Chemical Reactions

Chemical reactions make up the real "guts" of chemistry.
Think about it....what would chemistry be without them?

You know a chemical reaction occurs if:

• Color changes
• Heat content changes
• A gas is produced

Key points to remember when dealing with Chemical Reactions

Don't write a "wrong" equation. For example, 1 H₂ + O₂ à 1H₂O₂ (when what you want is water!)

Don't make a "correct" formula into a "wrong" one. For example, AlI₃ + Cl₂à AlCl₃ + I₂ (you wouldn't want to make the I₂ into an I₃!) Use your knowledge to write correct formulas (remember the diatomic molecules and write correct formulas using standard nomenclature rules). The meaning of numerical coefficients is important.

Coefficients show a ratio.

Coefficients show quantities of each type of particle represented in the equation.

Molecules represent covalent compounds, formula units represent ionic compounds and atoms are individual particles for the free elements.

The numerical coefficient represents a certain number of MOLES of the substance OR a certain number of molecules, formula units or atoms.

Single Replacement
A single element replaces an element in a compound. Remember, metals always replace metals and non-metals always replace non-metals!

Use your ACTIVITY SERIES SHEET!

Zn + 2HC l à ZnCl₂ + H₂
Replacement of hydrogen in water by metals:

Group A metal + H₂O àH₂ + metallic hydroxide

Group B metal + H₂OàH₂ + metallic oxide

Group C metal + H₂à no reaction

Double Replacement
An element from each of two compounds switch places. Either the positive ions switch places or the negative ions switch places. Note: it is sometimes easy to get this confused with a synthesis reactions but if you do, you will get one of the same products as you have reactants (an oxide).

2 Al(NO₃)₃ + 3 Na₂CO₃ à 1 Al₂(CO₃)₃ + 6 NaNO₃

H₂SO₄ + 2NaOH à Na₂SO₄ + 2H₂O
Remember that an acid plus a base will always give you a salt and water.

Decomposition
A compound breaks into parts.
2H₂O à 2H₂ + O₂
Some complications with heat:

Some acids, when heated, decompose into a non-metallic oxide and H₂O.
H₂SO₃ à SO₂ + H₂O

Metallic hydroxides, when heated, decompose into a metallic oxide and H₂O.
Ca(OH)₂ àCaO + H₂O

Metallic carbonates, when heated, decompose into a metallic oxide and CO₂.
Li₂CO₃ àLi₂O + CO₂

Metallic chlorates, when heated, decompose into metallic chlorides and O₂.
2KClO₃ à 2KCl + O₂

Metallic oxides, when heated, decompose into a metal and O₂ (See #6 on the Activity Series Sheet!)
2 HgO à 2 Hg + O₂

Synthesis

These reactions are the opposite of the decomposition reactions -- therefore, if you know your decomposition reactions, these synthesis reactions should be pretty easy to recognize. Again, use the Activity Series Sheet but pay particular attention to #4 there!

Elements are joined together.
2H₂ + O₂ à2H₂O

Compounds are joined together.
CaO (s) + H₂O (l)à Ca(OH)₂ (s)

Combustion
An organic compound (typically a hydrocarbon) combines with oxygen. Frankly, these reactions are the "easiest" to predict (because the substances made are always carbon dioxide and water!) but they are often a "pain" to balance.

CH₄ + 2O₂ àCO₂ + 2H₂O

2C₈H₁₈ + 25O₂ à 16CO₂ + 18H₂O

When you learn to recognize reaction types, you can predict the products of a reaction if the reactants are known. In fact, it really is imperative that you learn the fundamental types of chemical reactions (kind of like learning the irregular verb tenses in French! You just gotta sit down and do it!).

Predict the names of the products in the following reactions:
Magnesium + Oxygen à
Chlorine + Lithium Iodide à
Aluminum + Lead (II) Nitrate à
Calcium Oxide + Hydrochloric Acid à
Magnesium Phosphate + Lithium Sulfate à
Ammonium Phosphate + Barium Hydroxide à
C₄H₁₀+ Oxygen à
Sulfuric Acid + Potassiumà
Silver Nitrate + Potassium Chloride à