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What is a redox reaction?
- Oxidation is the loss of electrons
- Reduction is the gain of electrons
- Reduction and oxidation can happen at the same time
- An oxidising agent accepts electrons and gets reduced
- A reducing agent donates electrons and gets oxidised
What are two examples of redox reactions?
Explain the chlorine redox reaction.
- Chlorine gas is passed into a solution of an iron(II) salt
- The solution turns from green to yellow as iron(II) is oxidised to iron (III)
Explain the iron redox reaction.
- Iron atoms are oxidised to iron (II) ions when they react with dilute acid
- The iron atoms are oxidised by the hydrogen ions
- The hydrogen ions are reduced by the iron atoms
What is the displacement rule?
A more reactive metal will displace a less reactive metal from its compound
What do displacement reactions involve?
- One metal kicking another one out of a 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
In displacement reactions, what part is always reduced and what part is always oxidised?
- The metal ion is reduced
- The metal atom is oxidised
What type of reaction is the rusting of iron?
- A redox reaction
- Iron loses electrons when it reacts with oxygen
- Oxygen gains electrons when it reacts with iron
Describe the process of rusting iron.
- Rusting only happens when the iron is in contact with BOTH oxygen and water
- Rust is a form of hydrated iron (III) oxide
What is the word equation for rusting?
iron + oxygen + water -- hydrated iron (III) oxide
How can rusting be prevented using metals?
- Mixing iron with other metals to make alloys
- Steels are alloys of iron with carbon and small quantities of other metals
- One of the most common steels is stainless steel - a rustproof alloy of iron, carbon and chromium
How can rusting be prevented using paint?
- It acts as a barrier
- Keeps out the water, oxygen or both
- Ideal for large and small structures
- Can also make it look nice
How can rusting be prevented using oil or grease?
- Acts as a barrier
- Keeps out the water, oxygen or both
- Has to be used when moving parts are involved
- E.g. bike chains
How can rusting be prevented using tin?
- Tin painting is where a coat of tin is applied to the object
- E.g. food cans
- Acts as a barrier
- Stops water and oxygen from reaching the surface of the iron
- Only works as long as the tin remains intact
- If tin is scratched the iron will lose electrons in preference to the tin and the iron will rust even faster than if it was on its own
- Not always a good idea to buy reduced bashed tins of food as they could be starting to rust
How can rusting be prevented using zinc?
- Galvanising is where a coat of zinc is put onto the object
- The zinc acts as sacrificial protection - it's more reactive than iron so it'll lose electrons in preference to iron
- Zinc acts as a barrier
- Steel buckets and corrugated iron roofing are often galvanised
How can rusting be prevented using magnesium?
- Blocks of metal (e.g. magnesium) can be bolted to the iron
- Magnesium will lose electrons in preference to iron
- It's used on the hulls of ships, or on underground iron pipes
What is a sacrificial method?
Where you react a more reactive metal with the iron (less reactive)
What is electrolysis?
- The breaking down of a substance using electricity
- Electric current is passed through a molten or dissolved ionic compound, causing it to decompose
- This creates a flow of charge through the electrolyte
- The positive ions move towards the cathode and gain electrons
- The negative ions move towards the anode and lose electrons
- As the ions gain or lose electrons they become atoms or molecules and are discharged from the solution at the electrodes
What are the factors of an aqueous solution?
- Ions from the solute (the ionic compound)
- Hydrogen ions
- Hydroxide ions
- Hydrogen and hydroxide are from the water
If the ions have been discharged from the water, rather than the solute, what would the cathode and the anode produce?
- Cathode would produce hydrogen
- Anode would produce oxygen
If copper (II) sulfate is electrolysed what does it form?
Describe what happens when copper (II) sulfate is electrolysed.
- The cathode starts as a piece of carbon and gets coated with a layer of copper
- The copper ions are easier to discharge from the solution than the hydrogen ions
- They are attracted to the negative anode and are reduced to copper atoms
- the hydroxide ions are oxidised to oxygen and water at the anode
- Pure copper atoms bond to the cathode and from a think layer opf copper over the surface of the carbon electrode
Can ionic solids be electrolysed?
- The ions are in fixed positions and can't move
Can molten ionic compounds be electrolysed?
