Ralph & Bev Shock  

A chemical equation shows changes that take place during a chemical reaction. The equation can be described in words: Silver nitrate plus sodium chloride produces silver chloride plus sodium nitrate. However, it is faster to write:

AgNO₃ + NaCl --> AgCl + NaNO₃

The substances to the left of the arrow are called reactants. Substances to the right of the arrow are call products. The arrow itself means yields.

The states of the reactants may decide whether a reaction will take place. For example, when solid sodium chloride and silver nitrate are mixed, no reaction takes place. These substances must be in solutions for the reaction to take place. The products are silver chloride which forms a precipitate and sodium nitrate in solution. A precepitate is an insoluble product that either settles to the bottom of the reaction vessle or makes the solution cloudy.

To indicate the states of the reactants and products in and equation, state symbols are used. For the previous reaction we would use (aq) for aqueous solution and (s) for the solid precipitate formed.

AgNO₃(aq) + NaCl(aq) --> AgCl(s) + NaNO₃(aq)

Other state symbols include (g) for gaseous and (L) for liquid.

In chemical reactions , mass is neither created nor destroyed and must be accounted for. (Law of Conservation of Mass)

To conform to the law of conservation of mass, equations must be balanced. Chemical equations are balanced with coefficients (numbers placed in front of chemical formulas). Coefficients should represent the combination of lowest whole numbers which balance atoms on both sides of an equation. They show the relative amounts of each element or compound in the reaction.

EXAMPLE: 2KBr(aq) + Cl₂(aq) --> 2KCl(aq) + Br₂(l)

This equation shows that 2 formula units of potassium bromide reacts with one diatomic molecule of chlorine in water to form 2 formula units of potassium chloride in water and diatomic bromine as a liquid.

TO WRITE A BALANCED CHEMICAL EQUATION FOLLOW THESE STEPS:

1. NEVER CHANGE A FORMULA TO BALANCE AN EQUATION. IN OTHER WORDS DONT'S CHANGE SUBSCRIPTS!!
2. Write correct symbols and formulas for the substances in the reaction.
3. Check the number and kinds of atoms on each side to see whether the equation is balanced.
4. If the number of atoms are not in balance, use coefficients to balance the equation.
a. Balance the elements one at time. (If possible save hydrogen and oxygen to the last.)
b. If polyatomic ions are unchanged treat them as a single element. !!!!!! Treat polatomic ions as a single unit !!!!!!
c. When no coefficient is used it is assumed to be 1.
5. Make sure that all the coefficients are in the lowest possible whole number ratio.
Problem:

Example:

Aluminum sulfate and barium chloride react on aqueous solution to form barium sulfate and aluminum chloride. The barium sulfate forms a precipitate and aluminum chloride is soluble in water.
1. What is the charge of an aluminum ion?

2. What is the charge of sulfate polyatomic ion?

3. What is the charge of a barium ion?

4. What is the charge of a chloride ion?

5. Write the skeleton equation for this reaction.

Al₂(SO₄)₃ + BaCl₂ ---------> BaSO₄ + AlCl₃ .

6. Use coefficients to balance the equation and fill in state symbols.

Since there are two aluminum ions on the left, write a coefficient of 3 in front of aluminum chloride. This gives us 6 chlorine atoms so place a 3 infront of barium chloride. This gives us 3 barium atoms so we place a 3 infront of Bariums sulfate. This gave us 3 sulfates on the right which balances with the 3 in aluminum sulfate.

Al₂(SO₄)₃ + 3BaCl₂ ---------> 3BaSO₄ + 2AlCl₃ .

When doing chemical reactions it is important to be able to predict the outcome of the reactios. Based on some simple rules and example we can predict the products we get when a reaction is carried out. However, before we can predict our products we must correctly determine the formulas of our reactants. Lets review the the steps for writing formulas and what you will need to do inorder to predict your products.

To be successful in writing equations you must:
1. Know the symbols for the common elements.
2. Know the usual oxidation numbers of the common elements and complex ions.
3. Know the facts about the reaction for which a reaction is written.

I. Factors in writing chemical equations: (The following factors must be correct in every detail.)

A. The equation must represent the facts.
1. It must be a reaction that does occur.
2. It must be correct for each reactant and each product.

B. The equation must include the symbols and formulas of all elements and compounds which are used as reactants and which are formed as products. All formulas must be written correctly.

C. The law of conservation of atoms must be satisfied. This is done by adjusting the coefficients in front of the symbol. Under no circumstances do you change a formula while balancing an equation. Oxidation numbers are used to determine formulas! not balance equations.

