Working Out Formulae& Balancing Equations

(including electrolysis)

Valency

Using Valency

Writing Equations

Balancing Equations

END ALWAYS BRINGS YOU BACK HERE

Electrolysis half equations

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 Group Outer Electrons Valency

Sodium 1 1 1Chlorine 7 7 8 – 7 = 1

Oxygen 6 6 8 – 6 = 2

Magnesium 2 2 2

Nitrogen 5 5 8 – 5 = 3

Carbon 4 4 4

Here’s another method that works for ions…

For IONIC BONDS, the valency is just the size of the charge on the ion

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

Some transition metals can form more than one ion, so they have more

than one valency.

Iron forms Fe2+ and Fe3+

So it can have valency 2 or 3

You can use the same idea for ions made of more than one atom

Ammonium ion NH4+ has valency 1

Nitrate ion NO3 has valency 1

Sulphate ion SO42 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 Na SO4

Valency of sodium: 1

Valency of sulphate: 2

So Na2(SO4)1

Remove 1s: Na2SO4

Calcium oxide: Ca O

Valency of calcium: 2

Valency of oxygen: 2

So Ca2O2

Cancel: CaO

Iron(II) chloride Fe Cl

Valency of iron : 2

Valency of chloride : 1

So Fe1 Cl2

Remove 1s: Fe Cl2

DO NOT “MULTIPLY OUT THE BRACKETS” !

Write (NO3)2 not N2O6

Magnesium nitrate Mg NO3

Valency of magnesium : 2

Valency of nitrate : 1

So Mg1 (NO3)2

Remove 1s: Mg (NO3)2

WRITING EQUATIONS

Step 2: Write down the FORMULA of each of the chemicals in your word equation

The chemicals you start with

What you make in the reaction

Reactants Products

Step 1: Write a 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:

The REACTANTS

calcium carbonate

hydrochloric acid

+ calcium chloride

+ water +carbon dioxide

The PRODUCTS CaCO3 + HCl CaCl2 + H2O + CO2

Make sure you get the formulae right!No marks for balancing if the formulae are wrong!

(s) (aq) (aq) (l) (g)

Calcium carbonate is an insoluble solid

Hydrochloric acid, like other acids, is always a solution

Calcium chloride is soluble, so as there is water present, it is a solution

Water is a liquid (not a solution!)

Carbon dioxide is a gas It forms in bubbles

Now on to the harder bit – balancing the equation!

They all match. So it’s balanced!

Balancing EquationsWHAT 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 water2 waters2 2 = 4

2 from 2NaOH+ 2 from H2 makes 4

O 1 in each water2 waters. So 2

1 in each NaOH2NaOH. So 2

1 in each NaOH2NaOH. So 2

NaOH + H2SO4 Na2SO4 + H2O

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!

There are different numbers of Na and H

Na is only in one chemical each sideWe can balance them by putting 2 in front of NaOH

2

Now recalculate the numbers of atoms…

Left RightNa 1 2O 5 5H 3 2S 1 1

2

64

There are different numbers of O and H

H is in fewer different formulae.

We can balance H by putting 2 in front of H2O

2

Now recalculate the numbers of atoms…

46

THEY ALL MATCH! IT’S BALANCED!

Other examples…

Left RightCa 1 1C 1 1O 3 3H 1 2Cl 1 2

CaCO3 + HCl CaCl2 + H2O + CO2

2

2

Let’s balance the hydrogens:

We can do this by putting 2 in front of HCl2

Eureka ! That has automatically balanced the chlorines too. balanced

Left RightCl 2 1K 1 1I 1 2

Cl2 + KI KCl + I22

Let’s balance the chlorines :

We can do this by putting 2 in front of KCl

22

2

Unfortunately, this has also unbalanced the potassium.

However putting 2 in front of KI balances both K and I

22

balanced

Another example…

Al2O3 + HCl AlCl3 + H2O

None of the atoms are balanced!

