Chemistry C6

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Redox reactions

  • A loss of electrons= oxidation
  • gain of electrons= reduction
  • REDuction and OXidation happen at the same time
  • an oxidising agent accepts electrons and gets reduced
  • a reducing agent donates electrons and gets oxidised

examples of redox reactions:

  • iron atoms are oxidised to iron (II) ions when they react with dilute acid
  • the iron atoms lose electrons, they are oxidised by the hydrogen ions
  • the hydrogen ions gain electrons, they are reduced by the iron atoms
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Displacement

A more reactive metal will displace a less reactive metal from its compound

if you put a reactive metal into the solution of a dissolved metal compound, the reactive metal will replace the less reactive metal in the compound

(http://3.bp.blogspot.com/_zKYnjVTQROk/SRGqi4KQXqI/AAAAAAAAAwg/JDNA6RDfigE/s400/sci_dia_42.gif)

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Rusting of Iron

Iron and some steels will rust if they come in contact with both oxygen and water

Rust is a form of hydrated iron (III) oxide:

  • iron + oxygen + water ----> hydrated iron (III) oxide

rusting is a redox reaction because:

  •  the iron loses electrons when it reacts with oxygen and each Fe loses three electrons to become Fe3+- iron is oxidised
  • oxygen gains electrons when it reacts with iron, each O atom gains 2 electrons to become O2-, oxygen is reduced
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Ways of preventing rusting

Metals are combined with other things to prevent rust

  • alloys can be made by mixing iron with other metals
  • steels are alloys of iron with carbon- most common is stainless steel- rustproof alloy of iron, carbon and chromium

Oil, Grease and Paint prevent rusting

  • coating the iron with a barrier which keeps water, oxygen or both out
  • painting is ideal for large or small structures and can be colourful
  • oiling or greasing has to be used on moving  parts like bike chains

A coat of tin

  • tinning is where a coat if tin is applied to the object, it only works as long as the tin stays intact as scratched tins will reveal some iron and rust
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Sacrificial Method

You can place a more reactive metal with the iron to react with the water and oxygen instead

  • galvanising is where a coat of Zinc is put onto the object,
  • the zinc acts as protection as its more reactive so it'll lose electrons in preference to the iron
  • also acts as a barrier
  • blocks of metal e.g. magnesium can be bolted to the iron, the magnesium will lose electrons in preference to iron, it can be used on hulls of ships or on underground iron pipes
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Hydrogen and Oxygen give out energy when they reac

hydrogen plus a lighted splint gives a squeaky pop and oxygen relights a glowing splint

  • hydrogen and oxygen react to produce water
  • the reaction between hydrogen and oxygen is exothermic- it releases energy
  • you can get energy by reacting hydrogen and oxygen and it doesn't produce any nasty pollutants
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Fuel Cells

A fuel cell is an electrical cell that's supplied with a fuel and oxygen and uses energy from the reaction between them to generate and electrical voltage

  • fuel cells were developed in the 1960s as part of the space programme to provide electrical power on spacecraft
  • unlike a battery a fuel cell doesn't run down or need recharging, it'll produce energy in the form of electricity and heat as long as fuel is supplied
  • there are a few types of fuel cells using different fuels and different electrolytes
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Hydrogen - oxygen fuel cells

  • the electrolyte is often a solution of potassium hydroxide
  • electrodes porous carbon
  • hydrogen goes into the anode compartment
  • oxygen into the cathode compartment
  • at the -ve cathode oxygen gains electrons and reacts with water
    • O2 + 4e- + 2H2O ----> 4OH-
    • oxygen gas is gaining electrons= reduction
  • OH- ions in the electrolyte move to the positive anode
  • at the +ve anode hydrogen combines with the hydroxide ions to produce water and electrons,
    • hydrogen loses electrons- oxidation
    • 2H2 + 4oH- ----> 4H2O + 4 e-
  • The electrons flow through an external circuit from the anode to the cathode- electric current
  • overall reaction is hydrogen plus oxygen which gives water
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Advantages of Hydrogen-oxygen fuel cells

