Atomic structure, analysis and quantitative chemistry

Atomic structure, analysis and quantitative chemistry

1. Atomic Structure

   • Subatomic particles
     • Each atom consists of a nucleus containing protons and neutrons, with electrons arranged around it in energy levels
       • The relative mass of protons and neutrons is 1 whereas the relative mass is very small (1/1836)
         • The mass number of an atom is the total number of protons and neutrons it contains
         • The atomic number of an atom is the number of protons it contains
   • Isotopes
     • The atoms of a particular element will all have the same number of protons so they will have the same atomic number
       • However the same element can have a different number of neutrons so their mass numbers are different
         • These are called isotopes and they can have different properties for example some of them are radioactive
         • On the periodic table the mass number for elements with isotopes is averaged which is why, for example, chlorine has a mass number of 35.5

2. Analysing Substances

   • Paper Chromatography
     • Paper chromatography is used to analyse coloured substances such as the coloured pigments in plants and artificial used as food additives
       • It works because some of the coloured substances are better at dissolving in the liquid than they are at bonding with the paper so they travel further up the paper
         • Two substances are likely to be the same if they have the same colour and travel the same distance up the paper
   • Instrumental chromatography
     • This relies on machines which detect and identify elements or compounds
       • Instrumental techniques are generally fast accurate and sensitive
   • Gas Chromatography and Mass Spectrometry
     • Gas chromatography is done by a machine that separates a mixture of compounds
       • It consists of a long glass tube packed with solid material which is fitted to an oven
         • The Sample is dissolved into a solvent then injected into one end of the column
           • An unreactive gas- usually nitrogen- carries the sample through the column
             • Different substances in the sample travel through the column at different speeds and so become separated from each other
               • The separated substances leave the column one after the other, as they leave, they are detected by a detector
                 • The time taken for a substance to travel through the column is called its retention time
                   • A detector produces a graph where each substance is represented by a peak
                     • The number of peaks shows the number of compounds present in the sample
                     • The position of each peak shows the retention time for each compound
     • A mass spectrometer is able to identify substances very quickly and accurately
       • It can detect very small amounts and if linked with a GC machine it allows the separated substances leaving the column of the GC machine to be detected
         • It can also provide the relative formula mass of the substances separated by gas chromatography
           • The peak furthest to the right in a mass spectrum is called the molecular ion peak and its relative mass is the relative formula mass of the substance being analysed

3. Quantitative Chemistry

   • Percentage composition is just a way of to describe what proportions of the different elements there are in a compound
   • Conservation of mass
     • Mass is never lost or gained in chemical reactions: the total mass of the products at the end of the reaction is equal to the total mass of the reactants at the beginning
       • This principle lets you calculate the theoretical mass of product from a reaction however it is not always likely that you obtain this amount
         • This maybe because
           • Reversible reactions may not go to completion
           • Some product may be lost when it is removed from the reaction mixture
           • Some of the reactants may react in an unexpected way
         • The percentage yield is the ratio of actual yield to theoretical yield
   • Reversible reactions
     • Many reactions are irreversible- they go to completion and cannot be reversed easily
       • In reversible reactions the products can be reacted to produce the reactants
         • This is signified by a two way arrow