Reactions of Ions in Aqueous Solution (A Level only) (AQA A Level Chemistry)

Exam Questions

4 hours30 questions
1a4 marks

Name the four transition metal ions that you are expected to have detailed chemical knowledge of.

This knowledge includes:

  • The structure of their hexaaqua ions
  • The colours of the hexaaqua ion solutions
  • The reactions of the hexaaqua ion solutions with other reagents
  • The colour and state changes associated with these reactions
  • The structure of complexes that they form with other reagents
1b1 mark

When copper (II) sulfate dissolves in water, the conventional equation for this is represented as

CuSO4 (s) + aq → Cu2+ (aq) + SO4- (aq)

Write the equation for the same reaction showing the formation of the hexaaquacopper (II) ion.

1c3 marks

Draw the structure of the hexaaquacopper (II) ion complex.

1d2 marks

Hexaaquacobalt (II) ions react with an excess of hydrochloric acid in the following ligand substitution reaction.

[Co(H2O)6]2+ (aq) + 4Cl - (aq) ⇌ [CoCl4]2- (aq) + 6H2O (l)

Explain why the initial and final cobalt (II) complexes in the equation above have different shapes.

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2a1 mark

Define the term Brønsted-Lowry acid.

2b2 marks

Define the term Lewis base and suggest the relevance to transition metal complexes.

2c1 mark

Complex ions can have different pH values to one another.

State the reason why an aqueous solution containing [Fe(H2O)6]3+ ions has a lower pH than a solution containing [Fe(H2O)6]2+ ions.

2d1 mark

Aluminium can form a hydroxide complex, Al(H2O)3(OH)3 which is amphoteric.

Give the meaning of the word amphoteric.

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3a2 marks

Describe what would be observed when sodium hydroxide is added dropwise and then in excess to [Al(H2O)6]3+ solution.

3b2 marks

Aluminium can form a hydroxide complex, Al(H2O)3(OH)3 which is amphoteric.

Using an appropriate acid and an appropriate base, write full chemical equations to show the amphoteric nature of aluminium hydroxide, Al(H2O)3(OH)3.

3c2 marks

Two students carry out test-tube tests on two different aqueous solutions of transition elements X and Z.

The observations of the tests are shown in Table 1.

Table 1

Test

X (aq)

Z (aq)

1

NH3 (aq) added dropwise

Excess NH3 (aq) added

Green precipitate C 

Pale-brown solution D

Pale-brown precipitate E 

No further change

2

HNO3( aq)

followed by Ba(NO3)2 (aq)

No change

No change

No change 

White precipitate F

3

HNO3 (aq)

followed by AgNO3 (aq)

No change 

Yellow precipitate, G

No change 

No change


X
and Z and write their formulae.

3d3 marks

Transition metals form precipitates when the complex has no overall charge.

i)

Construct ionic equations for the formation of the products C and F from part (c).
State symbols are not required.

ii)

State what the purpose was of adding HNO3 (aq) first in Test 2 and Test 3?

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4a3 marks

This question is about the chemistry of iron compounds and complex ions.

Aqueous ammonia was added dropwise to an aqueous solution containing iron (II) sulfate. A green precipitate formed that turned brown on standing in air.

i)
Identify the green precipitate.

ii)
Explain why it turns brown on standing in air and identify the product formed.
4b1 mark

Write an equation to represent the reaction between aqueous iron (II) sulfate solution and dilute aqueous ammonia.

4c1 mark

State the role of ammonia in the reaction in part (b) and give the meaning of this term.

4d3 marks

Sodium carbonate can be added as a base to solutions containing complex ions.

State what would be observed when sodium carbonate is added to separate solutions containing [Fe(H2O)6]2+ and [Fe(H2O)6]3+ ions.

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5a4 marks

This question is about reactions of ions and compounds of transition elements.

Figure 1 shows reactions of the complex ion [Cu(H2O)6]2+.

Figure 1

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i)

Give the formulae, including state symbols, of the missing species responsible for the observations in Figure 1.

ii)
State the role of the ammonia in Reaction 1.

iii)
Name the type of reaction occurring in Reaction 2.
5b3 marks

An aqueous solution of copper (II) sulfate can react with carbonate, CO32- ions.

i)
Write an ionic equation to represent this reaction.

ii)
Describe what you would observe during this experiment.
5c3 marks

This question is about the test-tube reactions of [Fe(H2O)6]3+ ions in aqueous solution.

Give the following when sodium hydroxide solution is added to the solution: 

  • Colour of the original solution
  • Observations after reagent has been added
  • Equation
5d2 marks

Carbonate ions, CO32-, can be added as a base from a solution of sodium carbonate to transition metal complexes.

