6 Electrolysis

What is an electrolysis reaction?

• a non spontaneous, endothermic redox reaction that converts electrical energy into chemical energy and requires electricity for it to occur

→ opposite of a galvanic cell

• an external power source has to be applied to provide a flow of electrons that forces a non-spontaneous reaction to occur

→ the energy source must provide a potential greater than the cell potential

How can you predict electrolytic reactions?

• reactions occur between the strongest oxidant and the strongest reductant

→ for electrolytic reactions, the substances that formed the smallest positive gradient will occur

• There are two types of electrolytic cells: molten and aqueous

What is a molten electrolyte?

• Melted ionic compound with ions that are free to move and are in a liquid state

→ no water is present

→ this type of electrolyte is expensive because you need to maintain a high temperature to keep the electrolytes in its molten state

What is an aqueous electrolyte?

• Dissolved ion compound with ions that are free to move and are in the aqueous solution

  Substances include ions as well as water (where it can be oxidised or reduced)

What are reactive electrodes?

• There are instances where reactive electrodes are used and they participate in their reaction

→ a reactive anode may oxidise preferentially

What are the features of an electrolytic cell?

• anode: positively charged electrode where oxidation occur occurs

• cathode: negatively charge electrode reduction occur occurs

• electrolyte: chemical compound that allows the flow of electrical charge with a cell

• power supply: electrical device that provides electrical energy to a circuit acting as an electro pump

What is the electron flow for electrolytic cells and galvanic cells?

• galvanic: chemical energy → electrical energy

• electrolytic: electrical energy → chemical energy

what is and isn’t required in an electrolytic cell?

a salt bridge is not needed, however a power supply is required

How do you select electrolytes for an electrolytic cell?

• the electrolytes should minimise expenses and ensure that the correct reaction takes place

  An aqueous electrolyte can be used in a cell we both:

→ the desired oxidising agent is a stronger oxidising agent than water (> -0.83 V)

→ the desired reducing agent is a stronger reducing agent than water (< +1.23 V)

• Aqueous electrolytes are preferable because they are cheaper and less dangerous than molten electrolytes

What must you consider when you select electrolytes for an electrolytic cell?

• Any metal located under the water half-equation (> -0.83 V) cannot be obtained from an aqueous solution. It must be produced through electrolysis using a molten electrolyte.

How do you determine the minimum voltage required to energise an electrolytic reaction?

E∘ required > E∘ cathode − E ∘ anode

what are spectator ions?

• ions that are functionally inert

• only conduct a current

• doesn’t take part in the reaction

→ carbon in the form of platinum, graphite etc…

How do you find the minimum voltage of an electrolytic cell/ overall equation?

1. List species present

2. Circle species present on electrochemical series

3. Select the strongest oxidising agent which is reduced that the cathode

4. Select the strongest reducing agent which is oxidise at the anode → this reaction is flipped backwards

5. Write both chosen equations

6. Write overall equation - use single arrows

7. Determine minimum voltage

what are the factors affecting the electrolysis of solutions?

• the concentration of the electrolyte

• the nature of the electrolyte

• the nature of the electrodes.

what is the effect of concentration on electrolysis reactions?

When they reactant is of high concentration, it becomes more likely to react than it would understand conditions.

what is the effect of the nature of the electrode on electrolysis reactions?

•   Inert electrodes (e.g., graphite, platinum): Do not react. They simply pass electrons, allowing ions in the electrolyte to be oxidised at the anode (producing gas or non-metals) and reduced at the cathode.

•   Reactive electrodes (e.g., copper anode): The anode itself gets oxidised and dissolves into the solution (corrodes), while the cathode is where metal ions from the solution are deposited.

what effect does the electrolyte have on electrolysis reactions?

At the Cathode (Reduction): * If the electrolyte contains reactive metal ions (like Na+ or K+), water is reduced instead, producing hydrogen gas bubbles.

• If the electrolyte contains less reactive metal ions (like Cu2+ or Ag+), the metal is deposited on the electrode.

At the Anode (Oxidation):

• If the electrolyte has high concentrations of halide ions (like Cl−, Br−), halogen gas is produced.

• If it contains dilute solutions or polyatomic ions (like SO42−​), water is oxidized to produce oxygen gas bubbles.

what are the two faraday’s laws?

• CHARGE (Q) = CURRENT (L) X TIME (T)

• CHARGE (Q) = MOLES OF ELECTRONS (n(e-)) X FARADAYS CONSTANT (F)

→ c is charge in coulombs (C)

→ current is in amperes (A)

→ time is in seconds (s)

→ faradays constant = 96500 C/mol

what is electroplating?

• The process by which a layer of metal is deposited onto the surface of another metal by electrolysis

→ used to enhance the properties of objects e.g, improving electrical conductivity strength appearance and corrosion prevention

• The metal we are plating with is the anode

• The object we want to plate is the cathode

e.g, gold plated earrings - the anno is the gold and the earrings is the cathode

Give an example of silver plating a spoon

1. The power supply act as an electron pump, pushing electrons onto the negative electrode and removing electrons from the positive electrode

2. The silver is oxidised providing a constant supply of silver 2+ ions that migrate to the cathode where they are reduced inform a solid silver coating on the spoon.

what is electro-refining?

• the process of purifying and impure metal using electrolysis

• The impure copper is the anode

• The pure copper is the cathode

→ copper is oxidised at the anode and the pure copper is then electrochemically deposited from the electrolyte solution onto the cathode

→ since impurity such as nickel and sync are weaker oxygen than copper. They will remain as ions that stay in the solution.

what is a primary cell?

• a primary cell is an electrochemical (galvanic cell) that generate electricity, but cannot be recharged

what is a secondary cell?

Secondary cells, often referred to as rechargeable batteries, are devices that can be recharged when they become ‘flat’.

• During discharge they are galvanic cells that use spontaneous redox reactions to produce electricity.

• During recharge they become electrolytic cells, converting electrical energy back into chemical energy.

• the polarity must remain unchanged

what are some factors that impact the metal coating in electroplating?

• the solution

• the shape of the anode - must be similar to the anode for even coating

• type and concentration of the electrolyte

what is a membrane cell?

• an electrochemical cell used for th electrolysis of brine

what is brine?

• a high concentration solution of salt, usually sodium chloride in water

what is a discharge reaction?

• a spontaneous, galvanic cell that produces electricity

• negative anode, postive cathode

what is a recharge reaction?

• a non-spontaneous, electrolytic cell that produces chemical energy

• positive anode, negative cathode

how do you derive the recharging or the discharging processes from equations?

• for either the recharging or discharging process you swap the anode and cathode reactions and then reverse the products and reactants

what are fuel cells?

electrochemical galvanic cells that require a continuous supply of reactants

how can you tell if a reaction is discharge or recharge?

→ look at the external circuit and what its connected to → power supply = recharge

what is the voltage required for recharge?

• the voltage required for recharge must be HIGHER than the voltage produced by discharge

How do you find the half-equations from an overall discharge/recharge equation?

1. Find the oxidation number of the key elements on both sides of the equation

2. Identify the change via OIL RIG

3. Pull out the specific elements required and and balance the charge via KOHES, while simultaneously adhering to where it has OH- or H+ ions