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 Doc Brown's Chemistry KS4 science GCSE/IGCSE/O Level Chemistry Revision Notes

ELEMENTS, COMPOUNDS, MIXTURE  separation, CHEMICAL REACTIONS & EQUATIONS

(c) doc b

NaOH + HCl ===> NaCl + H2O

PART 3 How do we write chemical equations?, How do we construct equations from first principles? Examples of writing word equations and balanced symbol equations, what is the valency of an element? What are ionic equations? How do we write ionic equations? How can we use valency to work out a formula? - index of keywords-terms-phrases below

(c) doc b

Al2O3 + 3H2SO4 ==> Al2(SO4)3 + 3H2O

Al2O3(s) + 3H2SO4(aq) ==> Al2(SO4)3(aq) + 3H2O(l)  (with state symbols)

Part 1 Definitions in Chemistry, Elements, Compounds & Mixture pictures & Physical & Chemical Changes

Part 2 Methods of Separating Mixtures of substances

Part 3 How to write equations, valency, work out formula & name compounds (this page)

Alphabetical list of KEYWORDS for Parts 1-3: atom  *  balancing equations (work your way down the section carefully)  *  centrifuges/centrifuging  *  chemical reaction/change  *  chromatography (paper/thin layer)  *  compound  *  covalencycrystallisation  *  decanting/decantation  * displayed formula  *  distillation (simple/fractional)  *  element  *  equations  *  evaporation  *  filtration  *  formula  *  impure/pure  *  insoluble  *  ionic equations  *  ionic valency  *  iron-sulphur separation and heating experiment  *  magnet  *  mixture  *  molecule  *  naming compounds and ions  *  particle pictures of elements/compounds/mixtures  *  physical change  *  precipitation  *  products  *  pure substance  *  purification  *  reactants  *  sand/salt separation  *  separating funnel  *  separating mixtures  *  soluble/solution/solvent/solute  *  solvent extraction  *  symbols (for elements, formula, in equations)  *   state symbols  *  valency  * working out formulae  *

Spelling note

sulphur = sulfur, sulphuric acid = sulfuric acid, sulphate = sulfate, sulphite = sulfite, aluminium = aluminum (US)

State symbols used in chemical equations

(g) means gas, (l) means liquid, (s) means solid, and (aq) means aqueous solution or dissolved in water

 

    top3.1 THE CONSTRUCTION OF CHEMICAL EQUATIONS

How do we write equations?

"How to write and understand chemical equations" (3rd draft)

Seven equations are presented, but approached in the following way

The individual symbols and formulae are explained

The word equation is presented to summarise the change of reactants to products.

A balanced 'picture' equation which helps you understand reading formulae and atom counting to balance the equation.

The fully written out symbol equation with state symbols (often optional for starter students).

top3.1a Chemical Symbols and Formula 

For any reaction, what you start with are called the reactants, and what you form are called the products.

So any chemical equation shows in some way the overall chemical change of  ...

REACTANTS ==> PRODUCTS, which can be written in words or symbols/formulae.

It is most important you read about formula in an earlier section of this page.

Empirical formula and molecular formula are dealt with on another page.

In the equations outlined below several things have been deliberately simplified. This is to allow the 'starter' chemistry student to concentrate on understanding formulae and balancing chemical equations. Some teachers may disagree with this approach BUT my simplifications are:

The word 'molecule' is sometimes loosely used to mean a 'formula'.

The real 3D shape of the 'molecule' and the 'relative size' of the different element atoms is ignored.

If the compound is ionic, the ion structure and charge is ignored, its just treated as a 'formula'.

 

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3.1b Chemical word equations

==> means the direction of change from reactants == to ==> products

No symbols or numbers are used in word equations.

Always try to fit all the words neatly lined up from left to right, especially if its a long word equation.

 

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3.1c Balancing Symbol equations

Writing the correct symbol or formula for each equation component.

Numbers in a formula are written as subscripts after the number of atoms of the element concerned

e.g. H2SO4 means 2 H's, 1 S and 4 O's

or the subscript number can double, treble etc. a part of the formula

e.g. Ca(OH)2 means 1 Ca and 2 OH's (or 2 O's and 2 H's in total)

Numbers before a formula double or treble it etc.

e.g. 2NaCl means 2 Na's and 2 Cl's in total

or 2H2SO4 means 2 x H2SO4 =  4 H's, 2 S's and 8 O's in total

NOTE: If the number is 1 itself, by convention, no number is shown in a formula or before a formula.

Using numbers if necessary to balance the equation.

If all is correct, then the sum of atoms for each element should be the same on both side of the equation arrow .....

in other words: atoms of products = atoms of reactants

This is a chemical conservation law of atoms and later it may be described as the 'law of conservation of mass.

the 7 equations are first presented in 'picture' style and then written out fully with state symbols

The individual formulas involved and the word equations will be been presented in the examples below.