Write an ionic equation to represent the reaction of aqueous gallium (III) nitrate solution and sodium carbonate ions.

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1a3 marks

Reactions of aqueous copper ions are shown in Figure 1.

Figure 1

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i)

In reaction 1, the aqueous ammonia is added dropwise to the aqueous copper ions.
State the colour and formula of Precipitate A.

ii)
Give an equation for its formation.
1b3 marks

Use Figure 1 of part (a) to answer the following questions: 

i)
State the colour and the formula of Solution B.

ii)
Give an equation for its formation.
1c3 marks

Use Figure 1 of part (a) to answer the following questions:  

i)
State the colour of copper carbonate produced in Reaction 3.

ii)
Suggest a suitable reagent for Reaction 3.

iii)
Write an equation to represent this reaction.
1d3 marks

Bidentate ligands form two coordinate bonds to a central metal atom or ion in a complex. 1,2-diaminoethane is an example of a compound which is a bidentate ligand.  When added to [Cu(H2O)6]2+ all of the other ligands are replaced and two optical isomers are formed.

i)

Write an equation for the ligand substitution reaction that is taking place when [Cu(H2O)6]2+ reacts with 1,2-diaminoethane.

ii)
Draw the displayed structures of the two isomers produced.

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2a6 marks

A sample of iron (III) chloride is dissolved in water and a pale yellow solution is formed.

i)
Write the formula of the iron complex formed when iron (III) chloride is dissolved in water.

ii)

State what is observed when aqueous sodium hydroxide is added to the solution containing the iron complex.
Give an equation, including state symbols, for the reaction.

iii)

State two observations made when sodium carbonate solution is added to the solution containing the iron complex.
Give an equation, including state symbols, for the reaction.

2b5 marks

Tests were carried out to a solution of an unknown transition metal compound and the results are summarised in the table below:

Table 1

TEST 1

Sodium hydroxide solution is added

Green precipitate

TEST 2

Nitric acid and silver nitrate solution are added

White precipitate


Use the results in Table 1 to identify the compound present.

Explain your reasoning and use equations with state symbols to support your answer.

2c8 marks

Aluminium compounds dissolve in water to form colourless solutions containing hexaaquaaluminium (III) ions.  A chemist reacts separate samples of hexaaquaaluminium (III) ions with aqueous ammonia, NH3 and aqueous sodium hydroxide, NaOH.

Describe these reactions in detail, including the addition of excess volumes of aqueous ammonia, NH3 and aqueous sodium hydroxide, NaOH.

Your answer should include observations and equations. 

2d3 marks

Complete the following flow chart shown in Figure 1 by writing the correct formula for the copper complex formed at A, B and C.

Figure 1

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3a6 marks

Table 1 below show the reactions of some transition metal ion solutions with a few drops of aqueous sodium carbonate solution.

Results for the first ion have been summarised.  Complete the remainder of the table.

Table 1

Transition metal ion solution

Colour of precipitate formed

Formula of precipitate

Fe2+

Green

FeCO3

Cu2+

A

X

Fe3+

B

Y

Al3+

C

Z

3b3 marks

The simplest iron ions in solution are hexaaquairon (II) and hexaaquairon (III).

They both have acidic properties, but hexaaquairon (III) is more acidic. 

Explain why hexaaquairon (III) is more acidic, using an equation to support your answer.  

3c9 marks

The reactions of hexaaquaaluminium (III) [Al(H2O)6]3+ are shown in Figure 1.

For each of the Reactions 1 - 3, identify the formula of the aluminium product, state one observation and give an equation for the reaction which is occurring.

Figure 1

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3d2 marks

Ferric oxalate, also known as iron (III) oxalate, is a chemical compound composed of ferric ions and oxalate ligands. The anhydrous material is pale yellow; however, it may be hydrated to form several hydrates.  Oxalate ions can also be termed ethanedioate ions, C2O42-, and are bidentate ligands.

i)

Write an equation to show the formation of iron (III) oxalate from the reaction of aqueous iron (III) ions and excess ethanedioate ions.

ii)

State an observation if aqueous sodium hydroxide solution is added to the iron (III) oxalate solution.

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4a3 marks

A solution of cobalt chloride was reacted with a mixture of ammonia and ammonium chloride with a current of air blown throughout the mixture.  A compound Z was produced which contains a complex of cobalt.  The composition by mass in grams is shown in Table 1.

Table 1

Element

Composition by mass (g)

Co

11.78

N

14.00

H

3.00

Cl

21.3


Calculate the empirical formula of the compound.

