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Working Out Formulae
& Balancing Equations
(including electrolysis)
Valency
Writing Equations
Using Valency
Balancing Equations
Electrolysis half equations
END
ALWAYS BRINGS YOU
BACK HERE
VALENCY tells you how many BONDS an atom can form
You work the valency out like this…
Work out how many OUTER ELECTRONS the atom has
(That’s it’s GROUP NUMBER in the Periodic Table)
If it has 4 or less then THAT’S THE VALENCY
If it has 5 or more then VALENCY = 8 – OUTER ELECTRONS
Element
Sodium
Chlorine
Group
1
7
Outer Electrons
Valency
1
1
7
8–7=1
Magnesium
Nitrogen
2
5
2
5
2
8–5=3
Carbon
4
4
4
Oxygen
6
6
8–6=2
Here’s another method that works for ions…
For IONIC BONDS, the
valency is just the size
of the charge on the ion
Some transition metals
can form more than one
ion, so they have more
than one valency.
You can use the same
idea for ions made of
more than one atom
Sodium forms Na+ ion It has valency 1
Calcium forms Ca2+ ion It has valency 2
Oxygen forms O2- ion
It has valency 2
Chlorine forms Cl- ion
It has valency 1
Iron forms Fe2+ and Fe3+
So it can have valency 2 or 3
Ammonium ion NH4+
Nitrate ion NO3Sulphate ion SO42-
has valency 1
has valency 1
has valency 2
USING VALENCIES TO WORK OUT FORMULAE
1. Write down the two elements (or ions) in the compound
2. Write down the valency of each as a “small number”
next to the other one. Put brackets round any
complicated ions, like sulphate, nitrate etc.
3. Cancel if necessary, and remove any 1s
Sodium
sulphate
Iron(II)
chloride
Fe Na
Cl SO4
Calciumnitrate
oxide: Mg
Ca NO
O 3
Magnesium
Valencyofofiron
sodium:
1
Valency
:2
Valency
of calcium:: 22
Valency
of magnesium
Valencyofofchloride
sulphate:
Valency
: 12
Valency
of oxygen:
2
Valency
of nitrate
:1
So Fe
Na12Cl
(SO
)
So
2 4 1
So1 Ca
So Mg
(NO
2O
3)22
Remove1s:
1s:Fe
NaCl
2SO
Remove
2 4
Cancel:
Remove
1s: CaO
Mg (NO3)2
DO NOT “MULTIPLY OUT THE BRACKETS” !
Write (NO3)2 not N2O6
WRITING EQUATIONS
Step 1: Write a WORD EQUATION
The chemicals Reactants
you start with
Products What you make in
the reaction
Step 2: Write down the FORMULA of each of the
chemicals in your word equation
Step 3: Put in the STATE SYMBOLS
(s) for solid, (l) for liquid, (aq) for solution, (g) for gas
Eg: reaction of calcium carbonate with hydrochloric acid:
carbon
dioxide
calcium + hydrochloric
carbonate
acid
calcium
+ water +
chloride
The
REACTANTS
CaCO
HCl (aq)
3(s) +
PRODUCTS
CaCl2The
(aq) + H2O(l) + CO2 (g)
Make
sure
you
get
formulae
right!
Calcium
carbonate
isa as
Hydrochloric
acid,
like
other
Calcium
chloride
is
soluble,
Now on to
the
harder
bit
the
equation!
Carbon
Water
dioxide
is–the
abalancing
liquid
isso
gasthere
an is
insoluble
No marksisfor
balancing
if
the
formulae are wrong!
acids,
always
ainis
solution
water
present,
itsolid
a solution
It
(not
forms
a solution!)
bubbles
Balancing Equations
WHAT DOES IT MEAN?
Balancing an equation means making sure the numbers of
each type of atom are the same on each side
Let’s look at this balanced equation:
2Na + 2H2O  2NaOH + H2
Atom
Number on Left
Number on Right
Na
Have 2Na, so 2
H
2 in each water
2 waters
22=4
1 in each water
2 waters. So 2
1 in each NaOH
2NaOH. So 2
2 from 2NaOH
+ 2 from H2 makes 4
O
1 in each NaOH
2NaOH. So 2
They all match. So it’s balanced!
How to balance an equation
Step 1: Write down the unbalanced formula equation.
Step 2: Work out how many of each atom there are on each side.
(in your head, if it’s easy)
Step 3: Look for any atoms where there aren’t the same number
on each side
Step 4: Choose the “unbalanced atom” that’s in the smallest
number of different formulae.
Step 5: Balance it by putting a number IN FRONT of one of the
formulae (don’t change the actual formula!)
Step 6: Recalculate numbers of atoms – and repeat if needed!
2NaOH + H2SO4 Na2SO4 + 2H2O
Na
Left
2
1
5
6
3
4
1
There are different numbers of O and H
O
There are different numbers of Na and H
H is in fewer different formulae.
H
S
Na is only in one chemical each side
We can balance H by putting 2 in front of H2O
We can balance them by putting 2 in front of NaOH
Now
THEY
recalculate
ALL MATCH!
the numbers
IT’S BALANCED!
of atoms…
Right
2
56
2
4
1
Other examples…