  • much more efficient than power stations or batteries
  • in a fuel cell the electricity is generated directly from the reaction
  • there aren't lots of stages so fewer places for electricity to be lost as heat
  • there are no moving parts- energy isn't lost as friction
  • with hydrogen as the fuel, water is only product- no pollution
    • could be used in cars and lorries
    • could replace batteries which are polluting to dispose of because made of highly toxic metal compounds
    • its not the end of conventional power stations as they use electricity for electrolysis or to make hydrocarbons
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Hydrogen-oxygen fuel cells and spacecraft

  • used to provide electrical power
  • hydrogen and oxygen readily available from the spacecraft's rocket fuel tank
  • some products are used as drinking water- saves taking water with them
  • no waste products or pollutants
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The Car industry

  • developing fuel cells to replace conventional petrol/diesel engines
  • fuel cell vehicles don't produce any pollutants
    • no greenhouse gases
    • no nitrogen
    • no sulfur dioxide
    • no carbon monoxide
    • only products are water and heat
  • fuel cells could help countries become less dependant on crude oil
  • there are prototypes of fuel cell powered vacuum cleaners and vending machines
  • they are used to provide back up for hospitals and airports etc
  • mini fuel cells for mobile phones, laptops are in development
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Alcohols

Alcohols have an -OH and end in -ol

the general formula = CnH2n+1OH

so if an alcohol has 2 carbons its formula will be C2H5OH

naming system is same as for alkenes but -e replaced with -ol

meth= 1, eth- 2, prop= 3, but= 4, pent= 5

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Fermentation

fermentation converts sugars into ethanol

  • glucose -----> ethanol + carbon dioxide
  • C6H12O6 ----> 2C2H5OH + 2CO2

the reaction is brought about be enzymes (biological catalysts) found in yeast

  • the temperature needs to be carefully controlled, too cold, yeast inactive, too hot enzymes denatured- carried out at optimum temperature= 25'C- 50'C
  • important to prevent oxygen getting at the alcohol or the ethanol is turned into ethanoic acid
  • when the concentration of alcohol reaches about 10-20% the reaction stops, the yeast is killed by alcohol- mixture can be distilled to get pure alcohol
  • fermentation uses a renewable resource
  • its expensive to concentrate and purify the ethanol
  • the whole process is slow and inefficient
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Industrially made ethanol

ethene can be reacted with steam to produce ethanol

ethene (C2H4) react with steam (h20) to make ethanol (C2H5OH)

the reaction needs a temperature of 300'C and a pressure of 70 atmospheres, phosphoric acid is used as a catalyst

at the moment its a cheap process because ethene is cheap and not much is wasted

ethene is produced from crude oil which is non renewable resource which will run out so using ethene to make ethanol will become expensive

its made in a large chemical plant, so its continuous and quick

product is high quality

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Dehydrated Ethanol

Ethanol can be dehydrated to produce ethene

The plastics and polymers industry uses lots of ethene

countries which have no oil but plenty of land for growing crops for fermentation can make ethene through dehydration of ethanol

ethanol vapour is passed over a hot aluminium oxide catalyst

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Mining Salt

  • Salt is extracted in Britain from underground deposits left millions of years ago when ancient seas evaporated
  • massive deposits of this rock salt under Cheshire and Teeside 
  • Rock salt is a mixture of salt and impurities, its drilled, blasted and dug out and brought to the surface using machinery
  • it can also be mined by pumping hot water underground, the salt dissolves and the salt solution is forced to the surface by the pressure of the water
  • when the mining is finished it is important to fill the holes, if not the land could collapse and slide into the holes- subsidence
  • rock salt can be used in its raw state on roads to stop ice forming or salt can be filtered out to enhance the flavour in foods or for making chemicals
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Electrolysis of Brine

Concentrated brine (sodium chloride solution) is electrolysed industrially

there are 3 useful products:

  • Hydrogen gas- given off at the (-ve) cathode
  • chlorine gas- give off at (+ve) anode
  • sodium hydroxide (NaOH) is left in solution

these are collected and then used in all sorts of industries to make various products

the electrodes are made of an inert material so they shouldn't react with either the electrolyte or the products of the electrolysis

Half equations:

