Chemistry - C2.3 - How Much?

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C2.3.1 - The Mass of Atoms

  • Proton mass = neutron mass - relative mass of proton/neutron = 1
  • Mass of electron = very small, compared to proton/neutron - atom mass is mostly protons and neutrons - protons + neutrons = mass number
  • Atoms of same element have the same atomic number - no of protons ALWAYS = no of electrons, but no of neutrons varies - same element with different no of neutrons: isotopes
  • Mass number - atomic number = no of neutrons
  • Mass number: top number, atomic number: bottom number
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C2.3.2 - Masses of Atoms and Moles

  • Atoms are too small to weigh - so use relative atomic mass (Ar) in calculations - Ar shown on periodic table - substances usually weighed in grams in labs - Ar of element in grams = one mole of atoms in element
  • 12/6 C used as standard atom - compare all other atoms masses with it
  • Ar of an element - average value, depends on isotopes element contains - when rounded to a whole number, often = mass number of elements main isotope
  • Relative formula mass (Mr) of a substance - found by adding up Ar's of atoms in it's formula
  • Mr of substance in grams = one mole of substance - using substances moles helps working out how much of substance reacts or how much product will form
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C2.3.3 - Percentages and Formulae

  • Find % of elements in compound from formula of compound - Ar/Mr X100 - useful to decide if compound is suitable for a purpose or to identify compound
  • Empirical formula - simplist ratio of atoms or ions in compound - formula used for ionic compounds - for covalent compounds not always same as molecular formula - eg. molecule formula of ethane: C2H6, empirical formula; CH3
  • Calculate empirical formula from % composition - divide mass of each element in 100g by it's Ar - gives ratio of atoms -> convert to simplest whole number ratio
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C2.3.4 - Equations and Calculations

  • Chemical equations - show reactants and products of reaction, when balanced show amount of atoms, molecules or ions in reaction
  • Eg. 2Mg + O2 -> 2MgO:
    • 2 Mg atoms + 1 O molecule -> 2 Mg and 2 O ions
    • 2 moles of Mg atoms + 1 mole of O molecules -> 2 moles of MgO
    • 48g of Mg reacts with 32g of O to form 80g of MgO
    • (2xAr of Mg) + (2xAr of O) -> (2xMr of MgO)
    • (2x24g Mg) + (2x16g O) -> 80g MgO
    • 5g of Mg - work out mass of MgO - know 1g Mg = 80/48g of MgO: 5g Mg - 5 x 80/48g MgO = 8.33g of MgO
    • moles - 1 mole of Mg = 1 mole MgO
    • 5g Mg = 5/24 mole of Mg so will produce 5/24 mole of MgO
    • mass of 5/24 mole MgO = 5/24 x 40g = 8.33g of MgO
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C2.3.5 - The Yield of a Chemical Reaction

  • Yield of chemical process: how much you actually make - percentage yield compares amount made with maximum amount possible as a percentage
  • Percentage yield = amount made/maximum amount possible X100
  • Maximum amount possible calculated from balanced equation of reaction
  • Impossible to collect maximum amount possible - reactions may not go to completion, other reactions may happen, some reactant may be lost when separated or collected from apparatus
  • Reactions with high yields - used in industry to conserve resources and reduce waste - chemical processes should waste little energy as possible - helps reduce pollution and makes production more sustainable
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C2.3.6 - Reversible Reactions

  • Reversible reaction: products of reaction react to make reactants - reaction can go in both directions - represented with symbol: Equilibrium symbol (
  • Eg. Ammonium chloride Equilibrium symbol ( ammonia + hydrogen chloride - when heated, ammonium chloride produces ammonia and hydrogen chloride; when cooled, ammonia and hydrogen chloride produce ammonium chloride
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C2.3.7 - Analysing Substances

  • Substances added to food to improve quality: food additives - may be natural products or synthetic chemicals
  • Foods checked by chemical analysis so additives are safe and permitted - methods include paper chromatography and mass spectrometry
  • Paper chromatography - used to analyse artificial colours in food - spot of colour put on paper and solvent allowed to move through paper - colours move different distances depending on solubility
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C2.3.8 - Instrumental Analysis

  • Modern instrumental methods - rapid, accurate, sensitive using small samples, meaningful results quickly, expensive, special training needed
  • Samples for analysis often need separating so compounds can be identified - do this with gas chromatography linked to mass spectrometer
  • Gas chomatography - mixture carried by gas through column with solid particles, individual compounds = different speeds: come out at different times, amount leaving recorded against time - show number of compounds and retention time - compared to known times to be identified
  • Output can be directly linked to mass spectrometer (GC -MS) - gives further data to identify compounds
  • Mass spectrometer - gives relative molecular mass - individual compound: the peak with largest mass corresponds to ion with one electron removed: molecular ion peak - furthest to right on spectrum
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