Stoichiometry

Symbols & Formulae of Elements & Compounds#

• Each element is represented by its own unique symbol as seen on the Periodic Table e.g. H is hydrogen

• Where a symbol contains two letters, the first one is always in capital letters and the other is small, eg. sodium is Na, not NA

• Atoms combine together in fixed ratios that will give them full outer shells of electrons

• The chemical formula tells you the ratio of atoms

• Eg. H₂O is a compound containing 2 hydrogen atoms which combine with 1 oxygen atom

• The chemical formula can be deduced from the relative number of atoms present

• Eg. if a molecule contains 3 atoms of hydrogen and 1 atom of nitrogen then the formula would be NH₃

• Diagrams or models can also be used to represent the chemical formula

• The structural formula tells you the way in which the atoms in a particular molecule are bonded. This can be done by either a diagram (displayed formula) or written (simplified structural formula)

• The empirical formula tells you the simplest whole-number ratio of atoms in a compound

• The molecular formula tells you the actual number of atoms of each element in one molecule of the compound or element e.g. H₂ has 2 hydrogen atoms, HCl has 1 hydrogen atom and 1 chlorine atom

• The concept of valency is used to deduce the formulae of compounds

• Valency or combing power tells you how many bonds an atom can make with another atom

• Eg. carbon is in Group IV so a single carbon atom can make 4 single bonds or 2 double bonds

Deducing Formulae of Ionic Compounds#

• The formulae of these compounds can be calculated if you know the charge on the ions.

• Below are some common ions and their charges:

• For ionic compounds you have to balance the charge of each part by multiplying each ion until the sum of the charges = 0

• Example: what is the formula of aluminium sulfate?

  • Write out the formulae of each ion, including their charges
  • Al³⁺ SO₄^2-

• Balance the charges by multiplying them out:

  • Al³⁺ x 2 = +6 and SO₄^2- x 3 = -6; so +6 – 6 = 0

• So the formula is Al₂(SO₄)₃

Writing Word Equations & Balanced Equations#

• These show the reactants and products of a chemical reaction using their full chemical names

• The arrow (which is spoken as “goes to” or “produces”) implies the conversion of reactants into products

• Reaction conditions or the name of a catalyst can be written above the arrow

For compounds consisting of 2 atoms:

• If one is a metal and the other a nonmetal, then the name of the metal atom comes first and the ending of the second atom is replaced by adding –ide

  • Eg. NaCl which contains sodium and chlorine thus becomes sodium chloride

• If both atoms are nonmetals and one of those is hydrogen, then hydrogen comes first

  • Eg. hydrogen and chlorine combined is called hydrogen chloride

• For other combinations of nonmetals as a general rule, the element that has a lower Group number comes first in the name

  • Eg. carbon and oxygen combine to form CO₂ which is carbon dioxide since carbon is in Group 4 and oxygen in Group 6

For compounds that contain certain groups of atoms:

• There are common groups of atoms which occur regularly in chemistry

• Examples include the carbonate ion(CO₃^2-), sulfate ion (SO₄^2-), hydroxide ion (OH^–) and the nitrate ion (NO₃^–)

• When these ions form a compound with a metal atom, the name of the metal comes first

• Eg. KOH is potassium hydroxide, CaCO₃ is calcium carbonate

• These use the chemical symbols of each reactant and product

• When balancing equations, there needs to be the same number of atoms of each element on either side of the equation

• The following nonmetals must be written as molecules: H₂, N₂, O₂, F₂, Cl₂, Br₂ and I₂

• Work across the equation from left to right, checking one element after another

• If there is a group of atoms, for example a nitrate group (NO3–) that has not changed from one side to the other, then count the whole group as one entity rather than counting the individual atoms.

• There are equal numbers of each atom on either side of the reaction arrow so the equation is balanced.