Chemistry of Transition Elements - exam practice
Chemistry of Transition Elements
Question 1 of 13
When NH3(aq) is added to Cu2+(aq), dropwise at first and then in excess, two chemical reactions occur as shown.
For each reaction:
- describe what you would see
- write an equation.
When complex ion B, [Cu(NH3)4(H2O)2]2+, reacts with 1,2-diaminoethane (en), the ammonia molecules but not the water molecules are replaced.
Write an equation for this reaction.
en = 1,2-diaminoethane.
Suggest why the enthalpy change for the reaction in part b) is approximately zero.
Explain why the reaction in part b) is spontaneous despite having an enthalpy change that is approximately zero.
Ethanedioic acid, (COOH)2, is the active ingredient in a spray to treat infections of the outer ear.
A chemist carries out a redox titration using aqueous cerium(IV) sulfate, Ce(SO4)2(aq), to determine the percentage, by mass, of ethanedioic acid in the spray.
In the titration, Ce4+(aq) ions oxidise ethanedioic acid in acidic conditions, as shown by the equation below
2Ce4+(aq) + (COOH)2(aq) → 2Ce3+(aq) + 2CO2(g) + 2H+(aq)
Ce4+(aq) ions have a yellow colour. Ce3+(aq) ions are colourless.
Dilute sulfuric acid is added to 5.00 g of the spray to form a colourless solution which is made up to 250.0 cm3 with distilled water.
25.00 cm3 of this solution is titrated with 1.00 x 10-2 moldm-3 Ce(SO4)2 from the burette.
The titration is repeated until concordant titres are obtained.
The mean titre is 22.25 cm3.
What colour change would the chemist observe at the end point?
Calculate the percentage, by mass, of ethanedioic acid, (COOH)2, in the spray.
Give your answer to 3 significant figures.
A chemist uses the following procedure to analyse iron tablets containing iron in the form of iron(II) sulfate, FeSO4.
- Crush 1 iron tablet and transfer the resulting powder to a 250.0 cm3 volumetric flask.
- Rinse the apparatus used with dilute sulfuric acid transferring all washings to the volumetric flask.
- Add dilute sulfuric acid to make up the solution to exactly 250.0 cm3. Stopper the flask and invert it several times.
- Titrate 25.00 cm3 of this solution with 2.00 x 10-3 mol dm-3 potassium manganate(VII) solution.
- Repeat the titration until concordant results are obtained. The overall equation for the reaction occurring in the titration is:
MnO4-(aq) + 8H+(aq) + 5Fe2+(aq) ⇌ Mn2+(aq) + 4H2O(l) + 5Fe3+(aq)
The table below shows some standard electrode potential data.
Using data in the table, explain why the chemist used sulfuric acid and not hydrochloric acid in their titration.
Suggest why this titration does not require the addition of an indicator.
State the colour change at the end-point of this titration.
The chemist decides to take burette readings from the top of the meniscus rather than from the bottom.
Explain the effect of this, if any, on the titre values.
The chemist's titration readings are shown in the table below.
Complete the table.
Use the titration results in part e) and information from the procedure to calculate the mass of FeSO4 in 1 iron tablet.
Give your answer to 3 significant figures.
This question look at the ions and complexes of cobalt.
An octahedral complex ion Y, C9H30N6Co3+, exists as two optical isomers.
In complex ion Y, Co3+ is bonded to three molecules of a bidentate ligand L.
Write the molecular formula of the bidentate ligand L.
The diagram below shows the skeletal formula of L.
Explain how L is able to act as a bidentate ligand.
Draw a diagram that shows the shape of the complex ion Y and shows the type of bond between the ligand L and the cobalt.
The compounds [Co(NH3)5Br]SO4 and [Co(NH3)5SO4]Br are structural isomers.
Define the term structural isomer.
What is the oxidation number of cobalt in [Co(NH3)5Br]2+?
Ammonium hexachloroplatinate, (NH4)2PtCl6, is a complex of platinum used in platinum plating. Ammonium hexachloroplatinate contains the hexachloroplatinate ion.
Draw a 3D diagram to show the shape of a hexachloroplatinate ion.
On your diagram, show:
- the name of the shape
- the charge on the ion
- the value of the bond angle.
Platin, Pt(NH3)2Cl2, is a neutral complex of platinum(II).
Explain why Pt(NH3)2Cl2 has no overall charge.
Pt(NH3)2Cl2, displays cis-trans isomerism.
Draw the structure of cis-platin. State its shape and give a value of the Cl-Pt-Cl bond angle.
The use of cis-platin can cause side effects so oxaliplatin has been developed.
