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GCSE AQA Chemistry:Module Two

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Lizzy Corbett

on 27 December 2012

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Transcript of GCSE AQA Chemistry:Module Two

Chemistry - Module Two 1.Bonding and Calculations Atoms, Compounds and Isotopes Atoms Compounds Isotopes Electron Proton Neutron Atoms are looking for stability. They want to have a full outer shell like all of the noble gases.
Atomic Number and Mass Number Describes an atom
A molecule is a small group of atoms joined together - like hydrogen H2
Atoms are made up of protons, neutrons and electrons. Chemical Bonding Covalent Metalic Ionic Formula Masses (Mr and Ar) Mass Number: Number of protons and neutrons in the nucleus Atomic Number: Equal to the number of protons and electrons This is how Carbon is represented on the periodic table. To work out the number of neutrons, you subtract the atomic number from the mass number. Proton Neutron Electron 1 Small 1 Particle Mass NEUTRONS = MASS NUMBER-ATOMIC NUMBER 2. Reaction Rates, Salts and Electrolysis Compounds are formed when atoms of two or more elements are chemically combined together. Once reacted, it is difficult to separate the two original elements out again.
For Example...
Carbon Dioxide is a compound, formed from a chemical reaction between carbon and oxygen.

Carbon (C) + Oxygen (O2) = CO2 Dot Cross Diagram showing C02 Isotopes are different atomic forms of the same element, which have the SAME number of PROTONS but a DIFFERENT number of NUETRONS!

