AQA Chemistry C3 Part 2

This is the stuff you need to know for the second part of the chemistry specification (energy and chemical tests)

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  • Created by: Abbie
  • Created on: 23-05-12 16:31

Energy

Exothermic - gives out energy, less energy required to break bonds than released when forming them

Endothermic - takes in energy, more energy required to break bonds than is released when forming them

Energy is needed to break bonds & Energy is released when bonds are formed

You can measure energy transfer by taking the temp of the reactants before and the products after

BUT lose energy to surroundings

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Energy and Fuels

Fuels release energy - exothermic

Calorimetry - measures amount of energy produced when a fuel's burnt

Working out energy per gram of fuel:

  • Weigh the mass of spirit burner + lid before and after & find the difference
  • Measure the temp of the water before and after heating & find the difference
  • Takes 4.2J of energy to heat 1g of water by 1`C
  • 4.2 x amount of water in grams x difference of temp = y
  • y/difference of mass = energy per gram of fuel
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Bond Energies

Exothermic reactions - change in enthalpy(heat) is -ve

Endothermic reactions - change in enthalpy is +ve

Activation energy - min. energy needed to break the bonds & start the reaction

Catalysts lower activation energy BUT don't effect the overall energy change

Bond energy calculations, e.g. H2 + Cl2 --> 2HCl, H-H: +436kJ/mol, Cl-Cl: +242,       H-Cl: +431

  • 436 + 242 = 678kJ/mol needed to break the bonds
  • 431 x 2 = 862kJ/mol released when forming the bonds
  • 862 - 678 = 184kJ/mol: energy change
  • Because it's exothermic change in enthalpy is -184kJ/mol
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Energy and Food

1 calorie = the amount of energy needed to raise 1g of water by 1`C = 4.2joules

1 Calorie = the amount of energy needed to raise 1kg of water by 1`C = 4200joules

  • Fats and oils - high amount of energy
  • Carbohydrates - some energy but less than fats and oils
  • Proteins - same as carbohydrates but used for different things in body

Chemical reactions in your cells going on all the time need energy

BUT taking in more energy than you're using/ your body needs leads to obesity

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Testing for Positive Ions (Cations)

Flame tests:

  • Li+ - red
  • Na+ - yellowy/orange
  • K+ - lilac
  • Ca2+ - brick red
  • Ba2+ - green

Add NaOH:

  • Ca2+ & Mg2+ - white precipitate
  • Al2+ - white precipitate then dissolves in excess
  • Fe2+ - sludgy green precipitate
  • Fe3+ - red/brown precipitate
  • Cu2+ - blue precipitate

Ammonia - smells & turns red litmus paper blue

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Testing for Negative Ions (Anions)

Carbonates + acid:

  • CO2 - turns limewater cloudy
  • Copper carbonate - green > black
  • Zinc carbonate - white > yellow > white (when cooled)

Sulfates - add barium chloride & HCl, if a white precipitate then it's a sulfate

Halides - add nitric acid & silver nitrate:

  • Chloride - white precipitate
  • Bromide - cream precipitate
  • Iodide - yellow precipitate

Nitrates - add aluminium powder & NaOH, if ammonia then it's a nitrate 

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Testing for Organic Compounds

Burn with a yellowy/orange and/or blue flame & solid organic compounds will char

Unsaturated (C=C bonds) - decolourise bromine water

Emperical formula - e.g. 0.4g of hydrocarbon burnt. 1.1g of CO2 & 0.9g of H2O:

  • 1) Mass of CO2 - 1.1 x (12/44) = 0.3
  • Mass of H2O - 0.9 x (2/18) = 0.1
  • 2) Divide by relative atomic mass - 0.3/12 = 0.025moles
  • Divide by relative atomic mass - 0.1/1 = 0.1moles
  • 3) Divide both by smallest one - 0.025/0.025 = 1
  • Divide both by smallest one - 0.1/0.025 = 4
  • Ratio - 1:4
  • CH4
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Instrumental Methods

Advantages - by technicians not chemists, more accurate, faster

Disadvantages - expensive

Types:

  • Atomic Absorption Spectroscopy - identifies metals, like flame test, analyses patterns of light absorbed by metal
  • Infrared/UV Spectroscopy - identifies compounds, fingerprint of frequencies of infra-red/UV absorbed
  • NMR Spectroscopy - for organic compounds, shows what the hydrogen atoms are connected to
  • Gas-Liquid Chromatography - for gases & liquids, similar to paper chromatography
  • Mass Spectroscopy - for compounds & elements, finds mass - work out what's in there
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