Electrolysis

Question 1

What is the definition of electrolysis?

Answer 1

When substances are broken down by electricity

Question 2

What is the broad process of electrolysis?

Answer 2

When an electric current is passed through an electrolyte the different ions in the compound will move towards the different electrodes where they will react, causing the ionic compound to decompose into separate ions

Question 3

Why does electrolysis use molten ionic compounds instead of ionic solids?

Answer 3

Ionic solids cannot be electrolysed because the ions are in fixed positions and cannot move, but in a molten ionic compound the ions can move freely and conduct electricity

Question 4

What is the definition of an electrolyte in chemistry?

Answer 4

A substance that conducts electricity when dissolved in a solvent or molten due to the presence of free moving ions

Question 5

What is the purpose of the electrolyte in electrolysis?

Answer 5

The liquid that electrolysis occurs in that must contain delocalised ions

Question 6

What is an electrode?

Answer 6

A solid that can conduct electricity

Question 7

What is the purpose of the electrodes in electrolysis?

Answer 7

To carry charge into the electrolyte

Question 8

Describe the specific process of electrolysis

Answer 8

An electrical circuit containing an electrolyte and 2 electrodes is set up. The electrodes are submerged in the electrolyte and ions move from one electrode to the other, allowing the conduction of electricity through the circuit. When the ions react at the electrodes they are discharged and become the products of electrolysis

Question 9

What types of charge to the cathode and anode have and why?

Answer 9

The cathode has a negative charge and is called the cathode because it attracts cations and the anode has a positive charge and is called the anode because it attracts anions

Question 10

What type of reaction happens at the cathode and why?

Answer 10

A reduction reaction happens at the cathode because the positive ions gain electrons and reduction is gain of electrons

Question 11

What type of reaction happens at the anode and why?

Answer 11

Oxidation happens at the anode because the negative ions lose electrons and oxidation is loss of electrons

Question 12

Why do reactions happen at the electrodes?

Answer 12

So the ions can lose or gain electrons to become atoms

Question 13

What type of reaction does electrolysis always involve?

Answer 13

It always involves a redox reaction

Question 14

What happens to the ions in the electrolyte?

Answer 14

They dissociate

Question 15

What are inert electrodes usually made of?

Answer 15

Platinum or graphite

Question 16

What are non inert electrodes usually made of?

Answer 16

Metals that have ions in the electrolyte so they can take part in the reaction - usually metals like copper, zinc or silver

Question 17

What type of current does the power supply for electrolysis need to work?

Answer 17

It needs DC - direct current - because if it is AC - alternating current - then the charges will keep switching from positive to negative in the electrodes but DC means they will have a constant positive or negative charge. It also allows the ions to flow in one direction, for a chemical reaction to occur, and makes it easier to identify ions

Question 18

What are the steps to writing have half equation?

Answer 18

Write the symbol for the ion on the left, ensuring the charge is correct, draw an arrow and write the symbol of the same ion as an atom/neutral molecule, balance the equation, balance the charges of the equation by adding/subtracting electrons

Question 19

How do you show adding/subtracting electrons in a half equation?

Answer 19

+e- or -e-

Question 20

What must you ensure when writing half equations for reactions at the cathode/anode?

Answer 20

For the cathode half equation you must add electrons and for the anode half equation you must subtract electrons

Question 21

Why is the electrolysis of an aqueous ionic solution different to that of a molten ionic compound?

Answer 21

Aqueous means that the ions have been dissolved in water and because the electrolyte is ionised, the electrolyte will contain H+ and OH- ions as well as the ions you are electrolysing

Question 22

What are the rules for the products of the electrolysis of an aqueous solution at the anode?

Answer 22

At the anode if there are halide ions (Cl-, Br- and I-) in the solution the halogen is produced and if not then oxygen is produced

Question 23

What are the rules for the products of the electrolysis of an aqueous solution at the cathode?

Answer 23

If the metals are more reactive than hydrogen then hydrogen is produced and the metal will stay in the solution and if the metals are less reactive than hydrogen then the metal i produced and hydrogen will stay in the solution

Question 24

What is the half equation for when oxygen is produced at the anode?

Answer 24

4OH- –> O2 + 2H2O + 4e-

Question 25

What is the half equation for the reduction of hydrogen?

Answer 25

2H+ +2e- --> H2

Question 26

What do you need to do for the half equations of diatomic molecules?

Answer 26

If the equation looks like X+ +e- --> X2, add twos in front of the first 2 terms to get 2X+ +2e- --> X2

Question 27

What do inert electrodes do?

Answer 27

They provide a surface for the transfer of electrons

Question 28

What metals are less reactive than hydrogen?

Answer 28

Silver (Ag), Platinum (Pt), Copper (Cu) and Gold (Au)

Question 29

Describe and explain the test for hydrogen

Answer 29

Collect hydrogen gas in a test tube and hold a lit splint into the test tube. The hydrogen will cause it to extinguish with a squeaky pop. This is because the fire causes the gas to burn

Question 30

Describe and explain the test for oxygen

Answer 30

Expose a glowing splint to oxygen gas and it will relight because it provides the necessary oxidiser and supports combustion

Question 31

What will be the products if you carry out electrolysis on water and what is the ratio?

Answer 31

Hydrogen at the cathode and oxygen gas at the anode and the ratio of oxygen gas produced to hydrogen gas produced is 1:2 because of the formula of water. There is twice as many hydrogen atoms as there are oxygen atoms

Question 32

What would you observe when the reduction of copper is happening?

Answer 32

You would see clumps of copper forming at the cathode

Question 33

What would you observe happening in the oxidation of hydroxide?

Answer 33

You would observe oxygen gas bubbling out through the water. Water is the other product of the reaction but that would just become part of the electrolyte

Question 34

What do non-inert electrodes do?

Answer 34

They take part in the reaction and can lose/gain mass

Question 35

How can we purify copper through electrolysis?

Answer 35

Set up the electrolysis of copper sulfate but instead use copper electrodes

Question 36

What happens at the cathode during the purification of copper?

Answer 36

The cathode would attract the copper ions and they would be reduced and discharged - we would observe copper clumps forming on the cathode

Question 37

What happens at the anode during the purification of copper?

Answer 37

Something must be oxidised for electrolysis to work and when using non-inert electrodes copper is easier to oxidise than hydroxide and the copper atoms will lose electrons to become copper ions that will enter the electrolyte and move towards the cathode

Question 38

How does the purification of copper lead to mass changes?

Answer 38

The copper atoms at the anode become copper ions and move to the cathode; the cathode gains mass and the anode loses mass

Question 39

What is anode mud?

Answer 39

Impurities in the anode that fall to the bottom of the solution as the copper atoms are oxidised and flake off

Question 40

What is the mass change in the purification of copper proportional to?

Answer 40

It is proportional to the amount of voltage the electrolysis is running on, with a greater voltage resulting in a greater mass change

Question 41

How can electroplating be carried out through electrolysis?

Answer 41

The object to be plated is the cathode and the anode is the metal you want to plate it with and the electrolyte contains ions of the same metal. The metal ions will be reduced and form a thin silver layer on the object