Electrolysis II Flashcards
What is an electric current?
a flow of electrons or ions
What is a non-electrolyte?
a substance that does not undergo electrolysis
e.g. covalemt compounts as they do not release ions
What is an electrolyte?
liquids (molten compounds or compounds in solution) which are broken down when they conduct electricity
usually ionically bonded
Why must an electrolyte be in liquid form in order to conduct electricity?
so the ions can move
the movement of ions is responsible for the conduction of electricity
What is an electrode?
a fairly inert substance (e.g. carbon) that electricity passes into and out of
What is an anode?
a positive electrode
What is a cathode?
a negative electrode
How do you remember which electrode is positive and which electrode is negative?
PANC
Positive Anode Negative Cathode
What is an ion?
a charged particle (negative or positive)
What is electrolysis?
the breakdown of a substance (molten or in solution) by electricity
What are two metals commonly used for electrodes?
carbon
platinum
How do you make electrolytes?
by melting or dissolving ionic compounds
What does an electrolysis circuit need to be complete? In which direction do the electrons flow?
a circuit needs a flow of electrons to be complete
the electrons are taken away from the ions at a positive electrode (anode) and given to the other ions at the negative electrode (cathode)
What happens to ions as they gain or lose electrons?
they become atoms or molecules
How can you measure the conductivity of an electrolyte?
with a conductivity probe
when you place a conductivity probe in a non-electrolyte, no current flows, so you’ll get a reading of zero conductivity
How is lead (II) bromide bonded? What does this mean? What happens if it is melted?
lead (II) bromide is ionically bonded si the ions are not free to move when is a solid state because of its lattice structure - no current can occur
if melted, the electrostatic attraction is broken and the ions are free to move to the electrodes
What does the circuit for the electrolysis of lead (II) bromide look like?
What happens during the electrolysis of lead (II) bromide?
Nothing happens at all until the lead (II) bromide melts. Then:
the light bulb lights up, showing that electrons are flowing through it
there is bubbling around the anode (connected to the positive terminal of the power source) as red-brown bromine gas is given off
nothing seems to be happening at the cathode (connected to the negative terminal of the power source), but afterwards solid metallic lead is found underneath it (because it is no longer attracted to the cathode)
when you stop heating and the lead (II) bromide solidifies again, everything stops - there is no more bubbling and the bulb goes out
N.B. The power source must be switched on
In the electrolysis of lead (II) bromide, at which electrodes do the lead and the bromide ions go to?
the lead ions (positive) go to the cathode (negative)
the bromide ions (negative) go to the anode (positive)
this is because opposite charges attract
What happens to ions at electrodes?
they are discharged (they lose their charge either by giving up electrons or receiving electrons from the electrode)
Write the half equation for the reaction at the cathode during the electrolysis of lead (II) bromide
the positive ions are attarcted to the negative cathode
at the cathode, a lead ion accepts two electrons to become a lead atom
Pb2+ + 2 electrons- → Pb
Write the half equation for the reaction at the anode during the electrolysis of lead (II) bromide
the negative bromide ions are attracted to the positive anode
at the anode, two bromide ions lose one electron each and become a bromine molecul
2Br- → Br2 + 2 electrons-
Why don’t electrodes take part in electrolysis reactions?
they are made from inert (unreactive) material
At which electrode does oxidation and reduction happen? What is the word used to describe when these two happen at the same time
oxidation hsppens at the anode
reduction happens at the cathode
together, this is REDOX
Only Animas Reproduce Cats
Are all electrolysis reactions REDOX?
yes
What apparatus is used when carrying out an experiment to see which substances conduct electricity? How is this set up?
bunsen burner
heat proof mat
clay triangle
carbon electrodes
light bulb
chemicals
glass rod
crocodile clips
tripod
retort stand and clamps
battery
tongs
crucible
spatula
cable wires to create the circuit
N.B. Instead of a bulb, you can test for a current using an ammeter
What is the method for carrying out an experiment to see which substances conduct electricity? How is this set up?
- Take two spatials of your sample solid and place it into the crucible. Place the electrodes, which are part of a circuit connected to a lightbulb, into the sample to complete the circuit. If the lightbulb turns on then you do not need to heat the same - we can already see that it conducts electricity
- Heat the sample and observe whilst keeping the circuit complete. Take care that the wires do not become heated. If, after two minutes of strong heating, the lightbulb stays off then stop - this sample is not an electrolyte
What can we conclude about the electric conductivity of metals?
all metals conduct electricity in their solid state
only metals conduct electricity in their solid state, no compounds conduct electricity in their solid state
lead (II) bromide, potassium iodide, zinc chloride and potassium chloride are the only molten compounds that we tested that conducted electricity - this is because they are ionically bonded substances and, in their molten state, the ions are free to move
no covalent compounds conducted electricity in their molten state
What do metals have that allow them to conduct electricity?