- The ions can move freely and conduct electricity
- Always broken up into their elements
- Positive metal ions are reduced
- Negative metal ions are oxidised
Give the product produced at cathode and the half equation at cathode for the molten electrolyte 'lead iodide'.
Give the product produced at anode and the half equation at anode for the molten electrolyte 'lead iodide'.
Give the product produced at cathode and the half equation at cathode for the molten electrolyte 'potasssium chloride'.
Give the product produced at anode and the half equation at anode for the molten electrolyte 'potassium chloride'.
- 2Cl- -- Cl2 + 2e-
Give the product produced at cathode and the half equation at cathode for the molten electrolyte 'aluminium oxide'.
- Al3+ + 3e- -- Al
Give the product produced at anode and the half equation at anode for the molten electrolyte 'aluminium oxide'.
What increases the number of electrons transferred?
What is current?
- A flow of charge, and it's charge that determines how much product is formed during electrolysis
- More charge means more product
- Generally, charge is equal to current multiplied by time in seconds
- This means charge and amount of product created during electrolysis are directly proportional to the time taken and the current used
What to hydrogen and oxygen react to produce?
What is a fuel cell?
An electrical cells that's supplied with a fuel and oxygen and uses energy from the reaction between them to produce electrical energy efficiently
What is the hydrogen-oxygen fuel cell?
- Combines hydrogen and oxygen to produce head energy and nice clean water
- There are no pollutants to worry about
Describe the redox reaction hydrogen-oxygen fuel cells use.
- The electrolyte is often a solution of potassium hydroxide
- Electrodes often porous carbon with a catalyst
- Hydrogen goes into the anode compartment and oxygen goes into the cathode compartment
- At the cathode the oxygen is reduced
- At the anode the hydrogen is oxidised
- The electrons flow through an external circuit from the anode to the cathode - this is the electric current
- The overall reaction is hydrogen plus oxygen which gives water
What are the advantages of hydrogen-oxygen fuel cells?
- Much more efficient than power stations or batteries at producing electricity
- If heat produced is used, efficiency can be greater than 80%
- Electricity generated directly from the reaction
- Direct energy transfer
- Not a lot of stages so fewer places for energy to be lost as heat
- No moving parts - energy isn't lost through friction
- Only product is water - no pollution
What are the disadvantages of hydrogen-oxygen fuel cells?
- Producing the hydrogen needed to powwe the fuel cell requires a lot of energy
- Energy may have come from burning fossil fuels which causes pollution
- Hydrogen fuel cells often contain poisonous catalysts which eventually have to be disposed of
- Getting rid of the catalyst takes a lot of time and money and may cause environmental problems
How are hydrogen-oxygen fuel cells used in spacecraft?
- Provide electrical power
- Readily available from the rocket fuel tanks
- Lightweight and compact so they don't take up valuable room
- They don't have moving parts
- Some of the product is used as drinking water
- No other waste products or pollutants
What is the link between the car industry and fuel cells?
- The car industry is developing fuel cells to replace any conventional petrol/diesel engines
- Don't produce any conventional pollutants
- As hydrogen can be obtained by decomposing water, there is a lot left over for fuel
What are CFCs?
- Organic molecules containing carbon, chlorine and fluorine
- E.g. dichlorodifluoromethane
- Chemically inert (unreactive)
- Insoluble in water
- Low boiling points
What were CFCs used for?
- Coolants in fridges
- Air-conditoning systems
- Propellants in aerosol spray cans
When did scientists find chlorine helps destroy the ozone?
When did scientists find evidence of decreasing ozone levels in the atmosphere over Antartica?
When did some countries ban CFCs as aerosol propellants and what were they?
When were CFCs banned completely?
After the ozone hole was discovered
What is a free radical?
- When a covalent bond breaks evenly, each atom gets one of the shared electrons
- E.g. H-H -- H· + H·
- The H· is the free radical
What do Chlorine free radicals do?
Damage the ozone layer
What free radical process happens in the stratopshere?
The ultraviolet light causes the carbon-chlorine bonds in CFCs to break to form free radicals
In the free radical process, how do we make ordinary oxygen molecules and chlorine oxide?
Reacting the chlorine free radicals with the ozone
What is a key property of chlorine oxide?
It is very reactive
What are the products of a reaction between chlorine oxide and ozone?
