Electrolysis ppt

Let’s read!

Objectives

• To work out what happens to ions at each electrode. • To be able to write successful half-equations.

Electrolysis

Electrolysis

Lead bromide

Electrolysis is the break-down of a substance by

electricity

Electrolyte - a molten or aqueous solution through which an electrical current can flow.

Copy please

Electrode - an

electrical conductor which carries charge to or from a liquid undergoing

Electrolysis experiments

• Electrolysis only happens in: - molten ionic liquids or

- aqueous solutions containing ions.

• There must be a complete circuit.

1. Electrolytes contain

positive

and

negative

ions.

2. During electrolysis,

positive

and

negative

electrodes

are put into the electrolyte.

3. The

positive

electrode is called the

anode

.

4. The

negative

electrode is called the

cathode

.

5. The

negative

ions (called

anions

) are attracted to the

anode

.

6. At the

anode

, the

negative

ions lose electrons to

become

atoms/molecules

.

7. The

positive

ions (called

cations

) are attracted to the

cathode

.

8. At the

cathode

, the

positive

ions gain electrons to

become

atoms/molecules

1. Electrolytes contain

positive

and

negative

ions.

2. During electrolysis,

positive

and

negative

electrodes

are put into the electrolyte.

3. The

positive

electrode is called the

anode

.

4. The

negative

electrode is called the

cathode

.

5. The

negative

ions (called

anions

) are attracted to the

anode

.

6. At the

anode

, the

negative

ions lose electrons to

become

atoms/molecules

.

7. The

positive

ions (called

cations

) are attracted to the

cathode

.

8. At the

cathode

, the

positive

ions gain electrons to

become

atoms/molecules

At the electrodes

Cathode (-)

(negative electrode)

• Positive ions go here (cations).

• As metal ions are positive, they go to the cathode.

• Ions gain electrons. They are reduced and become

neutral atoms.

Anode (+)

(positive electrode)

• Negative ions go here (anions).

• As non-metal ions are

negative, they go to the

anode.

• Ions lose electrons. They are oxidised and become neutral atoms

Common ions

• Li

,

Na

,

K

,

• Mg

,

Ca

Zn

• Cu

,

Cu

,

Fe

, Fe

Al

• NH

(ammonium ion)

• F

, Cl

, Br

, I

-• O

S

2-• OH

(hydroxide ion), CO

Questions!

Half equations

• Show what happens at each electrode. • Are balanced equations.

• Consider the electrolysis of copper chloride:

Cu2+ _{+ 2e}- _{ Cu 2Cl}- _{- 2e}- _{ Cl}

Electrolysis of solutions –

Cathode

For solutions of highly reactive metals:

Electrolysis of solutions –

Anode

The product at

the anode depends on: The negative anions

Electrolysis of aluminium oxide

Electrolysis

Objectives

• recall that one Faraday represents one mole

of electrons

• calculate the amounts of the products of the

electrolysis of molten salts and aqueous

The Faraday

• A Faraday is one mole of electrons, and is

equivalent to 96 500C (Coulombs)

• A current of 1A = 1C per second flowing

• For example,

Cu

+ 2e  Cu

1 mole of Cu

ions reacts with 2 Faradays of

• Quantity of electricity in coulombs =

current in amps x time in seconds

Example

How much copper is deposited if a

current of 0.2 Amps is passed for 2

hours through a copper(II) sulphate

current of 0.2 Amps is passed for 2 hours

At the cathode:

Cu2+

(aq) + 2e-  Cu(s)

Q = I

x

t

= 0.2 x

(2 x 60 x 60)

=1440 Coulombs

1 mole electrons = 96500 Coulombs

Example

moles of electrons passed through circuit =

0.01492

Cu

(aq)

+ 2e-  Cu

• From equation, it takes two moles of

electrons to form one mole of copper

Example

• moles Cu = 0.00746

• mass of Cu

= moles x A

= 0.00746 x 64

‘How To” Guide

1. Write out relevant half equation

2. Work out coulombs of electrons flowing (Q = It)

3. Convert C into moles of electrons (Faradays) (Q/

96500)

4. Work out moles of product using ratio from

equation

• In an electrolysis of sodium chloride solution

experiment a current of 2 A was passed for 2

minutes.

– (a) Calculate the volume of chlorine gas

produced.

– (b) What volume of hydrogen would be formed?

– (c) In practice the measured volume of chlorine

– Electrode equations:

• (-) cathode 2H+ + 2e-  H 2

• (+) anode 2Cl-  Cl

2 + 2e

– (a) Calculate the volume of chlorine gas

produced.

• Q = I x t, so Q = 2 x 2 x 60 = 240 C

• 240 C = 240 / 96500 = 0.002487 mol electrons • this will produce 0.002487 / 2 = 0.001244 mol Cl₂

(two electrons/molecule)

• vol = mol x molar volume = 0.001244 x 24000 =

– (b) What volume of hydrogen would be

formed?

• 29.8 cm3 of H

2 because two electrons transferred per

molecule, same as chlorine.

– (c) In practice the measured volume of chlorine

can be less than the theoretical value. Why?

• In the electrolysis of molten sodium chloride

60 cm

of chlorine was produced.

– Calculate ...

– (a) how many moles of were chlorine produced?

– (b) what mass of sodium would be formed?

(a) how many moles of chlorine produced?

(b) what mass of sodium would be formed?

• from the electrode equations 2 mol sodium will be made for every mole of chlorine

• so 0.0025 x 2 = 0.005 mol sodium will be formed. A_r(Na) = 23

(c) for how long would a current of 3 A in the

electrolysis circuit have to flow to produce the

60cm

of chlorine?

• To produce 0.0025 mol of Cl₂ you need 0.005 mol of electrons

• 0.005 mol electrons = 0.005 x 96500 coulombs = 482.5 C

• Q = I x t, so 482.5 = 2 x t, therefore t = 482.5 / 3 =

Brine?

Electrolysis of brine

• Hydrogen is produced at the cathode

• Chlorine is produced at the anode

Electrolysis of brine

• Hydrogen is produced at the cathode

• Chlorine is produced at the anode

• The solution remaining is sodium hydroxide

Electrolysis of brine

• Hy

drogen is produced at the

cat

hode

• Chl

orine is produced at the

an

ode

• The solution remaining is sodium hydroxide

Cathy’s Ankles (

Cat

Hy

’s

An

Cl

)

Electrolysis of brine

• Cathode (-)

2H

(aq)

+ 2e

H

(SODIUM IS NOT FORMED (the sodium

ion is more stable than the hydrogen ion in

water H

O

H

+ OH

))

• Anode (+)

2Cl

Electrolysis of brine

• Cathode (-)

2H

(aq)

+ 2e

H

(SODIUM IS NOT FORMED (the sodium

ion is more stable than the hydrogen ion in

water H

O

H

+ OH

))

• Anode (+)

2Cl

-(aq)

– 2e

Cl

Chemicals from salt