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HOW TO WRITE CHEMICAL EQUATIONS & how to work out formulae from valencies
Doc Brown's Chemistry KS4 science GCSE/IGCSE/O Level Chemistry Revision Notes
ELEMENTS, COMPOUNDS, MIXTURE separation, CHEMICAL REACTIONS & EQUATIONS
NaOH + HCl ===> NaCl + H2O
PART 3 How do we write chemical equations?, How do we construct equations from first principles? How do we balance chemical equations? 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? Practice in balancing chemical equations How can we use valency to work out a formula? - index of keywords-terms-phrases below
Al2O3 + 3H2SO4 ==> Al2(SO4)3 + 3H2O
Al2O3(s) + 3H2SO4(aq) ==> Al2(SO4)3(aq) + 3H2O(l) (with state symbols)
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 * covalency * crystallisation * 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 *
State symbols used in chemical equations, to show the physical state of the reactants and products
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
Symbols and Formula
For any reaction, what you start with are called the reactants, and what you form are called the products.
It is most important you read about formula in an earlier section of this 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:
==> 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.
Writing the correct symbol or formula for each equation component.
Numbers before a formula double or treble it etc.
Using numbers if necessary to balance the equation, this is a matter of 'trial and error'.
If all is correct, then the sum of atoms for each element should be the same on both side of the equation arrow .....
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)
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!
Balancing equations example 3.1d(1)
A single symbol means an uncombined single atom of the element, or Fe 1 atom of iron, or S 1 atom of sulphur (2Fe would mean two atoms, 5S would mean five sulphur atoms etc.)
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
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
Atom balancing, sum left = sum right: 1Fe + 1S = (1Fe combined with 1S)
There is no need for any balancing numbers in this equation
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).
Balancing equations example 3.1d(2)
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
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
or the formula NaCl means 1 atom of sodium are combined with 1 atom chlorine to form the compound called sodium chloride
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
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
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.
There is no need for any balancing numbers in this equation
Balancing equations example 3.1d(3)
or the symbol Mg means 1 atom of the element called magnesium
or 2HCl means two separate molecules of the compound called hydrochloric acid (see example 2)
or the formula MgCl2 means 1 formula of the compound called magnesium chloride, made of one atom of magnesium and two atoms of chlorine.
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
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
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. You can only get the balance here by putting a 2 in front of the HCl formula because you need 2 Cl's to make the MgCl2.
Balancing equations example 3.1d(4)
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
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
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)
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
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
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
There is no need for any balancing numbers in this equation
Balancing equations example 3.1d(5)
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
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)
or 2H2O means two separate molecules of the compound called water (see also example 2)
methane + oxygen ==> carbon dioxide + water
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
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.
You can only get this to balance by having a 2 in front of the O2 and 2 in front of the CO2, you need an O2 to make CO2 and another O2 to convert the H4 into 2H2O
Balancing equations example 3.1d(6)
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.
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.
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.
or 2H2O meaning two molecules of the compound water (see also examples 2 and 5)
magnesium hydroxide + nitric acid ==> magnesium nitrate + water
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)
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
to balance this equation you need a 2 in front of the HNO3 and a 2 in front of the H2O, the 2s come from the 2 OH becoming 2 H2Os
Balancing equations example 3.1d(7)
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
or 3H2SO4 meaning three molecules of the compound called sulphuric acid (see also example 4)
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
or 3H2O means three separate molecules of the compound called water (see also examples 2 and 5)
aluminium oxide + sulphuric acid ==> aluminium sulphate + water
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!
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
This is quite an awkward equation to balance, a bit of real trial and error, but two 3s in the right place will do it.
The best clue here is that you need 3 x SO4 for the aluminium sulfate, so you need 3 of the H2SO4
GCSE-AS-A2-IB note: Aluminium sulfate is actually an ionic compound (Al3+)2(SO42-)3
The sign means a reversible reaction, it can be made to go the 'other way' if the conditions are changed. Example:
Extra NOTE 2 State Symbols
(g) means gas, (l) means liquid, (s) means solid
and (aq) means aqueous solution, which means the substance is 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.
e.g. any acid-alkali neutralisation involves the hydroxide ion is (base) and this accepts a proton from an acid.
(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'.
(3) Redox reaction analysis:
(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.
|3.2 VALENCY - COMBINING POWER - FORMULA DEDUCTION|
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.
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:
However it is easier perhaps? to grasp with ionic compound formulae.
3.2b 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).
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|
diagram on the left illustrates the three covalent examples above for
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 name1 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 oxide1 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) fluoride1 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
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.
See other web page for:
GCSE balancing and completing equation exercises:
GCSE 'name and formula' of a compound quizzes