Edexcel Chemistry Topics 3 & 4 (Obtaining and using metals (What is…
Edexcel Chemistry Topics 3 & 4
Acids and Alkalis
Acids in solution are a source of H+ ions and alkalis in solution are a source of OH- ions.
In an neutralisation reaction, the OH- and H+ ions react together to form water which decreases the concentration of H+ ions therefore increasing the pH. This occurs between an acid and a base
pH measures the concentration of Hydrogen ions in a solution or substance.
The higher conc. of H+ ions, the lower the pH but the higher the conc. of OH- ions, the higher the pH.
When you increase the conc. of H+ ions by a factor of ten then the pH will decrease by one.
Phenolphthalein goes colourless in acid and pink in alkalis.
Blue Litmus goes red in acidic solutions and stays blue in alkaline or neutral solutions.
Red Litmus goes blue in alkaline solutions and stays red in neutral or acidic solutions.
Methyl orange goes red in acidic solutions and goes yellow in alkaline or neutral solutions.
Strong acids fully dissociate in solution and a large amount of the acid molecules dissociate to release H+ ions whereas they only partially ionise in weak acids and a relatively low proportion of acid molecules ionise to release H+ ions.
Concentrated solutions mean that there are a lot of solute ions compared to the amount of solvent ions you began with. It essentially describes how watered down your solution is. Dilute solutions have less solute ions per fixed volume of solvent than concentrated solutions.
A base is a substance that reacts with an acid to form a salt ad water only. All alkalis are bases but not all bases are alkalis. Alkalis are simply soluble bases.
Metal + acid = salt + hydrogen
Metal oxide + acid = salt + water
Metal hydroxide + acid = salt + water
Metal carbonate + acid = salt + water + carbon dioxide
The chemical test for carbon doxide is (using limewater) to bubble the gas through limewater and if it turns cloudy then carbon dioxide is present.
The chemical test for hydrogen is to put a glowing splint into a test tube of gas that you suspect has hydrogen and if it burns with a squeaky pop then hydrogen is present.
The chemical test for oxygen is to put a glowing splint into a test tube of gas and if it relights then it has oxygen in it.
Precipitation method to make an insoluble salt (lead nitrate + sodium chlorine):
Add 1 spatula of lead nitrate to a test tube. Add water to it to dissolve it; this should be done in deionised water so there are no other ions in the solution. Shake it to make sure all the lead nitrate is fully dissolved. Repeat with one spatula of sodium chloride.
Tip the two solutions together to make them react. The lead chloride should precipitate.
Use filter paper and a funnel to filter the mixture to separate it, the liquid going into a conical flask.
Swill out the beaker with deionised water to make sure all the precipitate is removed and all you are left with in the conical is the sodium nitrate.
Then rinse the filter paper with more deionised water to make sure that all you are left with is the insoluble lead chloride.
Scrape that out and leave it on a tray in an oven to dry out.
Acid and Insoluble Base method to make a soluble salt:
You can make a soluble salt by reacting an acid with one of the ions you want in the salt and an insoluble base with the other (like a metal oxide or hydroxide).
Heat the acid in a water bath in a fume cupboard to speed up the reaction between the acid and the base.
Add the acid to the base, the base will need to be in excess so you know the neutralisation is fully competed. You can tell when it is reacted fully when all the excess sinks to the bottom.
Filter off the excess to leave only the salt and water.
Heat the solution using a Bunsen burner to allow some of the water to evaporate off. Leave the solution and for the salt to crystallise. Then filter off the solid salt and leave to dry.
all common sodium, potassium and ammonium salts are
all nitrates are soluble
common chlorides are soluble except those of silver and lead
common sulfates are soluble except those of lead, barium
common carbonates and hydroxides are insoluble except
those of sodium, potassium and ammonium
Acid/Alkali Titration method for making a soluble salt:
Because there is no indicator of when the reaction is finished and you can't add the alkali in excess as it is soluable, you must add an indicator but one with a single colour change like phenolphthalein or methyl orange.You must work out exactly how much alkali neutralises the acid therefore a titration must be used.
Measure out a set amount of acid in a conical flask using a pipette and add a few drops of indicator.
Fill a burette fully of alkali and slowly add it until the end point, where the solution changes colour- if using phenolphthalein this will be from colourless to pink- this is where the acid is fully neutralised.
You will likely need to repeat to find the exact volume.
Once you have found it, repeat with no indicator using those exact volumes of acid and alkali so that the indicator doesn't contaminate the salt.
The remaining solution will be a salt dissolved in water so evaporate the water off using a Bunsen burner and leave the salt to crystallise then filter it off and dry it.
