Why is cryolite used in the extraction of aluminium from aluminium oxide?
The melting point of aluminium oxide is very high, so melting it would be costly. Hence we use cryolite to decrease the melting point of aluminium oxide (from 2050° C to 850° C.)
Why does graphite have a high melting point?
Graphite has a giant ionic lattice.
Why can't covalent bonds undergo electrolysis?
Covalent compounds have no free ions that can move around and carry the charge.
Sodium chloride solution contains two types of positive ions, hydrogen ions (H+) and sodium ions (Na+).
Why is hydrogen produced at the negative electrode and not sodium?
Hydrogen is less reactive than sodium.
An object is electroplated with silver using an aqueous silver salt as the electrolyte. Which set of conditions is used?
a) the object to be electroplated is the anode and the other electrode is made from carbon
b) the object to be electroplated is the anode and the other electrode is made from silver
c) the object to be electroplated is the cathode and the other electrode is made from carbon
d) the object to be electroplated is the cathode and the other electrode is made from silver
d) the object to be electroplated is the cathode and the other electrode is made from silver
Cathode: the object
Anode: the metal
Electrolyte: the solution that contains the
metal used
i) What is the ionic equation for the reaction of chlorine with potassium iodide?
a) Cl2 + 2K → 2KCl
b) 2I⁻ + Cl2 → I2 + 2Cl⁻
c) I⁻ + Cl → I + Cl⁻
d) I⁻ + K+ → KI
ii) Why does a potassium iodide solution conduct electricity?
a) It contains a metal
b) It contains electrons which can move
c) It contains ions which can move
i) b)
ii) c)
In the electrolysis of aluminium oxide,
i) Oxygen is formed at the positive electrode. The oxygen then forms carbon dioxide. The equation for the reaction is shown below.
C + O2 → CO2
Complete the sentence. The name of the element which reacts with oxygen is .................................
ii) The positive electrode gets smaller. Suggest why.
i) Carbon
ii) reacts with oxygen or forms carbon dioxide accept burns
Write down the half equations of the electrolysis of molten lead(II) bromide!
PbBr2 🡪 Pb2+ + Br-
Anode (+) : 2Br- 🡪 Br2 + 2e- [100]
Cathode (-) : Pb2+ + 2e- 🡪 Pb [100]
Overall reaction: 2Br- + Pb2+ 🡪 Br2 + Pb
Write down the half equations of the electrolysis of sodium chloride solution!
Na+ Cl- H+ OH-
Anode(+): 2Cl- (aq) 🡪 Cl2 (g) + 2e- [100]
Cathode(-): 2H+ (aq) + e- 🡪 H2 (g) [100]
Overall reaction: 2Cl- + 2H+ 🡪 Cl2 + H2
i) What reaction takes place in the anode?
ii) What reaction takes place in the cathode?
i) Oxidation
ii) Reduction
State three uses of electroplating.
- electroplating is done to make metals more resistant to corrosion or damage
- improve the appearance of metals
- coating a cheaper metal with a more expensive one
i) What type of bonding is hydrogen fluoride?
a) covalent b) ionic c) metallic
ii) To allow electrolysis to take place the mixture of potassium fluoride and hydrogen fluoride must be ...
a) gas b) liquid c) solid
iii) The mixture of potassium fluoride and hydrogen fluoride contains fluoride ions (F–), hydrogen ions (H+) and potassium ions (K+). During electrolysis the element formed at the positive electrode is ...
i) covalent
ii) liquid
iii) fluorine
accept F / F2
do not accept fluoride
State the 3 products formed from the electrolysis of brine (concentrated sodium chloride) and their uses!
