IGCSE Online Tutorial - Jesmul Sir's Portal
In many reactions only certain ions change their 'chemical state' but other ions remain in exactly the same original physical and chemical state.
The ions that do not change physically or chemically are called spectator ions.
The ionic equation represents the 'actual' chemical change and omits the spectator ions.
Five types of examples of ionic equations are presented below including neutralisation, salt precipitation and redox equations.
(1) Acid-base reactions: Acids can be defined as proton donors. A base can be defined as a proton acceptor.
e.g. any acid-alkali neutralisation involves the hydroxide ion is (base) and this accepts a proton from an acid.
HCl(aq) + NaOH(aq) ==> NaCl(aq) + H2O(l) which can be re-written ionically as
H+Cl-(aq) + Na+OH-(aq) ==> Na+Cl-(aq) + H2O(l)
or: H+ + Cl-(aq) + Na+ + OH-(aq) ==> Na+ + Cl-(aq) + H2O(l)
H+(aq) + OH-(aq) ==> H2O(l) which is the ionic equation for neutralisation
the spectator ions are chloride Cl- and sodium Na+
(2) Insoluble salt formation: An insoluble salt is made by mixing two solutions of soluble compounds to form the insoluble compound in a process called 'precipitation'. A precipitation reaction is generally defined as 'the formation of an insoluble solid on mixing two solutions or a bubbling a gas into a solution'.
(a) Silver chloride is made by mixing solutions of solutions of silver nitrate and sodium chloride.
silver nitrate + sodium chloride ==> silver chloride + sodium nitrate
AgNO3(aq) + NaCl(aq) ==> AgCl(s) + NaNO3(aq)
in terms of ions it could be written as
Ag+NO3-(aq) + Na+Cl-(aq) ==> AgCl(s) + Na+NO3-(aq)
or: Ag+ + NO3-(aq) + Na+ + Cl-(aq) ==> AgCl(s) + Na+ + NO3-(aq)
but the spectator ions are nitrate NO3- and sodium Na+ which do not change at all,
so the ionic equation is simply: Ag+(aq) + Cl-(aq) ==> AgCl(s)
Note that ionic equations omit ions that do not change there chemical or physical state.
In this case the nitrate (NO3-(aq)) and sodium (Na+(aq)) ions do not change physically or chemically and are called spectator ions,
BUT the aqueous silver ion, Ag+(aq), combines with the aqueous chloride ion, Cl-(aq), to form the insoluble salt silver chloride, AgCl(s), thereby changing their states both chemically and physically.
(b) Lead(II) iodide, a yellow precipitate (insoluble in water!) can be made by mixing lead(II) nitrate solution with e.g. potassium iodide solution.
lead(II) nitrate + potassium iodide ==> lead(II) iodide + potassium nitrate
Pb(NO3)2(aq) + 2KI(aq) ==> PbI2(s) + 2KNO3(aq)
which can be written as
Pb2+(aq) + 2NO3-(aq) + 2K+(aq) + 2I-(aq) ==> PbI2(s) + 2K+(aq) + 2NO3-(aq)
the ionic equation is: Pb2+(aq) + 2I-(aq) ==> PbI2(s)
because the spectator ions are nitrate NO3- and potassium K+.
(c) Calcium carbonate, a white precipitate, forms on e.g. mixing calcium chloride and sodium carbonate solutions ...
calcium chloride + sodium carbonate ==> calcium carbonate + sodium chloride
CaCl2(aq) + Na2CO3(aq) ==> CaCO3(s) + 2NaCl(aq)
Ca2+(aq) + 2Cl-(aq) + 2Na+(aq) + CO32-(aq) ==> CaCO3(s) + 2Na+(aq) + 2Cl-(aq)
ionically: Ca2+(aq) + CO32-(aq) ==> CaCO3(s)
because the spectator ions are chloride Cl- and sodium Na+.
(d) Barium sulphate, a white precipitate, forms on mixing e.g. barium chloride and dilute sulphuric acid ...
barium chloride + sulphuric acid ==> barium sulphate + hydrochloric acid
BaCl2(aq) + H2SO4(aq) ==> BaSO4(s) + 2HCl(aq)
Ba2+(aq) + 2Cl-(aq) + 2H+(aq) + SO42-(aq) ==> BaSO4(s) + 2H+(aq) + 2Cl-(aq)
ionic equation: Ba2+(aq) + SO42-(aq) ==> BaSO4(s)
because the spectator ions are chloride Cl- and hydrogen H+.
