Elements from the sea

• Dynamic Equilibrium


  When the rate of forwards reactions equals the rate of backwards reactions. The concentration of each substance remains constant (not necessarily the same).

  -only occurs in closed systems
• How to calculate equilibrium constant

  Kc = the concentration of products/ concentration of reactants

  
• What happens to equilibrium when temperature increases?

  -equilibrium shifts in endothermic direction to oppose increase in temperature

  -Kc will decrease
• What happens to equilibrium when temperature decreases?

  -equilibrium shifts in exothermic direction to oppose decrease in temperature

  -Kc will increase
• Kc>>1


  -equilibrium lies well over to the right

  -many more products than reactants
• Kc<1


  -equilibrium lies slightly to the left

  -more reactants than products
• Kc=1


  equilibrium is in the middle
• What happens to Kc when there is a change in concentration?

  Nothing. Kc is unaffected.
• What happens to the equilibrium when pressure is increased?

  The equilibrium shifts to the side with the fewest number of gas particles to reduce the pressure.
• What happens to the equilibrium when pressure is decreased?

  The equilibrium shifts to the side with the most gas particles to increase the pressure
• Electrolysis


  The breaking down of a substance using electricity.
• What must the electrolyte be in electrolysis?

  an ionic compound that is molten or in an aqueous solution
• Anode


  + positive electrode

  + negative ions (anions) are attracted to the anode and give up electrons
• Cathode


  -negative electrode

  -positive ions (cations) are attracted to the cathode and receive electrons
• What are electrodes made of?

  Inert and conductive materials e.g. platinum or graphite
• What is the product at the cathode if the electrolyte is a group 1 or 2 metal or aluminium?

  Hydrogen

  2H2O(l) + 2e- -> 2OH-(aq) + H2 (g)
• What is the product at the anode if the electrolyte contains a halide?

  Halogen

  2Cl- (aq) -> Cl2 (g) + 2e-
• What is the product at the cathode in the electrolysis of metal that's not a group 1 or 2 metal or aluminium?

  Metal

  Cu2+(aq) + 2e- -> Cu(s)
• What is the product at the anode if the electrolyte contains a sulfate or nitrate?

  Oxygen

  2H2O(l) -> O2(g) + 4H+(aq) + 4e-
• What is the product at the cathode if the electrolyte is an acid?

  Hydrogen

  2H+(aq) + 2e- -> H2(g)
• What is the product at the anode if the electrolyte contains a hydroxide?

  4OH-(aq) -> O2(g) + 2H2O(l) + 4e-
• Electrolysis in the purification of metals (eg. Copper) at the ANODE

  -made from impure copper

  -copper loses 2 electrons

  - Cu(s) -> Cu2+(aq) + 2e-

  -the anode wears away and becomes lighter
• Electrolysis in the purification of metals (eg. Copper) at the CATHODE

  -made from pure copper

  -copper ions added to the solution from the anode gain electrons at the cathode to become copper again, this 'plates' the cathode

  - Cu2+(aq) + 2e- -> Cu(s)

  -the cathode will become heavier
• Brine


  -high concentration solution of salts

  -found in sea water (made when mineral rocks dissolve in water)

  -some halogens like chlorine can be extracted through the electrolysis of brine
• Chlorine extracted from the electrolysis of brine, anode

  Cl- ions from the salt make Cl2 gas

  2Cl-(aq) -> Cl2(g) + 2e-
• Chlorine extracted from the electrolysis of brine, cathode

  H+ ions from the water make H2 gas

  2H+(aq) + 2e- -> H2(g)
• Chlorine extracted from the electrolysis of brine, solution

  Sodium ions (Na+) are more reactive than hydrogen so remain in the solution. They react with hydroxide ions (OH- from the water) to make sodium hydroxide (NaOH)
• Why must the sodium chloride solutions in the electrolysis of brine be concentrated?

  If the solution is dilute the chloride ions don't release their electrons, instead OH- ions lose their electrons and water and oxygen are produced instead

  4OH-(aq) + 2H2O(l) + O2 + 4e-
• Extraction of bromine


  -uses a more reactive halogen to extract it from its brine

  -chlorine is more reactive than bromine

  -bromine is produced which is condensed and purified into a liquid

  2Br-(aq) + Cl2(g) -> Br2(g) + 2Cl-(aq)
• Extraction of iodine


  -uses a more reactive halogen to extract it from its brine

  -chlorine is more reactive than iodine

  -iodine is produced which is condensed and purified into a grey solid

  2I-(aq) + Cl2(g) -> I2(aq) + 2Cl-(aq)
• OIL RIG


  Oxidation is Loss of Electrons, Reduction is a Gain of Electrons
• Redox reaction


  Involves reduction and oxidation
• Reducing agents


  Lose electrons and are oxidise themselves
• Oxidising agents


  Gain electrons and are reduced themselves
• Oxidation number of every uncombined element

  Always 0

  e.g. Cl2, Fe, O2
• Oxidation number of an ion


  Always the charge on the ion

  e.g. Cl- = -1
• Oxidation number of groups 1, 2 and 3

  Group 1 always = 1

  Group 2 always = 2

  Group 3 always = 3

  e.g. in KCl, K = +1
• Oxidation state of hydrogen


  Always +1 except in hydroxides where it's -1
• Oxidation number of oxygen

  Always -2 (except it's -1 in peroxides and +2 in OF2)
• What happens to the oxidation number in reduction?

  It decreases