C2.3 atomic estruture, analysis and quantitive chemistry

C2.3 atomic estruture, analysis and quantitive chemistry

1. the relative masses of atoms can be used to calculate how much to react and how much we can produce because no atoms are gained or lost in chemical reactions. there are various methods used to analyse these substances

2. C2.3.1 atomic structure

   • atoms can be represented as shown:
   • the relative masses of protons, neutrons and electrons are: proton - 1 neutron - 1 electron 1/2000
   • the total number of protons and neutrons in an atom is called its mass number
   • atoms of the same element can have different numbers of neutrons; these atoms are called ISOTOPES of that element
   • the relative atomic mass of an element (Ar) compares the mass of atoms of the element with the 12C isotope. it is an average value for the isotope of the element
   • the relative formula mass (Mr) of a compound is the sum of the relative atomic masses of the atoms in the numbers shown in the formula
   • the relative formula mass of a substance in grams is known as one mole of that substance

3. C2.3.2 analysing substances

   • elements and compounds can be detected and identified using instrumental methods which are accurate, sensitive and rapid and are particularly useful when the amount of a sample is very small
   • chemical analysis can be used to identify additives in foods. artificial colours can be detected and identified by paper in chromatography
   • gas chromatography linked to mass spectroscopy (GC-MD) is an e.g. of an instrumental method:
     • it allows the separation of a mixture of compounds
       • different substances carried by a gas travel through a column packed with a solid material at different speeds so that they become separated
     • the time taken for a substance to travel through the column can be used to help identify the substance
       • the number of peaks on the output of a gas chromatograph shows the number of compounds present
       • the position of the peaks on the output indicates the retention time
     • the output from the gas chromatography column can be linked to a mass spectrometer which can be used to identify the substances leaving in the end of the column
       • a mass spectrometer can identify substances very quickly and accurately and can detect very small quantities
     • the mass spectrometer can also give the relative molecular mass of each of the substances separated in the column
       • the molecular mass is given by the molecular ion peak

4. C2.3.3 quantitive chemistry

   • the percentage of an element in a compound can be calculated from the relative mass of the element in the formula and thee relative formula mass of the compound
   • the empirical formula of a compound can be calculated from the masses or percentages of the elements in a compound
   • the masses of reactants can be calculated from balanced symbol equations
   • even though no atoms are gained or lost in a chemical reaction, it is not always possible to obtain the calculated amount of product because:
     • the reaction may not go to completion because it is reversible
     • some of the product may be lost when it is operated from the reaction mixture
     • some of the reactants may react in ways different from the expected reaction
   • the amount of product obtained is known as the yield. when compared with the maximum theoretical amount as a percentage it is called the PERCENTAGE YIELD
   • in some chemical reactions the products of the reaction can react to produce the original reactants. such reactions are called REVERSIBLE reactions and are represented:

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