Topic 5 - Electrolysis And Obtaining Metals Flashcards
What is electrolysis?
An electrical current transfers energy which allows it to decompose electrolytes
What is an electrolyte?
An ionic substance with freely moving ions
Can conduct electricity
What takes place at the cathode?
ReduCtion
Electrons on left of half equation
What happens at anode?
OxidAtion
Electrons on right of half equation
What is the Core Practical Electrolysis of copper sulfate solution?
Label one piece of copper foil the anode and the other the cathode
Set up an electrolysis circuit (put electrodes in copper sulfate solution)
Turn on power (alter resistor) so you have a current of 0.2 A
Record the current and leave for about 20mins
Turn off the power and remove electrodes from beaker
Wash electrodes with distilled water
Measure and record the mass of the electrodes and repeat experiment with different voltages
What happens when a molten salt is electrolysed?
Ions are discharged as atoms/molecules at the electrodes
How can you predict the electrolysis products of any molten salt?
The metal is produced at cathode and the non metal at anode
What else are attracted at the anode/cathode when it’s a salt solution?
Due to there being water, H ions are also attracted to Cathode and OH ions are attracted to anode.
However, copper ions e.t.c at the cathode are discharged more than hydrogen ions so copper is formed
Chloride ions at the anode are discharged more than the hydroxide ions so chlorine is formed as a gas
Why will the H ions or OH ions sometimes be produced?
They will discharge more easily if the metal/non metal is more reactive than hydrogen or hydroxide
E.g. h is discharged easier because na is more reactive
But cu is discharged because it’s not more reactive
Where can some excess mass be found?
Any impurities from anode e,t,c do not form ions and collect below the anode as sludge
What elements react with water/dilute acid a lot?
Potassium, sodium and calcium
They form hydrogen and a metal hydroxide with water
They react violently with dilute acids
except calcium which forms hydrogen and a salt solution
What elements react slowly with water/dilute acid?
Magnesium, aluminium, zinc, and iron
They react slowly with water by react with steam to form a hydrogen and a metal oxide
They form hydrogen and a salt solution when reacted with a dilute acid
What elements do not react with water or dilute acid?
Copper, silver and gold
In all of these reactions, what happens to the metal atoms?
They lose electrons to form positive ions (cations)
What is a displacement reaction?
When an element will react with an element below it in the reactivity series
I.e. zinc reacts with copper and displaces it but copper can’t displace zinc because it’s not as reactive as zinc
What is extraction?
Obtaining a metal from compounds (when reactive metals have reacted with other elements to form compounds in rock)
What is an ore?
A rock that contains enough of a compound to extract a metal for profit
How can they be extracted?
You obtain the element by heating the element oxide (I,e. You get iron by heating iron oxide) with carbon as carbon is more reactive and it will displace it
How do you extract ores, more reactive than carbon?
Electrolysis
How is this done?
Pass electricity through a molten ionic compound to decompose it into its elements
Why is electrolysis for extraction not the preferred way?
It’s really expensive to keep metal oxides molten for electrolysis as it requires loads of energy
So it’s only used of the element is more reactive than carbon
What are some examples of elements that have to be extracted through electrolysis?
Potassium, sodium, calcium, magnesium and aluminium
What are some examples of elements that can be extracted by heating carbon?
Zinc, iron and copper
How can silver / gold be extracted?
Don’t need to be as they’re found as an uncombined element
What is bioleaching?
Uses bacteria that has grown on a low grade ore
Bacteria produce a leachate (copper ions).
The copper is extracted from the leachate by displacing it using iron, then purified by electrolysis
Also be used for metals like zinc, nickel e.t.c
What is phytoextraction?
Growing plants that absorb metal compounds
When the plants absorb the compound, they are burnt to form ash, and the metal is extracted from ash
What are some advantages of bioleaching and phytoextraction?
Both: No harmful gases (e.g. sulfur dioxide) are produced
Causes less damage to landscape than mining
Conserved supplies of higher grade ores
Bio: Doesn’t require high temperatures
Phyto: can extract metals from contaminated soils
What are some disadvantages?
