3.2.3 - group 7, the halogens Flashcards
what is the trend in electronegativity of the halogens?
decreases down the group
the atomic radius increases as there are more energy levels, so the nucleus is less able to attract the bonding pair of electrons
what is the trend in melting and boiling point of the halogens?
increases down the group
molecules become larger meaning they have more electrons and the van der Waals forces between molecules become stronger
the forces therefore require more energy to overcome
what is the trend in oxidising ability of the halogens?
decreases down the group
how can you investigate the oxidising ability of the halogens?
using displacement (redox) reactions of halide ions by halogens - a halogen which is a strong oxidising agent will displace a halogen with a lower oxidising power from one of its compounds
use a spotting tile, chlorine, iodine and bromine solution, and the potassium halides for chlorine, bromine and iodine
what will the results be when investigating the oxidising ability of the halogens?
chlorine will displace bromide and iodide ions, bromine will displace iodide ions and iodine won’t displace anything
the solution colour shows which free halogen is present:
chlorine - pale green, may be colourless
bromine - yellow
iodine - brown, may be black solid
what would the ionic equation be for the reaction of chlorine and potassium bromide?
2Br^- (aq) + Cl2 (aq) –> 2Cl^- (aq) + Br2 (aq)
what would the half equations be for the reaction of chlorine and potassium bromide?
bromine is oxidised:
2Br^- (aq) –> Br2 (aq) + 2e^-
chlorine is reduced:
Cl2 (aq) + 2e^- –> 2Cl^- (aq)
how can you identify and distinguish between the halide ions?
by reacting them with acidified silver nitrate solution:
1. add a few drops of nitric acid to a potassium halide solution
2. add silver nitrate solution dropwise
why is nitric acid used when identifying halide ions?
it reacts with any carbonates present to stop the formation of silver carbonate precipitate (Ag2CO3), as this would mask the desired observations
what results will you see when halide ions react with acidified silver nitrate solution?
fluorides - no precipitate
chlorides - white precipitate (AgCl)
bromides - cream precipitate (AgBr)
iodides - yellow precipitate (AgI)
why are silver halide precipitates then treated with ammonia solution?
it differentiates between the precipitates if the colours look similar
what does each silver halide precipitate do when treated with ammonia solution?
silver chloride dissolves in dilute ammonia forming a complex ion
silver bromide dissolves in concentrated ammonia forming a complex ion
silver iodide is too insoluble to react with ammonia
what is the trend in solubility (most to least) of the silver halides in ammonia solution?
silver chloride, silver bromide, silver iodide
what is the trend in reducing ability of the halogens and why?
increases down the group
as the ions get bigger, the outer electron becomes less strongly attracted to the positive nucleus meaning it is easier for the electron to be donated
how can you investigate the reducing power of the halogens?
by reacting solid sodium halides with concentrated sulphuric acid:
1. place a spatula of the solid sodium halide in a test tube
2. add five drops of concentrated sulphuric acid
3. test the gaseous product with damp blue litmus paper and record the colour of the vapour and contents of the test tube
what are the different oxidation states sulfur may be in during the reaction of a sodium halide and sulfuric acid?
H2SO4 - +6
SO2 - +6
S - 0
H2S - -2
what products are always produced when a sodium halide reacts with concentrated sulfuric acid due to the displacement reaction?
the corresponding hydrogen halide gas and sodium hydrogen sulfate (NaHSO4, a white solid)
what 4 observations are always seen when a sodium halide reacts with concentrated sulfuric acid?
fizzing
white steamy fumes (the hydrogen halide gas)
a white solid (NaHSO4)
litmus paper goes red (acidic hydrogen halide gas produced)
what happens when fluoride and chloride ions react with sulfuric acid?
they aren’t strong enough reducing agents to reduce the S in the acid, so only acid-base reactions occur
what happens when bromide ions react with sulfuric acid?
after the acid-base reaction, bromide ions reduce the sulfur from +6 in the acid to +4 in SO2
what further observations are made when bromide ions react with concentrated sulfuric acid?
orange fumes (bromine)
colourless, acidic gas (SO2)
bromide ions and sulfuric acid - oxidation half equation?
2Br^- (s) –> Br2 (g) + 2e-
bromide ions and sulfuric acid - reduction half equation?
H2SO4 (l) + 2H^+ (g) + 2e^- –> SO2 (g) + 2H2O (l)
bromide ions and sulfuric acid - overall equation?
2Br^- (s) + H2SO4 (l) + 2H^+ (g) –> Br2 (g) + SO2 (g) + 2H2O (l)
what happens when iodide ions react with sulfuric acid?
as the strongest halide reducing agents, they reduce the sulfur romance +6 in the acid to +4 in SO2 to 0 in S to -2 in H2S
what further observations are made when iodide ions react with concentrated sulfuric acid?
black solid and purple fumes (iodine)
colourless, acidic gas (SO2)
yellow solid (sulfur)
gas with rotten egg smell (H2S)
iodide ions and sulfuric acid - oxidation half equation?
2I^- (s) –> I2 (g) + 2e^-
iodide ions and sulfuric acid - reduction half equations (production of SO2, S and H2S)?
H2SO4 (l) + 2H^+ + 2e^- –> SO2 (g) + 2H2O (l)
H2SO4 (l) + 6H^+ (g) + 6e^- –> S (s) + 4H2O (l)
H2SO4 (l) + 8H^+ (g) + 8e^- –> H2S (g) + 4H2O (l)
what is the general equation for the acid-base reaction when a sodium halide reacts with sulfuric acid?
NaX (s) + H2SO4 (l) –> NaHSO4 (s) + HX (g)