This means they have the same atomic number but a different mass number. Carbon-12 and Carbon-14 are examples of isotopes. Carbon 12
Mass number - 12
Atomic number - 6 Carbon 14
Mass number - 14
Atomic number - 6 As they have the same atomic number of 6 but different mass numbers(12&14) these are isotopes. Carbon-14 has two extra neutrons then Carbon-12. These are called ions. Metals form positive ions and non metals always form negative ions. neutrons - 6
protons - 6
electrons - 6 neutrons - 8
protons - 6
electrons - 6 Remember neutrons have a neutral charge so the overall charge of an isotope is still 0. A solid is represented by (s)
A liquid is represented by (l)
A gas is represented by (g)
A solution in water is represented by (aq) For the sake of stability
Covalent bonds are formed between two non metals
The atoms share electrons in order to complete their outer shell
The atoms all attain noble gas structure
The new particles formed are neutral molecules
SHARE ELECTRONS OUTER SHELL ONLY!!! Ionic bonding takes place between a metal and non metal
The metal atoms lose their outer shell electrons to form positive ions
The non-metal atoms gain these electrons to fill their outer shells and form negative ions.
The positive and negative ions attract each other in a lattice EXAMPLE: Sodium Chloride
Sodium loses an electron because its in group 1, leaving a +1 charge
Chlorine gains an electron because its in group 7, leaving a -1 charge
Both these ions have opposite charges, so are attracted to each other: creating an ionic bond. If they are in...
Group 1
Group 2
Group 4 They form...
+1 ions
+2 ions
+3 ions
+4 or -4 If they are in...
Group 5
Group 6
Group 7
Group 8 They form...
+3 or -3ions
-2 ions
-1 ions
noble gases Na loses electron Cl gains electron Rules for naming ionic compounds
Is there a metal and non metal in the compound
Name the metal
Name the non-metal
Use the ending 'ide' for simple non-metal compounds
If the compound includes oxygen use the ending 'ate' Rules for drawing ionic equations
Identify the symbol for the metal
Identify the symbol for the non metal
Balance out the negative and positive ions to give an overall charge of 0 For Example:
Hydrogen has one valance electron. Chlorine has seven valence electrons. So they share one of their outer electrons to give Chlorine a full shell of 8 and hydrogen a full shell of 2!
This is a single Covalent bond. Can be represented by the diagram on the right or H-CL For Example:
Oxygen molecule. Two atoms of oxygen, each with six valence electrons. They both need two more electrons, so share two of their electrons with each other, giving them both 8 valance electrons.
This is a double covalent bond. Can be represented by diagram on the right or 0=0 For Example:
Nitrogen molecule. Two atoms of nitrogen, each with 5 valence electrons. The both need three more electrons so share three of their valence electrons with each other, giving them both 8 valence electrons.
This is a tripple covalent bond. Can be represented by the diagram on the right or by N N YOU NEED TO KNOW HOW TO DRAW DOT CROSS DIAGRAMS. (WITH THE BRACKETS AROUND AND SHOWING THE CHARGE) Metals are hard, shiny, conduct electricity, malleable, ductile, high melting point, solids, crystals(giant structures) LATTICE OF POSITIVE METAL IONS SWIMMING IN A SEA OF DELOCALISED ELECTRONS For Example:
Magnesium (in group 2) has two valance electrons. So it loses two electrons. These electrons are delocalised (free) so float around in between the positive magnesium ions formed. For every atom of magnesium there must be two electrons. +2 +2 +2 +2 +2 +2 +2 +2 +2 Each atom has a +2 charge Each electron has a -1 charge (of course) There are 9 atoms with a +2 charge, which together create a +18 charge.
There are 18 electrons with a -1 charge, which together create a -18 charge.
Therefore, it still has an overall neutral charge of 0! Giant Metalic Structures Giant Covalent Structures Giant Ionic Structures Polymers New Materials/ Nano-chemistry Calculating Masses In Reactions Percentage Yield Reversible Reactions Rates of Reaction Measuring Rates of Reaction Collision Theory & Catalysts Acids and Alkalis Acids Reacting with Metals Oxides, Hydroxides and Ammonia Making Salts Electrolysis The atoms in metals are in layers, so can slide over each other easily making it easy to bend into shape.
They share their outer electrons with all the other atoms. Positive ions are in a sea of delocailised electrons, and the attraction between the positive ions and negative electrons create a strong metallic bond. Properties of Metals Strong
High Melting points
Good conductors of electricity and heat
Lustrous (shiny) In these substances, strong covalent bonds join atoms together in large numbers to make giant structures. They are macromolecules. FOR EXAMPLE:
Sand (silicon dioxide)
Graphite The bonds between the atoms are all very strong, so they have
High melting points
Hard (except graphite)
Don't conduct electricity (except graphite)
Insoluble in all solvents SiO2 C C Graphite Diamond Silicon Dioxide Each atom only has three covalent bonds, creating layers which are free to slide over each other. This makes graphite soft and slippy, the layers held so loosely you can rub them off on a piece of paper. (like a pencil) This is because of the weak intermolecular forces
Graphite is the only non metal that is a good conductor of heat and electricity. Each carbon atom has one delocalised electron and its this electron that is free too move around and pass on heat or electricity. Each carbon atom forms four covalent bonds, in a very rigid covalent structure. This structure makes diamond the hardest natural substance. It is used in drill tips and jewelery as it is pretty and sparkly. Silicon Dioxide is sometimes called silica, which is basically sand. Giant covalent structure of silicon and oxygen. x100Ionix100c compounds are made up from positive metal ions and negative no metal ions. They are arranged in a regular lattice so that each positive ion is surrounded by negative ions and each negative ion is surrounded by positive ions. The particles are held together by the strong attraction between positive and negative ions. Properties of Ionic substances
High melting and boiling points
Hard but brittle
Conduct electricity only when melted or dissolved in water
Many ionic substances are soluble in water ELECTROLYSIS CAN SEPARATE THEM Potassium chloride is an example of an ionic compound The Ar is the relative mass in the periodic table (mass number). You add all the values of Ar for each of the atoms in the substance to work out the formula mass/ Mr.
For Example
Hydrogen atoms have a mass number of 1 and there are 2 of them. So 1x2=2
Sulfur atoms have a mass number of 32 and there is just 1 atom. So 32x1=32
Oxygen atoms have a mass number of 16 and there are four of them. So 16x4=64
Then you just add them all together.
2+32+64=98g The formula mass for H2SO4 is 98g. If you need to find the percentage of a single element in a compound you use the following formula.
Percentage = mass number of element(Ar) x number of atoms
Formula Mass (Mr) x100 For Example: Find the percentage of nitrogen in ammonium nitrate (NH4NO3)
Ar of nitrogen = 14 Number of atoms =2 Mr of ammonium nitrate= 80
Percentage = 14 x 2
80 x100 Percentage = 35% Empirical Formula This is what shows you how many of each atom is in each compound - in the lowest possible ratio. For example C2H4 would have the empirical formula of CH2. To calculate the empirical formula, you must follow these steps.
Find the mass or percentage of each element present
