Hydrogen: burns with a 'pop' when lit using a lighted splint
Oxygen relights a glowing splint
Electrolysis:
Electrolysis is the decomposition of a liquid by passing an electric current through it
It is the flow of charge by moving ions. Moving ions discharge at electrodes - the cathode (-) and anode (+)
Electrolytes are ionic, charge moves through the molten liquid through the movement of ions
If the electrolyte solidifies then the ions are fixed and cannot move then the current can't flow
Positive ions (cations) move towards and discharge at the cathode
Negative ions (anions) move towards and discharge at the anode
Products of electrolysis:
Sodium hydroxide, NaOH: hydrogen at the cathode, oxygen at the anode
Sulphuric acid, H2SO4: hydrogen at the cathode, oxygen at the anode
Anode: 4OH- - 4e- → O2 + 2H2O
Cathode: 2H+ + 2e- → H2
Electrolysis of sodium chloride solution:
Copper (II) sulphate solution, CuSO4: copper is formed at and plates the cathode, oxygen is formed at the anode and the blue colour will slowly disappear
The reaction at the cathode (-) can be written as: Na+ + e- → Na
Here the sodium chloride splits up into ions, so the ions can move.
The positive Na+ ions migrate towards the negative cathode
Each Na+ ion gains one extra electron from the cathode
The reaction at the anode (+) can be written as: 2Cl- - 2e- → Cl2
Here, the negative Cl- ions migrate to the positive anode
Cl- ions are discharged as chlorine gas
Two Cl- ions each gain one electron and combine to form a chlorine molecule
Energy Transfers - fuel cells:
A fuel cell is a cell supplied with fuel and oxygen that uses the energy from the reaction between the fuel and oxygen to produce electrical energy efficiently
Fuel cells are used in spacecraft: - water produced is not wasted, the astronauts drink it - lightweight - compact - sturdy
Fuel cells in car industry are good because: - no CO2 emissions - less pollution - they use hydrogen which is widely available - there is a direct energy transfer with hydrogen-oxygen fuel cells - they weigh less than current car batteries
Disadvantages: - they use poisonous catalysts which have to be disposed of - hydrogen and oxygen production requires fossil fuels being burnt
Hydrogen-oxygen fuel cells - equations:
Hydrogen fuel cell: 2H2 + O2 → 2H2O
This is a exothermic reaction giving out heat, and chemical energy is converted directly to electrical energy
At negative electrode: 2H2 → 4H+ + 4e- this is oxidation loss of electrons
At positive anode: 4H+ + O2 + 4e- → 2H2O this is reduction gain of electrons
Redox reactions:
A reaction where electrons are gained and lost is a redox reaction
Rusting is a redox reaction, because iron loses electrons and oxygen gains electrons: iron + oxygen + water → hydrated iron (III) oxide
Galvanising stops material from rusting with a layer of zinc added to stop water and oxygen reaching it
Oxidations is losing electrons
-Reduction is gaining electrons +
An oxidising agent gains electrons from another substance and oxidises it
A reducing agent transfers electrons to another substance, reducing it
Displacement reactions:
In displacement reactions the more reactive metal swaps places with the less reactive metal
Reactivity series is needed to work out displacement reactions, e.g.: - Magnesium (most reactive) - Zinc - Iron - Tin (least reactive)
Methods of preventing rust:
Oil and grease
Paint
Galvanising
Sacrificial protection (e.g. coating iron with magnesium which is more reactive and will lose electrons instead of iron)
Alloying
Tin plate (which acts as a barrier. However, when it is scratched, the iron will lose electrons instead of the tin coating because iron is more reactive than tin)
Alcohols:
Ethanol C2H5OH is made by fermentation and hydrations
Fermentation is catalysed by the enzymes in yeast, if there is no oxygen present. The dilute liquid produced undergoes fractional distillation to produce ethanol
The temperature must be high enough for the enzymes to be active but not too high, as they can denature (so 25 °C - 50 °C is used). If air is present, ethanoic acid is produced instead of ethanol
Alcohols have the formula CnH2n+1OH, where n is the number of carbon atoms in the chain
Hydration is another way of making ethanol: ethene + water → ethanol C2H4 + H2O → C2H5OH
Ethanol is useful for alcoholic beverages, solvents and fuels for cars
Here are the advantages and disadvantages of producing ethanol with each method:
Hydration
Fermentation
Type of raw materials
Fossil fuels - non-renewable
Sugar from plants - renewable
Type of process
Continuous
Batch
Labour
Few workers needed
A lot of workers needed
Rate of reaction
Fast
Slow
Conditions needed
High temperature and pressure
Low temperature (35 °C), normal pressure
Purity of product
Pure
Impure - needs treatment
Energy needed
A lot
Little
CFCs:
Chlorofluorocarbons (CFCs) were originally thought to be totally safe, but we now know what they move upwards and attack the ozone layer
CFCs are organic molecules containing chlorine, fluorine and carbon atoms
CFCs can be replaced with alkanes or hydrofluorocarbons (HFCs) which don't damage the ozone layer
They are chemically inert, have low boiling points and are insoluble in water
The ozone layer:
The ozone layer, in the stratosphere, absorbs most high wavelength ultraviolet (UV) radiation from the sun. This is essential for humans as increased levels of UV light can increase the risk of sunburn, skin cancer, cataracts, and can accelerate the ageing of the skin
In the stratosphere, CFC molecules are broken down by the sun's UV radiation which breaks a bond, creating two chlorine radicals; Cl·