  • Cathode= 2H+ + 2e-  -----> H2
  • anode= 2Cl-  - 2e-   ------> Cl2
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Dilute Brine

  • A sodium chloride solution contains both sodium ions and chloride ions and a few hydrogen ions and hydroxide ions from the water
  • the OH- is the most easily discharged anion
  • so in dilute solution of brine the OH- ions are discharged before the chloride ions so oxygen is produced at the anode
    • 4OH-  - 4e- ----> O2  + 2H2O
  • in concentrated solution of brine there are lots of chloride ions and very few hydroxide ions so chlorine is formed at the anode
    • 2Cl-  -  2e-  -----> Cl2
  • of you electrolyte molten sodium chloride you get sodium at the cathode and chlorine at the anode
    • Na+  + e-  -----> Na
    • 2Cl-  -2e-  -----> Cl2
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Useful products from the electrolysis of brine

Chlorine

  • disinfectants
  • killing bacteria
  • household bleach- made reacting chlorine with sodium hydroxide
  • plastics
  • HCl
  • insecticides

Hydrogen

  • used in the haber process to make ammonia
  • used to change oils to fats for margerine

Sodium hydroxide

  • soap, ceramics, organic chemicals, paper pulp, oven cleaner, household bleach
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CFC's

Chlorofluorocarbons are organic molecules containing carbon, chlorine and fluorine

CFCs are non-toxic, non-flammable and non reactive, they are insoluble in water and have low boiling points

they were used as coolants in refrigerators and air conditioning systems

CFCs were also used as propellants in aerosol sprays

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Problem of CFCs

CFCs have been linked with the depletion of the ozone layer

  • Ozone is a form of oxygen O3
  • it is in the ozone layer in the stratosphere and does an important job of absorbing some of the UV rays from the sun
  • Ozone absorbs UV light and breaks it down: O3 + UV light ----> O + O2 then again
  • reducing the amount of ozone increased the amount of UV light passing through the atmosphere causing:
    • increased risk of sun burn
    • skin ages faster
    • more cases of skin cancer
    • increased risk of cataracts
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Countries banning CFCs

in 1978- USA, Canada, Sweden and Norway banned CFCs in aerosol propellants

after the ozone hole was discovered many countries got together and banned CFC's

some poorer countries haven't banned CFCs because of the costly alternatives

depletion of the ozone layer is a global problem, CFCs are easily blown by the wind all over the Earth and affect everyone

Poorer countries who sign up Montreal Protocol can get help with the cost of replacing CFCs

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Free Radicals

A covalent bond is one where 2 atoms share electrons

a covalent bond can break unevenly to form two ions

but a covalent bond can also break evenly- each atom gets one of the shared electrons

  • e.g. h-h ----> h. + h. the h. is called a free radical

the unpaired electrons make the free radical very very reactive

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Chlorine free radicals

  • ultraviolet light makes CFCs break up to form free radicals
    • CCl2F2 ----> CClF2. +Cl.
  • This happens high up in the atmosphere where the ultraviolet light from the sun is stronger
  • Chlorine free radicals from this reaction react with ozone turning it unto ordinary oxygen molecules
    • O3 + Cl. ----> ClO.  + O2
  • the chlorine oxide molecule ClO is very reactive so reacts with ozone to make 2 oxygen molecules and another Cl. free radical
    • ClO. + O3  ----> 2O2 + Cl.
  • this cl. free radical goes and reacts with another ozone molecule
  • its a chain reaction

CFCs don't attack ozone directly, they break up and form chlorine atoms which attack ozone. the chlorine atoms aren't used up so they continue to break down ozone

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CFCs in the Stratosphere

  • CFCs are not very reactive and will only break up to form chlorine atoms in the stratosphere where there is plenty of high-energy ultraviolet light around
  • CFCs in the atmosphere take a long time to be removed
  • each CFC molecule produced one chlorine atom which can react with lots of ozone molecules
  • even after all CFCs have been banned each molecule will stay around for a long time
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Safe Alternatives

  • Alkanes don't react with ozone
  • hydrofluorocarbons (HFCs) are compounds similar to CFCs but don't contain chlorine
  • scientists have investigated the compounds that could be produced by breakdown of HFCs in the upper atmosphere and none of them are able to attack ozone, suggesting they are safe
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Hard water