NEVER alter a formula to balance an equation! BUT use the CORRECT FORMULA and only put NUMBERS BEFORE THE FORMULA if needed to balance the number of atoms to balance the equation.

PRACTICE QUESTIONS - on words and symbol equations (on other web pages)

Multiple choice quiz on balancing numbers

Balancing number/formula-fill exercises

Reactions of acids with metals, oxides, hydroxides, carbonates and ammonia.

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3.1d EXAMPLES of CONSTRUCTING WORD or SYMBOL EQUATIONS

Remember from the 'Law of Conservation of Mass' the mass of products = mass of original reactants, which means that the number of atoms of each element in the reactants must be equal to those in the products and that is the basis of writing a correctly balanced symbol equation, BUT don't forget, you must write the correct formula for each species in the equation, otherwise you may write a correctly balanced equation which is totally wrong! so beware!

3.1d(1) A single symbol means an uncombined single atom of the element,(c) doc b or Fe 1 atom of iron,(c) doc b or S 1 atom of sulphur (2Fe would mean two atoms, 5S would mean five sulphur atoms etc.)

(c) doc b or the formula FeS means one atom of iron is chemically combined with 1 atom of sulphur to form the compound called iron sulphide

iron + sulphur ==> iron sulphide

(c) doc b

on average one atom of iron chemically combines with one atom of iron forming one molecule of iron sulphide

two elements chemically combining to form a new compound

Fe + S ==> FeS

Fe(s) + S(s) ==> FeS(s)   (with state symbols)

Atom balancing, sum left = sum right: 1Fe + 1S = (1Fe combined with 1S)

For a balanced equation on both sides of the equation you should have 1 iron atom and 1 sulfur atom combined in their particular way in the reactants or products

All the reactants (what you start with) and all the products (what is formed) are all solids in this case.

When first learning symbol equations you probably won't use state symbols like (s) at first (see end note).

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3.1d(2) (c) doc b or the formula NaOH means 1 atom of sodium is combined with 1 atom of oxygen and 1 atom of hydrogen to form the compound called sodium hydroxide

(c) doc b or the formula HCl means 1 atom of hydrogen is combined with 1 atom of chlorine to form 1 molecule of the compound called  hydrochloric acid

(c) doc b or the formula NaCl means 1 atom of sodium are combined with 1 atom chlorine to form the compound called sodium chloride

(c) doc b or the formula H2O means 2 atoms of hydrogen are chemically combined with 1 atom of oxygen to form the compound called water.

sodium hydroxide + hydrochloric acid ==> sodium chloride + water

(c) doc b

the reactants are one molecule of sodium hydroxide and one molecule of hydrochloric acid

the products are one molecule of sodium chloride and one molecule of water

all chemicals involved are compounds

NaOH + HCl ==> NaCl + H2O

NaOH(aq) + HCl(aq) ==> NaCl(aq) + H2O(l)   (with state symbols)

atom balancing, sum left = right: (1Na + 1O + 1H) + (1H + 1Cl) = (1Na + 1Cl) + (2H + 1O)

For a balanced equation on both sides of the equation you should have 1 sodium atom, 1 oxygen atom, 1 chlorine atom and 2 hydrogen atoms combined in their particular way in the reactants or products

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3.1d(3) (c) doc b or the symbol Mg means 1 atom of the element called magnesium

(c) doc b(c) doc b(c) doc b or 2HCl means two separate molecules of the compound called hydrochloric acid (see example 2)

(c) doc b or the formula MgCl2 means 1 formula of the compound called magnesium chloride, made of one atom of magnesium and two atoms of chlorine.

(c) doc b or the formula H2 means 1 molecule of the element called hydrogen made up of two joined hydrogen atoms

magnesium + hydrochloric acid ==> magnesium chloride + hydrogen

(c) doc b

one atom of magnesium reacts with two molecules of hydrochloric acid

the products are one molecule of magnesium chloride and one molecule of hydrogen

Mg and H-H are elements, H-Cl and Cl-Mg-Cl are compounds

Mg + 2HCl ==> MgCl2 + H2

Mg(s) + 2HCl(aq) ==> MgCl2(aq) + H2(g)   (with state symbols)

atom balancing, sum left = right: (1Mg) + 2 x (1H + 1Cl) = (1Mg + 2Cl) + (2H)

For a balanced equation on both sides of the equation you should have 1 magnesium atom, 2 hydrogen atoms and 2 chlorine atoms combined in their particular way in the reactants or products

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3.1d(4) (c) doc b or the formula CuCO3 means one formula of the compound called copper carbonate, made up of one atom of copper is combined with one atom of carbon and three atoms of oxygen to form the compound copper carbonate

