Halogens

Question 1

The following pairs of compounds can be distinguished by simple test-tube reactions.For each pair, give a suitable reagent that could be added separately to each compound to distinguish between them.Describe what you would observe in each case.AgBr(s) and AgI(s)ReagentObservation with AgBr(s)Observation with AgI(s)

Answer 1

concentrated sulfuric acid OR c(onc) H2SO4 (cream solid) turns orangeOR orange / red / brown fumes / gas / vapour(yellow solid) turns blackOR purple fumes / gas / vapourOR correct reference to H2S observation (eg bad egg smell)

Question 2

The following pairs of compounds can be distinguished by simple test-tube reactions.For each pair, give a suitable reagent that could be added separately to each compound to distinguish between them.Describe what you would observe in each case.HCl(aq) and HNO3(aq)Reagent ……………………………………………………………………………………………………………… Observation with HCl(aq) …………………………………………………………………………………………………………………………………………………………………………………………………………………..Observation with HNO3(aq) .

Answer 2

AgNO3 OR silver nitrate OR any soluble silver salt white precipitate or white solid / white suspensionremains colourless OR no reaction OR no (observed) change OR no precipitate

Question 3

The following pairs of compounds can be distinguished by simple test-tube reactions.For each pair, give a suitable reagent that could be added separately to each compound to distinguish between them.Cyclohexane and cyclohexeneReagent ……………………………………………………………………………………………………………… Observation with cyclohexane …………………………………………………………………………………………………………………………………………………………………………………………………………….Observation with cyclohexene .

Answer 3

Br2 OR bromine (water) OR bromine (in CCl4 / organic solvent)(stays) orange / red / yellow / brown / the sameOR no reaction OR no (observed) changeOR reference to colour going to cyclohexane layerdecolourised / goes colourless / loses its colour

Question 4

The following pairs of compounds can be distinguished by simple test-tube reactions.For each pair, give a suitable reagent that could be added separately to each compound to distinguish between them.Butanal and butanoneReagent ……………………………………………………………………………………………………………… Observation with butanal ……………………………………………………………………………………………………………………………………………………………………………………………………………………Observation with butanone .

Answer 4

acidified potassium manganate(VII) or KMnO4/H2SO4purple to colourless solution OR goes colourless(stays) purple or no reaction or no (observed) change

Question 5

Chlorine displaces iodine from aqueous potassium iodide. Write the simplest ionic equation for this reaction.Give one observation that you would make when this reaction occurs.In bright sunlight, chlorine reacts with water to form oxygen as one of the products. Write an equation for this reaction.

Answer 5

1⁄2Cl2 + I– —————>1⁄2 I2 + Cl– ORCl2 + 2I– —————->I2 + 2Cl–(Solution turns from colourless to) brown / red-brown solution2Cl2 + 2H2O ————->4HCl + O2

Question 6

Explain why chlorine has a lower boiling point than bromine (2)

Answer 6

Chlorine is smaller than bromine OR has fewer electrons/electron shellsandThe forces between chlorine / Cl2 molecules are weaker (than the forces between bromine / Br2 molecules leading to less energy needed to separate the molecules)

Question 7

Chlorine is a powerful oxidising agent.Write the simplest ionic equation for the reaction between chlorine and aqueous potassium bromide.State what is observed when this reaction occurs.

Answer 7

Cl2 + 2Br– ——>2Cl– + Br2 M2 solution goes orange / yellow ( from colourless)

Question 8

Write an equation for the reaction between chlorine and cold, dilute, aqueous sodium hydroxide.Give a major use for the solution that is formed by this reaction.Give the IUPAC name of the chlorine-containing compound formed in this reaction in which chlorine has an oxidation state of +1.

Answer 8

Cl2 + 2NaOH ——–>NaClO + NaCl + H2O (NaOCl)bleach or kills bacteria / bacteriacide / micro-organisms / microbessodium chlorate(I) ONLY

Question 9

Write an equation for the equilibrium reaction that occurs when chlorine gas reacts with cold water.Give one reason why chlorine is used for the treatment of drinking water even though the gas is very toxic.

Answer 9

Cl2 + H2O ——>HClO + HCl (HOCl)The (health) benefit outweighs the risk or wttea clear statement that once it has done its job, little of it remainsORused in (very) dilute concentrations / small amounts / low dose

Question 10

State how you could test a sample of water to show that it contains chloride ions.In your answer, give a reagent, one observation and the simplest ionic equation for the reaction with the reagent.

Answer 10

Silver nitrate OR AgNO3 (with or without nitric acid)white precipitate / white solidM3 Ag+ + Cl― AgCl

Question 11

A student knew that bromine can be used for killing microorganisms in swimming pool water.The following equilibrium is established when bromine is added to cold water.Br2(I) + H2O(I)  HBrO(aq) + H+(aq) + Br–(aq)Use Le Chatelier’s principle to explain why this equilibrium moves to the right when sodium hydroxide solution is added to a solution containing dissolved bromine.Deduce why bromine can be used for killing microorganisms in swimming pool water, even though bromine is toxic.

