OCR B (Salters) AS level Chemistry
UNIT 2 – F332
Elements from the sea
Halogens and Halides (group 7 chemistry)
* Halogen is the elemental molecule, eg/ Cl₂, Br₂
* Halide is in a compound, eg/ KBr, KCl
Appearance at room temp
| Pale yellow gas
| Green gas
| Dark red volatile liquid
| Shiny black solid – sublimes to purple gas
| Colour precipitate with silver nitrate
| Pale Yellow
‘Reduction is gain of electrons, oxidation is loss of electrons’
Overall reaction = 2KBr + Cl₂ 2KCl + Br₂
Halogens get more reactive going up the group, and in redox reactions the less reactive element come out of the compound/solution, and is oxidised. Oxidising agent is chlorine because it oxidises bromine, bromine is the reducing agent.
Half equations = 2Br⁻ Br₂ + 2e⁻
Cl₂ + 2e⁻ 2Cl⁻
This shows that Chlorine is reduced because it has gained electrons, and Bromine is oxidised because it has lost electrons.
| Oxidation number
| -2 (unless with Fluorine)
| -1 (unless with Fluorine or Oxygen, then +1)
A spectator ion, is used in the redox reaction, but is not reduced or oxidised itself.
Working out oxidations states is really just simple maths.
| Oxidation state of chlorine
| Name of ion
| Chlorate (I)
| Chlorate (III)
| Chlorate (V)
| Chlorate (VII)
Hydrogen becomes negative when it’s more electronegative then what it is reacting with, because it has a higher ability to draw electrons to itself.
Electrolysis is a form of redox reaction and may come up in the exam.
Example of this could be the electrolysis of potassium chloride: 2KCl Cl₂ + 2K
2K⁺ + 2e⁻ 2K
2Cl⁻ Cl₂ + 2e⁻
% Atom economy = (formula mass of useful products) / (formula mass of reactants) x 100
Electrolysis of water to produce hydrogen
2H₂O 2H₂ + O₂ Mr of 2H₂O = 36 Mr of 2H₂ = 4
(4/36) x 100 = 11.1%
Permanent dipole – Permanent dipole (when an element is more electronegative) ‘When two unlike atoms are convalently bonded, the shared electrons will be more strongly attracted to the atom of greater electronegativity. Such a bond is said to be polar. A polar bond results in the unequal sharing of the electrons in the bond’
Types of intermolecular bonds
* Instantaneous - Induced dipole (IDID) < 10KJ/mol
* Permanent dipole - Permanent dipole (PDPD) 10-20KJ/mol * Hydrogen bonding 10-40KJ/mol
Instantaneous – Induced dipoles (can occur in ALL molecules) * Occurs when more of the electrons happen to be at one end than the other, so one end is slightly (delta) negative and the other slightly (delta) positive. * Can cause induced dipoles if near other molecules.
* Permanent dipoles can also induce dipoles if near other molecules. * Larger molecules have more electrons so strength of IDID is greater. * Straight chained molecules have more ‘contact points’ so have more opportunities for IDID than branched molecules. * Weakest of intermolecular forces.
% yield = (actual yield) / (theoretical yield) x 100
100g of AgNO₃ (Mr = 169.9) reacts with something to produce Ag (Ar = 107.9). In the actual reaction 50.8g of Ag was produced. Calculate the percentage yield.
First we need to work out the theoretical yield.
Moles = 100/169.9 = 0.588 moles
Because 1 mole of AgNO₃ reacts to make 1 mole of Ag,
0.588 moles of Ag should be formed which =
(0.588 x 107.8) = 63.5g (theoretically)
Because only 50.8g was produced the % yield =
(50.8/63.5) x 100 = 80%
* Needs to be covalently...
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