(c) doc b or the formula H2SO4 means one formula of the compound called sulphuric acid, which is made up of two atoms of hydrogen, one atom of sulphur and four atoms of oxygen

(c) doc b or the formula CuSO4 means one formula of the compound called copper sulphate which is made up of one atom of copper, one atom of sulphur and four atoms of oxygen

H2O (example 2)

(c) doc b or the formula CO2 means one molecule of the compound called carbon dioxide which is a chemical combination of one atom of carbon and two atoms of oxygen.

  copper carbonate  + sulphuric acid ==> copper  sulphate + water + carbon dioxide

(c) doc b

the reactants are one formula of copper carbonate and one molecule of sulphuric acid

the products are one formula of copper sulphate, one molecule of water and one molecule of carbon dioxide

all molecules are compounds in this reaction

CuCO3 + H2SO4 ==> CuSO4 + H2O + CO2

CuCO3(s) + H2SO4(aq) ==> CuSO4(aq) + H2O(l) + CO2(g)  (with state symbols)

balancing sum left = sum right: (1Cu + 1C + 3O) + (2H + 1S + 4O) = (1Cu + 1S + 4O) + (2H + 1O) + (1C + 2O)

 For a balanced equation on both sides of the equation you should have 1 copper atom, 1 carbon atom, 7 oxygen atoms, 2 hydrogen atoms, 1 sulphur atom combined in their particular way in the reactants or products

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3.1d(5) (c) doc b or the formula CH4 means one molecule of the compound called methane which is made of one atom of carbon combined with four atoms of hydrogen

(c) doc b(c) doc b(c) doc b or 2O2 means two separate molecules of the element called oxygen, and each oxygen molecule consists of two atoms of oxygen

CO2 (see also example 4)

(c) doc b(c) doc b(c) doc b or 2H2O means two separate molecules of the compound called water (see also example 2)

methane + oxygen ==> carbon dioxide + water

(c) doc b

Using displayed formula the equation would look like this ...

... in which every individual atom is shown and how it is bonded ('connected') with other atoms in the molecule. All the dashes represent the covalent bonds between the atoms in the molecules.

one molecule of methane is completely burned by two molecules of oxygen

to form one molecule of carbon dioxide and two molecules of water

CH4 + 2O2 ==> CO2 + 2H2O

CH4(g) + 2O2(g) ==> CO2(g) + 2H2O(l)   (with state symbols)

atom balancing, sum left = sum right: (1C + 4H) + 2 x (2O) = (1C + 2O) + 2 x (2H + 1O)

For a balanced equation on both sides of the equation you should have 1 carbon atom, 4 hydrogen atoms, 4 oxygen atoms combined in their particular way in the reactants or products

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3.1d(6) (c) doc b or the formula Mg(OH)2 is the compound magnesium hydroxide made up of one magnesium, two oxygen and two hydrogen atoms BUT the OH is a particular combination called hydroxide  within a compound, so it is best to think of this compound as a combination of an Mg and two OH's, hence the use of the ( ). The subscripted 2 doubles everything in the brackets.

(c) doc b(c) doc b(c) doc b or 2HNO3 means two separate molecules of the compound nitric acid, each molecule is made up of one hydrogen atom, one nitrogen atom and three oxygen atoms.

(c) doc b or the formula Mg(NO3)2 is the compound magnesium nitrate, it consists of a magnesium (ion) and two 'nitrates' (ions), each nitrate consists of one nitrogen and three oxygen atoms, again the nitrate is a particular combination of atoms within a compound and hence the use of ( ) again.

(c) doc b(c) doc b(c) doc b or 2H2O meaning two molecules of the compound water (see also examples 2 and 5)

magnesium hydroxide + nitric acid ==> magnesium nitrate + water

(c) doc b

one formula of magnesium hydroxide reacts with two molecules of nitric acid to form one formula of magnesium nitrate and two molecules of water (all compounds)

Mg(OH)2 + 2HNO3 ==> Mg(NO3)2 + 2H2O

Mg(OH)2(aq) + 2HNO3(aq) ==> Mg(NO3)2(aq) + 2H2O(l)  (with state symbols)

atom balancing, sum left = sum right: (1Mg + 2O + 2H) + 2 x (1H + 1N + 3O) = (1Mg + 2N + 6O) + 2 x (2H + 1O)

For a balanced equation on both sides of the equation you should have 1 magnesium atom, 8 oxygen atoms, 4 hydrogen atoms, 2 nitrogen atoms combined in their particular way in the reactants or products

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3.1d(7) (c) doc b or the formula Al2O3 means one formula of the compound called aluminium oxide, made up of two atoms of aluminium Al and three atoms of oxygen O

(c) doc b(c) doc b(c) doc b(c) doc b(c) doc b or 3H2SO4 meaning three molecules of the compound called sulphuric acid (see also example 4)

