chemistry u3 aos 2 (electrolysis and faraday's laws)

electrolysis

= a process of passing through electric current into an electrolytic solution (conducting liquid) to make non-spontaneous redox reactions

• electrical energy → chemical energy

applications of electrolysis

• electroplating

• electrorefining

• rechargeable batteries

• hydrogen production from water

compare electrolytic cell and galvanic cell

similarities

• they are both electrochemical cell

• both have an anode for oxidation and a cathode for reduction

galvanic cell

• chemical energy → electrical energy

• produce electricity

• anode (-); cathode (+)

• spontaneous redox reactions

• two half cells are needed

electrolytic cell

• electrical energy → chemical energy

• consume electricity

• a battery/ energy source is needed

• anode (+); cathode (-)

• non-spontaneous redox reaction

• only one cell

why is cathode in electrolysis negative?

the power source forces electrons towards cathode, resulting in a negative charge

what would be/ how do we determine the required voltage applied for the electrolytic cell to operate?

greater than the cell emf/ potential difference, as it is a non-spontaneous reaction;

HOWEVER the voltage cannot be too high, or else side reactions may occur

** the calculated emf in electrolytic cell is always negative (coz non-spontaneous)

what is needed for electrolysis to occur?

free moving ions/ charged particles

• aqueous solution (water may undergo electrolysis)

• molten salt (no water); but more expensive

what to be considered when choosing species to react in your mini ECS?

• any repulsion? (same charge?)

• the strongest RA/ OA

• the concentrations of the RA/ OA (e.g. Cl^- VS H₂O)

• species running out, the next strongest RA/ OA will react

• any direct reactions between species

disadvantage and advantage of using molten salt for electrolysis

(disadvantage)

• high cost

(advantage)

• no water is present to be preferentially reduced

• any metals below Zn can only be produced via electrolysis of molten salt

ECS is determined under standard conditions (298K, 1M, 100kPa), how would different concentration of NaCl(aq) affect its electrolysis?

why nitrate ion is not reduced?

• reduction occurs at cathode (+) which will therefore repel the positive nitrate ion

• H⁺ ions are required for reduction of nitrate ions which is not present initially

choose using an aqueous solution/ molten salt to produce Mg and Cl₂ from MgCl₂

why does electrolysis sometimes yield different products to those predicted using ECS?

ECS is under standard conditions, sometimes conditions are not standard

use of sodium and chlorine

sodium

• raw material for bleach

• to produce number of organic chemicals

• used in nuclear sectors

chlorine

• disinfectant

• manufacture of plastics and pharmaceuticals

what is the purpose of adding chemical additives into the electrolytic cell with molten electrolyte? any criteria?

to lower the melting point, to reduce the energy used to melt the substance

these additives do not compete with the OA/RA or react within the cell to ensure the desired product is still obtained (e.g. CaCl in down cell, mixed with NaCl)

what’s the point of ‘electrolyte provides resistance to the electric current’?

this produces sufficient heat to keep the electrolyte molten

why is a low melting point of electrolyte desirable?

• lower energy required to melt

• lower the cost

what is the purpose of having a membrane/ screen/ mesh in an electrolytic cell?

to separate the (reactive) products, prevent them from reacting

how to abide with green chemistry principles when designing an electrolytic cell?

renewable energy sources can be used (e.g. solar power, wind power)

how does those chemical additives (e.g. cryolite for alumina) lower the m.p.?

• different sizes of particles

• alumina is less dense in cryolite than molten aluminium

• less energy to break the bonds

• lower the melting point

except from an energy source to keep the electrolyte molten, where else could we get the heat?

• the electrical resistance of the electrolyte to the electric current keeps the electrolyte molten

• any other subsequent reaction that is exothermic will contribute energy to maintain molten electrolyte

advantage of having unreactive electrodes

• avoid production of CO₂ for carbon electrode

• reduce the cost to replace the reactive electrodes

compare primary cell and secondary cell

(primary cell)

• chemical energy → electrical energy

• oxidation at anode (-); reduction at cathode (+)

• goes flat when the reaction reaches equilibrium must be discarded

• products migrate away from the electrodes

(secondary cell)

• both discharge and recharge

• discharge follows the rule for galvanic cell

• can be recharged to convert the products back to reactants

• recharge: electrical energy → chemical energy

• BECAUSE products from discharge remain in contact with electrodes

compare discharge (galvanic cell) and recharge (electrolysis)

(discharge)

• chemical energy → electrical energy

• oxidation at anode (-); reduction at cathode (+)

(recharge)

• electrical energy → chemical energy

• oxidation at anode (+); reduction at cathode (-)

• this is can happen coz the products from discharge remains in contact with the electrodes