Both: very slow
Bio: toxic substances and sulfuric acid can be produced by the process and damage the environment
Phytoextraction: more expensive that mining some ores
Growing plants is dependant on weather conditions
What is a redox reaction?
Reactions in which oxidation and reduction occur
How is aluminium obtained by removing the oxygen from aluminium oxide by electrolysis?
Aluminium ions are attracted to cathode where they gain electrons to form aluminium
The oxide ions are attracted to anode where they lose electrons to form oxygen
At high temperatures, the oxygen reacts with the graphite (carbon) anodes to form carbon dioxide.
How does corrosion occur?
When a metal reacts with oxygen, making the metal weaker over time
The metal gains oxygen so it is oxidised
Why is rusting?
When iron reacts with water and oxygen over time
Why do some metals corrode faster?
The more reactive it is, the more rapidly it corrodes
Why is gold used in jewellery?
It doesn’t corrode at all
How can some metals prevent themselves from corroding?
Metals like aluminium produce a protective oxide layer (tarnish) on its surface
What are some advantages of recycling?
Natural reserves of metal ores will last longer
Reduce need to mine ores (mining can damage landscape and create pollution and noise)
Less pollution produced because gases like sulfur dioxide are produced when metals are extracted from metal sulfide ores
Less energy to recycle than extracting
Less waste ends up in landfill
What are some disadvantages?
Sometimes it can cost more to collect, transport and sort the metals
Take mor energy transporting
What does the Life Cycle assessment do?
Helps people decide if it is worthwhile to manufacture or recycle a product
What is a reversible reaction?
When products react to reform the reactants
Both the forward and backward reaction occur at same time
What is dynamic equilibrium?
The reaction rate of the forward reaction is equal to the reaction rate of the backwards reaction (reversible reaction)
What system can dynamic equilibrium occur in?
A closed system - otherwise gas would escape and equilibrium wouldn’t be achieved
Why are the reaction conditions for the Haber process chosen?
They’re chosen to favour the forward reaction and to make the largest amount of product as cheaply as possible
What are the conditions for the Haber process?
450 degrees, pressure of 200 atmospheres and an iron catalyst
How does increasing the temperature affect the position of equilibrium?
Equilibrium position shifts in the endothermic direction (transfers energy from surroundings, cooling the surroundings down)
How does decreasing the temperature affect the position of equilibrium?
Equilibrium position shifts in the exothermic direction (transfer energy to the surroundings which heats them up)
How does increasing the gas pressure affect the position of equilibrium?
Equilibrium position shifts in direction with fewer gas molecules (this will reduce the pressure)
How does decreasing the gas pressure affect the position of equilibrium?
Equilibrium position shifts in direction that forms more gas molecules (increase pressure)
How does increasing the concentration affect the position of equilibrium?
Equilibrium position shifts in the direction that uses up the substance that’s been added
How does decreasing the concentration affect the position of equilibrium?
Equilibrium position shifts in the direction that forms more of the substance that’s been removed
What are the properties of transition metals?
Malleable (hammered or rolled without shattering)
Ductile (stretched out)
Good conductors of electricity
Shiny when polished
Why is iron used to make buildings/bridges/ships e.t.c?
It is malleable and strong
Why is copper used to make electrical Wires?
It is ductile and a good conductor of electricity
What chemical properties do transition metals have (compared to group 1 and 2 metals)
High melting points
High densities
What else do transition metals normally look like?
They’re normally coloured e.g. iron (111) oxide is brown
They’re normally used as catalysts e.g. iron is the catalyst that manufactures ammonia by the Haber process
What happens when metals react with oxygen in the air?
They oxidise to form metal oxides
What happens when reactive metals react with oxygen in the air?
They oxidise rapidly when their freshly cut
What happens when unreactive metals e.g. gold react with oxygen in the air?
They oxidise slowly
Gold e.t.c may not react with oxygen at all
How can rusting be prevented?