Answer 11

M1 The NaOH / OH– / (sodium) hydroxide reacts with / neutralises the H+ / acid / HBr (lowering its concentration)OR a correct neutralisation equation for H+ or HBr with NaOH or with hydroxide ionM2 Requires a correct statement for M1The (position of) equilibrium moves / shifts (from L to R)• to replace the H+ / acid / HBr that has been removed / lost• OR to increase the H+ / acid / HBr concentrationto make more H+ / acid / HBr / product(s) to oppose the loss of H+ / loss of product(s) to oppose the decrease in concentration of productsM3 The (health) benefit outweighs the risk or wtte ORa clear statement that once it has done its job, little of it remainsOR• OR product(s)used in (very) dilute concentrations / small amounts / low doses

Question 12

Iodine reacts with concentrated nitric acid to produce nitrogen dioxide (NO2). Give the oxidation state of iodine in each of the following.I2 HIO3

Answer 12

0+5

Question 13

Complete the balancing of the following equation.I2 + 10HNO3————–> ……….HIO3 + ……….NO2 + ……….H2O

Answer 13

2 + 10HNO3——————-> 2HIO3 + 10NO2 + 4H2O

Question 14

When concentrated sulfuric acid is added to potassium iodide, solid sulfur and a blacksolid are formed. Identify the black solid.Deduce the half-equation for the formation of sulfur from concentrated sulfuric acid.

Answer 14

Iodine I2H2SO4 + 6H+ + 6e– —-> S + 4H2OSO42- + 8H+ + 6e– —–> S + 4H2O

Question 15

When iodide ions react with concentrated sulfuric acid in a different redox reaction, theoxidation state of sulfur changes from +6 to –2. The reduction product of this reactionis a poisonous gas that has an unpleasant smell.Identify this gas.

Answer 15

hydrogen sulfide H2S

Question 16

A yellow precipitate is formed when silver nitrate solution, acidified with dilute nitricacid, is added to an aqueous solution containing iodide ions.Write the simplest ionic equation for the formation of the yellow precipitate.State what is observed when concentrated ammonia solution is added to thisprecipitate.

Answer 16

Ag+ + I – ————–> AgIThe (yellow) precipitate / solid / it does not dissolve / is insolublei.e. stays the same

Question 17

State why the silver nitrate is acidified when testing for iodide ions

Answer 17

The silver nitrate is acidified to• react with / remove (an)ions that would interfere with the test• prevent the formation of other silver precipitates /insoluble silver compounds that would interfere with thetest• remove (other) ions that react with the silver nitrate• react with / remove carbonate / hydroxide / sulfite (ions)

Question 18

Consider the following reaction in which iodide ions behave as reducing agents.Cl2(aq) + 2I– (aq) ————-> I2(aq) + 2Cl-(aq)In terms of electrons, state the meaning of the term reducing agent.Write a half-equation for the conversion of chlorine into chloride ions.

Answer 18

An electron donorCl2 + 2e – ——————> 2Cl –

Question 19

Suggest why iodide ions are stronger reducing agents than chloride ions

Answer 19

iodide ions are stronger reducing agents thanchloride ions, because Relative size of ions:Iodide ions / they are larger /have more electronlevels(shells) (than chloride ions) / larger atomic / ionicradiusOR electron to be lost/outer shell/level (of the iodide ion) isfurther the nucleusOR iodide ion(s) / they have greater / more shielding

Question 20

Chlorine reacts with cold water. Write an equation for this reaction.Give one large-scale application of the use of chlorine in water. Explain why it is used in this application even though chlorine is very toxic. Do not include cost.Example of applicationExplanation of use

Answer 20

Cl2 + H2O —–> HClO (hypochlorous acid) + HClin swimming pools or in drinking waterto sterilise / disinfect / sanitise water in water treatmentThe (health) benefit outweighs the risk OR a clear statement that once it has done its job, little of it remains OR used in (very) dilute concentrations / small amounts / low doses

Question 21

Two different chlorine-containing compounds are formed when chlorine reacts with cold, dilute sodium hydroxide solution. One of these compounds is sodium chloride. Name the other chlorine-containing compound formed.

Answer 21

Sodium chlorate(I) or sodium hypochlorite

Question 22

Chlorine is used in the extraction of bromine from seawater.Write the simplest ionic equation for the reaction of chlorine with bromide ions.

Answer 22

Cl2 + 2Br– ———> Br2 + 2Cl–

Question 23

Explain why bromine has a higher boiling point than chlorine.

Answer 23

Bromine is larger than chlorine OR has more electrons/electron shellsandThe forces between bromine / Br2 molecules are stronger (than the forces between chlorine / Cl2 molecules leading to more energy needed to separate the molecules)