Revision:AQA AS Chemistry Unit 2

Register

About Us | Help | Sign in

Create New Article | Editing Help

Advanced Search

Edit \| Hide
Welcome \| Study Help \| A Levels \| Applications, UCAS etc. \| University Discussion \| Careers \| General Discussion \| Have Your Say \| Health & Relationships \| Debate & Discussion \| Technology \| Sport \| Entertainment \| Chat \| Ask A Moderator

Revision:AQA AS Chemistry Unit 2

From The Student Room

Jump to:navigation, search

TSR Wiki > Study Help > Subjects and Revision > Revision Notes > Chemistry > AQA AS Chemistry Unit 2

AQA AS Chemistry Unit 2

1 Halogens
2 Energetics
3 Redox
4 Extraction of metals
5 Comments

Halogens

Reaction with concentrated Sulphuric acid

Sodium Chloride
- $\displaystyle \mathsf{NaCl + H_2SO_4} \longrightarrow \mathsf{HCl + NaHSO_4}$
Sodium Bromide
- $\displaystyle \mathsf{NaBr + H_2SO_4} \longrightarrow \mathsf{HBr + NaHSO_4}$
- then
- $\displaystyle \mathsf{2NaBr + 2H_2SO_4} \longrightarrow \mathsf{2H_2O + Br_2 + SO_2 + Na_2SO_4}$
Sodium Iodide
- $\displaystyle \mathsf{NaI +H_2SO_4} \longrightarrow \mathsf{HI +NaHSO_4}$
then
$\displaystyle \mathsf{8NaI + 5H_2SO_4} \longrightarrow \mathsf{4I_2 + H_2S + 4H_O + 4Na_2SO_4}$

Chlorine Water

Disproportionation – chlorine is simultaneously oxidised to a Cl^- ion and reduced to a ClO^- ion
- $\displaystyle \mathsf{Cl_2 + H_2O} \leftrightarrow \mathsf{HCl + HClO}$

Trends in chemical properties

chlorine will displace bromine and iodine
bromine will displace iodine but not chlorine
iodine will displace neither chlorine or bromine
Oxidising Power
- $\displaystyle \mathsf{F_2 > Cl_2 > Br_2 > I_2}$

Reactions with Cl2
- Halide ion
  1. Br^-
  2. I^-
- Observations
  1. yellow/brown
  2. brown colour, black ppt
- Conclusion
  1. Br₂ displaced
  2. I₂ displaced

Reactions with Br₂
- Halide ion
  1. Cl^-
  2. I^-
- Observations
  1. no change
  2. brown colour, black ppt
- Conclusion
  1. Cl₂ not displaced
  2. I₂ displaced

Reactions with I₂
- Halide ion
  1. Cl^-
  2. Br^-
- Observations
  1. no change
  2. no change
- Conclusion
  1. Cl₂ not displaced
  2. Br₂ not displaced

Trend in electronegativity

decreases as atomic number increases
- Because
- Atomic number increases
- attraction for bonding electron pair in covalent bond increase
- increase number of electron shells
- shielding increases
- electrons less strongly attracted
- atomic radius increases

Trend in boiling point

all exist as diatomic molecules
attraction = weak intermolecular forces
van der Waals forces
increase with size of molecule

Tests

Tests for halide ions
Reagents
- Silver nitrate solution
  - Fluoride ions
    - Silver fluoride is soluble
    - no ppt formed
  - Chloride ions
    - White ppt formed
    - soluble in dilute ammonia
  - Bromide ions
    - cream ppt formed
    - soluble in concentrated ammonia
  - Iodide ions
    - yellow ppt formed
    - insoluble in ammonia solution
- followed by ammonia solution

Energetics

$\displaystyle q = mc\delta T$

$\displaystyle \mathsf{heat\ energy\ (joules)} = \mathsf{mass \times heat\ capacity \times the\ change\ in\ temperature}$

$\displaystyle \mathsf{enthalpy\ change\ (kJ\ mol_{-1} )} = \frac{-q}{(1000 \times \mathsf{number\ of\ moles})}$

Standard enthalpy of formation

enthalpy change when 1 mol of a compound is formed from its elements under standard conditions
all reactants and products in their standard states

Standard enthalpy of combustion

enthalpy when 1 mol of a substance is completely burned in oxygen
under standard conditions and all reactants and products in their standard states

Standard enthalpy of neutralisation

enthalpy change that accompanies the neutralisation of an acid by a base to form 1 mol of H₂O
under standard conditions

Standard conditions

298 K
100kPa
1mol dm^-3

Redox

Oxidation

loss of electons
addition of oxygen

Reduction

gain of electrons
loss of oxygen

Reactions which involve both oxidation and reduction

Oxidising agent is reduced – an electron pair acceptor

Reducing agent is oxidised – an electron pair donor

Extraction of metals

Titanium

Batch process
Argon
- Inert atmosphere
- to prevent contamination of metal with oxygen or nitrogen
$\displaystyle \mathsf{TiO_2 + 2C + 2Cl_2} \longrightarrow \mathsf{TiCl_4 + 2CO}$
$\displaystyle \mathsf{TiCl_4 + 4Na} \longrightarrow \mathsf{4NaCl + Ti}$
economic factors
- chlorine and sodium have to be produced
- high temperatures involved
- argon atmosphere has to be contained

Aluminium

Bauxite
$\displaystyle \mathsf{Al_2O_3}$
bauxite dissolves in molten cryolite – to reduce costs from trying to melt just bauxite
At cathode
$\displaystyle \mathsf{Al^{3+} + 3e^-} \longrightarrow \mathsf{Al}$
At anode
$\displaystyle \mathsf{2O^{2-}} \longrightarrow \mathsf{4e^- + O_2}$

Iron

In blast furnace
- haematite
  - $\displaystyle \mathsf{Fe_2O_3}$
- coke
  - $\displaystyle \mathsf{C}$
- limestone
  - $\displaystyle \mathsf{CaCO_3}$
- hot air
$\displaystyle \mathsf{2C + O_2} \longrightarrow \mathsf{2CO}$
$\displaystyle \mathsf{Fe_2O_3 + 3CO} \longrightarrow \mathsf{2Fe + 3CO_2}$
$\displaystyle \mathsf{2Fe_2O_3 + 3C} \longrightarrow \mathsf{4Fe + 3CO_2}$
Impurities
- $\displaystyle \mathsf{SiO_2}$
- - $\displaystyle \mathsf{CaSiO_3}$ is slag
- sulphur impurities
  - $\displaystyle \mathsf{Mg + S} \longrightarrow \mathsf{MgS}$
- Phosphorus
- Manganese
Making steel
- O₂ under pressure blown on molten iron
- carbon oxidised to carbon monoxide

Economic factors and recycling

Purity required
Energy costs
Cost of reducing agent
- manufacturing chlorine and sodium
- electrolysis of brine water
Recycling aluminium
- cheaper to recycle aluminium than mine bauxite and electrolysis
- electrolysis needs high temperature and large electricity
- high resistance to corrosion
Recycling of iron
- Earths reservoirs of ore are saved
- energy of mining ore, transportation, smelting is greater than recycling