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Elements from the Sea. Chemical Storylines notes. ES1 Why is the sea so salty. Hydrothermal vents are geysers on the sea floor… They constantly spew out mineral-rich water They occur where tectonic plates move apart When they do, sea water enters the crack and gets heated by the mantle

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elements from the sea

Elements from the Sea

Chemical Storylines notes

es1 why is the sea so salty
ES1 Why is the sea so salty
  • Hydrothermal vents are geysers on the sea floor…
  • They constantly spew out mineral-rich water
  • They occur where tectonic plates move apart
  • When they do, sea water enters the crack and gets heated by the mantle
  • Hot water can dissolve more minerals
  • These then get carried into the sea when it ‘erupts’
  • Black smokers are the hottest vents
  • The ‘smoke’ they produce is often iron sulphide
  • This is insoluble and forms by a precipitation reaction…
  • …other ions such as chloride, bromide and some group 1 and 2 metals are more soluble and so stay in solution
CI5.1 “Ions in solids and solutions”
  • Hydrothermal vents account for much of the dissolved chemicals in the sea
  • Others will have reached the sea as rainwater has weathered rocks and eventually run into rivers
  • The constant evaporation of water has caused the salts to accumulate
  • Most seas have similar concentrations of dissolved minerals but the Dead Sea is different…
      • CI1.5 “Concentrations of solutions”
es2 the lowest point on earth
ES2 The lowest point on Earth
  • The lowest point on Earth is the Dead Sea(~400m below sea level)
  • There is no outflow and the surroundings are desert
  • Over thousands of years the resulting evaporation has led to huge accumulations of salts
  • The water contains about 350gdm-3 of salts compared to 40gdm-3 in the oceans
  • It is thought to contain about 43 billion tonnes of salts
  • It contains about 75 times the bromide ions of ocean water
  • A large chemical industry has developed as result
  • Israel is one of the largest bromine producers in the world
      • CI2.5 “Ionisation”
      • ES2.1 “Writing the formulae of ionic compounds”
      • ES2.2 “Solutions of ions”
      • CI3.1 revisit writing formulae (qq 10, 11)
bromine from sea water
Bromine from sea water
  • Uses of bromine:
    • Many pharmaceuticals – analgesics, sedatives, antihistamines
    • Flame retardants
  • Extracting bromine from bromide ions in the lab is simple;

Br-(aq) + Cl2(aq) Br2(aq) + 2Cl-(aq)

  • Extracting it from sea water is more complicated (see next slide)…
        • The sea water is acidified and warmed;
        • Chlorine (Cl2) is added (same reaction as above);
        • Steam is blown through the solution;
        • The vapours are then condensed and bromine forms a separate layer beneath the water (denser than water)
        • Br2 is then distilled and dried
  • This is an example of a very important type of reaction with involves one species being oxidised and one being reduced
  • Such reactions are called redox reactions…
      • CI9.1 ”Oxidation and Reduction”
      • CI11.4 ”Group 7 – The halogens”
      • CI 2.4 “ Sub shells and orbitals”
      • Assignment 3


Warm solution rich in Br- ions

Br2, Cl2 and steam





Acidic effluent solution

Br2 and some Cl2

Cl2 and some Br2



Damp Br2

Dil. sulphuric acid


98% sulphuric acid

Pure Br2

es3 an industrial case study how best to manufacture chlorine
ES3 An industrial case study – how best to manufacture chlorine?
  • Made by electrolysis of concentrated sodium chloride solution (brine) (NaCl (aq))
  • Can be extracted from the sea or from rock salt which can be mined or collected by pumping water into it
  • Electrolysis of brine produces two other very useful chemicals; hydrogen (H2)and sodium hydroxide (NaOH) (an alkali)
  • The industry is often referred to as the chlor-alkali industry
  • We will look at two ways of producing chlorine from brine both of which use electrolysis; the mercury cell and the membrane cell
      • CI15.1-15.6 “Greener Industry”

Mercury cell

  • First large-scale method used;
  • Uses toxic mercury and is more expensive to run than other methods
  • Will be phased out in Europe by 2020
Membrane Cell
  • Introduced in 1980s
  • Improvement on mercury cell because;
    • Lower energy requirements so lower

running costs

    • More Cl2 can be made in the same

factory space

    • No need for costly removal of mercury
    • Less pollution
  • Start up costs (fixed costs) are recouped within 5 years
  • Half equations are;
  • At +ve electrode (anode):

2Cl-(aq) Cl2(g) + 2e-

  • At -ve electrode (cathode):

2H2O(l) + 2e- 2OH-(aq) + H2(g)

  • Overall…

2Cl-(aq) + 2H2O(l) Cl2(g) + 2OH-(aq) + H2(g)

The cell must be designed to:
    • Keep Cl2 at +ve electrode away from OH- around –ve electrode
    • Minimise Cl- contaminating NaOH solution
    • Minimise OH- diffusing towards +ve electrode
    • Prevent Cl2 and H2 mixing which could lead to an explosion
  • This is achieved by using a

membrane made of


(PTFE or teflon)








  • The PTFE is modified so it has negatively charged side groups
  • This means the PTFE is a barrier to gases and liquids and even repels OH- ions
  • Therefore only sodium ions (Na+(aq)) are able to pass through……
      • Assignment 4 and 5







es4 from atomic bombs to drinking water
ES4 From atomic bombs to drinking water
  • Although halogens are hazardous to produce and transport, they are very important in our daily lives