• a source of electricity is supplied to convert electrical → chemical energy

• the electrical energy must be greater than the emf of the cell (overpotential)

when considering the secondary cell undergoing both discharge and recharge reactions, which remains no change? charge OR ‘anode’ ‘cathode’

the charge of each electrode remains unchanged

i.e. reaction at anode (during discharge) is then reversed and become a reaction at cathode (during recharge), while both electrodes in this case is still negative

combustion reaction for H₂

only water is produced (no CO₂) since there was no carbon provided initially

advantage and disadvantage of hydrogen as a fuel

(advantage)

• high energy density

• abundant on earth

• water is a sole product of its combustion

(disadvantage)

• it is not found as an element, energy is needed for production, which could be using non-renewable energy sources

• energy is needed to liquify it due to its very low boiling point

• high pressure is also needed to store hydrogen as gas

• the hydrogen is highly flammable and explosive, careful storage is needed

• also difficult to be transported safely as a gas/ liquid

green hydrogen VS brown hydrogen

(brown hydrogen)

• derived from fossil fuel

• produced using steam reforming of methane (also produces CO)

• production generates CO₂

• however this is the predominant type of hydrogen

(green hydrogen)

• produced by chemical processes that use renewable energy sources

• production is largely carbon neutral

(grey hydrogen)

• hydrogen derived from industrial processes

(blue hydrogen)

• derived from fossil fuels with carbon capture

ways to produce green hydrogen

• electrolysis of water using Hofmann voltameter

• PEM (polymer electrolyte membrane) electrolysis cell

• artificial photosynthesis

electrolysis of water using Hofmann voltameter

• a low concentration of H₂SO₄ (electrolyte) is added to water to allows conduction of electricity

  COZ (pure water CANNOT be electrolysed; free-moving charged particles are needed to conduct electricity)

why wouldn’t H₂SO₄ react in the Hofmann voltameter

the concentration of H₂SO₄ is too low for the ions to react

where is hydrogen produced

RHS, because the amount of gas produced is twice as much as that on the LHS

except from H₂SO₄ being added to conduct electricity in the electrolysis, what else can be added to increase the rate if gas production

salt

• increase salt concentration

• increase conductivity

• increase the rate of reaction

• increase rate of gas production

how can you make electrolysis of water produce green hydrogen?

• using renewable energy source (e.g. wind energy/ solar energy) to generate electricity for electrolysis of water

electrodes in PEM electrolytic cell

• expensive inert metals (e.g. Pt)

• porous, to allow passage of gases whilst preventing the flow of liquid

electrolyte in PEM electrolytic cell

• solid polymer membrane electrolyte

• allows movement of hydrogen ions from anode to cathode

• electrically insulating the electrodes

• separating the hydrogen gas and oxygen gas produced

any solution in PEM electrolytic cell?

no solutions, all solid, acidic cell, starts when water is pumped into the anode

advantage and disadvantages of PEM electrolytic cell

(advantage)

• pure hydrogen produced

• high rate of hydrogen production

(disadvantage)

• expensive (due to electrodes and electrolyte)

reactions in PEM electrolytic cell (cathode and anode)

reactions in Hofmann voltameter (cathode and anode)

artificial photosynthesis

• solar energy to produce hydrogen

• the photochemical cell resembles a solar panel operating in a solution

• the anode is coated with catalyst to absorb energy from sun

• the solar energy causes excitation of metal in electrode

• results in oxidation of water at anode to form oxygen gas and hydrogen ions

• hydrogen gas migrate to cathode where they are reduced

equations in artificial photosynthesis reaction (very similar/ same as that of PEM electrolytic cell)

** MAJOR DIFFERENCE:

• PEM electrolytic cell uses a battery/ external energy source e.g. solar/ wind

• artificial photosynthesis is initiated by sunlight directly (as it activates the anode with catalyst)

advantage of artificial photosynthesis

• higher efficiencies due to fewer energy conversions

• decrease in the need for large-scale storage tanks for electricity

according to the Faraday’s law, what is the amount of product formed dependent on?

• the size of electric current

• operating time

** Q = It

applications of electroplating

• appearance

• corrosion prevention (with a less reactive metal)

if nitrate ions in the electrolyte don’t react, what function do they perform?

to move towards anode to allow the current to flow

how does the concentration of metal ions change as the electroplating is operating

no change:

metal ions are generated at anode as the same rate as the metal ions are consumed at cathode, this maintains a constant concentration of metal ions

faraday’s first law

the mass of any substances deposited/ consumed at the electrode is directly proportional to the amount of electricity passed through the cell

faraday’s second law

for one mole of substance to be deposited/ consumed requires whole number of electrons (one, two, three or other moles)

how should the objected to be electroplated to be placed in the electrolyte?

fully immersed in the solution to ensure it is completely electroplated

why must electrolysis be used to extract lithium metal from its ore?1

lithium ion is a very weak OA that no stronger RA could reduce it through spontaneous reactions

what is the purpose of constantly blowing inert gas to the cathode during an electrolysis of molten MgCl

• since Mg has a property of reacting vigorously with oxygen

• the inert gas would prevent air from entering the cell to react with molten Mg