Using these solubility rules, you can predict what precipitate will form (if any) when reacting two soluable salts together and naming it. Eg. copper chloride + silver nitrate = copper nitrate + silver chloride (precipitate)
L- loss of electrons
G- gain of electrons
Electrolytes are ionic compounds dissolved in solution or in the molten state.
Electrolysis is the process of passing an electric current through an electrolyte to split up compounds.
The negative electrode is called a cathode and it causes positive ions or cations to move towards it during electrolysis. Reduction happens at the cathode.
The positive electrode is called an anode and it causes the negative ions or anions to move towards it during electrolysis. Oxidation happens at the anode.
As ions gain or lose electrons they form the uncharged substances and are discharged from the electrodes.
Get two inert electrodes
Clean them with emery paper
From this point on, be careful not to touch the surface of the electrodes as this would transfer grease back onto them.
Place both electrodes into a beaker filled with your electrolyte.
Connect the electrodes to a power supply using crocodile clips and wires. When you turn the power supply on, a current will run through it.
Put your ionic substance (which will become the electrolyte) in a crucible.
Heat it with a Bunsen burner until it is molten. Do this in a fume cupboard.
Once the solid's molten dip two clean, inert electrodes into the electrolyte.
Then connect the electrodes to a power supply using wires and clips, this will cause a current to flow through it.
The products of these solutions when undergoing electrolysis:
Copper chloride- Anode: chlorine. Cathode: copper.
Sodium sulfate- Anode: oxygen. Cathode: hydrogen.
Sodium chloride- Anode: chlorine. Cathode: hydrogen
Water acidified with sulfuric acid- Anode: oxygen. Cathode: hydrogen.
Molten lead bromide- Anode: bromine. Cathode: molten lead.
This happens because hydrogen will discharge in preference to a reactive metal but not to less reactive one like copper. If hydrogen is above the metal in the reactivity series then the metal will discharge but if it is higher than hydrogen, hydrogen will discharge.
Halogens discharge in preference to hydroxide ions but hydroxide ions discharge in preference to sulfate ions.
If it is a sulfate, oxygen will always form at the anode.
Half equations show what happens at each electrode and how the substances changes. For example, for lead bromide the half equations will look like this:
Pb^2+ + 2e^- = Pb
2Br^- = Br2 + 2e^-
Copper Electrodes in purifying copper:
If you set up electrochemical cell in the same way as the one above, but using copper electrodes in a solution of copper sulfate instead of inert electrodes, the result is different.
As the reaction continues, the mass of the anode will decrease because mass from the anode will be transferred to the cathode.
You can find how the mass has changed by weighing the electrodes before and after the experiment to determine the difference. The change at the cathode should be positive and the change at the anode should be negative.
When weighing the electrodes, they'll need to be dry.
Increasing the current will increase the rate of electrolysis. This will result in a larger change in the masses of the electrodes in the same amount of time.
The electrical supply works by pulling the copper atoms off the anode.
And offering electrons at the cathode.
Copper can be purified by using a lump of impure copper at the anode and a thin strip of pure copper at the cathode.
The impure copper anode is oxidised, dissolving into the electrolyte to form copper ions.
The copper ions are reduced at the pure copper cathode and added to it as a layer of pure copper.
Any impurities from the impure copper sink to the bottom of the cell, forming sludge.
Obtaining and using metals
What is the reactivity series?
Oxidation can be the reaction with of the adding of oxygen and reduction can be the removal of oxygen.
Metals below hydrogen on the reactivity series won't react with dilute acids.
Metals at the top of the series lose their electrons more easily to become cations and are also oxidised more easily.
Metals at the bottom don't give up their electrons as easily and are more resistant to oxidation than the metals higher up.
The more reactive a metal is, the faster the reaction will go and the more hydrogen will be produced. You can test this using the squeaky pop test. Very reactive metals will fizz vigorously, less reactive metals will bubble a bit and unreactive metals wont do anything at all.
Reactions with water also show reactivity.
Reactive metals will react if you put them in water ad will form a metal hydroxide and hydrogen However less reactive metals will need to react with steam as they do not react much in cold water and will form a metal oxide and hydrogen.
In displacement reactions, a more reactive element displaces a less reactive one. In metal displacement reactions, the more reactive metal loses electron and the less reactive metal gains electrons.
The more reactive metal is
, and the less reactive metal is
An ore is a rock that contains enough metal to make extraction worthwhile. Usually the ore is an oxide of the metal. Most ores are found in the Earth's crust they are then mined and the metal is extracted. Some unreactive metals such as gold or platinum are found uncombined in the Earth's crust but they usually need to be refined before they can be used.
