Additional Science: Chemistry

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  • Created by: Zahra
  • Created on: 05-05-13 20:02

The History of the Periodic Table:

  •  elements contain all the same types of atoms
     
  •  the modern Periodic Table is based on Dmitri Mendeleev's ideas (a Russian chemist)
     
  •  Mendeleev arranged elements in groups and periods, based on their relative atomic masses and their properties
     
  •  Whenever a new element was discovered, the element fitted Mendeleev's predictions
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Lines of discovery:

  •  When elements are heated they emit coloured flames
       - some elements emit distinctive flame colours, lithium salts produce a red flame
     
  • The coloured light can be split into a line spectrum that is unique to each element

  • The discovery of some of the elements happened because of the development of spectroscopy (i.e. Helium)

  • Helium was discovered when chemists looked at the line spectrum from the Sun 
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Inside the atom:

  • Atoms have a tiny, central nuclues that contains protons (positive) and electrons (negative)
  • Electrons travel around the outside of the atom in shells
  • All the atoms of a particular element have the same number of protons
  • Atoms have the same number of protons and electrons
  • The modern Periodic Table arranges atoms in order of their proton number
      - number of protons + number of neutrons = relative atomic mass
     
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Sorting electrons:

  • Electrons are arranged in shells around the nucleus
  • The first shell is closest to the nucleus and can hold a maximum of 2 electrons
  • The second and third shells are further away from the nucleus
  • The second shell holds a maximum of 8 electrons
  • The number of electrons in an atom = the number of protons
  • The number of electrons in an atom = the number of protons 
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Finding elements in the Periodic Table:

  • Elements are arranged in order of proton number
  • The rows across are called periods
  • The number of electrons in the outer shell increases across a period
  • As you move across a period (left to right), elements change from metal to non-metals and the elements gain an electron and a proton
  •  The group number = the number of electrons in the outer shell
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Group 1 - The Alkali metals:

  • A 'group' is a vertical column
  • All group 1 elements are metals and have 1 electron in the outer shell of their atoms
  • Group 1 elements are soft metals, can be cut with a knife and the surface quickly tarnishes in moist air - by reacting with Oxygen
  • Reactivity increases as you go down the group as atoms get bigger
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Group 7 - The Halogens:

  • The elements in group 7 are called the halogens
  • The halogens all contain diatomic molecules; this means that they have 2 atoms joined together in each molecule
  • The formulae of the halogens are: Chlorine Cl2, Bromine Br2, Iodine I2
  • As you go down the group, melting points and boiling points both increase
  • Group 7 elements are corrosive and toxic
  • Halogens are less reactive as you go down the group 
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Ionic compounds:

  • Ionic compounds: componds containing an element from group 1 and an element from group 7 (i.e. Sodium Chloride), which are solids with high melting points
  • Ionic componds contains charged particles (or ions), and they are arranged in a crystal lattice

(http://www.chemguide.co.uk/atoms/structures/naclexpl.GIF)

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Molecules in the air:

  • Dry air contains non-metal elements; nitrogen, oxygen and argon
     
  • The air is made of 78% Nitrogen, 21% Oxygen, 1% Argon and 0.04% Carbon Dioxide 
  • The atoms in the molecules are held together by covalent bonds 

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Ionic crystals:

  • Salts are ionic compounds. Ions have either a positive or a negative charge and are arranged in a giant 3-D pattern called a lattice
  • Ionic compounds have high melting points and high boiling points because a large amount of energy is needed to overcome the forces between ions in the lattice
  • Ionic compounds do not conduct electricity when solid because the ions are not free to move 
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Metals, minerals and ores:

  • Minerals are solids, with atoms or ions arranged in a regular arrangement or lattice
  • Minerals contain metals, rocks that contain metal minerals are called ores
     - some ores contain metal oxides
  • Reduction happens because the metal oxides loses oxygen; and oxidation happens because the carbon gains oxygen
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Atomic mass and formula mass:

  • The relative atomic mass of an atom is the mass of an atom compared to the mass of an atom of carbon, which is given the value 12
  • The relative formula mass of a compound is the sum of the relative atomic mass of all the atoms or ions shown in its formula
  • The gram formula mass of an element or compound is its relative atomic mass or relative formula mass in grams

