3.1.3 The halogens
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describe the trend in reactivity down g17 (7)
• reactivity decreases > atomic radius + electron shielding inc > nuclear attraction to outer shell e- dec > outer electron gained less easily so less reactive
describe the trend in bp down g17(7)
• increases > more electrons + stronger London forces > more energy required to break them • also elements change states from gas to solid
describe the trend in electronegativity down g17(7)
• decreases > more electron shells + electron shielding as greater atomic radius > nuclear attraction to a shared pair of electrons decrease
describe what happens when halogens undergo redox reactions
• halogen is reduced (gains e- and forms a 1- halide) • another species is oxidised (loses e-)
why are halogens oxidising agents
• they cause another species to be oxidised but is reduced itself
describe the halogen displacement reactions
• a more reactive halogen displaces a less reactive one from its compound
how can you tell solutions of iodine and bromine apart
• cyclohexane (organic non-polar solvent) added and mixture shaken > non-polar halogens dissolve more readily than in water • br = orange colour • iodine = deep violet
what is the colour of chlorine in water + cyclohexane
• both are pale green
what is the colour of bromine in water + cyclohexane
• both are orange
what is the colour of iodine in water + cyclohexane
• brown in water • violet in cyclohexane
when chlorine displaces bromine what colour solution forms
• orange colour from Br2 formation Cl2(aq) + 2Br-(aq) —> 2Cl-(aq) + Br2(aq)
when chlorine displaces iodine what colour solution forms
• violet from I2 formation Cl2(aq) + 2I-(aq) —> 2Cl-(aq) + I2(aq)
when bromine displaces iodine what colour solution forms
• violet from I2 formation Br2(aq) + 2I-(aq) —> 2Br-(aq) + 12(aq)
what is disproportionation
• a redox reaction in which the same element is both oxidised + reduced
the disproportionate reaction of chlorine with water is useful because
• it can be used as a disinfectant for drinking water • the chloric(l) acid and chlorate(l) ions kills bacteria + acts as a weak bleach
give the formula for chlorine reacting with water
Cl2(aq) + H2O(l) —> HClO(aq) + HCl(aq)
when universal indicator is added to HClO what colour does it turn
• first red due to acid > but then colour disappears as bleaching action takes place
reaction of chlorine + water is limited by low solubility of chlorine in water, to combat this what is used instead
• if water contains dissolved sodium hydroxide, much more chlorine dissolves
to make household bleach, what do you need to react + give full formula
• chlorine + cold dilute aqueous sodium hydroxide = soidum chlorate + sodium chloride + water Cl2(aq) + 2NaOH(aq) —> NaClO(aq) + NaCl(aq) + H2O(l)
what are some benefits of chlorine use
• ensures drinking water is safe + bacteria is killed
what are some risks of chlorine use
• chlorine is toxic gas • chlorine is respiratory irritant in small concs + can be fatal in large concs • if chlorine reacts with hydrocarbons, chlorinated hydrocarbons form which are suspected of forming cancer