group 7, the halogens (3.2.3) in papers 1 & 3

Question 1

what happens to the atomic radius when you go down group 7?

Answer 1

increases

Question 2

what happens to the electronegativity as you go down group 7?

Answer 2

• decreases
• as the halogen atom gets bigger, any bonding pair electrons gets further away from the nucleus and so is less strongly attracted towards it.

Question 3

state the appearance and state of fluorine, chlorine, bromine and iodine.

Answer 3

• F: very pale yellow gas
• Cl: greenish gas, colourless solution
• Br: red liquid, yellow solution
• I: grey solid, brown solution

Question 4

what happens to the bpt and mpt as you go down the group?

Answer 4

• increases
• As the molecules become larger they have
  more electrons and so have larger van der
  waals forces between the molecules. As the
  intermolecular forces get larger more energy has to be put into break the forces. This increases the melting and boiling points.

Question 5

what happens to the oxidising power as you go down group 7?

Answer 5

• decreases
• they oxidise other elements and are reduced in the process.
• they become halides after they take 1 electron.

Question 6

what colours are present when a halogen gets displaced?

Answer 6

• The colour of the solution in
  the test tube shows which free
  halogen is present in solution.
  Chlorine =very pale green
  solution (often colourless),
  Bromine = yellow solution
  Iodine = brown solution

Question 7

which halogen is the most effective oxidising agent?

Answer 7

• chlorine
• it displaces bromine and iodine
• incoming electrons will experience less shielding
• its the smallest halogen which makes it easier for the nucleus to attract an electron

Question 8

what are halides?

Answer 8

• reducing agents
• 2X -> X2 + 2e-

Question 9

whats the trend in reducing capabilities of halides?

Answer 9

• reducing power increases down a group, fluoride being very weak and iodide being very strong.
• as the number of shells in the ion increases, there is more shielding so the electrons become less strongly held.

Question 10

how far can hallides reduce H2SO4?

Answer 10

• KF: cant
• KCl: cant
• KBr: SO2
• KI: SO2, S, H2S

Question 12

whats the test for hallides?

Answer 12

• the use of nitric acid solution followed by aqeuous silver nitrate.

Question 13

why is HNO3 added in the test for halides?

Answer 13

to prevent the precipitation of any other silver salts.

Question 14

explain the obeservation of Cl in the halide test with AgNO3 and NH3?

Answer 14

• white percipitate is formed (Agcl)
• dissolved in the dilute NH3.
• AgCl is soluble in H20 and NH3.

Question 15

explain the obeservation of I- in the halide test with AgNO3 and NH3?

Answer 15

• yellow percipitate formed (AgI)
• didnt dissolve in either diluted or concentrated NH3.
• AgI is insoluble in both H20 and NH3.

Question 16

explain the obeservation of F in the halide test with AgNO3 and NH3?

Answer 16

• no reaction, stays colourless.
• didnt dissolve in any.
• AgF is soluble in both **H20 **and NH3

Question 17

explain the obeservation of Br in the halide test with AgNO3 and NH3?

Answer 17

• cream percipitate if formed (AgBr)
• dissolved in only concentrated NH3.
• AgBr is insoluble in water and dilute NH3 but soluble in concentrated NH3.

Question 18

explain the reaction of Cl with water when bubbled through.

Answer 18

• chlorate (I) ions form, acts as a bleach as an equilibrum is formed.
• ClO-
• its reduced and oxidised (disproportionation)
• Cl2 + H2O -> HClO + HCl , HClO is a mild oxidising agent and effective in killing bacteria.

Question 19

explain the reaction of Cl with a cold dilute alkali.

Answer 19

• disproportionation reaction
• Cl2 + 2NaOH -> NaCl + NaOCl (important oxidising agent and is the active ingredient in domestic bleach) + H20

Question 20

explain the reaction of Cl with water when sunlight is present.

Answer 20

• goes to completion and is instead a redox reaction.
• 2Cl2 + 2H20 -> O2 + 4HCl