Electrolysis II

Question 1

What is an electric current?

Answer 1

a flow of electrons or ions

Question 2

What is a non-electrolyte?

Answer 2

a substance that does not undergo electrolysis

e.g. covalemt compounts as they do not release ions

Question 3

What is an electrolyte?

Answer 3

liquids (molten compounds or compounds in solution) which are broken down when they conduct electricity

usually ionically bonded

Question 4

Why must an electrolyte be in liquid form in order to conduct electricity?

Answer 4

so the ions can move

the movement of ions is responsible for the conduction of electricity

Question 5

What is an electrode?

Answer 5

a fairly inert substance (e.g. carbon) that electricity passes into and out of

Question 6

What is an anode?

Answer 6

a positive electrode

Question 7

What is a cathode?

Answer 7

a negative electrode

Question 8

How do you remember which electrode is positive and which electrode is negative?

Answer 8

PANC

Positive Anode Negative Cathode

Question 9

What is an ion?

Answer 9

a charged particle (negative or positive)

Question 10

What is electrolysis?

Answer 10

the breakdown of a substance (molten or in solution) by electricity

Question 11

What are two metals commonly used for electrodes?

Answer 11

carbon

platinum

Question 12

How do you make electrolytes?

Answer 12

by melting or dissolving ionic compounds

Question 13

What does an electrolysis circuit need to be complete? In which direction do the electrons flow?

Answer 13

a circuit needs a flow of electrons to be complete

the electrons are taken away from the ions at a positive electrode (anode) and given to the other ions at the negative electrode (cathode)

Question 14

What happens to ions as they gain or lose electrons?

Answer 14

they become atoms or molecules

Question 15

How can you measure the conductivity of an electrolyte?

Answer 15

with a conductivity probe

when you place a conductivity probe in a non-electrolyte, no current flows, so you’ll get a reading of zero conductivity

Question 16

How is lead (II) bromide bonded? What does this mean? What happens if it is melted?

Answer 16

lead (II) bromide is ionically bonded si the ions are not free to move when is a solid state because of its lattice structure - no current can occur

if melted, the electrostatic attraction is broken and the ions are free to move to the electrodes

Question 17

What does the circuit for the electrolysis of lead (II) bromide look like?

Answer 17

Question 18

What happens during the electrolysis of lead (II) bromide?

Answer 18

Nothing happens at all until the lead (II) bromide melts. Then:

the light bulb lights up, showing that electrons are flowing through it

there is bubbling around the anode (connected to the positive terminal of the power source) as red-brown bromine gas is given off

nothing seems to be happening at the cathode (connected to the negative terminal of the power source), but afterwards solid metallic lead is found underneath it (because it is no longer attracted to the cathode)

when you stop heating and the lead (II) bromide solidifies again, everything stops - there is no more bubbling and the bulb goes out

N.B. The power source must be switched on

Question 19

In the electrolysis of lead (II) bromide, at which electrodes do the lead and the bromide ions go to?

Answer 19

the lead ions (positive) go to the cathode (negative)

the bromide ions (negative) go to the anode (positive)

this is because opposite charges attract

Question 20

What happens to ions at electrodes?

Answer 20

they are discharged (they lose their charge either by giving up electrons or receiving electrons from the electrode)

Question 21

Write the half equation for the reaction at the cathode during the electrolysis of lead (II) bromide

Answer 21

the positive ions are attarcted to the negative cathode

at the cathode, a lead ion accepts two electrons to become a lead atom

Pb²⁺ + 2 electrons^- → Pb

Question 22

Write the half equation for the reaction at the anode during the electrolysis of lead (II) bromide

Answer 22

the negative bromide ions are attracted to the positive anode

at the anode, two bromide ions lose one electron each and become a bromine molecul

2Br^- → Br₂ + 2 electrons^-

Question 23

Why don’t electrodes take part in electrolysis reactions?

Answer 23

they are made from inert (unreactive) material

Question 24

At which electrode does oxidation and reduction happen? What is the word used to describe when these two happen at the same time

Answer 24

oxidation hsppens at the anode

reduction happens at the cathode

together, this is REDOX

Only Animas Reproduce Cats

Question 25

Are all electrolysis reactions REDOX?

