Stoichiometry
Stoichiometry deals with the relative quantities of reactants and products in chemical reactions. In a balanced chemical reaction, the relations among quantities of reactants and products typically form a ratio of whole numbers. For example, in a reaction that forms ammonia (NH3), exactly one molecule of nitrogen (N2) reacts with three molecules of hydrogen (H2) to produce two molecules of NH3
N2 + 3H2 → 2NH3
Stoichiometry can be used to calculate quantities such as the amount of products (in mass, moles, volume, etc.) that can be produced with given reactants and percent yield (the percentage of the given reactant that is made into the product). Stoichiometry calculations can predict how elements and components diluted in a standard solution react in experimental conditions. Stoichiometry is founded on the law of conservation of mass: the mass of the reactants equals the mass of the products.
The Periodic Table and Charges:
| Group (Charge) |
1 (+1) |
2 (+2) |
Transition metals |
3 (+3) |
4 (±4) |
5 (-3) |
6 (-2) |
7 (-1) |
| Ions present |
Li+ Na+ K+ |
Be2+ Mg2+ Ca2+ Ba2+ |
Cu2+ / Cu+ Fe2+ / Fe3+ Zn2+ Ag+ |
Al3+ | C Si Pb2+ |
N3- P3- |
O2- S2- |
F- Cl- Br- I- |
Compound Ions:
They are like molecules but with a charge on them:
| Oxidation State | Name | Symbol |
| +1 | Ammonium Ion | NH4+ |
| -1 | Hydroxide Ion Nitrate Ion Nitrite Ion Manganate(VII) Oxide Ion Hydrogen Carbonate Ion |
OH- NO3- NO2- MnO4- HCO3- |
| -2 | Carbonate Ion Sulfate Ion Sulfite Ion Dichromate (Vi) Ion |
CO32- SO42- SO32- Cr2O72- |
| -3 | Phosphate Ion Phostphite Ion |
PO43- PO33- |
Acids and Their Formulae:
| Acid | Formula |
| Carbonic Acid | H2CO3 |
| Hydrochloric Acid | HCl |
| Nitric Acid | HNO3 |
| Phosphoric Acid | H3PO4 |
| Sulfuric Acid | H2SO4 |
Chemical Changes:
When chemical reactions take place, a certain change is certain. This change could be in color, energy or something like bubbles or a precipitate formation.
Color Change in Reactions:
When iron, which is grey, reacts with sulfur, which is yellow, a black solid, iron sulfide is formed. The color changed from grey and yellow to black.
| Iron | + | Sulpher | → | Iron Sulfide |
| (grey fillings) | (yellow powder) | (black solid) |
Energy Change in Reactions:
Energy is not necessarily heat, there are several forms of energy like Light and sound energy too.
When magnesium reacts with oxygen in air to make a combustion reaction, it burns with a very bright flame forming magnesium oxide. This bright flame is light energy which indicates a reaction took place.
Some reactions also release energy in form of sound like explosive reactions.
Effervescence in Reactions:
In some reactions, bubbles are formed rapidly. Those bubbles are indication that a gas was formed which is a new substance, this means a reaction took place.
When calcium carbonate reacts with hydrochloric acid, carbon dioxide gas is released in bubbles.
CaCO3 + 2HCl → CaCl2 + H2O + CO2
Precipitation in Reactions:
When two solutions react together, sometimes one of the products is an insoluble solid called precipitate. When silver nitrate solution reacts with a potassium chloride solution, white insoluble silver chloride is formed which is a precipitate.
AgNO3 + KCl → AgCl + KNO3
Writing and Balancing Equations:
Chemical reactions are always represented by chemical equations to show the reactants and the products. There are two types of chemical equations. These are word equations and symbolic equations.
For example if we want to represent the reaction between sodium and chlorine which produces sodium chloride as a word equation it will be like this:
Sodium + Chlorine → Sodium Chloride
If we want to represent the same reaction by a symbolic equation it will be like this:
Na + Cl2 → NaCl
The above symbolic equation however, is not chemically accurate. This is why chemists prefer to use what’s called a balanced symbolic equation. In this type of equation, we make sure that the number of atoms of each element in the reactants is the same in the products. If you look at the equation above you will find that we have one Na atom in the reactants and one Na+ ion in the product, but we have 2 Cl atoms in the reactants and only one Cl ion in the product. This is why this equation needs to be balanced as follows:
2Na + Cl2 → 2NaCl
Types of Chemical Reactions:
Combustion (oxidation) Reactions:
These are reactions between metals and oxygen or the oxidation of any other element.
| 2Mg | + | O2 | → | 2Li2O |
| (Magnesium) | (Oxygen) | (Magnesium Oxide) |
Hydration Reactions:
These are reactions between metals and water forming a metal hydroxide and hydrogen gas. It could also be a reaction between a salt and water forming a hydrated salt.
| 2Na | + | 2H2O | → | 2NaOH | + | H2 |
| (Sodium) | (Water) | (Sodium Hydroxide) | (Hydrogen) |
| CuSO4 | + | 5H2O | → | CuSO4.5H2O |
| (Copper Sulfate) | (Water) | (Hydrated Copper Sulfate) |
Displacement Reactions:
These are the reactions in which the more reactive metal displaces the less reactive positive ion. In the following reaction, sodium displaces Hydrogen ion from the hydrochloric acid because sodium is more reactive. The products are sodium chloride and hydrogen gas.
| 2Na | + | 2HCl | → | 2NaCl | + | H2 |
| (Sodium) | (Hydrochloric Acid) | (Sodium Chloride) | (Hydrogen) |
Neutralization Reactions:
These are reactions between an alkaline compound or and alkali and an acidic compound or an acid forming a salt, water and sometimes carbon dioxide.
In the following reaction sodium hydroxide is a base which reacts with sulfuric acid. Neutralization takes place producing a salt called sodium sulfate and water.
| 2NaOH | + | H2SO4 | → | Na2SO4 | + | 2H2O |
| (Sodium Hydroxide) | (Sulfuric Acid) | (Sodium Sulfate) | (Water) |
In the following reaction calcium carbonate which is a basic compound reacts with nitric acid forming a salt called calcium nitrate, water and carbon dioxide.
| CaCO3 | + | 2HNO3 | → | Ca(NO3)2 | + | H2O | + | CO2 |
| (Calcium Carbonate) | (Nitric Acid) | (Calcium Nitrate) | (Water) | (Carbon dioxide) |