Electrolysis

Question 1

Which electrode is the anode

Answer 1

The positive electrode that is connected to the positive terminal of the battery

Question 2

Define electrolyte

Answer 2

A compound which when molten or aqueous solution, conducts electricity with decomposition at the elctrodes

Question 3

Describe reduction at cathode with molten sodium chloride

Answer 3

Sodium ions are attracted to cathode. Sodium ions are discharged as they gain eletrons to form sodium metal. Silvery globules of molten sodium float to surface of electrolyte and burn in air with flashes of yellow light

Question 4

Describe reduction at cathode with molten sodium chloride

Answer 4

Chloride ions are attracted to anode. Chloride ions are discharged as they lose electrons to form chlorine gas. Effervescence occurs, greenish yellow pungent gas is evolved

Question 5

What is the state of metals at cathode during electrolysis of molten binary electrolyte

Answer 5

Liquid(solution is very hot due to being molten, hence metals melt)

Question 6

What type of reaction occurs at cathode/anode

Answer 6

Reduction/oxidation

Question 7

Why is ethanoic acid a weak electrolyte

Answer 7

Contains relatively low concentration of mobile ions

Question 8

What is the electrochemical series for cations

Answer 8

K, Na, Ca, Mg, Al, Zn, Fe, Pb, H, Cu, Ag, Au (same as reactivity series)

Question 9

What is electrochemical series for anions

Answer 9

F-, SO42-, NO3-, Cl-, Br-, I-, OH-
Cl to OH are almost equal in ease of discharge

Question 10

How does relative position of ions in electrochemical series affect ease of discharge

Answer 10

An ion lower in electrochemical series will have greater tendency to be discharged

Question 11

What is selective discharge of cations from reactivity

Answer 11

The less reactive a metal, the less stable its ion. The metal ion gains electrons more easily to form back the metal.

Question 12

Describe reaction at anode (dilute sulfuric acid)

Answer 12

Sulfate ions and hydroxide ions are attracted to anode. Hydroxide ions are preferentially discharged as they lose electrons more readily than sulfate ions to produce water and oxygen gas

Question 13

Describe reaction at anode (concentrated sodium chloride)

Answer 13

Chloride ions and hydroxide ions are attracted to anode. Chloride ions are preferentially discharged due to their much higher concentration than hydroxide ions, they lose electrons to produce chlorine gas.

Question 14

Describe reaction at cathode (concentrated sodium chloride solution)

Answer 14

Sodium ions and hydrogen ions are attracted to cathode. Hydrogen ions are preferentially discharged as they gain electrons more readily than sodium ions to produce hydrogen gas.

Question 15

What are changes to solution during electrolysis of concentrated sodium chloride solution

Answer 15

Concentration of hydrogen ions and chloride ions decreases as they are being discharged at the electrodes, while sodium ions and hydroxide ions remain in the solution. Sodium chloride solution gradually becomes alkaline sodium hydroxide solution and pH of solution increases

Question 16

What are changes to solution during electrolysis of copper(II) sulfate solution

Answer 16

Concentrations of copper(II) ions and hydroxide ions decrease as they are being discharged at the electrodes while hydrogen ions and sulfate ions remain in the solution. Blue colour of solution slowly fades due to loss of copper(II) ions. Copper(II) sulfate solution gradually becomes sulfuric acid, pH of solution decreases

Question 17

Reaction at anode during electrolysis of copper(II) sulfate solution using copper electrodes

Answer 17

Sulfate ions and hydroxide ions are attracted to the anode. Sulfate ions and hydroxide ions are not discharged. Instead, copper anode, being reactive electrode, loses electrons and undergoes oxidation to produce copper(II) ions. Copper anode becomes smaller in size as it forms copper(II) ions which go in the solution

Question 18

What are changes in solution during electrolysis of copper(II) sulfate solution using copper electrodes

Answer 18

For each copper(II) ion discharged at the cathode, another copper(II) ion is formed at the anode and goes into the solution. Concentration of copper(II) solution does not change and blue colour of solution remains the same

Question 19

During purification of impure copper, which electrode is pure and impure copper placed at

Answer 19

Impure copper is made the anode and pure copper is made the cathode. Electrolyte must contain Cu2+ ions

Question 20

Define electroplating

Answer 20

The coating of the surface of an object with a thin layer of metal by electrolysis

Question 21

What are requirements for electroplating of an object

Answer 21

Object to be plated is made the cathode
A piece of metal for electroplating is made anode
Aqueous solution of salt of the plating the metal is used as the electrolyte

Question 22

What is the net result of electroplating process

Answer 22

Transfer of plating metal from anode to the cathode
Thickness of plating can be controlled by adjusting magnitude of the current and duration of the process

Question 23

Which is positive and negative electrode in simple cell

Answer 23

More reactive metal is the negative electrode which loses electrons more readily and positive electrode is less reactive metal where ions in solution receive electrons from more reactive metal

Question 24

What happens at negative electrode of simple cell with zinc and copper

Answer 24

Zinc is more reactive than copper. Zinc atoms in electrode loses electrons and undergoes oxidation to form zinc ions. Zinc ions go into solution and electrons then flow along the connecting wire to copper electrode.

Question 25

How to change magnitude of voltage of simple cells

Answer 25

The further apart two metals are in the reactivity series, the higher the voltage produced. No current will flow if both electrons are made of the same metal

Question 26

Define fuel cell

Answer 26

A chemical cell in which reactants are continuously supplied to produce electricity directly

Question 27

What happens at negative electrode of fuel cell

Answer 27

In presence of platinum catalyst, hydrogen gas loses electrons and undergoes oxidation to form hydrogen ions. Electrons go through an external circuit to move to positive electrode, creating a flow of electricity. Hydrogen ions formed move through electrolyte towards the positive electrode

Question 28

What happens at positive electrode of fuel cell

Answer 28

Oxygen gas undergoes reduction by reacting with hydrogen ions and electrode to form water.

Question 29

What are advantages of using hydrogen as fuel

Answer 29

Combustion of hydrogen produces water, non-polluting product.
Hydrogen considered renewable fuel, can be obtained from electrolysis of water
Hydrogen is efficient source of energy, when burned it releases twice the amount of energy compared to many other fuels

Question 30

Disadvantages of hydrogen as fuel

Answer 30

Production of hydrogen can be costly, obtaining hydrogen from electrolysis of water is expensive and hydrogen currently obtained from non-renerable energy sources
Hydrogen is flammable and explosive. Speical precautions must be taken in storage and transport
Gas at room temperature, hydrogen requires large storage volumes compared to other fuels