Answer 25

yes

Question 26

What apparatus is used when carrying out an experiment to see which substances conduct electricity? How is this set up?

Answer 26

bunsen burner

heat proof mat

clay triangle

carbon electrodes

light bulb

chemicals

glass rod

crocodile clips

tripod

retort stand and clamps

battery

tongs

crucible

spatula

cable wires to create the circuit

N.B. Instead of a bulb, you can test for a current using an ammeter

Question 27

What is the method for carrying out an experiment to see which substances conduct electricity? How is this set up?

Answer 27

1. Take two spatials of your sample solid and place it into the crucible. Place the electrodes, which are part of a circuit connected to a lightbulb, into the sample to complete the circuit. If the lightbulb turns on then you do not need to heat the same - we can already see that it conducts electricity
2. Heat the sample and observe whilst keeping the circuit complete. Take care that the wires do not become heated. If, after two minutes of strong heating, the lightbulb stays off then stop - this sample is not an electrolyte

Question 28

What can we conclude about the electric conductivity of metals?

Answer 28

all metals conduct electricity in their solid state

only metals conduct electricity in their solid state, no compounds conduct electricity in their solid state

lead (II) bromide, potassium iodide, zinc chloride and potassium chloride are the only molten compounds that we tested that conducted electricity - this is because they are ionically bonded substances and, in their molten state, the ions are free to move

no covalent compounds conducted electricity in their molten state

Question 29

What do metals have that allow them to conduct electricity?

Answer 29

delocalised electrons

Question 30

Covalent compounds do/don’t have any charged particles. They consist of … or … which are electrically …. They can/cannot conduct electricity, even if we … or …

Answer 30

Covalent compounds don’t have any charged particles. They consist of atoms or molecules which are electrically neutral. They cannot conduct electricity, even if we melt or dissolve

Question 31

Positive metal ions are called … are attracted to the …

Negative non-metal ions are called … are attracted to the …

Answer 31

Positive metal ions are called anions are attracted to the cathode

Negative non-metal ions are called cations are attracted to the anode

Question 32

Anions become … at the cathode

Cations become … at the anode and usually form …

Answer 32

Anions become metal atoms at the cathode

Cations become non-metal atoms at the anode and usually form molecules

Question 33

Can all ionic compounds be electrolysed molten?

Answer 33

no

some break up into simpler compounds before their metlingpoint

others have such high meltimg points that it isn’t possible to melt them in the lab, although it can be done industrially

Question 34

Water is a weak/strong electrolyte

What does it ionise slightly to give?

Answer 34

Water is a weak electrolyte

it ionises very slightly to give hydrogen ions and hydroxide ions

H₂O _(l) ⇌ H⁺_(aq) + OH^- _(aq)

Question 35

What is present in aqueous solutions?

Answer 35

ions from the ionic compound

hydrogen (H⁺) ions from the water

hydroxide (OH^-) ions from the water

Question 36

When are hydrogen (H⁺) ions formed at the cathode?

Answer 36

if hydrogen ions and metal ions are present, if the metal ions are more reactive than the hydrogen ions, hydrogen gas will be produced at the cathode

if the metal ions are less reactive than the hydorgen ions, a solid layer of the pure metal will be produced instead

Question 37

When are hydroxide (OH^-) ions formed at the anode?

Answer 37

if hydroxide (OH^-) ions and halide ions are present, molecules of chlorine, bromine or iodine will be formed there

if no no halide ions are present, thern oxygen will be formed

Question 38

Describe what happens in the electrolysis of sulphuric acid (H₂SO₄)

Answer 38

Sulphuric acid (H₂SO₄) contains three different ions: SO₄^2-, H⁺ and OH^-

Hydrogen ions (from the water or sulphuric acid) accept electrons. So at the cathode, hydrogen gas is produced:

2H⁺ + 2e^- → H₂

Hydroxide ions lose electros more easily than sulphate ions. So at the anode, oxygen and water are produced:

4OH^- → O₂ + 2H₂O + 4e^-

Question 39

Describe what happens in the electrolysis of sodium chloride

Answer 39

A solution of sodium chloride (NaCl) contains four different ions: Na⁺, Cl^-, OH^-, H⁺

Hydrogen ions accept the electrons more easily than sodium ions. So at the cathode, hydrogen gas is produced:

