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sams chemistry > Group 7 The Halogens > Flashcards

Group 7 The Halogens Flashcards

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1
Q

Separate unlabelled solid samples of three anhydrous sodium compounds are provided for a student to identify.
These compounds are known to be sodium carbonate, sodium fluoride and sodium chloride but it is not known which sample is which.
Outline a logical sequence of test-tube reactions that the student could carry out to identify each of these compounds.
Include the observations the student would expect to make.
Give equations, including state symbols, for any reactions that would take place.

A

Stage 1 Suggested tests
1a Add named acid (HCL) to all 3
1b Add water / make into a solution
1c Add AgNO3
Ignore addition of NH3 / Ignore additional test for CO2 produced
Stage 2 Expected observations - conclusions
2a Na2CO3 will fizz with acid
2b NaCl gives white ppt with AgNO3
2c NaF shows no (visible) change / no ppt
Additional incorrect observations loses point
Stage 3 Equations – state symbols must match method
3a Na2CO3 + 2HNO3 → 2NaNO3 + CO2 + H2O … or ionic
3b AgNO3 + NaCl → AgCl + NaNO3
… or ionic
3c correct state symbols

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2
Q

Chlorine is used to decrease the numbers of microorganisms in water.
When chlorine is added to water, there is a redox reaction, as shown by the equation
Cl2 + H2O ⇌ HClO + HCl
(a) Deduce the oxidation state of chlorine in HClO and the oxidation state of chlorine in HCl
Oxidation state of chlorine in HClO _______________________________________
Oxidation state of chlorine in HCl _

A

Two correct Cl ox states: HClO = +1 HCl = -1

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3
Q

Chlorine is used to decrease the numbers of microorganisms in water.
When chlorine is added to water, there is a redox reaction, as shown by the equation
Cl2 + H2O ⇌ HClO + HCl
Give two half-equations to show the oxidation and reduction processes that occur in this redox reaction.
Oxidation half-equation ________________________________________________
Reduction half-equation

A 
Oxidation:
Cl2 + 2H2O → 2HClO + 2H+ + 2e–
Accept - 2e– on the other side
Allow Cl2 + 2H2O → 2ClO– + 2e–
1
Reduction:
Cl2 + 2H+ + 2e– → 2HCl
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4
Q

Which statement is correct about the reaction between concentrated sulfuric acid and solid sodium bromide?

A
Bromide ions are reduced.

B
Hydrogen bromide and sulfur are formed.

C
Sulfuric acid acts as an oxidising agent.

D
Bromine and hydrogen sulfide are formed.

A

C

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5
Q

Chlorine reacts with cold, dilute, aqueous sodium hydroxide.
Which is a complete list of the products?

A
Sodium chloride, sodium chlorate(I) and water

B
Sodium chlorate(I) and water

C
Sodium chloride, sodium chlorate(V) and water

D
Sodium chloride and sodium chlorate(I)

A

A

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6
Q

Which is the best technique to remove the silver chloride that forms when aqueous solutions of silver nitrate and sodium chloride react?

A
Refluxing

B
Evaporation

C
Filtration

D
Distillation

A

C

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7
Q

Which statement about astatine is correct?

A
Astatine has a greater electronegativity than bromine

B
Astatine is a better oxidising agent than bromine

C
Astatine has a greater boiling point than bromine

D
Astatine has a greater first ionisation energy than bromine

A

C

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8
Q

Which species is not produced by a redox reaction between solid sodium iodide and concentrated sulfuric acid?

A
Na2SO4

B
H2S

C
S

D
SO2

A

A

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9
Q

This question is about silver iodide.

(a) Define the term enthalpy of lattice formation.

A

Enthalpy change or heat energy change when 1 mol of solid ionic compound/substance or 1 mol of ionic lattice

is formed from its gaseous ions

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10
Q

A calculation of the enthalpy of lattice formation of silver iodide based on a perfect ionic model gives a smaller numerical value than the value calculated in part (b)
Explain this difference.

A

AgI contains covalent character

Forces / bonds (holding the lattice together) are stronger

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11
Q

Identify a reagent that could be used to indicate the presence of iodide ions in an aqueous solution and describe the observation made.
Reagent
Observation

A

AgNO3
yellow ppt

or

Cl2 or Br2
brown solution/black ppt

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12
Q

Concentrated sulfuric acid reacts with alkenes, alcohols and sodium halides.
(a) Name the mechanism for the reaction of concentrated sulfuric acid with an alkene.