For each element, divide this by the mass number (Mr)
Highlight the smallest number calculate in the previous stage and divide both the numbers calculated by this number.
Should give you the numbers whole - if not round them For Example: 20g of a compound of Silicon with hydrogen contains 17.5g of silicon. Find the empirical formula. Silicon Hydrogen Mass of element Relative Mass (Ar) Mass/Ar Divide by smallest Whole number ratio Empirical Formula 17.5 2.5 28 1 17.5 2.5
28 1 =0.625 =2.5 0.625 2.5
0.625 0.625 =1 =4 1 4 SiH4 The Mole To find the percentage yield, you divide the mass obtained in an experiment by the maximum mass which could have been obtained and multiply by one hundred to get your percentage. The maximum mass can be calculated easily by working out the Mr of the compound.
For Example: The maximum amount of alcohol which can be obtained by fermenting sugar is 4.6g. The amount obtained in an experiment was 3.5g. What is the percentage yield?
Answer = 3.5 x100 = 76.1%
4.6 In real life you never get 100% yield because the reaction is reversible, when you filter a liquid or remove solid particles, you always lose some of the products during this separation and also there can be unexpected reactions that use up some of the product. Percentage yield is important for sustainable development - making sure we don't use resources quicker then they can be replaced as there needs to be enough for future generations. So using little energy to create high percentage yield reactions is GOOD! Alot of energy for low percentage yield is BAD because you waste chemicals. The mole is an important quantity used in chemistry. What you need to know is
A mole of any kind of substance contains approximately 6x10^23 particles
The mass of one mole is its formula mass (Mr) in grams
One mole of any gas contains 24dm^3 (24 liters) at room temperature and pressure
For Example: One mole of Co2 (that has an Mr of 44) contains 6x10^23 molecules and weighs 44g. As a gas its volume at room temperature would be 24dm^3 DONT WORRY TOO MUCH ABOU|T THIS! Exothermic and Endothermic Reactions which take in energy are called endothermic. They cause a drop in temperature. Thermal decompositions are always endothermic. Energy is taken in from surroundings, making it colder.
Calcium Carbonate (limestone cycle)
Sports injury packs (more convenient) If a reaction is endothermic one way, it will be exothermic the other way Reversible reactions are reactions that can go back to its original form after the reaction - in other words it reacts twice. If its endothermic in one direction it will be exothermic and in the other it will be endothermic. The energy absorbed by the endothermic reaction is equal to the energy released during the exothermic reaction. A good example of the thermal decomposition of hydrated copper sulphate. For Example:
Hydrated Copper Sulphate anhydrous copper sulphate + water This weird double arrow represents a reversible reaction EXOTHERMIC ENDOTHERMIC 1. If you heat blue hydrated copper sulphate crystals it drives the water off and leaves white anhydrous copper sulphate powder. This is endothermic. 2. If you then add a couple of drops of water to the white powder you get the blue crystals back again. This is exothermic. Forces between molecules determine the properties of plastics. Strong covalent bonds hold the atoms together in long chains. But it's the bonds between the different molecule chains that determine the PROPERTIES of the plastic. Weak forces Individual tangled chains of polymers, held together by weak intermolecular forces, are free to slide over each other. THERMOSOFTENING POLYMERS don't have cross-linking between the chains. The forces between the chains are really easy to overcome so its dead easy to melt to plastic. When it cools, the polymer hardens into a new shape. You can melt these plastics and remould them as often as you like. Strong Forces
Some plastics have string intermolecular forces between the polymer chains, called crosslinks, that hold the chains firmly together. THERMOSETTING POLYMERS have crosslinks. These hold the chains together in a solid structure. The polymer doesn't soften when it's heated. Thermosetting polymers are the tough guys of the plastic world. They are strong, hard and rigid. Making Polymers to suit You....
The starting materials and reaction conditions will both effect the properties.
Two types of polythene can be made using different conditions.
Low density (LD) polythene is made by heating ethene to about 200c under high pressure. Its flexible and is used for bags and bottles
High density (HD) polythene is made at a lower temperature and pressure (with a catalyst) It's more rigid and is used for water tanks and drainpipes. Smart materials and nanoparticels Smart materials have strange properties:
They behave differently depending on the conditions
A good example is nitinol - a shape memory alloy - Its a metal alloy but when cool you can bend it and twist it like rubber. Bend it too far though and it stays bent. However, if you heat it to a certain temperature it returns to its remembered shape.
Its good for glass frames.
Used for dental braces. In the mouth it warms and tries to return to a remembered shape and so it gently pulls the teeth with it Nanoparticles are really small
small particles, 1-100 nanometres across
contain a few hundread atoms
Include fullerenes. These are molecules of carbon, shaped like hollow balls or closed tubes. The carbon atoms are arranged in hexagonal rings. They can be joined together to form nanotubes - hollow carbon tubes. All those covalent bonds make carbon nanotubes very strong. They can be used to reinforce graphite in tennis rackets.
Different properties from chemicals made from
Nanoscience has many uses The effects of temperature, concentration and surface area on the rate of reaction can be explained in terms of how often the particles collide. The more they collide the higher the reaction rate. HIGHER TEMPERATURE increases collisions
Particles move quicker as they have more kinetic energy so collide more often.
HIGHER CONCENTRATION increases collisions
More particles to react with. In a gas its the pressure that's increased. More squashed together so more frequent collisions.
LARGER SURFACE AREA increases collisions
Breaking up solids to smaller pieces means more surface area so more parts for the solution to react with. Higher temperature also increases the energy of the collisions; reactions can't happen if there isn't enough energy. The minimum amount of energy needed is called the activation energy. A catalyst is a substance which speeds up a reaction, without being changed or used up in the reaction. A solid catalyst works by giving the reacting particles a surface to stick to, increasing the number of successful collisions.
Catalysts reduce costs in industry
Increase rate of reaction which saves money because the plant doesn't have to operate as long to get the same amount.
Allows reaction to happen at lower temperature, reduces amount of energy needed(and the costs) so good for sustainable development.
Aren't used up in reaction so can be used again and again
BUT there are also disadvantages
expensive to buy and have to be removed and cleaned often
Different reactions use different catalysts, so if your making more than one product you need lots of catalysts.