2H⁺ + 2e^- → H₂

Chloride ions lose electrons more easily than hydroxide ions. So at the anode, chloride gas is produced:
2Cl^- → Cl₂ + 2e^-

Question 40

Describe what happens in the electroylsis of copper (II) sulphate

Answer 40

A solution of copper sulphate (CuSO₄) contains four different ions: Cu²⁺, SO₄²⁺, H⁺, OH^-

Copper ions accept electrons more easily than hydrogen ions. So at the cathode, copper metal is produced:

Cu²⁺ + 2e^- → Cu

Hydorxide ions lose electrons more easily than sulphate ions. So at the anode, oxygen and water are produced:

4OH^- → O₂ + 2H₂O + 4e^-

Question 41

What is the product and equation at the cathode and the anode of KI_(aq)?

Answer 41

Cathode:

product = hydrogen

equation = 2H⁺_(aq) + 2e^- → H_{2 (g)}

Anode:

product = iodine

equation = 2I^-_(aq) → I_{2 (aq)} + 2e^-

Question 42

What is the product and equation at the cathode and the anode of MgBr_{2 (aq)}?

Answer 42

Cathode:

product = hydrogen

equation = 2H⁺_(aq) + 2e^- → H_{2 (g)}

Anode:

product = bromine

equation = 2Br^-_(aq) → Br² _(aq) + 2e^-

Question 43

What is the product and equation at the cathode and the anode of H₂SO_{4 (aq)}?

Answer 43

Cathode:

product = hydrogen

equation = 2H⁺_(aq) + 2e^- → H_{2 (g)}

Anode:

product = oxygen

equation = 4OH^-_(aq) → 2H₂O _(l) + O_{2 (g)} + 4e^-

Question 44

What is the product and equation at the cathode and the anode of CuSO₄ (aq)?

Answer 44

Cathode:

product = copper

equation = Cu²⁺ _(aq) + 2e^- → Cu _(s)

Anode:

product = oxygen

equation = 4OH^-_(aq) → 2H₂O _(l) + O² _(g) + 4e^-

Question 45

What is the method for carrying out the electrolysis of copper chloride solution?

Answer 45

1. half-fill the beaker with copper chloride solution
2. connect the electrodes in a simple circuit with the power pack and lamp
3. start the current and observe what happens at the cathode and the anode
4. as soon as the observations are complete, the power pack should be switched off

Question 46

What is the name for a reaction where one substance breaks down into two or more substances?

Answer 46

decomposition

Question 47

What is a coulomb?

Answer 47

a measure of quantity of electricity

one coulomb = 6.2 x10⁸ electrons

Question 48

What is the rate of flow of electrical charge measured in?

Answer 48

amps (A)

Question 49

What is the equation for charge?

Answer 49

charge (Q) = current (I) x time (t)

Q is measured in coulombs (C)

I is measures in amperes (A)

t is measures in seconds (s)

Question 50

If 2 amps flow for 20 minutes, what is the charge?

Answer 50

Q = I x t

= 2 x (20 x 60)

= 2400 coulombs

Question 51

What is a faraday?

Answer 51

a quantity of electricity

one faraday (F) is the quantity of electricity which represents one mole of electrons passing a particular point in the circuit (6 x10²³ electrons)

one mole of electrons = 96,000 coulombs = one faraday

Question 52

What does the amount of product made by electrolysis depend on? How can this be increased?

Answer 52

the number of electrons that are transferred

if you increase the number of electrons, you increase the amount of substance produced

this can be achieved by electrolysing for a longer time or increasing the current

Question 53

How many moles of electrons does sodium need to make one mole of sodium atoms?

Answer 53

sodium ions need one electron to make a sodium atom

one mole of sodium ions need one mole of electrons (one faraday) to make one mole of sodium atoms

Na⁺ + e^- → Na

1 mole of sodium atoms + 1 mole of electrons → 1 mole of sodium atoms

Question 54

How many moles of electrons does zinc need to make one mole of zinc atoms?

Answer 54

zinc ions need two electrons to make a zinc atom

one mole of zinc ions need two moles of electrons (two faraday) to make one mole of zinc atoms

Zn²⁺ + 2e^- → Zn

1 mole of zinc atoms + 2 mole of electrons → 1 mole of zinc atoms

Question 55

How many moles of electrons does aluminum need to make one mole of aluminium atoms?

