4.2 Inorganic chemistry of the halogens

Describe and explain the trend in melting/boiling temperatures of the halogens.

Melting/boiling temperature increases down group 7. This is because: - Halogens exist as diatomic molecules therefore the strength of intermolecular forces determines the mps/bps. - Down group 7, the number of electrons increases - Therefore the strength of the London forces between molecules increases - These require more energy to overcome each time

What fluorine's physical state and appearance at room temperature?

Pale yellow GAS

What is chlorine's physical state and appearance at room temperature?

Pale green GAS

What is bromine's physical state and appearance at room temperature?

Orange/brown LIQUID which lets off brown fumes

What is iodine's physical state and appearance at room temperature?

Grey SOLID but often sublimates with little heat to form purple vapours

What is astatine's physical state and appearance at room temperature?

Black solid (radioactive)

Describe and explain the trend in electronegativity of the halogens.

- Electronegativity decreases down group 7. This is because: - As you go down group 7 the atomic radius increases, therefore the electrostatic attraction between the nucleus and bonding pairs is weaker - The shielding increases as you go down group 7 - These outweigh the increase in nuclear charge

Describe and explain the trend in reactivity of the halogens.

Reactivity decreases down group 7 This is because: - the electronegativity decreases down group 7 - therefore the ability to attract a bonding pair of electrons is less - therefore their oxidising power reduces

What reaction between groups 1 and 2 and the halogens will be the most vigorous?

Caesium/francium and fluorine

What type of reaction is the reaction between groups 1/2 metals and halogens?

Redox: - the halogens are reduced, hence oxidising agents - the metals are oxidised by the halogens

Describe the changes in oxidation numbers of a halogen reacting with group 1/2.

- Halogen is reduced from 0 to (-)1 - Metal is oxidised from 0 to (+)1 or (+2) depending on the group.

What is the order of displacement of the halogens?

- Chlorine displaces both bromine and iodine - Bromine displaces iodine but not chlorine - Iodine does not displace bromine or chlorine

In what conditions does halogen displacement occur?

In aqueous solution followed by the addition of an organic solvent such as cyclohexane.

Why does an organic solvent need to be added?

To make the colour changes more obvious, as colour changes of the halogens in aqueous solution tend to be very similar and hard to distinguish.

What is the colour of chlorine in aqueous solution?

Pale green

What is the colour of chlorine in hexane?

Pale green

What is the colour of bromine in aqueous solution?

orange

What is the colour of bromine in hexane?

Orange-red

What is the colour of iodine in aqueous solution?

brown

What is the colour of iodine in hexane?

purple / violet

Why does chlorine displace both bromine and iodine?

Chlorine is most electronegative: - it is the smallest atom, so the incoming electron gets closer to the nucleus and is more attracted to the nucleus - it has the smallest number of quantum shells therefore the least shielding, hence the bonding pair experiences the least repulsion

What is disproportionation?

A redox reaction in which the same species is both oxidised and reduced

Write an equation for the disproportionation of chlorine in water. With reference to oxidation states, why is this disproportionation?

Cl2 + H2O ——> HCl + HClO Chlorine is oxidised from 0 to (+)1 and reduced from 0 to (-)1 simultaneously.

What industrial benefit does chlorine have in terms of water treatment?

Chlorine is added to drinking water and pool water to kill pathogens to make it safe for consumption/swimming.

Write an equation for the disproportionation of chlorine with cold dilute NaOH. With reference to oxidation states, why is this disproportionation?

Cl2 + 2NaOH ——> NaCl + NaClO + H2O Chlorine is oxidised from 0 to (+)1 and reduced from 0 to (-)1 simultaneously.

What is the industrial purpose of sodium chlorate (I)?

Manufacture bleach (NaClO), a strong disinfectant.

Write an equation for the disproportionation of chlorine in hot, concentrated NaOH. With reference to oxidation states, why is this disproportionation?

3Cl2 + 6NaOH ——> 5NaCl + NaClO3 + 3H2O Chlorine is oxidised from 0 to (+)5 and reduced from 0 to (-)1 simultaneously.

What is the industrial purpose of sodium chlorate (V)?

Weed killer

What is the trend in the reducing power of the halide IONS?

Reducing power increases down Group 7 as electrons are easier to lose from larger ions.

What analogous reactions are used to determine the reducing power of halide IONS?

Reaction with concentrated H2SO4 in which sulfur is reduced to varying oxidation states based on the reducing power of the halide.

Write the equation for the reaction of sulfuric acid with chloride ions. How does this demonstrate its reducing power?

NaCl + H2SO4 ——> NaHSO4 + HCl Effectively, no redox reaction occurs here. H2SO4 deprotonates to HSO4- and H+ The chloride ion from HCl reacts with this proton and reforms the HCl and a salt. Demonstrating the poor reducing power of chloride ions as an acid-base reaction occurs instead of redox.

What observations can be made for this reaction?

Misty fumes

Write the equation for the reaction of sulfuric acid with bromide ions. Describe any redox reactions that occur as a result. How does this demonstrate its reducing power?

NaBr + H2SO4 ——> NaHSO4 + HBr (+ Br2 + SO2) One redox reaction occurs here. Sulfur is reduced from (+)6 to (+)4, therefore Br- ions are more powerful reducing agents than Cl-. HBr reacts with H2SO4 further which reduces the H2SO4 to form SO2, a colourless gas. Bromide ions in HBr are therefore oxidised to form bromine, a brown gas.

What are the observations for this reaction?

Misty fumes (HBr) Brown fumes (Br2) Colourless gas with choking smell (SO2)

Write the equation for the reaction of sulfuric acid with iodide ions. Describe any redox reactions that occur as a result. How does this demonstrate its reducing power?

NaI + H2SO4 ——> NaHSO4 + HI (+ I2 + SO2 + S+ H2S) Three redox reactions occur here. Sulfur is reduced 3 times from (+)6 to (+)4, 0 and (-)2, showing iodide ions are stronger reducing agents than bromide and chloride. HI reacts further with H2SO4, which reduces H2SO4 to SO2 and hence oxidises iodide ions in HI to iodine gas. HI reacts again with H2SO4 reducing it to sulfur and iodine gas. HI reacts again reducing H2SO4 to H2S and iodine gas.

Describe a chemical test for halide ions.

- add silver nitrate solution and nitric acid to the sample - silver halide precipitate will form - add dilute NH3 and observe the changes - add conc. NH3 and observe the changes

What is the positive test for chloride ions? Does this redissolve in dilute NH3? Does this redissolve in conc. NH3?

A white AgCl ppt. Yes Yes

What is the positive test for bromide ions? Does this redissolve in dilute NH3? Does this redissolve in conc. NH3?

A cream AgBr ppt. No Yes

What is the positive test for iodide ions? Does this redissolve in dilute NH3? Does this redissolve in conc. NH3?

A pale yellow AgI ppt No No

What is the general ionic equation for the precipitation reaction of a halide with AgNO3?

Ag+ + X- ——> AgX

Why do the precipitates redissolve? Give an exemplar equation using silver chloride including state symbols.

The formation of a complex ion. AgCl(s) + 2NH3(aq) ——> [Ag(NH3)2]+(aq) + Cl-(aq) The complex ion and chloride ions are colourless when aqueous.

What does a hydrogen halide reacting with water form?

Acidic solution, hyrdogen halides act as acids, donating a proton to water to form H3O+ and the halide ion. All solutions are colourless.

Why is the reaction of HF with water reversible, but the other hydrogen halides not?

HF is a weak acid HCl, HBr and HI are all strong acids

What does a hydrogen halide reacting with ammonia form?

Ammonium salt