From safer water to cleaner clothes

  • If we pass chlorine gas through cold sodium hydroxide we form sodium chlorate(I):

2NaOH + Cl2 NaCl + NaClO + H2O

  • NaClO is the active chemical in bleach
  • As both reactants are made by electrolysis, bleach is often made on the same site
  • 12% NaClO is used to kill bacteria in water purifying plants
  • 5% NaClO is used in household bleach
  • Bleach is an oxidising agent and breaks bonds in the coloured chemicals to form ones which are colourless
ES4.1 “Testing bleaches”
  • Chlorine has a poor public image;
    • Cl2 and phosgene (COCl2) were used in WW1
    • Organochlorine pesticides have polluted the land
    • Chlorofluorocarbons (CFCs) have polluted the atmosphere
  • However it is used in many ways that improve our quality of life;
    • Water treatment to kill bacteria, etc…
    • Plastics such as PVC
    • Manufacture of polyurethane
    • Solvents for removing grease from anything from clothes to metal
      • Assignment 6
      • CI3.1 “Why are bonds like bears?”
      • CI5.3 “Intermolecular forces”
      • ES4.2 “What do halogens look like?”
uses of other halogens
Uses of other halogens
  • Fluorine
    • First large scale use was to make UF6 for the Manhattan project which developed the atomic bomb
    • So reactive it is almost impossible to store
    • Has to be made and then used immediately
    • Used to make;
      • sodium fluoride (NaF) which is added to toothpaste and some water supplies
      • HCFCs for refrigeration
      • Teflon (PTFE)
  • Iodine
    • Dark grey solid, when heated it sublimes to form a purple vapour
    • Obtained from kelp (seaweed)
    • Uses:
      • Antiseptic
      • Needed in our bodies to produce hormones in our thyroid gland
    • Liquid at rtp
    • Toxic and makes skin blister if spilt
    • Transported in lead-lined steel tanks
      • ES4.3 “This liquid is dangerous”
    • Used for;
      • Flame retardants
      • Silver bromide (AgBr) is used in photographic film
      • Medicines
      • Dyes
      • Pesticides (e.g. bromomethane) (although it does also damage the ozone layer)
      • ES4.4 “Reactions of halogens and halides”
      • ES4.5 “Check your knowledge (part 1)”
es5 hydrochloric acid an industrial success
ES5 Hydrochloric Acid – an industrial success
  • HCl is often a secondary product.
  • A plant makes a particular chemical and a by-product is also made…
  • …this by-product is then used to make HCl (g) …
  • …which is then used to make HCl (aq) (hydrochloric acid)
  • A good example of this is at an electrolysis plant;

H2 + Cl2 2HCl

      • CI15.7 “Percentage yield and atom economy”
  • It is then dissolved in water to make hydrochloric acid at a concentration of about 30% by mass
  • It is cheaper to transport it conc. and dilute it at its destination
      • ES5.1 “Finding the concentration of an acid solution”
      • Assignment 8
      • ES5.2 “Manufacturing halogens and their compounds”
  • HCl (g) (is made of covalent molecules but dissolves in water to form hydrated ions H+ and Cl-
es6 treasures of the sea
ES6 Treasures of the sea
  • Everything from bacteria to humans produce organohalide compounds
  • To date chemists have recorded 4519 unique, naturally occurring ones. Examples include…
  • Epibatidine which is 500 times more potent than

morphine – produced by a frog the size of your

finger nail

  • Many sponges, corals and seaweed use organohalide compounds as part of their defence
  • Some sea slugs synthesise brominated compounds such as the bitter-tasting panacene to discourage predators
  • These consist of an alkane with one or more halogen atoms in the place of hydrogen atoms
  • They are made in the oceans (e.g. seaweeds make CH3Br), by vegetation and in forest fires
  • Halogenoalkanes can then react in a number of ways…
    • CH3Br can react with Cl- ions in sea water
    • One halogen can substitute for another…

2CH3Cl + I2 2CH3I + Cl2

    • These are often radical substitution reactions
    • They can react with water…

CH3Cl + H2O CH3OH + HCl

    • This is a hydrolysis reaction
During the 20th century halogenated compounds were used as refrigerants, aerosol propellants and solvents
  • Many of these were CFCs (chlorofluorocarbons)
  • The use of these is now being phased out as a result of their impact on the atmosphere (see next topic “Atmosphere”)
      • CI13.1 “Halogenoalkanes”
      • ES6.1 “Nucleophilic substitution mechanism”
  • The ease of making a halogenoalkane depends on which halogens and how many you want to use
      • ES6.2 “Halogenoalkanes reactivity”
  • Halogenoalkanes can be made by reacting an alcohol with the appropriate hydrogen halide
  • This is an example of a substitution reaction
ES6.3 “Making a halogenoalkane”
  • Small amount of chloromethane is also made by reacting methane with chlorine.
  • This produces four products with different numbers of chlorine atoms joined to the carbon
  • The mixture is then separated by fractional distillation
  • Main use nowadays is in manufacture of polymers such as PTFE (teflon)
      • Assignment 9