delocalised electrons
Covalent compounds do/don’t have any charged particles. They consist of … or … which are electrically …. They can/cannot conduct electricity, even if we … or …
Covalent compounds don’t have any charged particles. They consist of atoms or molecules which are electrically neutral. They cannot conduct electricity, even if we melt or dissolve
Positive metal ions are called … are attracted to the …
Negative non-metal ions are called … are attracted to the …
Positive metal ions are called anions are attracted to the cathode
Negative non-metal ions are called cations are attracted to the anode
Anions become … at the cathode
Cations become … at the anode and usually form …
Anions become metal atoms at the cathode
Cations become non-metal atoms at the anode and usually form molecules
Can all ionic compounds be electrolysed molten?
no
some break up into simpler compounds before their metlingpoint
others have such high meltimg points that it isn’t possible to melt them in the lab, although it can be done industrially
Water is a weak/strong electrolyte
What does it ionise slightly to give?
Water is a weak electrolyte
it ionises very slightly to give hydrogen ions and hydroxide ions
H2O (l) ⇌ H+(aq) + OH- (aq)
What is present in aqueous solutions?
ions from the ionic compound
hydrogen (H+) ions from the water
hydroxide (OH-) ions from the water
When are hydrogen (H+) ions formed at the cathode?
if hydrogen ions and metal ions are present, if the metal ions are more reactive than the hydrogen ions, hydrogen gas will be produced at the cathode
if the metal ions are less reactive than the hydorgen ions, a solid layer of the pure metal will be produced instead
When are hydroxide (OH-) ions formed at the anode?
if hydroxide (OH-) ions and halide ions are present, molecules of chlorine, bromine or iodine will be formed there
if no no halide ions are present, thern oxygen will be formed
Describe what happens in the electrolysis of sulphuric acid (H2SO4)
Sulphuric acid (H2SO4) contains three different ions: SO42-, H+ and OH-
Hydrogen ions (from the water or sulphuric acid) accept electrons. So at the cathode, hydrogen gas is produced:
2H+ + 2e- → H2
Hydroxide ions lose electros more easily than sulphate ions. So at the anode, oxygen and water are produced:
4OH- → O2 + 2H2O + 4e-
Describe what happens in the electrolysis of sodium chloride
A solution of sodium chloride (NaCl) contains four different ions: Na+, Cl-, OH-, H+
Hydrogen ions accept the electrons more easily than sodium ions. So at the cathode, hydrogen gas is produced:
2H+ + 2e- → H2
Chloride ions lose electrons more easily than hydroxide ions. So at the anode, chloride gas is produced:
2Cl- → Cl2 + 2e-
Describe what happens in the electroylsis of copper (II) sulphate
A solution of copper sulphate (CuSO4) contains four different ions: Cu2+, SO42+, H+, OH-
Copper ions accept electrons more easily than hydrogen ions. So at the cathode, copper metal is produced:
Cu2+ + 2e- → Cu
Hydorxide ions lose electrons more easily than sulphate ions. So at the anode, oxygen and water are produced:
4OH- → O2 + 2H2O + 4e-
What is the product and equation at the cathode and the anode of KI(aq)?
Cathode:
product = hydrogen
equation = 2H+(aq) + 2e- → H2 (g)
Anode:
product = iodine
equation = 2I-(aq) → I2 (aq) + 2e-
What is the product and equation at the cathode and the anode of MgBr2 (aq)?
Cathode:
product = hydrogen
equation = 2H+(aq) + 2e- → H2 (g)
Anode:
product = bromine
equation = 2Br-(aq) → Br2 (aq) + 2e-
What is the product and equation at the cathode and the anode of H2SO4 (aq)?
Cathode:
product = hydrogen
equation = 2H+(aq) + 2e- → H2 (g)
Anode:
product = oxygen
equation = 4OH-(aq) → 2H2O (l) + O2 (g) + 4e-
What is the product and equation at the cathode and the anode of CuSO4 (aq)?
Cathode:
product = copper
equation = Cu2+ (aq) + 2e- → Cu (s)
Anode:
product = oxygen
equation = 4OH-(aq) → 2H2O (l) + O2 (g) + 4e-
What is the method for carrying out the electrolysis of copper chloride solution?
- half-fill the beaker with copper chloride solution
- connect the electrodes in a simple circuit with the power pack and lamp
- start the current and observe what happens at the cathode and the anode
- as soon as the observations are complete, the power pack should be switched off
What is the name for a reaction where one substance breaks down into two or more substances?
decomposition
What is a coulomb?
a measure of quantity of electricity
one coulomb = 6.2 x108 electrons
What is the rate of flow of electrical charge measured in?
amps (A)
What is the equation for charge?
charge (Q) = current (I) x time (t)
Q is measured in coulombs (C)
I is measures in amperes (A)
t is measures in seconds (s)
If 2 amps flow for 20 minutes, what is the charge?
