Elements from the Sea OCR Chemistry


Ions and solids in solutions

Sodium chloride is an ionic lattice, made up of Cl - ions surrounded by six Na+ ions. The ions are held together by attractions as the ions are oppositely charged ions- a giant ionic lattice is built.

Thew lattice structure can be said to be simple cubic. Sometimes ionic crystals contain water molecules- known as water of crystallisation within the ionic lattice. The crystals are hydrated.

Water surrounds ions in trhe way it does because water is a polar molecule and has a bent shape. Positive hydrogen atoms attract to negative ions and vice versa. Each ion is surrounded by its own sphere of 5 water molecules- this is hydration.

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Ions and solids in solutions continued


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Ionic equations

If two reactants react together and form a solid, this is a precipitation reaction e.g.

Ag+ (aq) + NO3- (aq) + Na+ (aq) + Br- (aq) --> AgBr (s) + Na+ (aq) + NO3- (aq)

The Na and NO3 ions are spectator ions, so they can be removed from the equation so the ionic equation of the reaction would be:

Ag+ (aq) + Br- (aq) --> AgBr (s)

  • All nitrates are soluble in water
  • All chlorides are soluble in water except AgCl and PbCl2
  • All sulphates are soluble in water except BaSO4, PbSO4 and SrSO4
  • All sodium, potassium and ammonium salts are soluble in water
  • All carbonates are insoluble in water except (NH4)2CO3 and those of group 1 elements

Neutralisation reactions can be summarised in ionic equations

H+ (aq) + Cl- (aq) + Na+ (aq) + OH- (aq) --> H2O (l) + Na+ (aq) + Cl- (aq)

H+ (aq) + OH- (aq) --> H2O (l)

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Concentrations of solutions

Calculations of concentrations can be in gdm-3 e.g. 40g of sodium hydroxide in 2dm3 of solution has a concentration of 40/2 = 20gdm3.

However usually quantities are measured in moles, so concentration is moldm-3; the equations below show how to work out concentration, amount and volume:

Concentration = moles/volume

Moles = concentration x volume

Volume = moles/concentration

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Atoms and ions

The first ionisation enthalpy (ionisation energy) is the amount of energy needed to remove one electron from each of one mole of isolated gaseous atoms of an element.

It is difficult to remove an electron from the elements in group 0 as they have full outer shells and so are unreactive.

Group 1 elements have a low ionisation enthalpy as they only have one electron on the outer shell so it is easy to remove it.

The larger the atomic number of an element means that it is harder to remove electrons; as you go across a period the ionisation enthalpy also increases.

First ionisation enthalpy energies always decrease as you go down a group because there are more shells between the nucleus and the outermost electrons.

Ionisation enthalpies always increase as successive electrons are removed as you get closer to the nucleus, and the attraction of electrons to the nucleus is greater after each one is removed.

There is a big jump in ionisation enthalpy when an electron is removed from a full shell.

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Oxidation and reduction

Oxidation is loosing electrons, reduction is gaining elctrons ( OIL RIG).

Atoms in elements are always in an oxidation state of 0, and in compounds or ions, oxidation states are assigned to each atom or ion- the oxidation states must add up to 0 because compounds have no charge on them.

F is -1 and O is -2 (except in O2- and OF2), H is +1 and Cl is -1 (except when combined with O or F). 

Some elements can exist in more than one oxidation state in compounds, and there is a systematic name for the compound which includes the oxidation state e.g. FeO is iron (ll) oxide and Fe2O3 is iron (lll) oxide.

Oxyanions are negative and contain oxygen and another element. Their names end with "ate" e.g. ClO- is chlorate(l) and ClO3- is chlorate(v).

A displacement reaction occurs when a more reactive halogen is passed into a solution of less reactive halide ions e.g. Cl2 (g) + 2I- (aq) --> 2Cl- (aq) + I2 (aq)

Cl2 (aq) + 2e- --> 2Cl- (aq) THIS IS REDUCTION

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Electronic structure: sub-shells and orbitals

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Electronic structure: sub-shells and orbitals cont

  • An S sub-shell has 1 orbital holding a maximum of 2 electrons.
  • A P sub-shell has 3 orbitals holding a max of 6 electrons.
  • A D sub-shell has 5 orbitals holding a max of 10 electrons.
  • Electron distribution can be shown by electronic configurations, or by drawing arrows in boxes e.g. N = 1s2, 2s2, 2p3.
  • Group 1 and 2 elements will have one or two electrons in their outer shell which is an S orbital- these are in the S block.
  • Group 3,4,5,6,7 and 0 elements all have 3,4,5,6,7 or 8 electrons in their outer most shell which is a P shell- these are in the P block.
  • The transition metal block elements have electrons filling the D shell- these are in the D block.
  • Further transition metals have electrons filling the F sub shell- these are in the F block  
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