Chemistry C3

Ocr, Gateway chemistry C3

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Atoms

Nucleus contains protons and neutrons

Electrons move around the nucleus and occupy shells

protons have a mass of 1 and a charge of +1

Neutrons have a mass of 1 and a charge of 0

Electrons have a mass of 0.0005 and a charge of -1

The number of protons is equal to the number of electrons

Atomic number= number of protons

Mass number= total of protons and neutrons

number of neutrons= mass number-atomic number

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Isotopes, Elements and compounds

Isotopes= different atomic forms of the same element, which have the same number of protons but a different number of neutrons

  • must have same atomic number but different mass numbers

Elements consist of one type of atom only and it cannot be broken down chemically

Compounds are chemically bonded

  • two types of chemical bonding
    • ionic bonding- attraction between positive and negative particles called ions
    • covalent bonding- sharing a pair of electrons
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The periodic table

Elements are arranged in groups and periods in order of ascending atomic number

  • rows=periods
  • vertical columns= groups

Elements in the same group have the same number of electrons in their outer shell thats why they have similar properties

Period- shows how many electron shells

atomic structure= number of electrons in shells- 2 in first shell and 8 in all the rest

  • e.g. magnesium= 2.8.2
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Ionic Bonding

ions are formed when an atom loses or gains electrons

Atoms want full outer shells so lose and gain electrons to do this

The atom that gains=  - (minus)

the atom that loses= + (plus)

Ionic compounds have high melting points because they are giant structures containing millions of ions and ionic bonds which require alot of energy to break. They can conduct electricity when molten (ions are free to move)

Giant Ionic Lattices-

  • they have high melting and boiling points,
  • are soluble in water,
  • and conduct electricity in liquid form
  • e.g. NaCl and MgO
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Group 1- Alkali Metals

  • 1 electron in outer shell
  • As you go down the group they become more reactive as the outer electron is more easily lost because its furthur from the nucleus
  • low metal and boiling points
  • low density
  • very soft- can be cut with a knife
  • hydroxides of alkali metals dissolve in water toroduce an alkali p
  • loss of electrons= oxidation
    • ionic= Na- e- ------> Na+ or Li - e- -----> Li+
  • Alkali metals always lose an electron
  • reaction with cold water produces hydrogen gas
    • move around surface fizzing and reaction gets more reactive as go down
    • produce hydrogen-  test is lighted splint will make a squeaky pop
    • the solution will be alkaline which changes the univeral indicator to purple
    • form hydroxide in solution- OH- ions
    • e.g. sodium + water ------> sodium hydroxide + hydrogen

      2Na    + 2H2O ------> 2NaOH       +         H2

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Flame tests

1. Clean a wire loop using dilute HCl

2. dip in distilled water to remove acid

3. dip in powdered sample of compound being tested and place in blue bunsen flame and observe colour

Lithium= Red

Sodium= Orange/Yellow

Potassium= Lilac

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Electrolysis and the half equations

The electrolysis of sulfuric acid solution

  • water contains hydrogen and oxygen, but pure water doesn't conduct electricity well so sulfuric acid is added
    • H2O <----> H+  +   OH-
    • H2SO4 <----> H+  + SO4 2-
  • Hydrogen is produced at the negative cathode
    • positive ions are called cations as they are attracted to -ve cathode
    • the hydrogen ions are from sulfuric acid and water
    • half equation= 2H+ + 2e-   ----->  H2
  • Oxygen is produced at the positive anode-
    • -ve ions called anions because attracted to positve anode
    • all hydroxide ions come from water
    • half equation= 4OH-   ----->  2H2O + O2 + 4e-

1.at the cathode, 2 hydrogen ions accept 2 electrons to become 1 hydrogen molecule

2.at the anode, 4 hydroxide ions lose their electrons to make one oxygen and 2 water molecules