A

electrophilic addition

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13
Q

Explain why the product shown in your answer to part (b) is the major product.

A

(major) product formed via more stable carbocation OR secondary carbocation more stable (than primary)

Due to electron-releasing character / (positive) inductive effect of two alkyl / methyl groups (as opposed to one)

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14
Q

A by-product of the reaction of butan-2-ol with concentrated sulfuric acid has the molecular formula C4H8O
Name this by-product, identify the role of the sulfuric acid in its formation and suggest the name of a method that could be used to separate the products of this reaction.
By-product __________________________________________________________
___________________________________________________________________
Role of sulfuric acid ___________________________________________________
___________________________________________________________________
Name of separation method

A

butanone
ALLOW butan-2-one

oxidising agent
ALLOW electron acceptor but NOT electron pair acceptor

(fractional) distillation

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15
Q

A student was given a 50.0 g sample of solid silver chloride contaminated with solid silver carbonate.
The student suggested the following method to obtain the maximum amount of pure dry silver chloride from the sample:
1. Tip the solid into a boiling tube.
2. Add dilute nitric acid.
3. Allow the remaining solid to settle.
4. Decant off the liquid.
5. Leave the sample to dry on a shelf.
Identify any faults or omissions in the method suggested by the student.
Suggest improvements to the method, using commonly available laboratory equipment.
The following chemicals are also available:
distilled water, dilute solutions of NaOH, NH3, HCl, H2SO4

A

Stage 1 Apparatus
• Boiling tube likely to be too small for 50.0 g solid plus acid
• Use a (large / medium / 250 ml) beaker instead
Stage 2 AgCl Prep
• HNO3 will form soluble AgNO3 so max AgCl not obtained
• Use HCl instead
• Method doesn’t say how much acid should be used
Or
Use excess acid / Add acid until no more bubbles evolved
Stage 3 Making pure dry solid
• Decanting means solid wet / contaminated with HNO3 / acid or decanting means some solid lost
• Instead use filter and then wash with distilled water
(and then dry)

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16
Q

This question is about elements in Group 7 of the Periodic Table and their compounds.
(a) Bromine (Br2), strontium chloride (SrCl2) and iodine monochloride (ICl) all have similar Mr values.
Suggest, with reasons, the order of melting points for these three substances.

A

SrCl2> ICl > Br>

SrCl2 strong ionic bonds / (strong electrostatic attraction between opposite ions)

Lattice so many strong bonds to overcome

ICl has dipole-dipole between molecules – weaker than ionic bonds

Br2 has van der Waals forces between molecules – much weaker

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17
Q

Write an equation for the reaction of chlorine with cold water.
State a reason why chlorine is added to drinking water, and suggest a disadvantage of treating water in this way.

A

Cl2 + H2O ⇌ HCl + HClO

Kills bacteria

Wasteful as most potable water not used for drinking - used in washing clothes etc
OR Some people suffer eye irritation / Some people find the taste unpleasant
OR can react with organic compounds to produce harmful substances

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18
Q

Bromine reacts with phosphorus to form phosphorus tribromide.
Write an equation for this reaction and draw the shape of the phosphorus tribromide molecule formed.
Suggest the bond angle in phosphorus tribromide.

A 
6Br2 + P4 ⟶ 4PBr3
Accept 4P for P4
1
Pyramidal shown in a diagram (but the name of the shape isn’t needed)
1
100–108°
19
Q

Phosphorus pentabromide in the solid state consists of PBr4+ and Br − ions.
Draw the shape of the PBr4+ ion and suggest its bond angle.

A

Tetrahedral shown in a diagram (but the name of the shape isn’t needed)

109.5°

20
Q

Which equation represents a reaction that does take place?

A
Cl2 + 2NaI ⟶ 2NaCl + I2

B
Br2 + 2NaCl ⟶ 2NaBr + Cl2

C
NaCl + H2O ⟶ HCl + NaOH

D
2HCl + H2SO4 ⟶ Cl2 + SO2 + 2H2O

A

A

21
Q

Which species is the best oxidising agent?

