2.3 Group 7
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What is the trend in bpt down group 7? Why?
Increases down the group. Because: size of atom increases as more occupied electron shells —> stronger Van de Waals forces of attraction between molecules, take more energy to break
What is the trend in electronegativity down Group 7? Why?
Decreases Because: more occupied electron shells —> greater atomic radius and outer electrons are further from the positive charge of the nucleus —> lower force of attraction between the nucleus and electron pair in the covalent bond
What do you use to test for halide ions
Acidified AgNO3
Why do you add HNO3 ? Why not HCl?
To remove CO3 2- Adding HCl would add Cl- ions creating a false positive
Result and equation for Cl- test?
Ag+ + Cl- —> AgCl (s)
What is the result and equation for the test for Br
Cream ppt Ag+ + Br- —> AgBr (s)
What is the result and equation for the test for I-
Yellow ppt Ag+ + I- —> AgI (s)
What happens (+ equations) to each of the silver halide precipitates when dilute/conc ammonia are added ?
AgCl (s) - dissolves in both conc and dilute ammonia AgCl (s) + 2NH3 —> [Ag(NH3)2]+ (aq) + Cl- AgBr (s) - dissolves in conc ammonia AgBr (s) + 2NH3 —> [Ag(NH3)2]+ (aq) + Br- AgI (s) will not dissolve in either
What is the trend in oxidising ability down Group 7? Why?
Decreases down the group (Cl best, I worst) Because: Cl has fewest occupied electron shells, greatest force of attraction between outer electrons and nucleus, easiest to gain electrons and be reduced —> best oxidising agent
Write the equation for Cl2 oxidising 2I-
Cl2 + 2I- —> 2Cl- + I2
What is the trend in reducing ability down Group 7?
Increases down the group (Cl worst, I best) Because: I- has the most occupied electron shells, so outer electrons are further from the nucleus, weakest force of attraction between positive charge of nucleus and outer electrons —> easiest to be oxidised and lose electrons —> best reducing agent
What products are formed when I reacts with H2SO4? [4]
H2SO4 + 2I- —> SO4 2- + 2HI H2SO4 + 2H+ + 2I- —> SO2 + I2 + 2H2O (SO2 is a choking gas with a pungent smell) H2SO4 + 6H+ + 6I- —> S + 3I2 + 4H2O (S is a yellow solid) H2SO4 + 8H+ 8I- —> H2S + 4I2 + 4H2O (H2S smells of rotten eggs)
What are the products of Br + H2SO4?
HBr and SO2
Does Cl- reduce H2SO4?
No, not a powerful enough reducing agent, only HCl is formed
Why is Chlorine added to drinking water? Why is it safe?
Forms ClO- ions which kill all microorganisms in the water. Once it has done it’s job, little remains, and the health benefits outweigh the risks of using it
What are the potential risks of adding chlorine to water
Chlorine is toxic and damages the respiratory system in large enough quantities; can form carcinogens with hydrocarbons
Why is Ozone not used to purify water in the UK?
More expensive than Chlorine, evaporates from water more quickly
What is the reaction of Cl2 with water?
Cl2 (g) + H2O (l) —> HClO (aq) + HCl (aq)
What type of reaction is Chlorine with water?
Disproportionation; Chlorine is both oxidised and reduced
What are the two forms of the Chlorate ion?
ClO- is Chlorate (I) [ClO3]- is Chlorate (V)
What is the equation for making bleach?
Cl2 + 2NaOH —> NaCl + NaClO + H2O
Give the equation for the reaction of chlorine and water in the presence of sunlight
2Cl2 + 2H2O —> 4HCl + O2
What is desalination?
Converts saltwater into clean, potable water Either by reverse osmosis (using a smart membrane) or by vacuum distillation at low temperature and low pressure
What are the advantages and disadvantages of desalination?
Advantages - Safe, clean, drinkable water produced in places where it might not otherwise be available Disadvantages - uses lots of energy, reverse osmosis has low efficiency, can disturb marine ecosystems