electrolysis - unit 3 AOS 2

Question 1

why non spontaneous

Question 2

positive gradient

Question 3

why do we need more than the min valtage

Answer 3

• energy loss in the cell

Question 4

electrolysis

Answer 4

• the process in which a non-spontaneous chemical reaction occurs by passing an electric current through a substance in solution or molten state

Question 5

electrolytic cell

Answer 5

• an electric cell in which a non-spontaneous redox reaction is made to occur by the application of an external potential difference across the electrodes; also known as an electrolysis cell

Question 6

electrolytic cell features

Answer 6

• free moving ions - cations are attracted to the cathode and anions are attracted to the anode
• negative ions lose electrons at the anode
• positive ions gain electrons at the cathode
• electrons flow from the power source to the cathode

Question 7

aqueous electrolyte

Answer 7

• in an aqueous electrode, water is a possible reactant
• an aqueous solution can be used if the desired oxidising agents and reducing agents are stronger than water

Question 8

molten electrolyte

Answer 8

• melted ionic compound in a liquid state
• means water is not involves
• molten electrodes are not favourable because they require a high temperature to keep the electrolyte in its molten form.

Question 9

predicting products using the electrochemical series

Answer 9

• circle the species present on the electrochemical series
• select the strongest oxidising agent and the strongest reducing agent
• write the reduction and oxidation half equations (oxidation is reverse)
• write the overall equation

Question 10

determining valtage

Answer 10

min voltage required = E0 of oxidising reaction - E0 of reducing reaction

Question 11

two or more oxidising agents or two or more reducing agents

Answer 11

• the half reactions involving the strongest oxidising agent and the strongest reducing agents will be favoured.
• overtime, the reaction occurring may be replaced with ta different reaction

Question 12

reactive electrode

Answer 12

• if an anode can lose electrons and is a stronger reducing agent on the electrochemical series, it will be favoured.
• if a cathode can gain electrons and is a stronger oxidising agent on the electrochemical series, it will be favoured. however, metals have no tendency to gain electons

Question 13

inert electrode

Answer 13

• will not participate in the reaction

Question 14

additives in electrolysis

Answer 14

• Cryolite is an additive that can be used to lower the melting point of the
  electrolyte in the extraction of pure aluminium via electrolysis.

Question 15

discharge

Answer 15

• During discharge they are galvanic cells that use spontaneous redox reactions to produce electricity.
• oxidation occurs at the negative anode and reduction occurs at the positive cathode
• the discharge reaction is spontaneous

Question 16

secondary cell

Answer 16

• a rechargeable cell that works as both a galvanic cell and an electrolytic cell

Question 17

recharge

Answer 17

• During recharge they become electrolytic cells, converting electrical energy back into chemical energy.
• oxidation occurs at the positive anode and reduction occurs at the negative cathode
• the recharge reaction is non spontaneous

Question 18

energy transformations

Answer 18

• during discharge chemical energy is being converted into electrical energy
• during recharge, electrical energy is being converted into chemical energy

Question 19

secondary cell recharge requirements

Answer 19

• The cell must be connected to a power supply that provides a voltage above the
  required voltage.
• The products of the discharge half-reactions must remain in contact with the
  electrode at which they react.

Question 20

products of discharge reaction

Answer 20

• Once the required voltage threshold has been reached, a recharge reaction can only
  occur if the products of the discharge half-reactions are in contact with the electrode
  at which they react.
• Otherwise, they are unable to be electrolysed. For this reason, it
  is ideal for a secondary cell to contain predominantly aqueous species, since as longas the electrodes are submerged (figure 4), electrolytic reactions are predicted given
  a sufficient voltage.

Question 21

factors affecting longevity of cells

Answer 21

• Over time, as a battery undergoes numerous recharge and discharge cycles, a
  sufficient proportion of products of the discharge reaction may be unable to remain in contact with their relevant electrodes for the cell to operate as desired.

Question 22

green hydrogen

Answer 22

• hydrogen gas produced using renewable energy sources by splitting water into oxygen and hydrogen through electrolysis

Question 23

blue vs grey hydrogen

Answer 23

• produced from natural gas
• blue hydrogen production involves carbon dioxide carbon dioxide captured and stored

Question 24

black/brown hydrogen

Answer 24

• produced from coal via gasification
• releases greenhouse gases

Question 25

polymer electrolyte membrane

Answer 25

• also called proton exchange membrane, uses a proton-conducting polymer membrane as the electrolyte and connects two half-cells,
  but prevents the mixing of different molecules
• can produce green hydrogen when the energy used is solar power, wind power or biomass.
• PEM is selectively permeable to protons and does not conduct electrons. It helps ensure hydrogen and oxygen are produces separately
• oxidation of water: 2H2O(l) → 4H+(aq) + O2(g) + 4e−
• reduction of protons: 4H+(aq) + 4e− → 2H2(g)
• overall equation: 2H2O(l) → 2H2(g) + O2(g)

Question 26

artificial photosynthesis

Answer 26

• process that utilises human-made materials to capture sunlight and split water molecules to create hydrogen and oxygen

Question 27

sustainability of green hydrogen

Answer 27

Advantages
- hydrogen produced can be used in the deduction of carbon dioxide
- can be used for power generation, transportation, and industrial processes
- oxygen produced can be used in the medical industry
- it does not produce any greenhouse gases and does not require fossil fuels.

disadvantages
- energy efficiency: The process of producing green hydrogen can be energy-intensive, especially when using renewable energy sources that are costly to implement.
- infrastructure: There is currently limited infrastructure for producing, storing, and transporting hydrogen. This can make it challenging to scale up the use of hydrogen as a fuel source
cost: Cost: Currently, the cost of green hydrogen production is higher than that of
other conventional fuels. This can limit its use, especially in developing countries.
- water :Hydrogen production requires significant amounts of water, which can be a challenge in areas facing water scarcity or drought.

Question 28

Faradays first law

Answer 28

• The mass increase at the cathode of an electrolytic cell is directly proportional to the quantity of charge carried.

Question 29

Faradays second law

Answer 29

• To deposit 1 mole of substance at the cathode, 𝑥 moles of electrons (n(e−)) are required, where 𝑥 is the coefficient of electrons in the balanced reduction half-equation occurring in the cell.

Question 30

calculating Q, the amount of charge carries in an electrolytic cell

Answer 30

Q = It, where I is the amps and t is time in seconds

n(e-) = Q/F, where F is faradays constant which is 96500

Question 31

PEM evaluation and features

Answer 31

positive anode where oxidation of water occurs

negative cathode where protons flow to

proton exchange membrane selectively permeable to protons and so blocks electrons and other gases

high efficiency due to catalysts and low operating temperatures

advantages
- pure hydrogen
- can produce green hydrogen when powered by wind or solar energy
- high efficiency due to catalysts

disadvantages
- PEM can degrade over time
- requires lots of water