Electrolysis Expanded Flashcards
What is electrolysis
the process of electricity splitting an ionic compound into its component metal and non metal
the compound MUST contain ions therefore
only ionic compounds can be used
the compound’s ions MUST be free to move therefore
the compound must be MOLTEN (l) or in a SOLUTION (aq)
the anode is
POSITIVE
the cathode is
NEGATIVE
positively charged ions
are always attracted to the CATHODE as opposite charges attract
negatively charged ions
are attracted to the ANODE as opposite charges attract
Passing an electric current through ionic substances that are molten or in a solution does what?
breaks them down into elements, process called electrolysis
Equations for electrolysis of CuCl2
at cathode: Cu[2+] (aq) + 2e- –> Cu (s) therefore electrons are gained i.e. reduction
at anode: 2Cl[-] (aq) –> Cl2 (g) + 2e- therefore electrons are lost i.e. oxidation
In electrolysis, a liquid ionic substance such as lead bromide is needed to conduct electricity and allow current to flow around the electrolytic cell, what is there name given to these conductive substances
electrolyte
how can you test whether a particular substance can be used as an electrolyte
make a complete circuit with a battery and a bulb and if the bulb lights up then it is a conductor therefore suitable as an electrolyte
what flows around the circuit in electrolysis
electrons
always answer the substance names for electrolysis with what
the name and ions i.e. bromide IONS
how to explain that a substance can conduct electricity
as it has free delocalised electrons that electricity can pass through
how can ionic structures only work in electrolysis if they are molten
as they have a lattice structure without delocalised electrons therefore the ions are not able to move freely and carry charge
what if a substance cannot be easily dissolved in water
heat it until it reaches its melting point so it will become molten
a NEGATIVE reaction in electrolysis is a
reduction reaction
a POSITIVE reaction in electrolysis is a
oxidation reaction
if a circuit is in use via electrolysis of molten compounds the circuit will stop working if
the heat source is removed and the molten substance solidifies as electrons will no longer be carried around the circuit
usually at the negative terminal connected to the power source in electrolysis i.e. the cathode but soon after a metallic build up is found underneath it and on the anode
a gas is formed usually of the non metal substance
the circuit goes from
negative and around to positive
when electrons are picked up from the cathode by the positive metal they are replaced with
the electric pump of the circuit which is filled with more electrons
when electrons are taken away from the anode by the negative non metal they are pumped away by
the electric pump of the circuit which will take the the extra electrons via the appliance by the power source
electrons flow in the external circuit because of the … changes to the ions arriving at the electrodes and we say that the ions are … at the electrodes
chemical
discharged
what does discharging an ion mean
it means that it loses its charge, either giving up its electrons or receiving electrons from it
positive ions are known as
cations because they are attracted to the cathode
negative ions are known as
anions because they are attracted to the anode
some substances cannot be electrolysed molten
as they dont melt, but just break up or their melting point is too high for labs
when electrolysing something in an aqueous solution what is produced at the cathode
hydrogen, this comes from the water in the solution as it is a very weak electrolyte and only ionises slightly to give hydrogen ions and hydroxide ions
what is the equation for electrolysing water
H2O H[+] (aq) + OH[-] (aq)
why do monatomic and simple molecular substances not conduct electricity
as they have no free electrons and do not form ions
explain why giant covalent substances apart from graphite do not conduct electricity
as all the electrons on the outer shell are bonded to the atom
Rules of electrolysis of solutions at the anode
if it is a carbonate nitrate or sulphate, then the ions stay in the solution and oxygen is given off, but if it is a chloride bromide or iodide then the OH[-] stays in the solution and the halogen is given off
Rules of electrolysis of solutions at the cathode
- if the metal ion present is LESS reactive than hydrogen then the METAL is formed
- if the metal ion present is MORE reactive than hydrogen then HYDROGEN is formed
if the compound contains oxygen then what will be produced at the anode
oxygen
if the substance is aqueous then either hydrogen or hydroxide will be left in the pot but if it is molten then
nothing is left in the pot
how can salt be extracted from underground deposits
by solution mining, and pumping very hot water under pressure down into the salt deposit, when the salt dissolves into it, the solution is pumped back to the surface
what can concentrated salt be electrolysed to produce
three useful chemicals: sodium hydroxide, chlorine, hydrogen
explain the left side of the diaphragm cell
concentrated sodium chloride goes in through the top half of that side, passes by a titanium anode, chlorine goes out through the top
explain the right side of the diaphragm cell
sodium hydroxide solution contaminated with sodium chloride goes down the bottom half of that side, passes by a steel cathode, hydrogen goes out through the top
explain the whole diaphragm cell
- designed to keep the products apart
- if the chlorine make contact with sodium hydroxide, bleach is made