A
Cl2

B
Cl−

C
Br 2

D
Br−

A

A

22
Q

The following tests were carried out to identify an unknown green salt Y.
An aqueous solution of Y gave a cream precipitate of compound A when reacted with silver nitrate solution.
Compound A gave a colourless solution when reacted with concentrated ammonia solution.
Another aqueous solution of Y gave a green precipitate B when reacted with sodium carbonate solution.
The green precipitate B was filtered and dried and then reacted with sulfuric acid to give a pale green solution containing compound C and a colourless gas D.
(a) Identify by name or formula the compounds A, B, C, D and Y.
Identity of A _________________________________________________________
Identity of B _________________________________________________________
Identity of C _________________________________________________________
Identity of D _________________________________________________________
Identity of Y

A 
A  Silver bromide / AgBr
1
B  Iron(II) carbonate / FeCO3
1
C  Iron(II) sulphate / FeSO4
1
D  Carbon dioxide / CO2
1
Y  Iron(II) bromide / FeBr2
23
Q

Write the simplest ionic equation for the reaction of silver nitrate solution with the anion that is present in compound Y. (FeBR2)

A

Ag+ + Br− ⟶ AgBr

24
Q

Write the simplest ionic equation for the reaction that occurs between the green precipitate B and sulfuric acid

A

2H+ + CO32− ⟶ H2O + CO2

25
Q

Which statement is not correct about the addition of chlorine to water?

A
Chlorine can react with water to form an alkaline solution.

B
Chlorine can react with water to produce chloride ions and oxygen.

C
Chlorine can be added to drinking water to kill bacteria.

D
Chlorine can react with water to produce chloride ions and chlorate(I) ions

A

A

26
Q

his question is about the chemical properties of chlorine, sodium chloride and sodium bromide.
(a) Sodium bromide reacts with concentrated sulfuric acid in a different way from sodium chloride.
Write an equation for this reaction of sodium bromide and explain why bromide ions react differently from chloride ions.

A

2NaBr + 2H2SO4 Na2SO4 + Br2 + SO2 + 2H2O
Allow ionic equation
2Br– + 2H2SO4 Br2 + SO42– + SO2 + 2H2O
1
Br– ions are bigger than Cl– ions
1
Therefore Br– ions more easily oxidised / lose an electron more easily (than Cl– ions)

27
Q

A colourless solution contains a mixture of sodium chloride and sodium bromide.
Using aqueous silver nitrate and any other reagents of your choice, develop a procedure to prepare a pure sample of silver bromide from this mixture.
Explain each step in the procedure and illustrate your explanations with equations, where appropriate

A

Stage 1: formation of precipitates
• Add silver nitrate
• to form precipitates of AgCl and AgBr
• AgNO3 + NaCl → AgCl + NaNO3
• AgNO3 + NaBr → AgBr + NaNO3
Stage 2: selective dissolving of AgCl
• Add excess of dilute ammonia to the mixture of precipitates
• the silver chloride precipitate dissolves
• AgCl + 2NH3 → Ag(NH3)2+ + Cl−
Stage 3: separation and purification of AgBr
• Filter off the remaining silver bromide precipitate
• Wash to remove soluble compounds
• Dry to remove water

28
Q

Write an ionic equation for the reaction between chlorine and cold dilute sodium hydroxide solution.
Give the oxidation state of chlorine in each of the chlorine-containing ions formed.

A

Cl2 + 2HO– OCl– + Cl– + H2O

OCl– is +1
Cl– is –1

29
Q

Which of these species is the best reducing agent?

A

Cl2

B

Cl−

C

I2

D

I−

A

D

30
Q

Which of these substances reacts most rapidly to produce a silver halide precipitate with acidified silver nitrate?

A

CH3Br

B

CH3Cl

C

CH3F

D

CH3l

A

D

31
Q

A Grignard reagent is a compound in which magnesium is bonded to an alkyl group (R) and a halogen (X). It can be represented by the formula RMgX.
A Grignard reagent is formed by the reaction of magnesium metal with a haloalkane using dry ethoxyethane as a solvent. Ethoxyethane has a boiling point of 35 ºC, forms a dense vapour and is highly flammable.
(a) Give one reason why a hot water bath is used rather than direct heating with a Bunsen burner when preparing the Grignard reagent.

A

Ethoxyethane is flammable

32
Q

A Grignard reagent is a compound in which magnesium is bonded to an alkyl group (R) and a halogen (X). It can be represented by the formula RMgX.
A Grignard reagent is formed by the reaction of magnesium metal with a haloalkane using dry ethoxyethane as a solvent. Ethoxyethane has a boiling point of 35 ºC, forms a dense vapour and is highly flammable.
Grignard reagents react with water.
Suggest one reason why the ethoxyethane protects the Grignard reagent from reacting with water vapour in the air.