Can be poisoned by impurities and stop working. PH scale goes from 1 to 14 and is a measure of how acidic or alkaline a solution is. Strong acid is 0, strong alkali is 14, neutral is 7. The dye in an indicator changes color depending on the the PH of the substance. Universal indicator is a combination of dyes which give the colors shown above. It good for estimating the PH of a substance. Car battery acid/Stomach acid Vinegar/lemon juice Acid rain normal rain NEUTRAL PURE WATER Washing up liquid Pancreatic juice Soap powder Bleach Caustic Soda Acids and bases neutralise each other.
An ACID is a substance with a PH less than 7 that forms H+ ions in water.
A BASE is a substance with a PH more than 7.
An ALKALI is a BASE the dissolves in water and forms OH- ions in water.
So H+ ions make something acidic and OH- ions makes something alkaline The reaction between acids and bases is called neutralisation.
H+ + OH- H2O (l)
The substance made is neutral. An indicator can be used to show when the reaction has finished because it will turn green. Metals react with acids to form salts.
The more reactive the metal, the faster the reaction will occur. Very reactive metals like sodium react explosively.
Copper doesn't react at all with dilute acids because its less reactive then hydrogen
The speed of the reaction is indicated by the rate the bubbles of gas are given off
The hydrogen is confirmed by the burning splint test giving you a squeaky pop.
The name of the salt produced depends on the metal and acid used
hydrochloric acid + magnesium magnesium chloride +hydrogen
hydrochloric acid + aluminium aluminium chloride +hydrogen
hydrochloric acid + zinc zinc chloride +hydrogen
sulfuric acid + magnesium magnesium sulfate + hydrogen
sulfuric acid + aluminium aluminium sulfate + hydrogen
sulfuric acid + zinc zinc sulfate +hydrogen All metal oxides and hydroxides react with acid to make salt and water. Some are soluble and some are in soluble forms of the oxide and hydroxide. This is a neutralisation reaction.
The metal and acid you use decides what salt your produce
Hydrochloric acid + Copper oxide Copper Chloride + water
Hydrochloric acid + Sodium hydroxide Sodium Chloride + water
Sulfuric acid + Zinc oxide Zinc Sulfate + water
Sulfuric acid + Calcium hydroxide Calcium Sulfate +water
Nitric acid + Magnesium oxide Magnesium Nitrate + water
Nitric acid + Potassium hydroxide Potassium Nitrate + water Ammonia
Ammonia dissolves in water to make an alkaline solution. When it reacts with nitric acid, you get a neutral salt - ammonium nitrate.
Ammonia + Nitric acid Ammonium Nitrate
This is different to most neutralisation reactions because no water is produced.
Ammonium nitrate is a good fertiliser because it has nitrogen from two sources so is very rich in nitrogen. Plants need nitrogen to make protein. The rate of the reaction can be simplified by the speed of which the reactants are converted into the products. In the chemical industry it is vital for conditions to be carefully controlled. WHY? To make a profit in industry the products must be cheap and made quickly and safely. There are four things that can effect the rate of reaction Temperature Catalyst Surface Area Concentration As the temperature increases, the rate of reaction also increases. This is because the particles will gain more energy and collide with each other more often. A catalyst will speed up a chemical reaction. They are not actually used up in the reaction itself so in theory can be re-used. A drawback of catalysts is they can be expensive. Catalysts provide and alternative pathway to the products (example of of Hill - rather go through it then over it). They reduce the activation energy needed. As we increase the concentration the rate of reaction increases. This is because there are no more reactant particles available to collide with each other The larger the surface area, the quicker the reaction. This is because more of the reactant is exposed to react together, meaning particles will collide more frequently. Marble powder reacts faster then marble chips because they have a larger surface area. Reactions which give out energy are called exothermic. They cause the temperature to rise as energy is transferred to the surroundings. All combustion (burning) reactions, Neutralization reactions and many oxidation reactions are exothermic. These reactions have many everyday uses...
Hand warmers
Self heating cans In some reactions the energy released when new bonds are formed is more than the energy needed to break the bonds. In other reactions the energy needed to break the bonds in the reactants is MORE than the energy released when new bonds are formed in the products, Hand Warmers - Re-useable and Disposable Disposable
A disposable hand warmer works by using the oxidation of iron to release energy. The iron turns into hydrated iron oxide in an exothermic reaction. Sodium Chloride is used as a catalyst.
Advantage - lasts a few hours
Disadvantage - can only be used once Re-useable
A re-useable handwarmer works by the formation of crystals from a solution of salt. Sodium methonate(salt) is dissolved in water by heating. A small metal disc inside starts exothermic reaction - when pressed small particles of metal are scraped off. These start of crystalisation which gives off heat energy. Crystals then redissolve in water.
Advantage - Reused
Disadvantage - Only lasts an hour This is a typical graph depicting the rate of a reaction.
Y axis = loss/decrease/change in mass
X axis = time
Steeper curve = highest rate of reaction
If the line flattens off sooner it means the reactants are used up quicker Rate of Reaction = Amount of reactant used OR amount of product formed
Time We can measure the rate of reaction using three methods.
1- Decreasing Light intensity (precipitation)
This is where the reaction produces a cloudy substance. You observe a mark through the solution - like a cross - and see how long it takes to disappear. Not accurate though.
2-Measuring the decrease of mass
If gas is given off you can carry out the experiment by weighing the start reactants. As the reaction occurs, the mass will decrease, the faster it drops it quicker the reaction. Most accurate of the three methods
3-Measuring the volume of gas given off
You use a gas syringe to measure the volume of gas given off at certain points in the experiment. The more gas given off in period of time, faster the reaction. Hydrochloric Acid and Marble Chips Sodium Thiosulfate and hydrochloric acid Measuring Mass and Effect of Concentration Decomposition of Hydrogen Peroxide Measure volume of gas at regular intervals
Make table of readings then a graph, x axis is time y axis is volume
Repeat experiment with same acid and mass of marble chips but crunch them up (increase surface area)
Repeat again with powder marble chips Measuring Volume of Gas and Effect Of Surface Area Measuring Volume of Gas and Effect Of Surface Area Precipitation and Effect of Temperature Measuring Volume of Gas and Effect Of Catalysts Two chemicals are clear but react to give a yellow precipitate of sulfur.
Repeated at different temperatures
Depth of liquid must be the same With manganese oxide catalyst speeds up reaction of water back to hydrogen and oxygen.
Oxygen gas is given off
Use different catalysts Gives off hydrogen gas which can be measured (decrease on the scales) There are thee ways we can make salts...
1. Reacting a metal and acid metal+acid salt+hydrogen
You have to pick an acid and metal, for example copper oxide and hydrochloric acid. You add the metal to the acid and it will dissolve. When the acid is neutralised the solid will sink to the bottom of the flask. You then filter out this excess solid to leave the salt solution. You evaporate some of the water off and then leave it to slowly evaporate as bigger crystals are formed. This is called crystallisation.