(c) doc b or the formula Al2(SO4)3 means one formula of the compound called aluminium sulphate, it consists of two aluminium, three sulphur and twelve oxygen atoms BUT the SO4 is a particular grouping called sulphate, so it is best to think of the compound as a combination of two Al's and three SO4's

(c) doc b(c) doc b(c) doc b(c) doc b(c) doc b or 3H2O means three separate molecules of the compound called water (see also examples 2 and 5)

aluminium oxide + sulphuric acid ==> aluminium sulphate + water

(c) doc b

one formula of aluminium oxide reacts with three molecules of sulphuric acid

to form one formula of aluminium sulphate and three molecules of water

note the first use of numbers (3) for the sulphuric acid and water!

so picture three of them in your head, otherwise the picture gets a bit big!

Al2O3 + 3H2SO4 ==> Al2(SO4)3 + 3H2O

Al2O3(s) + 3H2SO4(aq) ==> Al2(SO4)3(aq) + 3H2O(l)   (with state symbols)

atom balancing, sum left = sum right: (2Al + 3O) + 3 x (2H + 1S + 4O) = (2Al + 3S + 12O) + 3 x (2H + 1O)

For a balanced equation on both sides of the equation you should have 2 aluminium atoms, 15 oxygen atoms, 6 hydrogen atoms, 3 sulfur atoms combined in their particular way in the reactants or products

GCSE-AS-A2-IB note: Aluminium sulfate is actually an ionic compound (Al3+)2(SO42-)3

topNOTE 1: (c) doc b means a reversible reaction, it can be made to go the 'other way' if the conditions are changed. Example:

nitrogen + hydrogen (c) doc b ammonia

N2(g) + 3H2(g) (c) doc b 2NH3(g)   (with state symbols)

balancing: 2 nitrogen's and 6 hydrogen's on both sides of equation

Note 2 on the state symbols X(?) of reactants or products in equations

(g) means gas, (l) means liquid, (s) means solid

and (aq) means aqueous solution or dissolved in water

e.g. carbon dioxide gas CO2(g), liquid water H2O(l), solid sodium chloride 'salt' NaCl(s)

and copper sulphate solution CuSO4(aq)

3.1e IONIC EQUATIONS  (for higher GCSE and AS students)

What is an 'ionic equation'? How do we construct and write ionic equations?

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.

top(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+ 

top(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.

top(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+.

top(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+.

top(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:

top(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

top(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

top(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

top(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)

top(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+

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 3.2 VALENCY - COMBINING POWER - FORMULA DEDUCTION

3.2a Introduction

What is valency? How do you use valency to work out the formula of a compound?

The valency of an atom or group of atoms is its numerical combining power with other atoms or groups of atoms.

i.e. its numerical capacity to combine with other 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 atom 'A' x valency of 'A' = number of atom '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.

number of ion 'A' x charge of ion 'A' = number of ion 'B' x charge of ion 'B' (you ignore charge sign)

Example: As difficult an example as any you will have to work out!

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 

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table of ions, names and symbols (c) doc b3.2b Examples of COVALENT and IONIC COMPOUND FORMULAE

Selected combining power of ions (table left) valency = numerical ion charge value and examples of covalent combining power of atoms ie valencies (selection 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)

 

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3.2c 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 

(c) doc b

The diagram on the left illustrates the three covalent examples above for

methane CH4

nitrogen trichloride NCl3

carbon dioxide CO2

6 more examples of working out an ionic formula

numerically charge = valency of A or B to deduce the formula

valency or ionic charge = the combining power of the ion

'molecular' or ionic style of formula and compound name

1 of K+ balances 1 of Br- because 1 x 1 = 1 x 1 gives KBr or K+Br-  potassium bromide

2 of Na+ balances 1 of O2- because 2 x 1 = 1 x 2 gives Na2O or (Na+)2O2-  sodium oxide

1 of Mg2+ balances 2 of Cl- because 1 x 2 = 2 x 1 gives MgCl2 or Mg2+(Cl-)2  magnesium chloride

1 of Fe3+ balances 3 of F- because 1 x 3 = 3 x 1 gives FeF3 or   Fe3+(F-)3  iron(III) fluoride

1 of Ca2+ balances 2 of NO3- because 1 x 2 = 2 x 1 gives Ca(NO3)2 or Ca2+(NO3-)2  calcium nitrate

2 of Fe3+ balances 3 of SO42- because 2 x 3 = 3 x 2 gives Fe2(SO4)3 or (Fe3+)2(SO42-)3  iron(III) sulphate

 
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3.3 KS3-GCSE note on naming compounds

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

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(c) doc b KS3 Science GCSE/IGCSE Chemistry States of Matter - kinetic particle theory of Gases, Liquids and Solids revision notes

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(c) doc b GCSE/IGCSE Chemical Bonding revision notes (ionic, covalent, metallic etc.)

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