Metals can be extracted from ores in two ways: electrolysis and burning with carbon.
You can extract metals using burning by carbon if they are lower than it on the reactivity series as it would be reduced and oxygen would be removed from it. Eg. iron oxide would be reduced to make iron.This is because carbon can only remove oxygen away from metals which are lower on the reactivity series than itself.
Electrolysis must be used when the element is higher on the reactivity series than carbon. The ore must be made molten and undergo electrolysis. Oxygen will be discharged at the cathode and the metal at the anode. Electrolysis is a much more expensive process than heating with carbon because it requires large amounts of electricity which is expensive.
Many rich ores are limited and low grade ores are plentiful as they have been deemed not worthwhile to extract from so scientists are looking into way to extract from low grade ores.
There are two ways: phytoextraction and bioleaching.
Bioleaching require bacteria to separate the metals from the ores. The bacteria get energy from the bonds in between the atoms in the ore, separating the metal from the ore in the process. The leachate contains metal ions which can be extracted using electrolysis or displacement.
Phytoextraction involves plants growing in soil that contains metal compounds. The compounds can't be used by the plant so they build up, the plants can be harvested and burned in a furnace. The ash contains metal ions which can be extracted by displacement or electrolysis.
These methods of extraction have less impact on the environment but are slow.
Extracting raw metals can take lost of energy which comes from burning fossil fuels. These are a finite source and will be used up. Recycling materials saves energy and only uses a small fraction of the energy needed to extract and refine materials from the beginning. It is particularly important to recycle rare materials. Extracting damages the environment as mines are destroying habitats and are an eyesore. Recycling more metals means we don't need more mines.
It also cuts own the rubbish that gets sent to landfills, these take up space and pollute he surroundings.
Recycling has important economic benefits. Saving energy means saving money. It's very beneficial to recycle materials that are expensive to extract or buy. Recycling is a big industry that creates lots of jobs. the materials to b e recycled need to be transported and processed at recycling centres. Jobs are created in every stage of the process; more than in disposing of waste at landfills.
An LCA is a Life Cycle Assessment and it looks at each stage of the life of a product and works out the potential environmental impact of it.
It looks at the
choice of material
and how these come to be. Eg. are they extracted using electrolysis or mined and how much energy do they processes take.
: how much energy does this require, how much pollution is caused, how many waste products are formed and how can you dispose of them.
: using the product could also damage the environment. Paint gives off toxic fumes, burning fuels releases greenhouse gases and fertilisers can leach into streams and rivers and cause damage to ecosystems.
: how is the product disposed of- is it recycled or disposed of in a landfill. Is it incinerated, which causes air pollution.
Reversible reactions and equilibria
Reversible reactions are ones signified by using this symbol: ⇌.
The direction of the reaction can be changed by changing the conditions. The products can react to for the reactants in reversible reactions.
Dynamic equilibrium is the state achieved where the forward reaction takes place at the same
as the backwards direction. The concentrations of the reactants and products have reached a balance and won't change At equilibrium both reactions are taking place so there is no overall effect. Equilibrium can only be reached in a closed system so nothing escapes. This doesn't mean that the amounts of each product and reactant is the same, it could 'lie to the left' or 'lie to the right', the position of equilibrium relies on the conditions of the reaction.
The Haber Process is a reversible reaction of nitrogen and hydrogen to form ammonia: N2 + 3H2 ⇌ 2NH3.
The Nitrogen is obtained from the air (about 73% nitrogen) and the hydrogen is extracted from hydrocarbons such as natural gas or crude oil. The Haber Process is carried out at 450 degrees, 200 atmospheres and with an iron catalyst in order to obtain as much yield as possible. It can reach dynamic equilibrium.
The position of equilibrium can change depending on the temperature, pressure (for reactions involving gases) and concentration (for reactions involving liquids). This is know as Le Chatelier's principal.
: If you decrease the temperature then it will more in the exothermic direction to produce more heat and if you increase the temperature, equilibrium will move in the endothermic direction to absorb the extra heat.
: If you increase the pressure, the equilibrium will move towards the direction will fewer moles of gas to reduce pressure. If you decrease the pressure then equilibrium will move in the direction of more moles of gas to increase pressure.
: If you increase the concentration of the reactants then equilibrium will move to the right to use up the reactants and make more products. If you increase the concentrations of he products then equilibrium will move to the left to use up the products and make more reactants. Decreasing the concentration will have the opposite effect.