2. Reacting an acid and alkali acid+alkali salt+water
You can't use the method above because you cant tell when the reaction has finished and you cant just add more and filter it out! You have to add exactly the right amount of alkali to neutralise the acid - by using an indicator. The you repeat is using the amount you just worked out to neutralise it without using an indicator. Then you evaporate the water and crystalise the salt like normal.

If the salt you want to make is insoluble you can use a precipitation reaction. You need to pick two solutions that contain the ions you need. For example if wanted to make lead chloride you need a solution that contains lead ions (lead nitrate) and one containing chloride ions (sodium chloride). Once the salt has precipitated out and is at the bottom of the flask, all you have to do is filter it out, wash it and then dry it on filter paper.
These reactions can be used to remove poisonous ions like lead from drinking water. It can also treat effluent (sewage). SOLUBLE SOLUBLE INSOLUBLE It requires a liquid to conduct the electricity - called the electrolyte.
Has free ions in this liquid which is the reason it conducts electricity
Involves oxidation and reducation
Reduction is gain of electrons (RIG)
Oxidation is loss of electrons (OIL) If you pass an electric current through an ionic substance that's molten or a solution, it breaks it down into the elements it's made of. Negative Electrode - Cathode Positive Electrode - Annode Positive ions are attracted to the negative electrode (cathode) Here they gain electrons (reduction)