Cl 🡪 bleach
H 🡪 mix with vegetable oil to make margarine
NaOH 🡪 make soap, paper, bleach
Aluminium is produced by electrolysis of a molten mixture of aluminium oxide and cryolite.
i) Name a gas produced at the positive electrode. [100]
ii) Aluminium ions move to the negative electrode. Explain why. [100]
iii) At the negative electrode, the aluminium ions gain electrons to produce aluminium. What is this type of reaction called? [100]
i) Oxygen gas (accept carbon dioxide)
ii) Aluminium ions are positive so they are attracted to the cathode
iii) Reduction
Electrolysis is used to extract magnesium metal from magnesium chloride.
i) What must be done to solid magnesium chloride to allow it to conduct electricity?
ii) Why do the magnesium ions move to the negative electrode?
iii) Name the product formed at the positive electrode
i) melt
allow add to / dissolve in water
allow heat until liquid
allow turn it to liquid / make it molten
ignore heat
ii) they are positive OR opposite charges or opposites attract
do not accept electrodes attracting
do not accept positive electrons
iii) chlorine
accept Cl2
do not accept chloride
Alloys are used to make dental braces and coins.
(i) Nitinol is an alloy used in dental braces.
Why is Nitinol used in dental braces?
(ii) Suggest one reason why coins are not made of pure copper. Do not give cost as a reason.
iii) Some coins are made from an alloy of aluminium. Complete the sentence. Aluminium is manufactured by the electrolysis of a molten mixture of cryolite and
iv) Banks keep coins in poly(ethene) bags. These bags are made from low-density poly(ethene). High-density poly(ethene) can also be made from the same monomer. How can the same reaction produce two different products?
i) can return to its original shape
accept shape memory alloy
accept smart alloy
ignore other properties
ii) (pure copper is too) soft
accept converse
accept malleable or bends
accept copper is running out
ignore references to strength and weakness
(iii) aluminium oxide
accept alumina
accept Al2O3
ignore bauxite / aluminium ore
iv) any one from:
• different conditions
• different catalyst
• different pressure
allow different concentration
• different temperature.
do not accept different monomers
Explain how electricity is conducted in a metal. To gain full marks you must include a description of the structure and bonding of a metal.
giant structure / lattice / layers / close packed
first 3 marks can be obtained from a suitably labelled diagram
incorrect structure or bonding or particle = max 3
made up of atoms / positive ions
with delocalized / free electrons
so electrons can move / flow through the metal
accept so electrons can carry charge through the metal
accept so electrons can form a current
Why do carbon anodes need to be replaced regularly in the electrolysis of aluminium oxide?
Carbon anodes react with the oxygen in the anode, so the carbon anodes gradually burn away and need to be replaced regularly.
i) What can be done to covalent compounds so that they can undergo electrolysis?
ii) Give an example of a covalent compound that can undergo electrolysis!
i) React (or ionise) in water to form ions
ii) HCl solution
Why are alloys harder than pure metals?
either metal (atoms) in layers / rows
accept metal (atoms) all the same size
allow metal (atoms) in lines
or alloy (atoms) not in layers / rows
accept different sizes of atoms in alloy
allow alloy (atoms) not in lines
Metals are extracted from their ores. Many copper ores contain only 2% of copper compounds. Copper is now extracted from ores containing a low percentage of copper compounds.
Suggest two reasons why. [250 each]
any two from:
• copper / ores are running out / harder to find
• there are no / very small amounts of high-grade copper ores left
• copper metal is in demand
• copper is expensive
• now economical to extract copper from low-grade ores
it = copper
allow new methods of extraction e.g. bioleaching and phytomining
allow high-grade ores are running out
Graphite is often used as an electrode in electrolysis because it can conduct electricity as it has free electrons. Explain how graphite has free electrons.
In graphite, the carbon atoms are arranged in a hexagonal structure and each carbon atom is attached to three other carbon atoms. We know that carbon has four valence electrons in its outermost shell. In graphite, each carbon is bonded to three other carbon atoms hence leaving behind a free electron.
Fluoride ions are sometimes added to drinking water. It is thought that these ions help to reduce tooth decay.
i) Which question cannot be answered by scientific investigation alone?
a) Do fluoride ions in drinking water reduce tooth decay?
b) Are fluoride ions in drinking water harmful to health?
c) Should fluoride ions be added to drinking water?
ii) Explain why you have chosen this question.
i) c) Should fluoride ions be added to drinking water?[200]
ii) any one from: [300]
• not enough reliable/valid evidence
• may be other factors involved
• it is an opinion / choice / belief / ethics issue
• it can’t be scientifically investigated allow can’t do an experiment
ignore test
mark independently of (c) (i)