(3) Redox reaction analysis:
(a) magnesium + iron(II) sulphate ==> magnesium sulphate + iron
Mg(s) + FeSO4(aq) ==> MgSO4(aq) + Fe(s)
this is the 'ordinary molecular' equation for a typical metal displacement reaction, but this does not really show what happens in terms of atoms, ions and electrons, so we use ionic equations like the one shown below.
Mg(s) + Fe2+SO42-(aq) ==> Mg2+SO42-(aq) + Fe(s)
The sulphate ion SO42-(aq) is the spectator ion, because it doesn't change in the reaction and can be omitted from the ionic equation. No electrons show up in the full equations because electrons lost by Mg must equal the electrons gained by Fe.
so the ionic-redox equation is
Mg(s) + Fe2+(aq) ==> Mg2+(aq) + Fe(s)
Mg oxidised by electron loss, Fe2+ reduced by electron gain
(b) zinc + hydrochloric acid ==> zinc chloride + hydrogen
Zn(s) + 2HCl(aq) ==> ZnCl2(aq) + H2(g)
Zn(s) + 2H+(aq) + 2Cl-(aq) ==> Zn2+(aq) + 2Cl-(aq) + H2(g)
the chloride ion Cl- is the spectator ion
Zn(s) + 2H+(aq) ==> Zn2+(aq) + H2(g)
Zinc atoms, Zn, oxidised by electron loss and hydrogen ions, H+, are reduced by electron gain
(c) copper + silver nitrate ==> silver + copper(II) nitrate
Cu(s) + 2AgNO3(aq) ==> 2Ag + Cu(NO3)2(aq)
the nitrate ion NO3- is the spectator ion
Cu(s) + 2Ag+(aq) ==> 2Ag(s) + Cu2+(aq)
Cu oxidised by electron loss, Ag+ reduced by electron gain
(d) halogen (more reactive) + halide salt (of less reactive halogen) ==> halide salt (of more reactive halogen) + halogen (less reactive)
X2(aq) + 2K+Y(aq) ==> 2K+X(aq) + Y2(aq)
X2(aq) + 2Y-(aq) ==> 2X-(aq) + Y2(aq)
the potassium ion K+ is the spectator ion
halogen X is more reactive than halogen Y, F > Cl > Br > I)
X is the oxidising agent (electron acceptor, so is reduced)
KY or Y- is the reducing agent (electron donor, so is oxidised)
(4) Ion Exchange Resins: Ion exchange polymer resin columns hold hydrogen ions or sodium ions. These can be replaced by calcium and magnesium ions when hard water passes down the column. The calcium or magnesium ions are held on the negatively charged resin. The freed hydrogen or sodium ions do not form a scum with soap.
e.g. 2[resin]-H+(s) + Ca2+(aq) ==> [resin]-Ca2+[resin]-(s) + 2H+(aq)
or 2[resin]-Na+(s) + Mg2+(aq) ==> [resin]-Mg2+[resin]-(s) + 2Na+(aq) etc.
(5) Scum formation with hard water: On mixing hard water with soaps made from the sodium salts of fatty acids, insoluble calcium or magnesium salts of the soap are formed as a grey precipitate ...
CaSO4(aq) + 2C17H35COONa(aq) ==> (C17H35COO)2Ca(s for scum!) + Na2SO4(aq)
or more simply ionically: Ca2+(aq) + 2C17H35COO-(aq) ==> (C17H35COO-)2Ca2+(s)
the spectator ions are SO42- and Na+
The valency of an atom or group of atoms is its numerical combining power with other atoms or groups of atoms.
The theory behind this, is all about stable electron structures!
The combining power or valency is related to the number of outer electrons.
You need to consult the page on 'Bonding' to get the electronic background.
A group of atoms, which is part of a formula, with a definite composition, is sometimes referred to as a radical.
In the case of ions, the charge on the ion is its valency or combining power (list below).
To work out a formula by combining 'A' with 'B' the rule is:
number of 'A' x valency of 'A' = number of 'B' x valency of 'B',
However it is easier perhaps to grasp with ionic compound formulae.