Q = I x t
= 2 x (20 x 60)
= 2400 coulombs
What is a faraday?
a quantity of electricity
one faraday (F) is the quantity of electricity which represents one mole of electrons passing a particular point in the circuit (6 x1023 electrons)
one mole of electrons = 96,000 coulombs = one faraday
What does the amount of product made by electrolysis depend on? How can this be increased?
the number of electrons that are transferred
if you increase the number of electrons, you increase the amount of substance produced
this can be achieved by electrolysing for a longer time or increasing the current
How many moles of electrons does sodium need to make one mole of sodium atoms?
sodium ions need one electron to make a sodium atom
one mole of sodium ions need one mole of electrons (one faraday) to make one mole of sodium atoms
Na+ + e- → Na
1 mole of sodium atoms + 1 mole of electrons → 1 mole of sodium atoms
How many moles of electrons does zinc need to make one mole of zinc atoms?
zinc ions need two electrons to make a zinc atom
one mole of zinc ions need two moles of electrons (two faraday) to make one mole of zinc atoms
Zn2+ + 2e- → Zn
1 mole of zinc atoms + 2 mole of electrons → 1 mole of zinc atoms
How many moles of electrons does aluminum need to make one mole of aluminium atoms?
aluminium ions need three electrons to make an aluminium atom
one mole of aluminum ions need three moles of electrons (thre faraday) to make one mole of aluminiumatoms
Al3+ + 3e- → Al
1 mole of aluminium atoms + 3 mole of electrons → 1 mole of aluminium atoms
What mass of aluminium will be deposited if a current of 1.5A is passed through aluminum oxide (dissolved in moleten cyrolite) for 40 minutes?
1. Find the total charge passed
Charge passed = 1.5 A x (40x60)s = 3600C
2. Convert this to moles of electrons (faradays) (divide by the charge equivalent of one faraday_
number of moled of electrons = 3600C ÷ 96000C C/mol = 0.0375 mol
3. Write out the ion-electron equation and deduce the number of moles of substance per moles of electrons (molar ratio)
Al3+ + 3e- → Al
e : Al
3 : 1
0.0375 : (0.0375 ÷ 3)
mols of Al = 0.0125
4. Convert moles of electrons passed into mass of product
mass of Al = moles x Ar
= 0.0125 x 27
= 0.0338g
During the electrolysis of copper (II) sulphate solution using carbon electrodes, copper is deposited on the cathode according to the equation:
Cu2+ (aq) + 2e- → Cu (s)
Calculate the gain in mass at the cathode if a current of 0.50 amps flows for 1 hour
(RAM: Cu = 64; 1 faraday = 96,000 coulombs)
1. Q = I x t
= 0.50 A x (60 x 60) s
= 1800 C
2. number of moles of electrons = 1800 ÷ 96,000 C/mol = 0.01875 moles of electrons
3. Cu : electrons
1 : 2
(0.01875 ÷ 2) : 0.01875
moles of Cu = 0.009375
4. mass of Cu = moles x Ar
= 0.009375 x 64
= 0.6 g
During the electrolysis of lead (II) nitrate solution, lead is deposited at the cathode and oxygen is released from the anode. If a current of 0.350 amps flows for 1000s, calculate the mass of lead deposited
(RAM: Pb = 207, 1 faraday = 96,000 coloumbs)
Pb2+ (aq) + 2e- → Pb (s)
1. Q = I x t
= 0.350 x 1000
= 350
2. number of moles of electrons = 350 ÷ 96,000 C/mol = 0.00365
3. Pb : electrons
1 : 2
(0.00365 ÷ 2) : 0.00365
moles of Pb = 7/3840
4. mass of Pb = moles x Ar
= 7/3840 x 207
= 0.377 g
During the electrolysis of lead (II) nitrate solution, lead is deposited at the cathode and oxygen is released from the anode. If a current of 0.350 amps flows for 1000s, calculate the volume of oxygen (measured at room temperature and pressure) produced
(the molar volume of gas is 24,000 cm3 at rtp, 1 faraday = 96,000 coloumbs,)
4OH-(aq) → 2H2O (l) + O2 (g) + 4e-
1. Q = I x t
= 0.350 x 1000
= 350
2. number of moles of electrons = 350 ÷ 96,000 C/mol = 0.00365
3. O2 : electrons
1 : 4
(0.00365 ÷ 4) : 0.00365
moles of O2 = 73/80,000
4. volume = moles x 24 dm3
= 73/80,000 x 24
= 0.0219 dm3 / 21.9 cm3
Some copper (II) suplhate solution was electrolysed using a pure copper cathode and an impure copper anode. Copper is lost from the anode and deposited on the cathode. Insolutble impurities in the anode form a sludge underneath the anode.