Left
1
1
3
12
12
CaCO3 + 2 HCl  CaCl2 + H2O + CO2
Ca
C
Let’s balance the hydrogens:
balancedO
H
We can do this by putting 2 in front of HCl
Cl
Right
1
1
3
2
2
Eureka ! That has automatically balanced the chlorines too.
Cl2 +2 KI  2 KCl + I2
Let’s balance the chlorines :
We can do this by putting 2 in front of KCl
Cl
K
I
Left
2
12
2
1
Right
2
1
12
2

Unfortunately, this has also unbalanced the potassium.
However putting 2 in front of KI balances both K and I
balanced
Another example…
Al2O3 + 6HCl  2AlCl3 + 3H2O
Now
balance
oxygens:
Nonelet’s
of the
atomsthe
arehydrogens:
balanced!
We
can
thisin
byjust
putting
3 in front of HCl
6
H2O
They
all do
occur
two chemicals
Al
O
H
Cl
Recalculate…
Choose
one to balance…
Recalculate…
We can balanceTHEY
Al byALL
putting
2 in front
of AlCl3
MATCH!
IT’S BALANCED!
Recalculate…
Left
2
3
16
16
Right
12
13
26
36
An awkward one!
2Al +3Cl2 2AlCl3
Now
we must
balance
the aluminiums
Chlorines
aren’t
balanced.
Butcan
howdocan
we
the balancing?
We
this
bydo
putting
2 in front of Al
Al
Cl
Left
1
2
2
6
Right
12
36
We
haven’t got “nice” numbers!
Recalculate…
This is like finding
“lowest
common
denominator” in fractions.
THEYthe
ALL
MATCH!
IT’S BALANCED!
We have two chlorines on one side, and three on the other.
We find the smallest number two and three go into – that’s six.
If you like maths, you could try balancing ones like this using fractions
So we need
aimtofor
six
instead.
You’dto
need
use
1½chlorines on each side
To do that, we put 3 in front of Cl2 and 2 in front of AlCl3
Recalculate…
Here
are Cu2+ ions moving
ELECTROLYSIS
When electrons are gained
Cu2+
to the negative electrode.
byPositive
a positive
ion,
name
ions in
the the
solution
are
of
the chemical
change
is –
attracted
to negative
electrode
2+
The electrode is negative because it Cu
REDUCTION.
opposite
charges attract.
has too many electrons
REDUCTION
IS THEthe
GAIN
As they get close,
ions
OFelectrons
ELECTRONS.
gain
from the
electrode and the Cu2+ is
THE COPPER
ION HAS
neutralised.
BEEN REDUCED
2e
e- go to ion
Cu2+
Cu
+ Cu2+  Cu
e- go to ion.
This makes
copper
TWO
AARE
NEUTRAL
ELECTRONS
ADDED
ATOM
TOthe
OFelement,
FROM
THE
THE
ELEMENT
THE
COPPER
CATHODE
COPPER.
ION
which
covers
the
electrode.
Cu2+
e- go to cell
Cl-
Clthe ion loses an e
ClCl2
the ion loses an e
Cl-
This
Here is
are
what
negative
happens
chloride
at theions
positive
When electrons are lost by a
attracted
electrodetowards
when chloride
the positive
ions, Clnegative ion, the name of the
electrode.
are present
Opposite
in thecharges
electrolyte
attract.
chemical change is OXIDATION.
As they get close, each Cl- ion loses
OXIDATION
THE
LOSS
OF
This
electrodewhich
isIS
positive
because
some
an
electron
goes
onto
the
electrons
have been removed by the cell.
ELECTRONS
electrode.
The ion becomes electrically neutral
THE CHLORIDE ION HAS BEEN
We have made chlorine the element. The
OXIDISED
neutral
atoms join in pairs to make chlorine
molecules, Cl2 which bubble off as a gas.
2Cl -
2e  Cl2
Cl-
Cl-
TWO CHLORIDE
THE
A TWO
NEUTRAL
IONS,
ELECTRONS
CHLORINE
EACH WITH
LOST
AN BY
EXTRA
MOLECULE
THE IONS GO
ELECTRON
TO THE ELECTRODE
Half equations for reduction at the negative electrode.
click3+
for+
solution
Cu2+ +2e  Cu
Al
3e  Al
the ion GAINS electrons
from the electrode
Ag+ +1e  Ag
Pb2+ +2e  Pb
Here are different ions that might be in a solution.
You need
to be
able
balance
the halfwould
equations.
In these
cases,
metallic
elements
appear
+ to
click
for solution

at the negative electrode.
2
2 H +2 e
H
Half equations for
oxidation at the positive electrode.
+
H is present in all acids and hydrogen gas
evolved
from the electrode.
Here electrons
are lostisby
the ion.
-
2Cl - 2 e  Cl
2 Br
- 2 e  Br
Cu - 2 e Cu
2 are produced, they bond together
2
When gaseous elements
in pairs to make a molecule.
2+
2

The balancing needs
 to2 include this.
2O
- 4e
O
In a special case, a positive copper electrode dissolves in a solution of
copper sulphate. Electrons are lost by the copper metal.