4b3 marks

A solution of cobalt (II) chloride reacts with concentrated hydrochloric acid forming a complex which is tetrahedral and reacts with ethanedioate ions C2O42- to form a complex which is octahedral.

Deduce the formula of the two complexes and give a reason for the coordination numbers being different.

4c4 marks

The addition of aqueous sodium hydroxide a solution containing P produces a precipitate iron (II) hydroxide.  This is then filtered and slowly turns into a brown solid Q on the filter paper.

i)
State the colour of the iron (II) hydroxide.

ii)
Give the formula of Q.

iii)
What does the iron (II) hydroxide react with to produce Q.

iv)
What type of reaction occurs in the formation of Q.
4d4 marks

Aqueous copper (II) ions will react with aqueous ammonia to form a precipitate X. When excess ammonia is added and the precipitate dissolves to form a solution Y. State the colour and formulas for X and Y.

Using an equation explain how ammonia is acting as a Bronsted-Lowry base in the reaction to form X.

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5a4 marks

Aqueous iron (II) ions are reacted separately with aqueous ammonia solution and aqueous sodium carbonate solution.

Using equations describe the observations that would be associated with each of the reactions.

5b4 marks

A range of complexes are given in the table below.

Table 1

W

X

Y

Z

[Cu(H2O)6]2+

[Al(H2O)6]3+

[Fe(H2O)6]2+

[Cu(NH3)4(H2O)6]2+


Use the information in the table to answer the following questions.

i)
What is observed when aqueous sodium hydroxide solution is added to complex X.

ii)
State which complexes are blue.

iii)
What is observed when aqueous ammonia solution is added to complex Y.

iv)

What is observed when aqueous sodium carbonate solution is added to complex W.

5c4 marks

The transition metals ion solutions have some similarities with how they will react to form different complexes.  Figure 1 shows a series of reactions which aqueous nickel (II) ions undergo.

Figure 1

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Using your knowledge of the transition metal ions and their solutions answer the following questions.

i)
State the coordination number of the complex [Ni(edta)]2-.

ii)
State the oxidation state of nickel in [Ni(H2NCH2CH2NH2)3]2+.

iii)
Name the reagent that would cause reaction A.

iv)
State the shape of the [Ni(NH3)6]2+ complex.
5d6 marks

When solutions of aqueous iron (II) and aqueous iron (III) ions are individually reacted with aqueous sodium carbonate, different reactions occur.  Using equations and relative acidity explain the difference in the reactions.

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1a6 marks

Describe, with the use of equations, the reactions of aqueous copper (II) ions with:

  • aqueous sodium hydroxide
  • excess aqueous ammonia
  • hydrochloric acid
1b2 marks
i)
State how ammonia behaves as a Lewis base in the reaction forming [Cu(NH3)4(H2O)2]2+ in part (a). 

ii)

The overall reaction of Cu2+(aq) with NH3 is made up of four separate equilibrium reactions.

Write the equilibrium constant, Kc for the final reaction forming [Cu(NH3)4(H2O)2]2+

1c1 mark

[CuCl4]3– can be produced from heating [CuCl4]2– with copper metal.

Deduce the role of copper metal in this reaction.

1d4 marks
i)

In a separate reaction, copper (I) oxide reacts with hot dilute sulfuric acid in a disproportionation reaction. A blue solution, C, and a brown solid, D, are formed.

Identify the products C and D and include equations and any changes of oxidation numbers.

ii)
State the shorthand electron configuration of the copper ion formed in product C

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2a2 marks

Aluminium can form metal complex ions, even though it is not a transition metal.

State the equation and observations when aqueous ammonia is added dropwise to [Al(H2O)6]3+.

2b3 marks

Diaminoethane, H2NCH2CH2NH2, like ammonia, can react with aluminium chloride solution to form a white precipitate.

i)
Write an equation for the reaction between an excess of aqueous diaminoethane and aqueous aluminium chloride solution.

ii)

State the role of diaminoethane in the reaction.

2c2 marks

Aluminium hydroxide, Al(H2O)3(OH)3, can be classified as amphoteric.

Write two equations to prove this statement.

2d6 marks

All metal-aqua ions behave as weak acids if the charge on the metal ion is +2 or +3. The Ka for [M(H2O)6]3+ varies between 10-2 to 10-5 (pKa = 2 to 5) and the Ka for [M(H2O)6]2+ varies between 10-6 to 10-11 (pKa = 6 to 11).

i)
Write a Ka expression showing the equilibrium established when [Al(H2O)6]3+ dissolves in water to form [Al(H2O)5(OH)]2+

ii)
Explain why the pKa for [M(H2O)6]3+ ions is lower than the pKa for [M(H2O)6]2+ ions.