Electron taken from the cathode by ion Negative ions are attracted to the positive electrode (annode) Here they loose electrons (oxidation)

Electron given to the annode by the ion + + + - - - - Examples:
Lead Bromide
Lead is collected at the cathode (negative) and bromine gas is released at the annode (positive) Each atom of lead (pb2+) gains two electrons and each atom of bromine (Br-1) looses one electron.

Reactivity affects the products formed in electrolysis. There isn't always just two free ions - like the lead and the bromine above. If something is dissolved in water, you also have H+ and OH- ions to add the situation.
At the negative electrode you have metal ions and h+ ions, if the metal is more reactive then hydrogen it will stay in the solution and hydrogen will gain the electron to from hydrogen gas.
At the positive electrode if you have OH- and either Cl- Br- or I- (a halide) then chlorine bromine or iodine will be formed. If not then oxygen will be formed.
Sodium Chloride
Produces three useful products - hydrogen, chlorine and sodium hydroxide.
At the negative electrode, two hydrogen IONS gain an electron each to become one hydrogen MOLECULE
At the positive electrode two chloride ions loose one electron each to become one chlorine MOLECULE
The sodium ions stay in the solution because they are more reactive then hydrogen. OH- (hydroxide) ions from the water are also left over. Therefore sodium hydroxide is formed in the solution. Half Equations Half equations show the reactions in terms of the electrodes. The main thing is to make sure the number of electrons is the same for both half equations. The half equations for sodium chloride are...
Negative electrode: 2H+ + 2e- H2
This basically means that two hydrogen ions with a plus one charge gain two electrons with a negative charge to make a hydrogen molecule with no charge.
Positive electrode: 2Cl- - 2e- Cl2
This basically means that two chlorine ions with a negative one charge loose two electrons to make a chlorine molecule with no charge. Extraction of Aluminum Electroplating Electroplating uses electrolysis to coat a surface of one metal with another metal. The negative electrode is the metal object you want to plate (add the coat to)
The positive electrode is the pure metal you want it to be plated with (covered in) You need the electrolyte to contain the ions of the same element of the positive electrode (plating/covering metal)

For example
To electroplate silver onto a brass cup you'd make the brass cup the negative electrode (to attract the positive silver ions) and the lump of pure silver at the positive electrode. You would have to have a solution that also contained silver, like silver nitrate.

There are lots of different uses of electroplating
Decoration: silver is attractive but very expensive. It's much cheaper to plate boring brass with silver then to make it from solid silver.
Conduction: Metals like copper conduct electricity so is used to electroplate metals for electric circuits and computers. Electrolysis is used to extract aluminium from its ore. It's a very abundant metal but always found in natural components. Its main ore is bauxite and after mining and purifying you have a white powder left which is aluminium oxide. The aluminium has to be extracted via electrolysis.

Cryolite is used to lower the temperature and cost.
Aluminium oxide has a very high melting point of over 2000c so melting would be very expensive
Instead of melting, auminium oxide is dissolved in molten cryolite (less common ore of aluminium) This brings down the melting point to 900c which saves money and is easier.
The electrodes are made of carbon - graphite
Aluminium forms at the negative electrode and can be collected.
Oxygen forms at the positive electrode. The oxygen reacts with the carbon electrode to make CO2 carbon dioxide. This is why the positive electrode has to be replaced regulary - it is gradually worn away because oxygen reacts with it.
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