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Extracting Aluminium

Electrolysis removes aluminium from its ore bauxite

The ore is mixed with cryolite to lower the temperature (900'C) and costs

The electrodes are made of graphite- a good conductor of electricity

bauxite + cryolite= electrolyte

  • the positively charged aluminium ions are attracted to the cathode
  • oxygen forms at the anode- causing the carbon to be worn away so need replacing
  • It is a redox reaction
  • at the cathode (-ve)
    • Al3+  +  3e-  -----> Al  (reduction= gain of electrons)
  • At the anode (+ve)
    • 2O2-  -----> O2 + 4e-  (oxidation= loss of electrons)
  • aluminium oxide -----> aluminium + oxygen

its expensive- lots of electricity, heat for electrolyte, anode replacement

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Covalent Bonding

Covalent bonding= sharing electrons

non metals don't want to give up electrons so ionic bonds are impossible so they share electrons

They have low melting and boiling points because the intermolecular forces are weak so the bonds are broken quicker and easier.

covalent bonds have specific properties:

  • form molecules
  • intramolecular forces are strong (within molecules)
  • intermolecular forces are weak (between molecules)
  • low melting and boiling points
  • at room tempurature most are gases
  • do not have charged particles that are free to move so cannot conduct electricity
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Groups and Periods

groups define how many in outer shell

period defines how many shells

group-  1 - 1 in outer shell

    7 - 7 in outer shell

    8 - 8 in outer shell

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Group 7- The Halogens

have 7 in outer shell- similar properties

They become less reactive as you go down as the outer shell is furthur from the nucleus so the force of attraction is weaker

Halogens can form covalent bonds with themselves or ionic bonds with alkali metals

Halogens are keen to gain an electron, the gain of electrons is reduction

  • Cl2 + 2e-  -----> 2Cl-

The halogens react with alkali metals to form salts called metal halides

  • sodium + chloride -----> sodium chloride

More reactive halogens will displace less reactive ones, most reactive=F, least reactive= At

  • Chlorine + Potassium iodide ----> iodine + potassium chloride
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Metals

  • Have a giant structure held together with metallic bonds
  • have high melting and boiling points and a high density
    • they are lustrous (shiny)
    • there's a strong attraction between the free electrons causing very strong metallic bonds
  • they are strong, bendy and malleable
    • high tensile strength
    • can be hammered into a different strength
  • good conductors of heat and electricity
    • have free electrons so can carry current
    • can carry heat energy
  • Are ductile because layer of atoms can slide over eachother

different metals are used for different things e.g.; zinc is used to stop steel on ships being eroded

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Superconductors

a superconductor= when a metal becomes cold enough and resistance disappears completely which means no electrical energy is wasted and you can take the battery out the the circuit will continue to flow forever

You can make

  • power cables
  • really strong electromagnets
  • electronic circuits that work really fast

BUT.. they have to be really cold, less than -265'C which is very hard and expensive

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Transition Metals

= metals in the middle of the periodic table e.g. gold, copper etc

they are all hard except mercury

they are less reactive than alkali metals

The compounds are colourful:

  • iron (II)= light green
  • Iron (III)= orange/brown
  • Copper= blue

make good catalysts- iron used in haber process, nickel used in hydrogenation of alkenes

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Thermal decomposition

Thermal decomposition is when a substance breaks down into a simpler substance when it is heated

Transition metal carbonates break down on heating- usually break down to form a metal oxide and carbon dioxide

  • copper carbonate -----> copper oxide + carbon dioxide

Use limewater to test for carbon dioxide

  • bubble gas through limewater and if carbon dioxide is present the limewater goes milky
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Precipitation Reactions

= when two solutions react and an insoluble solid forms

the solid is said to 'precipitate out' but it is also called the 'precipitate'

you can use precipitation to test for transition metal ions as some have distinct colours

  • copper hydroxide= blue
  • Iron (II) hydroxide= dark grey/ green
  • Iron (III) hydroxide= orange
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