Keeping air away by storing the metal in a (unreactive) atmosphere of nitrogen or argon
How else can rusting be prevented (water)?
By prevent water from reaching it
For example, a desiccant powder that absorbs water vapour
What are some simple ways to prevent rusting?
Painting, oiling, greasing or coating with plastic
What is sacrificial protection?
Doesn’t rely on keeping air or water away
A piece of magnesium or zinc is attached to the iron/steel object
Magnesium and zinc oxidise more easily than iron so the oxygen reacts with the piece of magnesium/zinc rather than the iron/steel
This continues until the sacrifices metal corrodes away
Why does a metal oxides more easily than others?
The more reactive a metal is, the more easily oxidised it becomes and the more easily it loses its electrons
(OxidAtion is loss)
What is electroplating?
Coating the surface of one metal with a thin layer of another metals
Why is it good for jewellery?
Gold and silver are attractive but v expensive so solider or gold can be electroplated onto cheaper metals e.g. copper which produces attractive jewellery but for cheaper
How may it improve a metal object’s ability to resist corrosion?
Chromium is a transition metal that resists corrosion
Objects e.g vehicle parts that are made from steel may be ‘chrome plated’ using electroplating
The thin layer of chromium stops air and water from reaching the steel below, preventing rusting
Why will cables and electronic equipment use gold-plated connectors?
Gold is the third best electrical conductor (after silver and copper) however it doesn’t tarnish unlike silver and copper
What do you need to electroplate an object?
An anode (positively charged electrode) A cathode (negatively charged electrode) An electrolyte (solution contains ions of the plating metal)
How is an metal electroplated (e,g. Copper ring with silver)?
A direct current flows through the electrodes and the electrolyte
The silver ions in the electrolyte move to the negatively charged copper ring
They gain electrons and are deposited as silver atoms
At the silver anode, silver atoms lose electrons to become silver ions which go into the electrolyte
Longer the current flow, the thicker the layer of silver on copper ring
What is galvanising?
Coating iron/steel objected with zinc
How does this work?
The thin layer of zinc improves corrosion resistance by stopping the water reaching the iron/steel and acts as a sacrificial metal
This sacrificial protection carries on even if the zinc layer is damaged
He can something be galvanised?
Carried out by electroplating
Dipping the object in molten zinc
How are food cans protected?
Their made from steel but their inner surface is protected from rusting by electroplating with tin
Tin doesn’t react with air or water at room temp
However what happens if the tin layer is damaged?
The steel can will rust faster
This is because iron is more reactive than tin so the iron Will act as a sacrificial metal to protect the tin
What is an alloy?
A mixture of a metal element with one or more other elements
What are alloy steels?
They are made deliberately by adding other elements to iron
Why do stainless steels revisit rusting?
They contain chromium which reacts with the oxygen in the air to form chromium oxide.
This layer is thick enough to stop air and water but thin enough to be transparent
If the layer is scratched, more chromium reacts to replace the layer
How do you make the metal/alloy stronger or harder?
You increase the carbon content of steel
What is mild steel (low carbon content) used for?
Building material and car parts
Why do car manufactures use strong steels?
Although their more difficult to press into shape, car manufacturers use them to produce strong but relatively light weight car bodies
How are the atoms arranged in a solid pure metal (no alloys)?
They are all the same size and are arranged regularly and in layers
These layers move past each other of enough force is applied
This is why metals are malleable and ductile
How are atoms arranged in an alloy?
The atoms of other elements may be different sizes
They distort the regular structure making it more difficult for layers to slide past each other
This is why they are stronger
What are the properties of gold and copper?
Both resist corrosion
Both malleable and ductile and good conductors
Both good for electrical wiring
Gold is more expensive so copper is chosen for most electrical wiring
Gold is used in tiny amounts e.g. memory chips
How do alloys have more useful properties than the elements 5ey contain?
Brass is alloy of copper and zinc
Copper and brass both resist corrosion
Copper is a better electrical conductor but brass is a lot stronger
So brass is preferred when it comes to electrical plug pins
What does a low density mean?
Light weight