- Two oxygen molecules
- A chlorine free radical
Do CFCs directly attack the ozone?
CFCs do not attack ozone directly. They break up and form chlorine atoms (chlorine free radicals) which attack ozone. The chlorine atoms aren't used up, so they can carry on breaking down ozone.
Are CFCs reactive?
No, not very
Can CFCs be removed from the stratosphere easily?
- They take a long time to be removed
What are some safe alternatives to CFCs?
Why are alkanes a better alternative to CFCs?
They don't react with ozone
What is a HFC?
- Compounds very similar to CFCs
- Contain no chlorine
Why are HFCs a better alternative to CFCs?
- They don't contain chlorine
- They don't therefore attack the ozone
What does hard water make?
What is limescale?
- Calcium carbonate
- Form on the insides of pipes, boilers and kettles
- Thermal insulator
Why is it bad for a kettle to have a build up of limescale?
It takes longer to boil
Why is it bad for pipes to have a build up of limescale?
Eventually, if the build up gets too bad, the pipes can get blocked
What is hardness of water caused by?
- Calcium ions
- Magnesium ions
- Or both
What does magnesium sulfate do?
- Dissolve in water
- Calcium sulfate does too but only a little bit
What does calcium carbonate do?
- Commonly exists as chalk, limestone or marble
- Doesn't dissolve in water
- Will react with acids
- Rain water is slightly acidic so it can react with it
Why is rainwater slighly acidic?
- The carbon dioxide from the air dissolves in it
- Forms carbonic acid
- CO2 + H2O -- H2CO3
What do calcium carbonate and rainwater form?
- Calcium hydrogencarbonate
- H2CO3 + CaCO3 -- Ca(HCO3)2)
- Carbon dioxide + water + calcium carbonate -- calcium hydrogencarbonate
How can temporary hardness be removed?
What causes temporary hardness?
What causes permanent hardness?
Dissolved calcium sulfate (among other things)
What happens when boiling temporary hardness?
The calcium hydrogencarbonate decomposes to form insoluble CaCO3
How can both types of hardness be removed?
- Adding washing soda - sodium carbonate
- Ion exchange resin
How does sodium carbonate remove the hardness?
Carbonate ions join onto calcium ions and ake insoluble precipitate of calcium carbonate
How does ion exchange resin remove the hardness?
Loads of sodium ions (or hydrogen ions) whihc it exchanges for calcium or magnesium ions
What is the method for comparing the hardness of water samples?
- Add 100cm3 of water to a conical flask
- Add 1cm3 soap solution to the water
- Put a bung in and shake
- Repeat until a good lasting lather is formed
- Record how much soap was needed
What is a good lasting lather?
One where the bubbles cover the surface for at least 30 seconds
What is the general formula for alcohol?
What is fermentation used to produce?
- Converts sugars into ethanol
How if the fermentation reaction brought about?
By enzymes found in yeasts
What is the ideal temperature for fermentation?
- If too cold, the yeast is inactive, so reaction is really slow
- If too hot, the enzymes in the yeast are denatured
- The optimum temperature is between 25oC and 50oC
Why is it important to prevent oxygen from reaching the alcohol?
Oxygen converts ethanol to ethanoic acid (which is what you get in vinegar)
Once the reaction stops, how can we get pure ethanol?
It can be distilled using fractional distillation
How does industry make ethanol?
By hydrating ethene
How does hydrating ethene produce ethanol?
Ethene will react with steam to produce ethanol
What are the perfect conditions for producing ethanol by hydrating ethene?
- Temperature of 300oC
- Pressure of 70 atmospheres
- Phosphoric acid catalyst
Fermentation VS hydration
Fermentation is usually a batch process which is slow and inefficient. Ethene hydration uses a continuous process so the ethanol is made more quickly. Hydration requires much harsher reaction conditions so is a more expensive process to run
Fermentation VS hydration
The ethanol made by fermentation is a renewable fuel. It's made from renewable resources so won't run out. The ethene produced in the hydration reaction is a non-renewable fuel. It's produced from crude oil, which will one day run out
Fermentation VS hydration
The ethanol made by fermentation isn't very pure and has to be purified by distilled before it's used. The ethanol made by hydration is of much higher purity
Fermentation VS hydration
Fermentation has a lower atom economy than hydration as not all of the atoms in the reactants are used to make the ethanol
Fermentation VS hydration
The yield of a hydration reaction is very low but by recycling any unused reactants you can achieve yields of up to 95%. The yields achieved by using fermentation are much lower
What do animal fats and oils include?