These radicals start a chain reaction: They react with ozone (O3, a form of oxygen), which creates more chlorine radicals. Each radical can break down more than 100,000 ozone molecules, they are stopped with termination reactions such as Cl· + Cl· → Cl2
Symbol equations for the chain reaction are: - Cl· + O3 → OCl· + O2 - OCl· + O3 → Cl· + 2O2 These combine to form 2O3 → 3O2; ozone breaking down into oxygen
CFCs can last over 20 years before they are broken down by UV radiation, so they will continue to deplete ozone, even though they are now banned
Hardness of water:
Rainwater can be slightly acidic, containing dissolved carbon dioxide
If rainwater flows over rock and dissolves some of it, it can become hard - hardness is caused by dissolved calcium and magnesium ions in water
Calcium carbonate, such as from chalk, marble and limestone react with water and carbon dioxide, forming calcium hydrogencarbonate, which dissolves to form temporary hard water
When calcium sulfate rock dissolves, the water becomes permanently hard
To measure water hardness, the amount of soap required to produce a lather can be measured. If a large amount is required, the water is harder
However, both hard and soft water lather well with soapless detergents
When a soap flake is shaken in water, calcium ions in the water react with the soap to turn it into scum. Eventually, all calcium ions have reacted, and any further soap produces a stable lather
Softening water:
Boiling can be used to remove temporary hardness; calcium hydrogencarbonate decomposes in hot water to form calcium carbonate (limescale), water and carbon dioxide: Ca(HCO3)2 → CaCO3 + CO2 + H2O
Heating also remove soluble calcium ions from the water, changing them to insoluble calcium carbonate
Calcium sulfate is too stable to be removed by heating
However, permanent hardness can be removed by ion-exchange columns: water flows over beads of solid resin, which trap calcium ions, exchanging them for sodium ions, removing temporary and permanent hardness
Washing soda (sodium carbonate - Na2CO3) can soften temporary and permanent hard water; when it dissolves it reacts with calcium sulfate in water to form insoluble calcium carbonate, locking up the calcium ions: CaSO4 + Na2CO3 → Na2SO4 → Na2SO4 + CaCO3
Fats and oils:
Natural fats and oils are important raw materials for the chemical industry
Vegetable oils can be used to make bio-diesel, an alternative to fuel diesel produced from crude oil
Oils are liquids at room temperature, but fats are solids
Fats and oils are esters, they have chains of carbon atoms
If there are some double bonds bonding carbon atoms, then it is unsaturated. If there are only single bonds between carbon atoms, the compounds are saturated
Bromine water is orange, but it turns colourless when mixed with unsaturated compounds, in an addition reaction with double bonds, forming a dibromo compound which is colourless
Vegetable oils can be used to make margarine. They are unsaturated. They are 'hardened', making them saturated: - Hydrogen is bubbled through the oil at around 200 °C with a nickel catalyst - This hydrogen reacts with the carbon-carbon double bonds turning them into single bonds
Saturated fats and oils come from animals, unsaturated often come from plants
By having a diet rich in unsaturated fats and oils, the chance of heart disease is reduced as there is less cholesterol which can cause it
Compounds with more than one carbon-carbon double bond are called polyunsaturated
Oil and water do not dissolve in each other, but disperse into small droplets to form an emulsion. Milk is an oil-in-water emulsion, cold cream and margarine are water-in-oil emulsions
Saponification:
Saponification is splitting up natural oils with alkalis
To make soap and glycerol, fats and oils can be split using hot sodium hydroxide: - fat + sodium hydroxide → soap + glycerol
Here, the ester forms one glycerol molecule and three soap molecules. This is a hydrolysis reaction, a reaction of alkaline water
Washing powder ingredients:
Active detergent to do the cleaning
Water softener to soften hard water
Bleaches to remove coloured stains
Optical brighteners to make clothes whiter
Enzymes to remove food stains
Washing-up liquid ingredients:
Active detergent to do the cleaning
Water to thin out the detergent so it can be dispensed easily
Colouring agent and fragrance to improve attractiveness of product
Rinse agent to help water drain off washed items
Low temperature washing:
Washing clothes at 40 °C is better for the environment than at higher temperatures
Additionally, many dyes are damaged easily by high temperatures
Detergents:
Detergents lift grease stains off into water
They have a hydrophilic head (attracted to water/dissolving in water) and hydrophobic tail (intermolecularly bonding in/dissolving in grease)
Detergents form strong intermolecular bonds with the oil, and water, so it can be lifted off the cloth
The hydrophobic part forms strong intermolecular forces with the grease, and eventually the grease has so many detergents it is lifted off the cloth
Dry cleaning:
Fabrics which will be damaged if washed in water must be dry cleaned, using an organic solvent. It is dry because the liquid is not in water
How it works: - Grease molecules join other grease molecules with weak intermolecular forces - Solvent molecules join other solvent molecules with weak intermolecular forces - However, solvent molecules form strong intermolecular forces with grease molecules, so the solvent surrounds the grease