  • won't easily form a lather with soap
  • makes scum
  • hardwater forms limescale on the inside of pipes, boilers and kettles
  • Limescale is a thermal conductor, meaning that a kettle with limescale on the heating element takes longer to boil
  • scale can eventually block pipes
  • hard water causes scum to form on the surface of tea
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Cause of Hard water

Hard water contains calcium ions (ca2+) or magnesium ions (Mg2+) or both, as water flows over rocks and through soils containing calcium and magnesium compounds, these ions dissolve in it

  • magnesium sulfate MgSO4 dissolves in water
  • Calcium Carbonate found in chalk, limestone or marble doesn't dissolve in water but reacts with acids. CO2 from air dissolves in rainwater so is slightly acidic meaning that the calcium carbonate can react with rainwater to form calcium hydrogencarbonate which is soluble
  • Carbon dioxide + water + calcium carbonate -----> calcium hydrogencarbonate
  • CO2 + H2O + CaCO3 -----> Ca(HCO3)2
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Temporary hardness

Temporary- caused by the hydrogencarbonate HCO3- ion in Ca(HCO3)2

Permanent hardness- caused by calcium sulfate

Temporary hardness is removed by boiling- hydrogencarbonate decomposes to form insoluble CaCO3

  • calcium hydrogencarbonate ----> calcium carbonate + water + carbon dioxide

both types of hardness are removed by adding washing soda- sodium carbonate

  • the carbonate ions join onto the calcium ions and make an insoluble precipitate

both types of hardness can also be removed by ion exchange columns as have lots of sodium ions and exchange them for calcium or magnesium

  • resin contains Na+ ion as hard water flows through the Ca2+ exchange places, Ca2+ remain in resin and Na+ in the water
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Removal of Limescale

Limescale is calcium carbonate and can be dissolved by acid (what commercial descalers)

  • carbonates react with many acids to produce a soluble salt, water and carbon dioxide
  • limescale should be removed from a kettle using a weak acid so it doesn't react with the electric element in the kettle
  • the H+ ion from the acid reacts with the calcium carbonate
    • CaCO3 + 2H+ ---> Ca2+ + H2O + CO2
  • you can use vinegar (ethanoic acid 2CH3COOH) to remove limescale
    • calcium carbonate+ethanoic acid---> calcium ethanoate+water+carbon dioxide
    • CaCO3 + 2CH3COOH ---> Ca(CH3COO)2 + H2O + CO2
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Method for testing hardness

1. add 100cm3 of water into a conical flask

2. add 1cm3 soap solution to the water, put in a bung and shake

3. repeat until a good lasting lather (1cm deep) has formed

4. record how much soap was needed

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Fats and oils

  • animal fats and oils= lard, blubber, ghee and cod liver oil
  • plant fats and oils= walnut oil, coconut oil, olive oil and soya oil
  • fats are solid and oils are liquid at room temperature
  • fats and oils are esters, an ester is what you get when you react an acid with an alcohol
  • fats and oils are produced when glycerol reacts with fatty acids
  • natural fats and oils are important raw materials for the chemical industry e.g. paints, machine lubricants, detergents and cosmetics 
  • they can be used as alternatives to chemicals made from crude oil
  • they are esters joined together by ester linkages
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Emulsions

Oils don't mix with water- they are immiscible

you can mix oil with water to make an emulsion,

you have to shake the two liquids vigorously, this will break up the oil into very small droplets which disperse in water

Milk is a oil-in-water emulsion there is less oil than water

Butter is a water-in-oil emulsion, there is a lot more oil than water

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Biodiesel

Vegetable oils such as rapeseed oil and soybean oil can be processed and turned into fats

vegetable oil provides a lot of energy

biodiesel has similar properties to ordinary diesel fuel- it burns in the same way and can be used instead of diesel