- Lard (pork fat)
- Blubber (whale fat)
- Ghee (butter oil)
- Cod liver oil
What do plant fats and oils include?
- Walnut oil
- Coconut oil
- Olive oil
- Soya oil
At room temperature, what state is a fat?
At room temperature, what state is an oil?
What is an ester?
What you get when you react an acid with an alcohol
What makes fats and oils esters?
They are made when glycerol (alcohol) reacts with fatty acids (acid)
What can the natural fats and oils be used for in chemical industry?
- Machine lubricants
What can natural fats and oils be used as an alternative for?
Chemicals made from crude oil
Give an example of an oil-in-water emulsion.
Give an example of a water-in-oil emulsion.
- More oil than water
Give two examples of vegetable oils.
Why are vegetable oils used for fuels?
- Vegetable oil provides a lot of energy
- Biodiesel is an example of a vegetable oil fuel
What are some properties of biodiesel?
- Similar to those of diesel
- Burns in the same way
- Can be used as an alternative to diesel fuel
How do vegetable oils make soap?
They react with an alkali
How do natural fats and oils make soap?
- Boiled up with sodium hydroxide
- Hot sodium hydroxide spits up the fats and oils to produce a soap and glycerol
What is the process where natural fats and oils make soap called?
What is the stage when the chemical reaction breaks up the fat or oil to release glycerol and fatty acids called?
- Means breaking apart with water
What is the word equation of making soap?
fat + sodium hydroxide -- soap + glycerol
What is the only thing saturated oils and fats have?
Single C-C bonds
What do unsaturated oils and fats contain?
At least one C=C double bond in their carbon chains
What is a monousaturated fat?
Contains one C=C double bond somewhere in their carbon chains
What is a polyunsaturated fat?
Contain more than one C=C double bond
How can we detect C=C double bonds?
Using bromine water
What will happen to the bromine water if there is an unsaturated fat or oil?
- It will decolourise
- An addition reaction takes place at the double bond and a colourless dibromo compound is formed
What will happen to the bromine water if there is a saturated fat or oil?
- They don't have any double bonds so they won't react
- Bromine water will stay orange
How can we harden unsaturated vegetable oils?
Process called hydrogenation
What is the process of hydrogenation?
- Unsaturated fats react with hydrogen in the presence of a nickel catalyst at about 60oC
- The hydrogen reacts with the double-bonded carbons and opens out the double bonds
What is margarine made out of?
- Partially hydrogenated vegetable oil
- Turning all the double bonds into single bonds would make it too hard and not easy to spread
What tends to be unsaturated?
What tends to be saturated?
- Unsaturated - vegetable fats and oils
- Saturated - Animal fats and oils
Why are saturated fats less healthy than non-saturated fats?
- They increase the amount of cholesterol in the blood
- This can block up the arteries and increase the risk of heart disease
- Natural unsaturated oils reduce the amount of cholesterol in the blood
What effect do partly hydrogenated vegetable oils have on cholesterol?
- They decrease the 'bad' cholesterol
- They decrease the 'good' cholesterol
- Eating these foods can increase heart disease
How does dry cleaning remove stains?
How does a solvent work?
- There are weak intermolecular forces between the solvent molecules
- There are also weak intermolecular forces between the molecules of grease
- When the solvent is applied to the clothes, intermolecular forces are formed between the solvent and grease molecules so the grease molecules are surrounded by molecules of solvent
- When the solvent is removed, the grease is removed with it and the clothes are left squeaky clean
What do biological detergents contain?
- Enzymes which are biological catalysts
- Help break down some of the larger insoluble molecules into smaller soluble molecules which can be easily removed
What are the perfect conditions for biological detergents?
- Lower temperatures
- Turning down the washing machine from 40oC to 30oC decreases the energy by about 40%
- At higher temperatures, the enzymes can be denatured
- Don't work as well at temperatures above 40oC
- Cooler temperature means you can wash more delicate clothes too