6.3 Reactions of Ions in Aqueous Solution (A Level only)

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⁺.

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

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

Write two equations to prove this statement.

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

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.

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 C
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 C
The colour of the solution changes from blue to pink

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, CrO₄^2-, by hydrogen peroxide solution.

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. 

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.

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

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.

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]⁺.

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, FeO₄^2- are stable in acidic conditions.

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

FeO₄^2- (aq) + 8H⁺ + 3e^- ⇌ Fe³⁺ (aq) + 4H₂O (l)             E^ꝋ = +2.20 V

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

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.

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

Draw the 3D representations of these isomers.

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.

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

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

Table 1

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

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

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. 

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

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

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.  

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

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, C₂O₄^2-, and are bidentate ligands.

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

Calculate the empirical formula of the compound.

A solution of cobalt (II) chloride reacts with concentrated hydrochloric acid forming a complex which is tetrahedral and reacts with ethanedioate ions C₂O₄^2- to form a complex which is octahedral.

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

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.

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.

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.

A range of complexes are given in the table below.

Table 1

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

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

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

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.