Unit 3 AOS 2 Flashcards
Electrolysis is used for (4)
extracting metals, making jewellery, protecting metal, recharging batteries
Define electrolysis
process in which a current is used to make a non-spontaneous redox reaction occur.
How does electrolysis work
electricity passes through a molten ionic compound or through an electrolyte, external power source forces electrons to move in the reverse direction so reactions are reversed
Define electrolytes
liquids that can conduct electricity
Where do positive ions gain electrons
cathode
Where do negative ions lose electrons
anode
Define 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.
Features of an electrolytic cell
electrolyte solution with free-moving ions which allow electrons to flow through external circuit, two electrodes, external source of electrons
What type of current is the electron flow
DC
Polarity of cathode in electrolysis
negative
Polarity of anode in electrolysis
positive
What occurs at anode and cathode (reduction/oxidation)
reduction at cathode, oxidation at anode
Where are cations and anions attracted
cations to cathode, anions to anode
Why is cathode negative
the external DC source forces electrons onto it.
Why is the anode positive
The anode is positive because the DC source withdraws electrons from it.
How are polarities determined in galvanic cell
spontaneous reaction
Factors affecting the electrolysis (3)
concentration of electrolyte, nature of the electrolyte, the nature of the electrodes
What happens to E naught when conditions aren’t standard
the standard the E naught values change
Why is electrolysis important
electrolysis provides the only practical way to prepare many metals
What reactive gases are prepared through electrolysis
fluorine, chlorine, sodium hydroxide
Define electroplating
thin coating of metal is applied to more common base metal through electrolysis
Why is electroplating used
to make metal durable, protection from corrosion, attractive
Factors affecting quality of metal coating
concentration of cations to be reduced, unwanted side reactions must be avoided, type and concentration of electrolyte needs careful consideration, solution must contain compounds to control the acidity and increase the conductivity, it needs to be considered whether the metal coating makes colour brighter and smoother, in many electroplating cells the anode must be shaped like the object at the cathode to achieve an even metal coating.
What might happen in electroplating if conditions aren’t satisfactory
metal coating powdery and drops off
Why must cleaning acids and bases be replaced
contamination/dirty
Name for when using electrolysis to extract metal
electro-refining
What is Hall-Heroult method used for
aluminium production using molten aluminium
Why can’t the aluminium be aqueous
water would be reduced
Membrane cell
electrolysis of brine (aqueous NaCl) - to produce Cl, H gases and NaOH. Semi-permeable plastic membrane separates anode half-cell from cathode half-cell, and traps chloride and water but not sodium and hydroxide.
Faraday’s 1st law of electrolysis
the amount of any substance deposited, evolved or dissolved at an electrode during electrolysis is directly proportional to the quantity of electric charge passed through the cell.
What is an ammeter
measures rate at which charge flows in a circuit
How many electrons per Q
6.24*10^18 electrons
Faraday’s constant
A constant that represents the amount of electric charge carried by 1 mole of electrons.
Faraday’s constant
96500 C mol-1
Faraday’s 2nd law of electrolysis
to produce 1 mol of a substance by electrolysis, a certain number of moles of electrons (Faraday’s) must be consumed according to the relevant half equations
What is a lead-acid accumulator
A battery with lead electrodes using dilute sulfuric acid as the electrolyte; each cell generates about 2 volts.
Define recharging
Forcing electrons to travel in the reverse direction and, because the discharge products are still in contact with the electrodes, the original reactions are reversed.
What is a lithium ion cell
A battery where lithium ions move from the negative electrode to the positive electrode during discharge and back when charging.
What happens to lithium ion cells at high temp
‘thermal runaway’
Most common secondary cell
is lead-acid battery
Silver and gold are… (poor or good conductors)
poor conductors
How to find Kc when reaction reversed
1/Kc
How to find Kc when reaction coefficient doubled
Kc^2
How to find Kc when reaction coefficient halved
square root Kc
Formula to find charge
Q=It
What does Q stand for
charge in C
What does I stand for
current in amps
What does t stand for
time in seconds
Formula for moles and Q
n=Q/f
Factors affecting battery life (4)
increase in temperature, overcharging (may not fully recharged/discharged, or reduce insulation - short circuits and danger of explosion), products of discharge fall from electrodes and when no contact reverse reaction cannot occur, corrosion due to acidic environment or constant use