- if chlorine makes contact with hydrogen, the mixture made would violently explode on exposure to heat/sunlight giving hydrogen chloride
in a diaphragm cell what happens at the titanium anode
chloride ions are discharged to produce chlorine gas
equation: 2Cl[-] (aq) –> Cl2 (g) + 2e[-]
in a diaphragm cell what happens at the steel cathode
it is too difficult to discharge sodium ions, so hydrogen ions from the water are discharged instead to produce hydrogen gas
equation: 2H[+] (aq) + 2e[-] –> H2 (g)
what happens to replace the discharged hydrogen
more water keeps splitting but hydroxide ions are also formed after each split, thus having a build up of sodium and hydroxide ions in the right compartment
with residue build up in the right compartment of a diaphragm cell from sodium and hydroxide what happens
sodium hydroxide solution is formed, which is then contaminated with unchanged sodium chloride, the solution is concentrated via evaporation, but most sodium chloride crystallises into salt, this can be recycled by redissolving it into water
uses of sodium hydroxide
- the purification of bauxite to make aluminium oxide for production of aluminium
- to help make paper; helps break wood down into pulp
- to help make soap; reacts with animal and vegetable fats and oils to make compounds
- to help make bleach; it is formed when reacted with chlorine in the cold
uses of chlorine
- sterilising water to make it safe to drink
- making hydrochloric acid by a controlled reaction with hydrogen
- making bleach
the ions in the solution of the electrolysis of Brine (chloralkali industry)
sodium ions and chloride ions from NaCl:
NaCl –> Na+ + Cl-
hydrogen ions and hydroxide ions from water:
H2O –> H+ + OH-
the positive ions in the electrolysis of brine: Na+ and H+ go to what
the cathode, here the ion of the least reactive element is discharged so the reaction is:
2H+ + 2e- –> H2(g)
the negative ions in the electrolysis of brine: Cl- and OH- go to what
the anode, here the ion of the least reactive element is discharge so the reaction is:
2Cl- –> Cl2(g) + 2e-
what are the products of electrolysing sodium chloride solution
- hydrogen
- chlorine
- sodium hydroxide left in the solution
- remember CatHy’s AnCl
uses of hydrogen
- making margarine
- hydrochloric acid
- as a non-polluting fuel
Faraday’s first law
The amount of substance produced by electrolyse is proportional to the quantity of electricity used
Faraday’s second law
For a given quantity of electricity the quantity of substance produced is proportional to its mass of the substance
Quantity of electricity =
charge
Charge in Coulombs equation (C)
= I x t
Current in Amps (I)
Time in seconds (s)
what does one faraday (F) mean
the charge of one mole of electrons
what is one faraday (F) equivalent to
96, 000 Coulombs
number of moles of electrons equation
Q = nF
charge in Coulombs (C) = number of moles of electrons x Faraday constant (96,000Cmol-1) never use in actual Faradays
OR
n= Q/F
What mass of aluminium will be deposited if a current of 1.5A is passed through aluminium oxide (dissolved in molten cryolite) for 40 minutes? [Data: Al=27 RAM; 1 Faraday = 96,000 C/mol]
1) Find the total charge passed Charge passes = 1.5A x 40 x 60s = 3600C 2) Convert this to moles of electrons number of moles of electrons = 3600C/96000 C/mol 3) Write out cathode equation and deduce molar ration of electrons to substance, and therefore moles to substance. Equation is: Al3+ + 3e- --> Al Molar Ratio 3:1 moles: 0.0375 ---> 0.0375/3 4. Convert moles of product into mass of product mass = moles x RAM RAM Al = 27 Mass = 0.0125 x 27 = 0.3389
Diatomic bonds: molecules with 2 bonds
H, N, O, F, Cl, Br, I
What volume of hydrogen gas would be produced from the electrolysis of some hydrochloric acid by a current of 0.5A for 2.5 hours
Q = 0.5 x 9000 Q = 4500 C n = 4500/96000 = 0.046875 2H+ + 2e- --> H2 = 2:1 0.046875/2 = 0.0234375 ANS x 24 = 0.5625 dm(cb)
what is contact process
the industrial method of producing sulphuric acid
3 stages of contact process
1) making sulphur dioxide
2) making sulphur sulphur trioxide
3) making the sulphuric acid
Stage 1 Method 1 of the contact process with balanced equation
raw materials of sulphur and air:
S(s) + O2(g) –> SO2(g)
Stage 1 Method 2 of the contact process with balanced equation
raw materials of sulphide ores and air:
4FeS2(s) + 11O2(g) –> 2Fe2)3(s) +8SO2(g)
Stage 2 of the contact process and balanced equation:
the reaction is … this means that at the same time as the forward reaction producing products there is also a … reaction which produces the …
reversible backward reactants 2SO2(g) + O2(g) 2SO3(g) triangle H = -196kJ mol-1 = exothermic
conditions and explanation for the contact process:
Catalyst
Temperature
Pressure
Catalyst:
Vanadium(V) oxide –> has no effect on the percentage conversion, speeds up reaction. without it, the reaction would be extremely slow
Temperature:
450 degrees C –> forward reaction exothermic, higher conversions if at a lower temperature, but low temperature = slow reaction –> still 99.5% conversion rate
Pressure:
1-2atm –> 3 gas molecules left side, only 2 on right side, high pressures would increase product but not economically worthwhile
Stage 3 of the contact process and balanced equation
sulphur trioxide is not reacted with water to produce sulphuric acid as it produces an uncontrollable fog of concentrated sulphuric acid
sulphur trioxide is reacted with sulphuric acid to form fuming sulphuric
this is then reacted with water to produce sulphuric acid
1) H2SO4(l) + SO3(g) –> H2S2O2(l)
2) H2S2O2(l) + H2O (l) –> 2H2SO4(l) (converted into twice as much sulphuric acid
