Representing reactions
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C2.01 Word equations
If a reaction occurs between magnesium and oxygen, magnesium
oxide is produced, here is the word equation for this reaction: -
magnesium + oxygen --> magnesium oxide
Some other examples are:
hydrochloric acid + calcium carbonate --> calcium chloride+
carbon dioxide+ water
sodium + water --> sodium hydroxide + hydrogen
hydrochloric acid + sodium hydroxide --> sodium chloride
+ water
Task C2.01 Write word equations for the reactions
in which the following compounds form from a halogen and another suitable
element: hydrogen fluoride, hydrogen chloride, iron III chloride, iron III
bromide, sodium chloride, copper chloride.
C2.02 Formulae
The formula of an element or compound is simply the
symbol
of each element present and numbers to show how many atoms are present. Carbon dioxide has the formula
CO2.
This means
that it has one carbon atom and two oxygen
atoms in each molecule.
| substance |
formula |
substance |
formula |
| methane |
CH4 |
bromine |
Br2 |
| ethane |
C2H6 |
hydrogen |
H2 |
| propane |
C3H8 |
ethanol |
C2H5OH |
| butane |
C4H10 |
glucose |
C6H12O6 |
| oxygen |
O2 |
ammonia |
NH3 |
| carbon |
C |
nitric acid |
HNO3 |
| carbon monoxide |
CO |
ammonium nitrate |
NH4NO3 |
| carbon dioxide |
CO2 |
calcium carbonate |
CaCO3 |
| ethene |
C2H4 |
calcium oxide |
CaO |
| propene |
C3H6 |
calcium hydroxide |
Ca(OH)2 |
| water |
H2O |
hydrochloric acid |
HCl |
| sodium hydroxide |
NaOH |
sodium chloride |
NaCl |
| copper carbonate |
CuCO3 |
copper oxide |
CuO |
Task C2.02.1 State the name and the
number of atoms of each element in the formulae above.
Formulae can be worked out from valency.
| Valency |
atom or ion with that valency |
| 1 |
hydrogen, group 1 e.g. sodium and potassium, group
7e.g. fluorine and chlorine, ammonium NH4+,
hydroxide OH-, nitrate NO3- |
| 2 |
group 2 e.g. magnesium and calcium, group 6,
sulphate SO42-, carbonate CO32-,
copper , II Cu2+ iron II Fe2+ |
| 3 |
group 3 e.g. aluminium, group 5, phosphate PO43-,
iron III Fe3+ |
| 4 |
group 4 e.g. carbon |
name aluminium oxide
valency
3
2
symbols
Al
O
formula Al2O3
Task C2.02.2 Use valency to work
out the formulae of the following compounds: sodium chloride, potassium bromide,
magnesium oxide, calcium sulphide, aluminium nitride, calcium iodide, lithium
oxide, aluminium chloride, aluminium sulphide, magnesium nitride.
C2.03 Calculating relative formula mass
Add up the relative atomic mass (found in periodic table) of each atom in
the compound.
e.g. Al203
relative atomic masses of Al = 27, O = 16 (found in periodic table). The
formula shows 2 atoms of aluminium and 3 atoms of oxygen so:
formula mass of = (2*27) + (3*16) =54 + 48 =
102
Task C2.03 Work out the relative formula masses of the
following: MgO, FeS, O2, H2O, CaBr2, Na2S,
CaCO3, NaOH, HCl, (NH4)2SO4. Relative
atomic masses Mg=24, O=16, Fe=56, S=32, Ca=40, Br=80, C=12, Na=23, H=1, Cl=35.5.
C2.04 Simple balanced equations
It is possible to write balanced equations for
reactions. For example substances such as hydrogen and magnesium combine with
oxygen. One method to write them is:
Write a word equation first.
Magnesium + oxygen --> magnesium oxide
Write in the formulae of the substances used.
Mg + O2 --> MgO
Balance the equation so that each element has the same number of atoms on each
side.