In the electrically balanced stable formula, the total positive ionic charge must equal the total negative ionic charge. Example:
Aluminium oxide consists of aluminium ions Al3+ and oxide ions O2-
number of Al3+ x charge on Al3+ = number of O2- x charge on O2-
the simplest numbers are 2 of Al3+ x 3 = 3 of O2- x 2 (total 6+ balances total 6-)
so the simplest whole number formula for aluminium oxide is Al2O3
Examples of ionic combining power of ions (table right) valency = numerical charge value. Examples of covalent combining power of atoms (valencies below)
Hydrogen H (1)
Chlorine Cl and other halogens (1)
Oxygen O and sulphur S (2)
Boron B and aluminium Al (3)
Nitrogen (3, 4, 5)
Carbon C and silicon Si (4)
Phosphorus (P 3,5)
Examples of working out covalent formulae | ||
'A' (valency) | 'B' (valency) | deduced formula of A + B |
1 of carbon C (4) | balances 4 of hydrogen H (1) | 1 x 4 = 4 x 1 = CH4 |
1 of nitrogen (3) | balances 3 of chlorine Cl (1) | 1 x 3 = 3 x 1 = NCl3 |
1 of carbon C (4) | balances 2 of oxygen O (2) | 1 x 4 = 2 x 2 = CO2 |
The
diagram on the left illustrates the three covalent examples above for
methane CH4 nitrogen trichloride NCl3 carbon dioxide CO2 |
||
Examples of working out ionic formulae | ||
numerically charge = valency of A or B to deduce the formula, 'molecular' or ionic style and compound name | ||
2 of Na+ balances 1 of O2- because 2 x 1 = 1 x 2 = Na2O or (Na+)2O2- sodium oxide | ||
1 of Mg2+ balances 2 of Cl- because 1 x 2 = 2 x 1 = MgCl2 or Mg2+(Cl-)2 magnesium chloride | ||
1 of Fe3+ balances 3 of F- because 1 x 3 = 3 x 1 = FeF3 or Fe3+(F-)3 iron(III) fluoride | ||
1 of Ca2+ balances 2 of NO3- because 1 x 2 = 2 x 1 = Ca(NO3)2 or Ca2+(NO3-)2 calcium nitrate | ||
2 of Fe3+ balances 3 of SO42- because 2 x 3 = 3 x 2 = Fe2(SO4)3 or (Fe3+)2(SO42-)3 iron(III) sulphate |
When combined with other elements in simple compounds the name of the non-metallic element changes slightly from ...??? to ...ide.
Sulphur forms a sulphide (ion S2-), oxygen forms an oxide (ion O2-), fluorine forms a fluoride (ion F-), chlorine forms a chloride (ion Cl-), bromine a bromide (ion Br-) and iodine an iodide (ion I-).
The other element at the start of the compound name e.g. hydrogen or a metal like sodium, potassium, magnesium, calcium, etc. usually remains unchanged in simple compounds at KS3-GCSE level.
So typical compound names are, sodium sulphide, hydrogen sulphide, magnesium oxide, potassium fluoride, hydrogen chloride, sodium chloride, calcium bromide, magnesium iodide etc.
However, even at GCSE level the complications will arise e.g.
(i) Where an element can form two different compounds with different formulae with the same element there needs to be a way of expressing it in the name as well as in the formula e.g.
iron(II) chloride, FeCl2 and iron(III) chloride, FeCl3
copper(I) oxide, Cu2O and copper(II) oxide, CuO
Hear chlorine has a combining power of 1 (valency 1) and oxygen 2 in both compounds.
However, iron can have a valency of 2 or 3 and copper 1 or 2 and these also correspond numerically to the charge on the metal ions in such compounds e.g. Fe2+ and Fe3+, Cu+ and Cu2+.
Therefore the 'Roman numerals' number in () gives the valency of the element in that particular compound. At a higher academic level this is known as the oxidation state.
(ii) When the non-metal is combined with oxygen to form a negative ion (anion) ion which combines with a positive ion (cation) from hydrogen or a metal, then the end of the 2nd part of the name ends in ...ate or ...ite e.g.
NO3 in a compound formula is nitrate e.g. KNO3, potassium nitrate.
SO3 in a formula is sulphite, e.g. Na2SO3, sodium sulphite,
SO4 is sulphate, e.g. MgSO4, magnesium sulphate,
PO4 is phosphate, e.g. Na2HPO4, disodium hydrogen phosphate