Section 4 Elements From The Sea Flashcards
(20 cards)
Rules for cathode product in aqueous solution?
- If metal is less reactive than hydrogen ( copper, silver, gold, platinum, mercury), then the metal will form.
- If metal is more reactive than hydrogen ( group 1 and 2 and aluminium), hydrogen gas will form.
Rules for anode product in aqueous solution?
- If solution doesn’t contain a halide, oxygen will form.
- If solution is concentrated and contains a halide, then the halogen will be formed.
- If the solution is dilute and contains a halide, oxygen will form.
Why does chlorine displace bromine and iodine?
It is more reactive. Because as you go down the group, atoms become larger, so their outer electrons are further from the nucleus. The outer electrons are also shielded from the attraction of the positive nucleus because there are more inner electrons. This makes it harder for larger atoms to attract the electron needed to form an ion.
What are iodine-sodium thiosulfate titration for and what is the first step in them?
To find the concentration of an oxidising agent ( the more concentrated an oxidising agent is, the more ions will be oxidised by a certain volume of it.)
Step 1: Use sample of oxidising agent to oxidise as much iodide as possible.
1. Measure out 25cm³ potassium iodate (v) ( the oxidising agent)
2. Add this to an excess of acidic potassium iodide solution. The iodate (v) ions will oxidise some iodide ions to iodine:
IO3- + 5I- + 6H+ —>3I2 + 3H2O
How do you find how many moles have been produced of iodine in step 2 of the iodine-sodium thiosulfate titration?
This can be done by titrating the solution with sodium thiosulfate.
1. Pour all solution from 1 into flask.
2. Add sodium thiosulfate from burette into solution.
3. When solution reaches straw yellow colour, add 2cm³ starch solution. It will turn blue, indicating that iodine is still present.
4. Add sodium thiosulfate drop by drop until blue colour disappears.
5. When this happens, all iodine has reacted and number of moles of iodine can be calculated.
E.g. solution from step 1 reacted with 11.1 cm³ of 0.12 sodium thiosulfate.
I₂ + 2 S₂O₃²⁻ → 2 I⁻ + S₄O₆²⁻
Moles thiosulfate= 0.12 × 11.1/1000 = 1.332× 10-³
So moles of iodine = 1.332× 10-³ ÷2 = 6.66×10-⁴
How can the concentration of the oxidising agent be calculated from moles of iodine?
Original equation = IO3- + 5I- + 6H+ –> 3I2 + 3H2O
25 cm³ potassium iodate (v) makes 6.66 ×10-⁴ moles I2. So 6.66×10-⁴ ÷ 3 = 2.22 × 10-⁴ moles iodate (v)
So 2.22 × 10-⁴= conc× 25/1000
Conc = 2.22×10-⁴ ÷ 25 ×1000 = 8.88×10-³ mol dm-³
Appearance of each of the halogens at RTP and volatility trend of halogens?
F2= pale yellow gas
Cl2= yellow-green gas
Br2= red-brown liquid
I2= shiny grey solid
Volatility decreases down group.
Colour of halogens in water and hexane?
Cl2: water= colourless hexane= colourless
Br2: water= yellow/orange hexane= orange/red
I2: water= brown hexane= pink/violet
What acid can be added to an ionic halide to make a hydrogen halide?
Phosphoric acid. ( H3PO4)
Which acid can be added to ionic halides that will produce hydrogen chloride but not hydrogen bromide or hydrogen iodide?
Sulfuric acid (H2SO4)
When sodium bromide/iodide react with H2SO4, the bromide/iodide ions are oxidised to bromine/iodine gas.
Because iodine and bromide are strong enough reducing agents to reduce Sulfuric acid.
Thermal stability trend of hydrogen halides and why?
Decreases down halides.
Because halogen atoms get bugger down the group, so bonding electrons are further away from nucleus and shielded by more inner electron shells.
Why are hydrogen halides acidic?
When dissolved in water, they dissociate. So molecule splits apart to form hydrogen ion and halide ion.
Hydrogen ions make the solutions acidic.
Hydrogen halides reaction with ammonia?
Ammonia accepts proton to form NH4+. Ammonium ions then bond with negative halide ion to produce an Ammonium halide.
E.g. HF + NH3 > NH4F
Same for all halides
Hydrogen halides reaction with sulfuric acid?
HF and HCl don’t react
HBr reduces H2SO4 to SO2
HI reduces H2SO4 to H2S
Silver halides reaction with ammonia solution?
AgCl: ppt dissolves in dilute ammonia to give colourless solution.
AgBr: ppt remains unchanged in dilute ammonia, but will dissolve in concentrated ammonia solution to give a colourless solution.
AgI: ppt does not dissolve, even in concentrated ammonia solution.
Uses of chlorine and why is it risky to store and transport?
Uses: water sterilisation by killing disease-causing microorganisms, bleach
Storing and transporting: chlorine is very toxic and corrosive. Must be kept away from flammable materials. Must be transported as a liquid under pressure in small cylinders.
Atom economy=
Molecular mass desired product/ molecular mass of all products ×100
Define dynamic equilibrium.
When the rate of forward and backward reactions are equal, in a closed system.
Affect of pressure on equilibrium?
- Increasing pressure shifts equilibrium to side with fewer gas molecules.
- Decreasing pressure shifts equilibrium to side with more gas molecules.
Affect of temperature on equilibrium position?
- Increasing temperature will shift in endothermic direction to absorb heat.
- Decreasing the temperature renews heat. The equilibrium shifts in the exothermic direction to replace the heat.
