Summary notes for CIE IGCSE Chemistry Topic 4: Stoichiometry
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Course
Chemistry
Institution
GCSE
Complete revision notes for Topic 4 of the CIE IGCSE Chemistry course: Stoichiometry. Explanations with diagrams for every specification point. These notes are written for candidates taking the Extended paper.
Use the symbols of the elements and write the formulae of simple compounds
Deduce the formula of a simple compound from a model or a diagrammatic representation
Types of chemical formulae
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
Example: butane
Structural formula (displayed)
Structural formula (simplified)
CH3CH2CH2CH3
Molecular formula
C4H10
Empirical formula
C2 H 5
,Deduce the formula of a simple compound from the relative numbers of atoms present
Using valency to determine formulae
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
E.g. carbon is in Group IV so a single carbon atom can make 4 single bonds or 2 double
bonds
The following valencies apply to elements in each group:
How to use in practice
Example: what is the formula of aluminium sulphide?
Write out the symbols of each element and write their combining powers
underneath:
Al S
3 2
The formula is then calculated by cross multiplying each atom with the number opposite,
hence the formula for aluminium sulphide is Al2S3
Determine the formula of an ionic compound from the charges on the ions present
The formulae of these compounds can be calculated if you know the charge on the ions.
Below are some common ions and their charges:
, When combining these ions, you need to find the lowest common denominator and then
multiply the ENTIRE ion within the compound.
Construct word equations and simple balanced chemical 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
Naming compounds
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
o E.g. 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
o E.g. 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
o E.g. 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:
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