The structure of oxaliplatin is shown below.
Draw the structures of the two bidentate ligands in oxaliplatin.
Steel pipelines carrying natural gas can corrode to form ‘green rust’, Fe(OH)2.
After some time, green rust can change into ‘black rust’ via the equation shown below.
3Fe(OH)2 ➔ Fe2+Fe3+2O4 + H2 + 2H2O
Use oxidation states to show that this is a redox reaction.
If seawater is present, the green rust is often oxidised to [FeCl4]- ions.
Draw diagrams to show two possible shapes for [FeCl4]-.
Name the shapes.
State the type of bonding between the chloride ligands and the iron ion in [FeCl4]-.
Cu+ also forms complexes with chloride ligands.
One example is the colourless complex [CuCl2]-.
Explain why [CuCl2]- ions are colourless.
The complex [Co(NH3)4Cl2]+ shows isomerism.
Suggest the type of isomerism shown in [Co(NH3)4Cl2]+ and draw 3D structures of the two isomers.
The diagram below shows the structure of ethane-1,2-diamine, en.
en is an example of a bidentate ligand.
Explain what is meant by the term bidentate ligand.
[Cr(en)2Cl2]+ shows geometrical and optical isomerism.
Complete the 3D diagrams to show the 3 stereoisomers of [Cr(en)2Cl2]+.
Using the model answer diagram, give the letters of the two isomers of [Cr(en )2Cl2]+ which show optical isomerism.
Using the model answer diagram, give the letter of the isomer which has no overall dipole moment.
Explain what is meant by the term transition element.
Draw electrons in the boxes below to show the electronic configurations of Cu, Cu+ and Cu2+.
Complete the table by predicting the appropriate metal ions, co-ordination numbers, formulae and charges for the complexes A-D.
Transition metals have characteristic properties due to their electron arrangements.
Give two characteristic properties of transition metals.
Sketch the shapes of the following atomic orbitals in the table below.
Give the electronic configurations of a chromium atom, a Cr2+ ion and a Cr3+ ion.
Explain why chromium has variable oxidation states.
Explain why complexes formed from transition metal ions are coloured.
This diagram represents the energy change that occurs when a d electron in a transition metal ion is excited by visible light.
The energy change shown in the diagram represents the energy of green light and results in a solution that appears violet.
Violet light has a higher frequency than green light.
Suggest how the energy change, ΔE,will differ for a transition metal ion that forms a green solution.
Explain your answer.
State 3 different features of transition metal complexes that cause a change in the value of ΔE.
The equation below represents the dissociation of [Fe(H2O)6]3+ ions in aqueous solution.
[Fe(H2O)6]3+(aq) ⇌ [Fe(H2O)5OH]2+(aq) + H+(aq)
The Ka for this dissociation is 6.00 × 10-3 mol dm-3
Calculate the pH of a 1.50 x 10-2 mol dm-3 solution of [Fe(H2O)6]3+.
Give your answer to 2 decimal places.
The diagram below shows some reactions of iron ions in aqueous solution.
Give the formulae of A and B and state their colours.
The green precipitate A, Fe(OH)2, turned brown on standing in air.
Explain why.
When concentatd hydrochloric acid is added to aqueous Cu2+ ions, the following equilibirum is established.
[Cu(H2O)6]2+ + 4Cl- ⇌ [CuCl4]2- + 6H2O
State the colours of the [Cu(H2O)6]2+ and [CuCl4]2- complex ions.
Draw the shapes of the [Cu(H2O)6]2+ and [CuCl4]2- complex ions.
Give the name of the shape for each ion.
State the coordination numbers of the copper ion in [Cu(H2O)6]2+ and [CuCl4]2-.
Explain the change in coordination number in this equilibrium.
In aqueous solution, the [Cu(H2O)6]2+ ion reacts with an excess of ethane-1,2-diamine (en) to form the complex ion Z.
Write an equation for this reaction.
en = 1,2-diaminoethane
Explain, in terms of the chelate effect, why the complex ion Z is formed in preference to the [Cu(H2O)6]2+ complex ion.
Compound X is a green solid containing one cation and one anion. It dissolves in water to form a green solution.
Give the formula of two cations which could be responsible for the green colour in the solid.
A student adds dilute sodium hydroxide solution, drop by drop, to an aqueous solution of X.
A green precipitate forms, which dissolves in excess sodium hydroxide solution to form a dark green solution.
Give the formula of the cation in X.
Give a formula for the green precipitate.
Give a formula for the species present in the dark green solution.
The student adds a few drops of acidified potassium mangantate(VII) solution to another sample of a solution of X in a test tube.
State the colour change that occurs.