electrolysis - unit 3 AOS 2 Flashcards
why non spontaneous
positive gradient
why do we need more than the min valtage
- energy loss in the cell
electrolysis
- the process in which a non-spontaneous chemical reaction occurs by passing an electric current through a substance in solution or molten state
electrolytic cell
- 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
electrolytic cell features
- 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
aqueous electrolyte
- 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
molten electrolyte
- 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.
predicting products using the electrochemical series
- 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
determining valtage
min voltage required = E0 of oxidising reaction - E0 of reducing reaction
two or more oxidising agents or two or more reducing agents
- 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
reactive electrode
- 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
inert electrode
- will not participate in the reaction
additives in electrolysis
- Cryolite is an additive that can be used to lower the melting point of the
electrolyte in the extraction of pure aluminium via electrolysis.
discharge
- 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
secondary cell
- a rechargeable cell that works as both a galvanic cell and an electrolytic cell
recharge
- 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
energy transformations
- during discharge chemical energy is being converted into electrical energy
- during recharge, electrical energy is being converted into chemical energy
secondary cell recharge requirements
- 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.
products of discharge reaction
- 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.
factors affecting longevity of cells
- 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.
green hydrogen
- hydrogen gas produced using renewable energy sources by splitting water into oxygen and hydrogen through electrolysis
blue vs grey hydrogen
- produced from natural gas
- blue hydrogen production involves carbon dioxide carbon dioxide captured and stored
black/brown hydrogen
- produced from coal via gasification
- releases greenhouse gases
polymer electrolyte membrane
- 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)
artificial photosynthesis
- process that utilises human-made materials to capture sunlight and split water molecules to create hydrogen and oxygen
sustainability of green hydrogen
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.
Faradays first law
- The mass increase at the cathode of an electrolytic cell is directly proportional to the quantity of charge carried.
Faradays second law
- 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.
calculating Q, the amount of charge carries in an electrolytic cell
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
PEM evaluation and features
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