Answer 55

aluminium ions need three electrons to make an aluminium atom

one mole of aluminum ions need three moles of electrons (thre faraday) to make one mole of aluminiumatoms

Al³⁺ + 3e^- → Al

1 mole of aluminium atoms + 3 mole of electrons → 1 mole of aluminium atoms

Question 56

What mass of aluminium will be deposited if a current of 1.5A is passed through aluminum oxide (dissolved in moleten cyrolite) for 40 minutes?

Answer 56

1. Find the total charge passed

Charge passed = 1.5 A x (40x60)s = 3600C

2. Convert this to moles of electrons (faradays) (divide by the charge equivalent of one faraday_

number of moled of electrons = 3600C ÷ 96000C C/mol = 0.0375 mol

3. Write out the ion-electron equation and deduce the number of moles of substance per moles of electrons (molar ratio)

Al³⁺ + 3e^- → Al

e : Al

3 : 1

0.0375 : (0.0375 ÷ 3)

mols of Al = 0.0125

4. Convert moles of electrons passed into mass of product

mass of Al = moles x Ar

= 0.0125 x 27

= 0.0338g

Question 57

During the electrolysis of copper (II) sulphate solution using carbon electrodes, copper is deposited on the cathode according to the equation:

Cu²⁺ _(aq) + 2e^- → Cu _(s)

Calculate the gain in mass at the cathode if a current of 0.50 amps flows for 1 hour

(RAM: Cu = 64; 1 faraday = 96,000 coulombs)

Answer 57

1. Q = I x t

= 0.50 A x (60 x 60) s

= 1800 C

2. number of moles of electrons = 1800 ÷ 96,000 C/mol = 0.01875 moles of electrons

3. Cu : electrons

1 : 2

(0.01875 ÷ 2) : 0.01875

moles of Cu = 0.009375

4. mass of Cu = moles x Ar

= 0.009375 x 64

= 0.6 g

Question 58

During the electrolysis of lead (II) nitrate solution, lead is deposited at the cathode and oxygen is released from the anode. If a current of 0.350 amps flows for 1000s, calculate the mass of lead deposited

(RAM: Pb = 207, 1 faraday = 96,000 coloumbs)

Pb²⁺ _(aq) + 2e^- → Pb _(s)

Answer 58

1. Q = I x t

= 0.350 x 1000

= 350

2. number of moles of electrons = 350 ÷ 96,000 C/mol = 0.00365

3. Pb : electrons

1 : 2

(0.00365 ÷ 2) : 0.00365

moles of Pb = 7/3840

4. mass of Pb = moles x Ar

= 7/3840 x 207

= 0.377 g

Question 59

During the electrolysis of lead (II) nitrate solution, lead is deposited at the cathode and oxygen is released from the anode. If a current of 0.350 amps flows for 1000s, calculate the volume of oxygen (measured at room temperature and pressure) produced

(the molar volume of gas is 24,000 cm³ at rtp, 1 faraday = 96,000 coloumbs,)

4OH^-_(aq) → 2H₂O _(l) + O_{2 (g)} + 4e^-

Answer 59

1. Q = I x t

= 0.350 x 1000

= 350

2. number of moles of electrons = 350 ÷ 96,000 C/mol = 0.00365

3. O₂ : electrons

1 : 4

(0.00365 ÷ 4) : 0.00365

moles of O₂ = 73/80,000

4. volume = moles x 24 dm³

= 73/80,000 x 24

= 0.0219 dm³ / 21.9 cm³

Question 60

Some copper (II) suplhate solution was electrolysed using a pure copper cathode and an impure copper anode. Copper is lost from the anode and deposited on the cathode. Insolutble impurities in the anode form a sludge underneath the anode.