They all occur in just two chemicals

Choose one to balance…

2 3

We can balance Al by putting 2 in front of AlCl3

Recalculate…

Now let’s balance the oxygens:

We can do this by putting 3 in front of H2O

Recalculate…

6

Now let’s balance the hydrogens:

We can do this by putting 6 in front of HCl

Recalculate…

THEY ALL MATCH! IT’S BALANCED!

Left RightAl 2 1O 3 1H 1 2Cl 1 3

2

663

66

An awkward one!

Al + Cl2 AlCl3

Chlorines aren’t balanced.

This is like finding the “lowest common denominator” in fractions.

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.

So we need to aim for six chlorines on each side

To do that, we put 3 in front of Cl2 and 2 in front of AlCl3

3 2

But how can we do the balancing?

We haven’t got “nice” numbers!

Recalculate…

Now we must balance the aluminiums

We can do this by putting 2 in front of Al

Left RightAl 1 1Cl 2 36 6

22

Recalculate…

2

THEY ALL MATCH! IT’S BALANCED!

If you like maths, you could try balancing ones like this using fractions

instead. You’d need to use 1½

Cu2+

Cu2+

Cu2+

Cu2+

As they get close, the ions gain electrons from the

electrode and the Cu2+ is neutralised.

Here are Cu2+ ions moving to the negative electrode.

Positive ions in the solution are attracted to negative electrode –

opposite charges attract.The electrode is negative because it

has too many electrons

e- go to ion.2e- + Cu2+ Cu

When electrons are gained by a positive ion, the name of the chemical change is

REDUCTION.

REDUCTION IS THE GAIN OF ELECTRONS.

THE COPPER ION HAS BEEN REDUCED

Cu

This makes copper the element, which covers the electrode.

e- go to ion

TWO ELECTRONS FROM THE CATHODE

ARE ADDED TO THE COPPER IONA NEUTRAL ATOM OF

THE ELEMENT COPPER.

ELECTROLYSIS

This is what happens at the positive

electrode when chloride ions, Cl- are present in the electrolyte

This electrode is positive because some electrons have been removed by the cell.

Here are negative chloride ionsattracted towards the positive electrode. Opposite charges attract.

As they get close, each Cl- ion loses an electron which goes onto the electrode. The ion becomes electrically neutral

the ion loses an e-

the ion loses an e-

Cl-

Cl-

Cl-

Cl-

Cl-

Cl-

Cl2

e- go to cell

We have made chlorine the element. The neutral atoms join in pairs to make chlorine molecules, Cl2 which bubble off as a gas.

2Cl- - 2e Cl2

TWO CHLORIDE IONS, EACH WITH AN

EXTRA ELECTRON

THE TWO ELECTRONS LOST BY THE IONS GO TO THE ELECTRODE

A NEUTRAL CHLORINE MOLECULE

When electrons are lost by a negative ion, the name of the chemical change is OXIDATION.

OXIDATION IS THE LOSS OF ELECTRONS

THE CHLORIDE ION HAS BEEN OXIDISED

Half equations for reduction at the negative electrode.

Cu2+ + e Cu click for solution2 Al3+ + e Al3

Ag+ + e Ag1 Pb2+ + e Pb2

H+ + e H2click for solution

H+ is present in all acids and hydrogen gas is evolved from the electrode.

2 2 In these cases, metallic elements would appear at the negative electrode.

Half equations for oxidation at the positive electrode.

Here electrons are lost by the ion. When gaseous elements are produced, they bond together in pairs to make a molecule.The balancing needs to include this.

Cl- - e Cl22 2 Br- - e Br22 2

O2- - e O22 4 Cu - e Cu2+

In a special case, a positive copper electrode dissolves in a solution of copper sulphate. Electrons are lost by the copper metal.

2

Here are different ions that might be in a solution.You need to be able to balance the half equations.

the ion GAINS electrons from the electrode