2Mg + O2 --> 2MgO
sodium + oxygen --> sodium oxide
4Na{s} + O2{g} --> 2Na2O{s}
(word equation)
hydrogen + oxygen ---> water
(formulae)
H2 + O2
-----> H2O
(balance)
2H2 + O2
-----> 2H2O
Task C2.04
C2.05 State symbols
The state symbols are put in a balanced equation to show whether something is a solid,
liquid, gas or dissolved in water (aqueous solution).
The symbols for these are:
| state |
Symbol |
| Solid |
(s) |
| Liquid |
(l) |
| Gas |
(g) |
| Aqueous |
(aq) |
Magnesium + oxygen --> magnesium oxide
2Mg{s} + O2{g} --> 2MgO{s}
hydrochloric acid + calcium carbonate --> calcium chloride + carbon
dioxide + water
2HCl (aq) + CaCO3 ---> CaCl2(aq) + CO2(aq) +
H20(l)
Task C2.05
C2.06 Balanced equations and ionic equations
Ionic equations only show ions which change in a reaction and ignore those
which do not change. E.g.
word equation
hydrochloric acid + sodium hydroxide --> sodium chloride + water
balanced chemical equation
HCl(aq) + NaOH(aq) ---> NaCl(aq) + H2O(l)
ionic equation
H+(aq) + OH-(aq) ---> H2O(l)
E.g. in the electrolysis of sodium chloride
Cl-(aq) --> Cl(g) + e-
2Cl(g) --> Cl2(g)
C2.07 Working out formulae from reacting masses
elements
reacting
magnesium chlorine
symbols of
elements
Mg
Cl
masses reacting (from experiment)
2.4g
7.1g
molar mass (look up relative atomic
24g/mol
35.5g/mol
mass in periodic table)
amounts (amount = mass/molar mass)
2.4g/24g/mol 7.1g/35.5g/mol
=
0.1mol
0.2mol
ratio of atoms (divide by
smallest)
1
: 2
formula
MgCl2
Task C2.07
Work out formulae of compounds formed when the following react:
56g of iron and 32g of sulphur (Fe =56, S =32)
2g of hydrogen and 16g of oxygen (H=1, O=16)
14g of lithium and 16g of oxygen (Li=7)
32g of copper and 8g of oxygen (Cu=64)
6.4g of copper and 0.8g of oxygen.
C2.08 Calculating reacting masses using equations
You can work out ratio of the masses of products and reactants
by simply multiplying the number of moles shown in the equation by the
formula mass of each substance.
Example 1: What mass of magnesium oxide can be made from 12g of
magnesium? Relative atomic masses are Mg =24, O = 16.
equation 2Mg(s) + O2(g) -->
2MgO(s)
formula 2*24 1(16*2)
2(24+16)
masses =48
=32 =80
reacting 48g of Mg
forms 80g of MgO
masses
1g of Mg forms 80/48 g
of MgO
12g of Mg forms
12*80/48 g of MgO = 20g
Example 2: What mass of magnesium oxide can be made from 12g of
magnesium?
equation 2Mg(s) + O2(g) -->
2MgO(s)
amounts 2 moles 1 mole 2
moles
masses 2*24 1{16*2}
2{24+16}
=48g
=32g =80g
so
48g Mg forms 80g
MgO
1g Mg
forms 80/48 g MgO
12g Mg forms
12*80/48 g MgO = 20g
Also note that the ratio of amounts of reactants and
products in the equation above can be written as:
Amount of Mg/amount of O2 =2/1
Or
Amount of O2/amount of MgO = 1/2
Task C2.08
problem C2.08 You decide to travel from
London to Delhi for a holiday and hire your own Airbus 319 jet. You fly
the 2183 miles to Cairo first and make 9.4 tons of carbon dioxide. After
seeing the Pryramids you fly the 1018 miles to Riyadh making 4.5 tons of carbon
dioxide. After a brief stop in the Saudi Capital you fly on the 1900 miles
to Delhi making 8.1 tons of carbon dioxide. Finally you return to London
travelling 4171 miles and releasing 22 tons of carbon dioxide. The jet
burns the fuel kerosine C15H32 in the reaction
C15H32 + 23O2 --> 15CO2 +16H2O
(a) How much carbon dioxide was released into the atmosphere during the trip?
(b) What mass of kerosine was burnt?
(c) What effect does the trip have of the environment?