Stoichiometry

Stoichiometry is the study of quantitative relationships between substances undergoing chemical changes.

History[edit]

In the late 1700s a wealthy French nobleman named Antoine Lavoisier began a significant series of careful, quantitative studies of chemical reactions. He observed what countless subsequent chemists have verified: The total mass of all substances present after a chemical reaction is the same as the total mass before the reaction. This fact is known as the Law of Conservation of Mass, one of the fundamental laws of chemistry.

In 1789, Lavoisier published a textbook of chemistry in which he stated:

We may lay it down as an incontestable axiom that, in all the operations of art and nature, nothing is created; an equal quantity of matter exists both before and after the experiment.^[1]

Chemists today state the law in a simpler and more cautious fashion: There is no detectable change in mass in any chemical reaction. This modification was due to the evidence that when an object loses energy it loses mass and when ever it gains energy it gains mass. These changes in mass are too small to detect in chemical reactions. However, for nuclear reactions, such as those involved in a nuclear reactor or in a hydrogen bomb, the energy changes are enormously larger; in these reactions there are detectable changes in mass.

The Chemical Equation[edit]

Chemical reactions can be represented in a concise way using chemical equations. For example, when hydrogen, H₂, burns it reacts with molecular oxygen, O₂, in the air to form water, H₂O. The chemical equation for the reaction is below:

2 H₂+ O₂ → 2 H₂O

The + sign means "reacts with" and the arrow is read as "produces." The chemical formulas on the left of the arrow represent the starting substances, called reactants. The substances produced in a reaction, called products, are shown to the right of the arrow. The numbers in the front of the formulas are called coefficients.

Because atoms are neither created nor destroyed in any non nuclear reaction, an equation must have an equal number of atoms of each element on each side of the arrow. When this condition is met, the equation is said to be balanced. The number of atoms of an element is the product of the coefficient in front of a formula and the subscript for that element in the formula. Thus 2H₂O contains 2 x 3 = 4 hydrogen atoms and 2 x 1 = 2 oxygen atoms.

Common Types of Chemical Reactions[edit]

Combination reactions[edit]

In a combination reaction two reactants combine to form a single product. Many elements react with one another in this fashion to form compounds.

A + B → AB

C (s) + O₂ (g) → CO₂ (g)

N₂ (g) + 3H₂ (g) → 2NH₃ (g)

CaO (s) + H₂O (l) → Ca(OH)₂ (s)

Decomposition reactions[edit]

A single reactant breaks apart to form two or more substances. Many compounds behave in this fashion when energy is added to the system (heating for example).

AB → A + B

2KClO₃ (s) → 2KCl (s) + 3O₂ (g)

CaCO₃ (s) → CaO (s) + CO₂ (g)

Single Displacement reactions[edit]

One element replaces another in a compound. (The elements are often hydrogen or a metal.)

A + BX → AX + B

Fe (s) + 2HCl (aq) → FeCl₂ (aq) + H₂ (g)

Zn (s) + CuSO₄ (aq) → ZnSO₄ (aq) + Cu (s)

2Na (s) + 2H₂O (l) → 2NaOH (aq) + H₂ (g)

Double Displacement (metathesis) reactions[edit]

Atoms or ions exchange partners. There are two kinds of reactions of this type, precipitation and acid-base reactions.

AX + BY → AY + BX

Precipitation

BaBr₂ (aq) + K₂SO₄ (aq) → 2KBr (aq) + BaSO₄ (s)

Acid-Base

Ca(OH)₂ (aq) + 2HCl (aq) → CaCl₂ (aq) + 2H₂O (l)

References[edit]