When elements chemically combine, they form compounds that can be represented by chemical formulae.
The combining power tells you how many bonds an atom can make with another atom or how many electrons its atoms will gain/lose/share to form a compound.
The combining power of group I is 1
The combining power of group II is 2
The combining power of group III is 3
The combining power of group IV is 4
The combining power of group V is 3-
The combining power of group VI is 2-
The combining power of group VII is 1-
Hydrogen has a combining power of 1
To work out chemical formula of a simple compound:
Write down the name of the compound
Write down the chemical symbol for the elements in the compound
Find combining power of each element
If number cancels out, do so
Swap combining power
If number is 1, don’t write it down
Elements bonded together is a radical
E.g. Copper (II) sulfate, the sulfate is a radical
What are radicals with the combining power of -1?
Hydroxide (OH-)
Hydrogen-carbonate (HCO3^-)
Nitrate (NO3^-)
Ammonium (NH4^+)
What are radicals with the combining power of 2?
Carbonate (CO3^2-)
Sulfate (SO4^3-)
What are radicals with the combining power of 3?
Phosphate (PO4^3-)
If formulas contains more than one radical unit, put it in brackets
E.g. Calcium + hydroxide
Ca = 2 and OH = 1
Ca(OH)2
Formula of an ionic compound can be worked out from ions present
E.g. Sodium + chloride
Sodium = 1+ and chlorine = 1-, cancels out
NaCl
Lead (II) + bromide
Lead = 2+ and Bromine = 1-, does not cancel out and needs 2 of 1- charge to cancel out
PbBr2
The reactants are the starting chemicals
The products are the finishing chemicals
To write a chemical equation:
Write down the word equation
Write down the symbols for elements and formulae for compounds
Balance the equation to make sure there are same amount of atoms on each side
Include state symbols
Some elements are diatomic, existing as 2 atoms
What are the diatomic elements?
Hydrogen
Oxygen
Nitrogen
Chlorine
Bromine
Iodine
Ionic equations show reactions involving ions (atoms or radicals that have gained or lost electrons)
In ionic reactions, ions play no part and are spectator ions
To find Mr, add up the mass of all electrons
Mole is the amount of substance
Avogrado constant is one mole of atoms, which is 6 x 10^23 atoms
Relative atomic mass tells you the mass of a mole of atoms in an element
E.g. A mole of carbon has a mass of 12g
What is the formula for moles?
Moles = mass / relative atomic mass (Mr)
Chemical equations allows quantities of reactants and products to be linked together
It tells you how much of the product you can expect to make from a fixed amount of reactants
In a balanced equation, number in front of symbol or formula indicates number of moles present
Number of moles can be converted into mass in grams
A solution is made when a solute dissolves in a solvent
The concentration of a solution depends on how much a solute is dissolved in how much solvent
How do you find moles in solution?
Moles = concentration (mol/dm3) * volume (dm3)
1dm3 = 1000cm3
What is the equation to find moles in gases?
Moles = volume / 24,000cm3
OR 24dm3
In reactions involving gas, it is better to use volume than mass
One mole of gas occupies same volume under conditions of temperature and pressure