Cathode equation:

Cu²⁺ _(aq) + 2e^- → Cu _(s)

Anode equation:

Cu _(s) → Cu²⁺ _(aq) + 2e^-

a) What mass of copper will be deposited on the cathode if 0.40 amps flows for 75 minutes? (RAM: Cu = 64, 1 faraday = 96,000 coulombs)
b) If the anode was found to have lost 0.80 g during the experiment, calculate the percentage purity of the impure copper anode, assuming that only insoluble impurities were present

Answer 60

a) 1. Q = I x t

= 0.40 x (75 x 60)

= 1800

2. number of moles of electrons = 1800 ÷ 96,000 C/mol = 0.01875

3. Cu : electrons

1 : 2

(0.01875 ÷ 2) : 0.01875

moles of Cu = 3/320

4. mass = moles x Ar

= 3/320 x 64

= 0.6 g

b) purity ÷ impurity x 100

0.6 ÷ 0.80 x 100

= 75%

Question 61

Aluminium is manufactured by electrolysing a solution of aluminium oxide, Al₂O₃, in molten cryolite. The electrode equation is:
Al³⁺ _(l) + 3e^- → Al _(l)

A typical cell produces 1 tonne (1000kg) of aluminum every 24 hours. What current (in amps) is needed to produce this amount of aluminium?

(RAM: Al = 27, 1 faraday = 96,000 coulombs)

Answer 61

work backwards

1. moles of aluminium = mass ÷ Ar

= (1000 x 1000) ÷ 27

= 37037.03704

2. Al : electrons

1 : 3

37037.03704 : (37037.03704 x 3)

3. moles of electrons = 111111.111111

4. charge = moles of electrons x 96,000 C/mol

= 111111.111111 x 96,000

= 10666666666

5. I = Q ÷ t

= 10666666666 ÷ (24 x 60 x 60)

= 123456.7901 amps

Question 62

Two solutions were elctrolysed in series. One beaker contained chromium (III) sulphate solution, and the other cobalt (II) sulphate solution. 0.295 g of cobalt was deposited on the cathode in the beaker containing cobalt (II) sulphate. The electrode equations are:

Cr³⁺ _(aq) + 3e^- → Cr _(s)

Co²⁺ _(aq) + 2e^- → Co _(s)

a) Calculate the quantity of electricity that flowed during the experiment

(RAM: Cr = 52, Co = 59, 1 faraday - 96,000 coulombs)

b) Calculate the mass of chronium deposited on the cathode in the other beaker

Answer 62

work backwards

a) 1. moles of cobalt = mass ÷ Ar

= 0.295 ÷ 59

= 1/200

2. Co : electrons

1 : 2

1/200 : (1/200 x 2)

moles of electrons = 0.01

3. charge = moles of electrons x 96,000 C/mol

= 0.01 x 96,000

= 960 coulombs

b) If 960 coulombs passed through in a) ten exactly the same amount passed through the rest of the circuit

1. the charge is the same (960 coulombs)

2. the moles of electrons are the same (0.01)

3. Cr : electrons

1 : 3

(0.01 ÷ 3) : 0.01

moles of Cr = 1/300

4. mass = moles x Ar

= 1/300 x 52

= 0.1733333 g

Question 63

Copper (II) sulphate solution and lead (II) nitrate solution were electrolysed in two beakers connected in series. If 0.64 g of copper was deposited at the cathode in one beaker, calculate the mass of lead deposited in the other one.

(RAM: Cu = 64, Pb = 207, 1 faraday = 96,000 coulombs)

Cu²⁺ _(aq) + 2e^- → Cu _(s)

Pb²⁺ _(aq) + 2e^- → Pb _(s)

Answer 63

First, find the quantity of electricity (coulombs) that flowed during the experiment

1. moles of copper = mass ÷ Ar

= 0.64 ÷ 64

= 0.01

2. Cu : e

1 : 2

0.01 : (0.01 x 2)

moles of electrons = 0.02

3. charge = moles of electrons x 96,000 C/mol

= 1920 coulombs

Second, we know that the same charge passed through the rest of the circuit

1. we know the charge = 1920 coulombs

2. we know the number of electrons = 0.02

3. Pb : e

1 : 2

(0.02 ÷ 2) : 0.02

moles of Pb = 0.01

4. mass of Pb = moles x Ar

= 0.01 x 207

= 2.07 g

Question 64

How is hydrogen chloride gas converted into hydrochloric acid?

Answer 64

HCl dissolves in water to from the aqueous solution; hydrochloric acid

Question 65

Why is chlorine sometimes added to water supplies?

Answer 65

it acts as a water treatment to sterilise water