3.2.3.1 Trends in properties
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How can you identify halide ions?
All metal halides, except fluorides, react with silver ions in aqueous solution. Solution is first acidified by adding dilute nitric acid. The nitric acid reacts with and removes impurities which would of produced a confusing precipitate Add a few drops of silver nitrate solution The colour of the precipitate indicates the halide ion
Why does bromine dissolve into hexane rather than water?
Strength of the bonding between solvent molecules is similar to the strength of bonding between simple covalent molecules and the solvent Strength of bonds between water and bromine isn’t sufficient to overcome the hydrogen bonding
What are the properties of hydrogen halides?
All have the formula HX All dissolve in water All form acidic solutions due to formation of H+ ions Are all gases
What are the colours and natural states of the halogens?
Fluorine = pale yellow gas Chlorine = green gas Bromine = red - brown liquid Iodine = black solid
What is the trend for bond energies of the halogens in their natural state?
Decreases down the group However F - F is weaker than chlorine and bromine
What is the trend in electronegativities of the halogens?
Decreases in electronegativity down the group This is because the shared pair of electrons becomes further from the nucleus the further down the group you get
What is the trend in melting and boiling points of the halogens?
Increase going down the group This is due to increased size which increases the van der Waals forces Additionally they become more volatile up the group
What is the trend in oxidising ability of halogens?
Oxidising ability increases going up the group Fluorine = one of the most powerful oxidising agent one
What happens in a halogen displacement reaction?
A more reactive halogen displacing a halide in a compound This is a redox reaction A halogen will displace the ion of a halogen below it in the periodic table
What happens to halide ions in reactions?
They act as reducing agents They lose electrons to become halogen molecules
What is the trend in reducing ability of halide ions?
Increasing reducing power down the group This is because the ions are larger so are further from the nucleus and loses the electron more easily
How does sodium chloride react with concentrated sulfuric acid?
NaCl + H2SO4 = NaHSO4 + HCl Chloride is a weak reducing agent Similar reaction happens with sodium fluoride to produce hydrogen fluoride
How does sodium bromide react with concentrated sulfuric acid?
Two reactions occur: NaBr + H2SO4 = NaHSO4 + HBr 2H+ ➕ 2Br- ➕ H2SO4 = SO2 ➕ 2H2O ➕ Br2 Bromide are strong reducing agents to reduce sulfuric acid Redox reaction Exothermic
How does sodium iodide react with concentrated sulfuric acid?
NaI + H2SO4 = NaHSO4 + HI Iodide is a stronger reducing agent so it reduces sulfuric acid even further 8H+ ➕ 8I- ➕ H2SO4 = H2S ➕ 4H2O ➕ 4I2
What happens when you add a metal halide (except fluorides) to silver ions in solution?
A precipitate of an insoluble silver halide is formed Dilute nitric acid is first added to the halide solution to remove any carbonate or hydroxide ions which would confuse the result
What are the results of the halide ion test?
Silver fluoride: soluble in water Silver chloride: white precipitate, dissolves in dilute ammonia Silver bromide: cream precipitate, dissolves in concentrated ammonia Silver iodide: pale yellow precipitate, insoluble in concentrated ammonia
How does chlorine react with water?
Reversible reaction Cl2 + H2O = HClO + HCl Disproportionation reaction
How does chlorine react with water in sunlight?
2Cl2 + 2H2O = 4HCl + O2
What is an alternative to the direct chlorination of swimming pools?
Add solid sodium or calcium chlorate NaClO ➕ H2O = Na+ ➕ OH- ➕ HClO Reversible reaction
How does chlorine react with dilute sodium hydroxide?
Cl2 + 2NaOH = NaClO + NaCl + H2O Other halogens behave similarly