Cathode equation:
Cu2+ (aq) + 2e- → Cu (s)
Anode equation:
Cu (s) → Cu2+ (aq) + 2e-
a) What mass of copper will be deposited on the cathode if 0.40 amps flows for 75 minutes? (RAM: Cu = 64, 1 faraday = 96,000 coulombs)
b) If the anode was found to have lost 0.80 g during the experiment, calculate the percentage purity of the impure copper anode, assuming that only insoluble impurities were present
a) 1. Q = I x t
= 0.40 x (75 x 60)
= 1800
2. number of moles of electrons = 1800 ÷ 96,000 C/mol = 0.01875
3. Cu : electrons
1 : 2
(0.01875 ÷ 2) : 0.01875
moles of Cu = 3/320
4. mass = moles x Ar
= 3/320 x 64
= 0.6 g
b) purity ÷ impurity x 100
0.6 ÷ 0.80 x 100
= 75%
Aluminium is manufactured by electrolysing a solution of aluminium oxide, Al2O3, in molten cryolite. The electrode equation is:
Al3+ (l) + 3e- → Al (l)
A typical cell produces 1 tonne (1000kg) of aluminum every 24 hours. What current (in amps) is needed to produce this amount of aluminium?
(RAM: Al = 27, 1 faraday = 96,000 coulombs)
work backwards
1. moles of aluminium = mass ÷ Ar
= (1000 x 1000) ÷ 27
= 37037.03704
2. Al : electrons
1 : 3
37037.03704 : (37037.03704 x 3)
3. moles of electrons = 111111.111111
4. charge = moles of electrons x 96,000 C/mol
= 111111.111111 x 96,000
= 10666666666
5. I = Q ÷ t
= 10666666666 ÷ (24 x 60 x 60)
= 123456.7901 amps
Two solutions were elctrolysed in series. One beaker contained chromium (III) sulphate solution, and the other cobalt (II) sulphate solution. 0.295 g of cobalt was deposited on the cathode in the beaker containing cobalt (II) sulphate. The electrode equations are:
Cr3+ (aq) + 3e- → Cr (s)
Co2+ (aq) + 2e- → Co (s)
a) Calculate the quantity of electricity that flowed during the experiment
(RAM: Cr = 52, Co = 59, 1 faraday - 96,000 coulombs)
b) Calculate the mass of chronium deposited on the cathode in the other beaker
work backwards
a) 1. moles of cobalt = mass ÷ Ar
= 0.295 ÷ 59
= 1/200
2. Co : electrons
1 : 2
1/200 : (1/200 x 2)
moles of electrons = 0.01
3. charge = moles of electrons x 96,000 C/mol
= 0.01 x 96,000
= 960 coulombs
b) If 960 coulombs passed through in a) ten exactly the same amount passed through the rest of the circuit
1. the charge is the same (960 coulombs)
2. the moles of electrons are the same (0.01)
3. Cr : electrons
1 : 3
(0.01 ÷ 3) : 0.01
moles of Cr = 1/300
4. mass = moles x Ar
= 1/300 x 52
= 0.1733333 g
Copper (II) sulphate solution and lead (II) nitrate solution were electrolysed in two beakers connected in series. If 0.64 g of copper was deposited at the cathode in one beaker, calculate the mass of lead deposited in the other one.
(RAM: Cu = 64, Pb = 207, 1 faraday = 96,000 coulombs)
Cu2+ (aq) + 2e- → Cu (s)
Pb2+ (aq) + 2e- → Pb (s)
First, find the quantity of electricity (coulombs) that flowed during the experiment
1. moles of copper = mass ÷ Ar
= 0.64 ÷ 64
= 0.01
2. Cu : e
1 : 2
0.01 : (0.01 x 2)
moles of electrons = 0.02
3. charge = moles of electrons x 96,000 C/mol
= 1920 coulombs
Second, we know that the same charge passed through the rest of the circuit
1. we know the charge = 1920 coulombs
2. we know the number of electrons = 0.02
3. Pb : e
1 : 2
(0.02 ÷ 2) : 0.02
moles of Pb = 0.01
4. mass of Pb = moles x Ar
= 0.01 x 207
= 2.07 g
How is hydrogen chloride gas converted into hydrochloric acid?
HCl dissolves in water to from the aqueous solution; hydrochloric acid
Why is chlorine sometimes added to water supplies?
it acts as a water treatment to sterilise water
