The halogens & qualitative analysis of ions
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What colour are the halogens in their standard states?
• Flourine - pale yellow gas • Chlorine - green gas • Bromine - red liquid that gives off brown/orange fumes • Iodine - shiny grey solid
Explain the trend in boiling and melting points of the halogens.
• Trend: Melting and boiling points increase. • Down the group, molecles become larger. This leads to them having more electrons, so more London Forces between each molecule. • More energy is then required to break these London Forces, so melting and boiling points increase.
In inorganic solution, what colours will the halogens display?
• Chlorine - very pale green • Bromine - yellow solution • Iodine - blue/black solution
What halogens will chlorine displace?
Bromine and iodine
What halogens will bromine displace?
Only iodine
What halogens will iodine displace?
None.
State and explain the trend in the reactivity of halogens.
• Trend: Reactivity decreases down the group • The size of each atom increases down the group, as such shielding increases. This causes the atoms to less easily accept an electron, decreasing their reactivity.
What are the colours of the halogens when dissolved in a cyclohexane solvent?
• Chlorine = colourless • Bromine = yellow • Iodine = purple or violet
What is the general equation for the displacement of any halogen?
X₂ + 2Y⁻ -> 2X⁻ + Y₂ Where X is the more reactive halogen
What is meant by disproportination?
When an element is simltaneously oxidised and reduced in a reaction
• What products are made when chlorine reacts with water? • How is this reaction disproportination?
• HClO + HCl • Chlorine is reduced from 0 in Cl₂ -> -1 in HCl • Chlorine is oxidised from 0 in Cl₂ -> +1 in HClO
State a use of chlorine.
Water treatment, to kill bacteria in drinking and pool water
Halogens react with bases. • State the equation for when chlorine reacts with Sodium Hydroxide. • State the conditions required for this reaction. • Explain why the reaction is considered to be disproportination.
• Cl₂ + 2NaOH -> NaCl(aq) + NaClO(aq) + H₂O • Cold, aqueous sodium hydroxide • Chlorine is simltaneously oxidised and reduced. 0 in Cl₂ -> -1 in NaCl (reduction) and 0 in Cl₂ -> +1 in NaClO (oxidation)
How do we test for the prescene of halide ions in an unknown solution and what observation returns a positive result?
• Add nitric acid and then silver nitrate drop by drop. • Chlorides produce a white ppt • Bromides produce a yellow ppt • Iodides produce a pale yellow ppt
How can ammonia be used to differenciate between different solutions when the colours appear similar?
• AgCl will dissolve in dilute ammonia to form Cl- ions • AgBr will dissolve in concentrated ammonia to form Br- ions • AgI will not dissolve. It is insoluble.
When testing for ions, what does the CaSH acronym stand for?
Carbonates > Sulphates > Halides The order of testing for inorganic ions, to remove false positive results
Write a full method to find the identity of 4 unknown solutions. Each solution contains either….. * Sodium carbonate, Na₂CO₃ * Ammonium chloride, NH₄Cl * Sodium chloride, NaCl * Sodium sulphate, Na₂SO₄
• Add all 4 solutions to a test tube. • Add nitric acid to each test tube. The solution that effervesces (fizzes) is the metal carbonate. • Then, add barium nitrate to the 3 remaining test tubes. The solution that forms a white ppt is the metal sulphate. • Add silver nitrate to the final 2 test tubes. White ppts should form as both solutions should contain chlorine ions. • To distinguish between the Ammonium salt and metal salt, add sodium hydroxide and place it in a warm water bath. After 10 or so minutes, place a piece of damp litmus paper over the top of each test tube. If it turns blue, ammonia is present.