C6 CHEMISTRY OUT THERE

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  • Created by: Ellie
  • Created on: 11-05-13 15:33

ELECTROLYSIS

  • the charge moves through the electrolyte by movement of ions
  • if the electrolyte solidifies then the ions can't move so the current doesn't flow
  • Electrolysis: the flow of charge by moving ions
  • positive ions go to cathode and gain electrons, negative ions go to anode and lose electrons
  • the amount of substance formed in electrolysis increases as time or currrent increases
  • electrolysis of NaOH or H2SO4 are: C = H2 and A = 2H20 + O2
  • ^ in the electrolysis of NaOH, hydrogen  is discharged instead of sodium because sodium is higher up in the reactivity series
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ENERGY TRANSFERS

fuel cells in a spaceshipwater not wasted - drunk by astronauts, save carrying batteries which are heavy, they are compact, there are no moving parts.

fuel cells in a carno carbon emission, renewable and less polution, main product of hydrogen fuel cell is water - water is not a pollutant, large source of hydrogen available by decomposing water.

Advantages of fuel cells: direct energy transfer, useful mobile energy sources, fewer production stages so less energy wasted, less polluting, last longer than rechargeable batteries, weigh less

Disadvantages of fuel cells: poisonous catalysts are used that have to disposed of, fossil fuels burnt to produce oxygen and hydrogen needed

  • when fuel reacts with oxygen, energy is released - exothermic 
  • fuel cell converts chemical energy into electrical energy directly so no heat is given out
  • reaction in hydrogen-oxygen fuel cell: 2H2 + O2 --> 2H2O
  • when the fuel reacts some of the chemical energy is given out, this is shown in an energy-level diagram
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REDOX REACTIONS

Redox reaction: where electrons are gained and lost (eg rusting)

  • rusting: iron + oxygen + water --> hydrated iron(III) oxide
  • galvanising protects iron from rusting by covering it with a thin layer of zinc (zinc = sacrificial)

An oxidising agent takes electrons away so the substance is oxidised, a reducing agent pushes electrons onto another substance so it gains.

  • redox reactions can be pushed in either direction - oxidation or reduction
  • iron is normally oxidised but is sometimes reduced
  • iron ions can be forced either way
  • chlorine is like oxygen in that it is normally reduced
  • Reactivity series is needed to work out displacement reactions, the most reactive displaces the less reactive
  • displacement reaction: reactive metal element + less reactive metal compound --> reactive metal compound + less reactive metal element
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ALCOHOLS

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

Ethanol is made by the fermentation of glucose solution:

  • reaction catalyzed by enzymes in yeast (only happen if no oxygen is present, if it is then ethanoic acid is made instead)
  • dilute liquid is produced which undergoes fractional distillation to produce ethanol
  • if temp is too low, enzymes in yeast are inactive, if temp too high, enzymes are denatured

Formula = CnH2n+1OH

  • ethanol used for fuel is renewable because the plants used for the fermentation process are fast-growing
  • ethanol used in industry is made from ethene which is made from fossil fuels so is non-renewable, it is a hydration reaction
  • hydration reaction: ethene + water --> ethanol
  • C2H4 + H2O --> C2H5OH
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ALCOHOLS 2

Advantages and disadvantages of ethanol production:

Fermentation:

  • less energy needed
  • batch processing
  • sustainable
  • fractional distillation needed to purify
  • low yield and atom economy

Hydration:

  • more energy needed for hot catalyst
  • continuous processing
  • finite resource
  • relatively pure on production
  • high yield and atom economy
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DEPLETION OF THE OZONE LAYER

  • the ozone layer is in the stratosphere - here it absorbs most of the ultraviolet radiation from the sun - the more depleted the ozone layer becomes, the more UV light can get to earth
  • UV radiation has the right frequency to make the ozone molecules vibrate, energy from UV radiation is converted into movement energy inside each molecule, the thicker the ozone layer = more UV radiation absorbed

CFC's:

  • can be replaced with alkanes or HFC's (don't contain chlorine)
  • UV radiation breaks down CFC molecules to give highly reactive chlorine radicals, these then react with ozone molecules to turn the ozone back into oxygen gas, chlorine atoms are regenerated so they can react with more ozone molecules
  • when a covalent bond breaks it splits into equal halves 
  • each radical sets off a chain reaction:
  • 1. UV breaks a bond in CFC molecules to form chlorine radicals
  • 2. chlorine radicals react with ozone molecules giving more chlorine radicals    2O3 --> 3O2
  • 3. one possible termination reaction is Cl + Cl --> Cl2
  • CFC's take 20-50 years before being broken down
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HARDNESS OF WATER

Temporary hardness is formed when a calcium carbonate (chalk, limestone and marble for example) and carbon dioxide dissolve into rainwater to form calcium hydogen carbonate

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

Permanent hardness is produced when calcium sulfate rock dissolves.

  • Water hardness can be measured by how much soap is needed to produce a lather, the more soap needed for all the calcium ions to react so a lather can be formed, the harder the water is
  • temporary hardness can be removed by boiling because calcium hydrogencarbonate decomposes easily (thermal decomposition: Ca(HCO3)2 --> CaCO3 + CO2 + H2O)
  • if water is boiled, and the soap experiment is done, if less soap is needed compared to before boiling then the hardness was temporary
  • permanent hardness is not affected by boiling because calcium sulfate is too stable
  • ion-exchange resins remove both types of hardness because the water flows over beads of solid resin that trap the calcium ions on its surface and exchanges them for sodium ions
  • washing powder can soften both types of water hardness
  • CaSO4 (from hard water) + Na2SO3 (from washing powder) --> Na2SO4 + CaCO3
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NATURAL FATS AND OILS

  • if all carbon atoms are linked by single bonds, the compound it saturated
  • if there is one carbon-carbon double bond, the compound is unsaturated, if there is more than one double bond, the compound is polyunsaturated
  • to test for unsaturation, shake oil with bromine water, unsaturated will decolourise because double bonds react with bromine molecules to make a dibromo compound (colourless), saturated will stay orange because they lack carbon-carbon bonds

Making margerine from vegetable oil:

  • harden the oils turning them into saturated compounds
  • hydrogen bubbled through oil at 200c using nickel catalyst
  • hydrogen reacts with carbon-carbon double bonds and turns them into single bonds

Oil and water = immiscible liquids, they don't dissolve in each other, just disperse into tiny droplets to form an emulsion (milk = oil in water, margerine = water in oil)

Saponification: reaction splitting up natural oils with alkalis (fat + sodium hydroxide --> soap + glycerol) - ester forms one glycerol molecule and 3 soap molecules = hydrolysis reaction

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DETERGENTS

  • washing clothes at low temp is good because dyes are damaged at high temps and it's better for the environment

How a detergent works:

  • form strong intermolecular bonds with both oil and water
  • hydrophillic forms bonds with water molecules
  • hydrophobic forms bonds with molecules of oil and grease
  • contains enzyme that denatures at high temp

Dry cleaning: dry-cleaning machines use organic solvents to wash clothes, not water

How dry cleaning works:

  • molecules of grease form bonds with other molecules of grease
  • molecules of solvent form bonds with other molecules of solvent
  • molecules of water are held together by strong intermolecular forces
  • molecules of water can't form intermolecular bonds with grease because both bonds are too strong by themselves but the solvent can form bonds with both
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