> Salt (Metal Ion⁺ + Nonmetal Ion^-)

Example: Calcium metal is heated in the presence of nitrogen gas:

                Ca + N₂ ---> Ca₃N₂ 

Example: Solid potassium is added to a flask of oxygen gas:

                K + O₂ ---> KO₂

II. Combustion

Substance + Oxygen gas --> Oxide of Element

Example: Solid copper(II) sulfide is heated strongly in oxygen gas:

                CuS + O₂ ---> CuO +SO₂

Example: Methanol is burned completely in air:

                CH₃OH + O₂ ---> CO₂ + H₂O

              All alchols (along with hydrocarbons) burn in oxygen to produce CO₂ and H₂O.

III. Single Reactants (Decomposition)

AB --> A + B

When there is only one reactant, this usually means that it will undergo Decompositon.  These reactions usually produce simple salts and orxide gasses.

Example: Solid calcium carbonate is heated.

                CaCO₃ ---> CaO + CO₂

Example: A piece of solid potassium nitrate is heated.

                KNO₃ ---> KNO₂ + O₂

IV. Look for Water As A Reactant (Three rules)

Pure Metal (or Metal Hydride) + H₂O --> Base + H₂

Example:  Calcium Metal is added to distilled water.

                Ca + H₂O  ---> Ca(OH)₂ + H₂

Metal Oxide + H₂O --> Base

Example: Solid lithium oxide is added to water.

                Li₂O + H₂O ---> Li⁺ + OH^-

Nonmetal + H₂O --> Acid

Example: Carbon dioxide gas is bubbled through water.

                CO₂ + H₂O ---> H₂CO₃

V. Look for an Acid Base Neutralization (Four Rules)

See Acids & Bases Section for rules

VI. Mixture of Two Salt Solutions

The trick to this section is to be familar with your solubility rules.  If you know these, this section will not be hard at all.

Example: A solution of barium chloride is mixed with a solution of silver (I) sulfate.

                Ba²⁺ + Cl^- + Ag⁺ + SO₄²⁺ ---> BaSO₄ + AgCl

Example: A solution of ammonium sulfide is added to a solution of magnesium iodide.

                Mg²⁺ + S^2- ---> MgS

VII. A Solid Transition Metal Placed in Solution (Two Rules)

This will be a redox reaction with the solid metal being oxidized.

Neutral transition metal salt solution

Example: Solid managanese flakes are placed in a solution of copper (II) sulfate.

                Mn + Cu²⁺ ---> Mn²⁺ + Cu

Strong oxoacid solution

Example: A piece of silver is placed in dilute nitric acid.

                Ag + H⁺ + NO₃^- ---> Ag⁺ + NO + H₂O

VIII. Transition Metals Ions With Ammonia, Hydroxide, Cyanide, or Thiocyanide

Example: Excess ammonia is added to a solution of silver nitrate

                Ag⁺ + NH₃ ---> Ag(NH₃)₂⁺

Special thanks to the Cliffs AP Chemistry book and The Princeton Review  for the information on this section.