II. Procedures for writing chemical equations.
A. Know the facts. What is reacting? What will be produced from that reaction?

B. Write each formula correctly.

C. Balance the equation as to atoms. (Law of conservation of matter- you must end up with the same number of atoms of each element as you have in the reactants.)

Change the coefficients (the numbers before the formulas) to balance.
Examples:

H₂ + O₂ --> H₂O

balanced: 2H₂ + O₂ --> 2H₂O

S + O₂ --> SO₂

balanced: S + O₂ --> SO₂

Zn + HCl --> ZnCl₂ + H2

balanced: Zn + 2HCl --> ZnCl₂ + H₂

Al₂(SO₄)₃ + Ca(OH)₂ --> CaSO₄ + Al(OH)₃

balanced: Al₂(SO₄)₃ + 3Ca(OH)₂ --> 3CaSO₄ + 2Al(OH)₃

HOW DO YOU DETERMINE THE PRODUCTS OF A REACTION ARE?
The following information gives you the four major types of chemical reactions and the subgroups.

The major headings are the types of reaction, and the subtype or subgroup is the "reason" for the reaction.

GENERAL TYPES OF CHEMICAL REACTIONS:
1. Composition (combination, synthesis): Two simple substances combine to make a more complex substance. A + X --> AX
SUBTYPES

a. Two elements combine to form a compound.
2Na + Cl₂ --> 2 NaCl

b. Water and a nonmetal oxide combine to form an acid.
H₂O + SO₂ --> H₂SO₃

c. Water and a metal oxide combine to form a metal hydroxide (a base).
H₂O + CaO --> Ca(OH)₂

d. Carbon dioxide and a metal oxide combine to form a metal carbonate.
CO₂ + CaO --> CaCO₃

e. Metal oxide and nonmetal oxide combine to form a metal with a polyatomic ion. (Similar to reaction b)
Ag₂O +SO₂ --> Ag₂SO₃




2. Decomposition: One substance breaks into two or more simpler substances. AX --> A + X
a. Metallic carbonates when heated break into metal oxides and carbon dioxide.
CaCO₃ + Heat --> CaO +CO₂

b. Metallic hydroxides when heated break into metallic oxides and water.
Ca(OH)₂ + Heat --> CaO + H₂O

c. Metallic chlorates when heated break into metallic chlorides and oxygen.
2KClO₃ + Heat --> 2KCl + 3O₂

d. Some oxyacids when heated decompose into nonmetal oxides and water. (Memorize these two.)
H₂CO3 --> H₂O + CO₂ H₂SO₃ --> H₂O + SO₂

e. Some oxides of less active metals when heated decompose.
2HgO + Heat --> 2Hg + O₂ 2PbO₂ + Heat --> 2PbO + O₂

f. Electrolysis will drive some decompositions.
2NaCl(l) + DC current --> 2Na + Cl2

3. Replacement (single replacement): An element reacts with a compound in solution to replace a less active similar element from the compound.
AX + B --> BX + A AX + Y --> AY + X

a. Replacement of a metal in a compound by a more reactive metal.
2Au(NO₃)₃ + 3Pb --> 3Pb(NO₃)₂ + 2Au

b. Repacement of hydrogen in water by very active metals to produce hydrogen gas and the metal hydroxide.
2Na + 2H₂O --> H₂ + 2NaOH

c. Replacement of hydrogen in acids by metals.
6HCl + 2Fe --> 2FeCl₃ + H₂

d. Replacement of a halogen in a compound by a more reactive halogen.
2NaI + Cl₂ --> 2NaCl + I₂

4. Ionic (double replacement, double displacement): Two ionic compounds are brought together, normally in water solution, and the positive ion of the first combines with the negative ion of the second and vice versa. For this type of reaction to proceed ions must be removed from solution. This can be accomplished by three types of reactions.

a. An insoluble product (a precipitate) is produced.
HCl(aq) + AgNO₃(aq) --> AgCl(s) + HNO₃(aq)

b. A product is produced which will decompose into a gas.
2HCl(aq) + CaCO₃(s) --> CaCl₂(aq) + H₂O(l) + CO₂(g)

c. Ions combine to form a molecular product, usually water.
HCl(aq) + NaOH(aq) --> NaCl(aq) + H₂O(l)

5. Combustion reation: Organic compound such a carbohydrate or hydrocarbon combine with oxygen to form carbon dioxide and water.

octane + oxygen ----> water and carbon dioxide
2C₈H₁₈ + 25O₂ ----------- 16CO₂ + 18H₂0