  • Percentage of metal in minerals: total mass of metal atoms/gram formula mass x 100% 
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Using electrolysis:

  • Electrolysis is passing an electric current through an ionic compound when it is either melted or dissolved in water
  • The compound is called the electrolyte because it conducts electricity
  •  Electrolytes break down or decompose, as the electricity passes through
  • Electrolysis is used to extract more reactive metals (e.g. aluminium) because their oxides cannot be reduced by carbon
  • Metals form at the negative electrode because positive metal ions are attracted to the negatively charged electrode (cathode)
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Metals:

  • Metals: are strong, can be hammered and bent into different shapes (they are malleable), have high melting points and are a good conductor of electricity
  • Atoms in metals are held together by metallic bonds - the atoms are arranged in a regular pattern in a giant lattice

  • Some metals are poisonous - leadm mercury and cadmium
      - waste poisonous metals from mines destroys habitats and damages soil and water sources
     
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Acids and Alkalis:

  • Indicators turn into different colours in acids and alkalis - Lithium is red in acids and is blue in alkalis; univeral indicators is orange or red in acids and green to blue in alkalis
  • Sodium hydroxide, potassium hydroxide and calcium hydroxide are common alkalis
  • The pH scale is a measure of how strong an acid or alkali is, using universal indicator or a pH meter
     - the colour of the universal indicator can be compared to a colour chart to find the pH number
  • Acids have a pH number of less than 7, and alkalis have a pH number of more than 7

     (http://t0.gstatic.com/images?q=tbn:ANd9GcTbBslB7_tyWkIIuoqBTAgMmuVH__EFkVN2GNOpBqv8ui-NgLF_)

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Reactions of acids:

  • Acids react with many metals and metal compounds to make salt
  • Acids react with many metals to form a salt and hydrogen gas
     - e.g. calcium + hydrochloric acid makes calcium chloride + hydrogen
  • Acids react with metal oxides and hydroxides to form a salt and water
     - e.g. magneisum oxide + sulfuric acid makes magnesium sulfate + water
  • Acids react with metal carbonates to form a salt, water and carbon dioxide gas
     - e.g. calcium carbonate + hydrochloric acid makes calcium chloride + water + carbon dioxide
  • Reactions of acids can be shown using symbol equations with state symbols: they can be either solid (s), liquid (l), gas (g) or solution in water (aq) 
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Explaining neutralisation:

  • When an acid reacts with an alkali, a salt and water are always made
  • All acids contain hydrogen ions (H+) when they are dissolved in water. The pH of an acid is related to the concentration of H+ ions in the acid solution
  • All alkalis contain hydroxide ions, OH-, when they are dissolved in water                          

    Energy changes:

  • Exothermic reactions gives out heat energy. The temperature of the surroundings rises
  • Endothermic reactions take in heat energy. The temperature of the surroundings falls 
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Separating and purifying:

  • A pure substance has nothing else mixed with it
  • Filtration can be used to seperate a solid from a liquid or from a solution

  • Crystallisation is used to purify impure solid crystals:
    1. Dissolving - dissolve the product in a small amount of hot water
    2. Filtering - filtering off any starts impurities that do not dissolve, the filtered solution is called a filtrate
    3. Evaporating - the filtrate starts to crystallise as some of the water evaporates off
    4.Filtering - filter off the crystals, leaving any soluble impurities in the solution
    5. Drying - dry the crystals in a dessicator or oven

    Percentage yield:

  • The percentage yield at the end of an experiment is worked out from the actual yield and the theoretical yield
     
  • Percentage yield = actual yield/theoretical yield x 100% 
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Rates of reaction:

  • The rate of reaction is the amount of a product produced or the amount of reactant used up in a certain time, it is usually measured as the amount per second
  • If the reaction makes a gas, the rate can be followed by:
    - measuring the volume of gas made at set times, e.g. every 30 secs
    - measuring the decrease in mass of the flask as the gas leaves the reaction

  • If the reaction makes a solid, the rate can be followed by measuring the time taken until you cannot see a cross underneath the flask or beaker

    Changing the rates of reactions:

  • A catalyst is a substance that speeds up a chemical reaction but is not used up

  • Reactions are faster when:
    - the temperature of the reactants increases
    - the size of solid particles are smaller (increases the surface area)
    - the concentration of reactants in solution increases
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