A

Ethoxyethane (vapour) forms a protective layer

33
Q

A Grignard reagent is a compound in which magnesium is bonded to an alkyl group (R) and a halogen (X). It can be represented by the formula RMgX.
A Grignard reagent is formed by the reaction of magnesium metal with a haloalkane using dry ethoxyethane as a solvent. Ethoxyethane has a boiling point of 35 ºC, forms a dense vapour and is highly flammable.
A Grignard reagent can be used to introduce an extra carbon atom into an organic molecule.
For example, propanone (CH3COCH3) reacts with CH3MgBr in a two-stage process to form 2-methylpropan-2-ol, (CH3)3COH. The isomer 2-methylpropan-1-ol is not formed in this process.
Suggest a suitable reagent and the associated observations that could be used to distinguish between 2-methylpropan-2-ol and its isomer 2-methylpropan-1-ol.

A

Acidified potassium dichromate(VI)/K2Cr2O7 M1
If incomplete, lose M1 but mark on. If incorrect CE = 0/3.
Accept acidified sodium dichromate / Na2Cr2O7
1
Observation with 2-methylpropan-2-ol – no colour change M2
Accept “stays orange”.
1
Observation with 2-methylpropan-1-ol – (orange to) green

34
Q

At the end of the preparation of the alcohol, the bromine from the Grignard reagent remains as the bromide ion.
Suggest reagents and observations to confirm that this halide ion is Br– and not I–I

A

i. (Acidified) silver nitrate (solution)
Allow a chemically correct alternative reagent.
1
ii. Cream / off-white precipitate
Allow this alternative with correct observation.
1
Dilute ammonia solution OR Conc ammonia solution
Allow this alternative with correct confirmatory reagent or procedure.
1
some ppt dissolves or ppt dissolves

35
Q

Silver cyanide can be precipitated from sodium cyanide solution by adding an excess of silver nitrate solution.
Describe how you would obtain a pure dry sample of silver cyanide from this mixture.

A 
Filter
Must be in this order
1
Wash (the residue) with water
1
Dry by pressing between filter paper or in air
36
Q

A student oxidised a solution of hydrochloric acid with a few drops of sodium chlorate(l) solution. The reaction mixture effervesced and turned pale green. The gas formed bleached universal indicator paper.
(a) Write a half-equation for the oxidation of chloride ions.

A

2Cl− → Cl2 + 2e(−)

37
Q

Write a half-equation for the reduction of chlorate(l) ions to chlorine in acidic conditions

A

2ClO− + 4H+ + 2e− → Cl2 + 2H2O

38
Q

A solution of sodium chlorate(l) was added to a colourless solution of potassium iodide.
Suggest what is observed.
Explain the reaction that leads to this observation.

A 
Goes brown (or shades of brown)
Allow black ppt/solid but NOT black solution or purple
1
Due to iodine or I3−
Correct ½ equation scores M2 and M3
1
Because I− oxidised
39
Q

Explain the trend in electronegativities shown by the halogens.

A

Increasing atomic radius / shielding / number of shells / size (down group) or reverse argument
NOT ‘molecules’
1
Decreasing attraction of nucleus/protons for shared (electron) pair / bond electrons

40
Q

Explain, in terms of electron transfer, the meaning of the term oxidising agent.

A

Electron acceptor / species that accepts electrons / species that gains electrons

41
Q

Solid sodium halides react with concentrated sulfuric acid.
(i) A sample of solid sodium iodide is reacted with concentrated sulfuric acid.
A black solid forms and hydrogen sulfide gas is produced.
Write a half-equation for the reaction of sulfuric acid to form hydrogen sulfide.

A

H2SO4 + 8H+ + 8e(-) → H2S + 4H2O

42
Q

Solid sodium halides react with concentrated sulfuric acid.
(i) A sample of solid sodium iodide is reacted with concentrated sulfuric acid.
A black solid forms and hydrogen sulfide gas is produced.

Write a half-equation for the formation of the black solid.

A

2I– → I2 + 2e(-)

43
Q

Suggest one reason why the reaction of sodium fluoride with concentrated sulfuric acid is different from the reaction with sodium iodide.

A

Fluoride less powerful reducing agent (than iodide)
OR
Fluoride less easily oxidised than iodide
Or reverse argument in either case

44
Q

Chlorine reacts with water to form an equilibrium mixture containing hydrochloric acid and chloric(I) acid.
(i) Write an equation for the formation of this equilibrium mixture.

A

Cl2 + H2O ⇌ 2H+ + Cl- + ClO-/HCl + HOCl

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