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3a8 marks

A student states that when aqueous [Cu(H2O)6]2+ ions are reacted with dropwise then excess ammonia, the [Cu(H2O)6]2+ ions would undergo complete ligand substitution of H2O ligands for NH3. The final complex produced would not have a change in overall charge, coordination number or shape.

Explain if you agree with their conclusion.

Include equations, observations and 3D drawings of the initial and final cis and trans, complexes in your answer.

3b3 marks

Cobalt (II) forms complex ions with water ligands and with chloride ligands.

With water ligands, cobalt (II) forms a pink octahedral complex ion, [Co(H2O)6]2+.
With chloride ligands, cobalt (II) forms a blue tetrahedral complex ion.

A student dissolves cobalt (II) sulfate in water in a boiling tube and a pink solution forms.

Experiment 1

The student places the boiling tube in a water bath at 100 degreeC
Concentrated hydrochloric acid is added dropwise
The colour of the solution changes from pink to blue

Experiment 2

The student places the boiling tube from experiment 1 in an ice / water bath at 0 degreeC
The colour of the solution changes from blue to pink

i)
Write the equilibrium equation for the reaction that takes place when the colour of the solution changes.

ii)
Explain the observations and predict whether the formation of the blue colour is exothermic or endothermic.
3c3 marks

This is a question about chromium (III) and chromium (VI) compounds.

The chromium (III) complex, [Cr(OH)6]3−, can be oxidised to chromate (VI) ions, CrO42-, by hydrogen peroxide solution.

i)
Deduce the ionic equation for this reaction, which takes place in alkaline conditions.

ii)

If the solution of chromate (VI) ions is then acidified, the colour of the solution changes to orange as dichromate (VI) ions form. Write the equation for this reaction.

3d2 marks

State with the use of an equation, what happens to the equilibrium when OH- ions are added to dichromate (VI) ions in a solution of high pH. 

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4a5 marks

A student performed the following tests to identify an unknown white salt X.

  1. Silver nitrate solution was added to an aqueous solution of X which gave a yellow precipitate of compound A.
  2. Concentrated ammonia solution was added and compound A did not dissolve.
  3. Sodium carbonate solution was added to another aqueous solution of X which gave a white precipitate B.
  4. The white precipitate B was filtered and dried and then reacted with sulfuric acid to give a colourless solution containing compound C and a colourless gas D.

State the formulas the compounds A, B, C, D and X.

4b1 mark

Write an equation to represent the reaction of aqueous compound X with sodium carbonate solution.

4c5 marks

Explain why different precipitates are produced when [Fe(H2O)6]2+ and [Fe(H2O)6]3+ ions react with aqueous sodium carbonate solution.

Include in your answer the colour of the precipitates formed in both reactions and the formula of the precipitate produced from the reaction of hexaaquairon (II) complex.

4d5 marks

State what you would observe when an excess of dilute ammonia is added to separate aqueous solutions containing [Cu(H2O)6]2+ and [Ag(H2O)4]+.

i)

In your answer state the initial colour of the solution and the final observation that you would make.

In each reaction, write an overall equation for the formation of the final product from the initial aqueous metal ion.

ii)

State the bond angles in the complex formed when excess of dilute ammonia reacts with [Ag(H2O)4]+.

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5a4 marks

Iron is a transition element that forms a number of ions with the iron in different oxidation states.

Deduce, using the data shown below, whether ferrate ions, FeO42- are stable in acidic conditions.

O2 (g) + 4H+ (aq) + 4e- 2H2O (l)                                 E = +1.23 V

FeO42- (aq) + 8H+ + 3e- Fe3+ (aq) + 4H2O (l)             E = +2.20 V

Include in your answer an overall ionic equation for any reaction that takes place.

5b3 marks

Hexaamminecobalt (II) ions react with an excess of H2NCH2CH2NH2 in a ligand substitution reaction.

Explain, with the aid of an equation, what will happen to the entropy change of the reaction of hexaamminecobalt (II) ions with H2NCH2CH2NH2.

5c2 marks

The complex CoCl2(H2NCH2CH2NH2)2 is a neutral molecule and exhibits two types of stereoisomerism.

Draw the 3D representations of these isomers.

5d2 marks

A student reacted aqueous CoCl2(H2NCH2CH2NH2)2 with aqueous silver nitrate followed by dilute aqueous ammonia solution. They predicted the silver nitrate would form a white precipitate and which would then not dissolve in dilute aqueous ammonia.

State and explain whether you agree with their predictions.

Include any relevant ionic equations to support your conclusion.

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