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Making Soap

Vegetable oils react with alkali to make soap

natural fats and oils are boiled up with sodium hydroxide, the hot sodium hydroxide splits up the fats and oils to produce soap and glycerol

process= saponification

the chemical reaction first breaks up the fat or oil to release glycerol and fatty acids- hydrolysis then the fatty acids react with the sodium hydroxide to make soap

word equation= fat + sodium hydroxide ----> soap + glycerol

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Unsaturated Oils

  • Oils and fats contain long-chain molecules with lots of carbon atoms
  • oils and fats are either saturated or unsaturated
  • unsaturated oils contain double bonds between some of the carbon atoms
  • C=C double bonds can be detected by reacting bromine-
    • an unsaturated oil will decolourise bromine water
  • Monounsaturated fats contain one C=C double bond
  • Polyunsaturated fats contain more than one C=C double bond
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Unsaturated oils can be Hydrogenated

  • unsaturated vegetable oils are liquid at room temperature
  • can be hardened by reacting them with hydrogen in the presence of a nickel catalyst at about 60'C- hydrogenation, the hydrogen reacts with the double bonds
  • hydrogenated oils have higher melting points so are more solid at room temperature- used for making cakes and spreads
  • margarine is made from partially hydrogenated vegetable oil
    • most of the double bonds are single to make it spreadable
    • if all double bonds were single it would be difficult to spread and would have the wrong consistency
  • partially hydrogenated vegetable oils are used instead of butter in processed foods e.g. biscuits, because they are a lot cheaper and can be kept longer
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Vegetable oil affects on health

  • vegetable fats and oils tend to be unsaturated while animal fats tend to be saturated
  • saturated fats are less healthy because they block up arteries with build up of cholesterol and put you at risk of heart disease
  • natural unsaturated fats such as olive oil and sunflower oil reduce the amount of blood cholesterol
  • partially hydrogenated vegetable oil increases the amount of 'bad' cholesterol in the blood which decreases the amount of 'good' cholesterol
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Drugs

Drugs are externally administered

they are taken into our bodies by swallowing, injecting or inhaling

once a drug is inside our body it modifies or affects one or more of the chemical reactions in the body

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Analgesics

Analgesics are painkillers e.g. aspirin, paracetamol and ibuprofen

the chemicals used to make analgesics must be very pure because any impurity could cause unwanted and dangerous side effects

aspirin= 9 carbons, 8 hydrogens, 4 oxygens, C9H8O4

Paracetamol= 8 carbon, 9 hydrogen, 2 oxygen, 1 nitrogen, C8H9O2N

Ibuprofen= 13 carbon, 18 hydrogen, 2 oxygen, C13H18O2

similarities:

  • all have a benzene ring
  • paracetamol and aspirin have a -COCH3 group
  • aspirin and ibuprofen have a -COOH group
  • paracetamol is different as it contains a nitrogen atom and the OH group is attached directly to the benzene ring
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Dangers of painkillers

aspirin overdose:

  • can lower blood pressure and raise heart rate and can cause breathing problems
  • can irritate the stomach causing nausea, vomiting and internal bleeding
  • can be fatal

paracetamol overdose:

  • causes liver damage
  • 20-30 tablets can be fatal
  • takes 4-6 days to show symptoms
  • cannot be treated by doctors if too late
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Manufacturing Aspirin

Was originally discovered in willow bark but is now synthetically made

its made using salicylic acid,  equation:

salicylic acid + ethanoic anhydride ----> aspirin + ethanoic acid

C7H6O3 + C4H6O3 -----> C9H8O4 +C2H4O2

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Soluble aspirin

Soluble aspirin is an ionic compound which is more soluble in water because of its positive and negative charges

it is easier to take as it can be drunk

its is absorbed into the blood quicker so will give speedier relief of symptoms

Aspirin has non- analgesic effects:

  • reduces temperature of body quickly
  • can thin blood- prevent strokes and heart attacks
  • aggravates stomach ulcers
  • can cause headaches, dizziness and ringing in the ears
  • can get reyes syndrome
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Comments

Gemz

Thankyou very much this was great! I was just wondering, did this cover the whole OCR sylabus? Gemz :)

UP THE BORO

Gemz wrote:

Thankyou very much this was great! I was just wondering, did this cover the whole OCR sylabus? Gemz :)

what do you think idiot

NoirRose

Tad bit hash :(

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