Exam

• give the definition of a ligand (1)

  an atom, ion or molecule which can donate a lone electron pair
• give the definition of a complex ion (1)

  a central metal ion / species surrounded by coordinately bonded ligands
• give the definition of the coordination number


  the number of coordinate bonds formed to a central metal ion
• State the origin of the colour of transition-metal complexes. (2)

  -Electron transitions/electrons excited in d shell (1) or d-d transition

  -(Energy in) visible range (1)
• Give three changes to a transition-metal complex which result in a change in colour (3)

  Change 1: (Different) oxidation states (1)

  Change 2 : (Different) ligands (1)

  Change 3: (Different) co-ordination number (1)
• You are provided with a 1.00 mol dm-3 solution of iron(III) ions and a visible-light spectrophotometer (colorimeter). Outline a plan for experiments using this solution and this apparatus which would enable you to determine the concentration of iron(III) ions in a solution of unknown concentration. (5)


  1. Add an appropriate (or a given correct) ligand to intensify colour (1)

  e.g. thiocyanate (CNS)— or bipyridyl

  2. Make up solutions of known concentration (1)

  3. Measure absorption or transmission (1)

  4. Plot graph of results or calibration curve (1)

  5. Measure absorption of unknown and (1) compare
• Explain the meaning of the term bidentate ligand. (1)

  Two electron pair donor atoms
• Explain why metal(II) ions do not usually form octahedral complexes when chloride ions are the only ligands.

  Chloride or Cl- big or large or repel
• Suggest why a reaction between S2O8^2- and Cl- is slow?

  (Both) ions are negative or ions repel hence a high Ea
• Chloride ions form the tetrahedral complex ion [AlCl4]- but fluoride ions form the octahedral complex ion [AlF6]3-. Suggest a reason for this difference. (1)

  F- is smaller than Cl-
• Give one example of a linear complex ion formed by a transition metal. (1)

  [Ag(NH3)2]+
• In catalytic converters which clean up petrol engine exhaust gases, a catalyst promotes the reduction of nitrogen oxides using another polluting gas as reductant. State a suitable catalyst for this task, identify the reductant, and write an equation for the reaction that results. (3)

  Pt or Rh or Pt/Rh

  CO

  2CO + 2NO → N2 + 2CO2
• State what is meant by the term homogeneous as applied to a catalyst. (1)

  A catalyst in the same phase/phase as the reactants
• State what is meant by the term autocatalysis. (1)

  A reaction in which a product acts as a catalyst
• Identify a transition metal used in catalytic converters and state how the converter is

  constructed to maximise the effect of the catalyst. (3)

  Pt, Pd or Rh

  Deposited on a ceramic honeycomb or matrix or mesh or sponge

  To increase surface area of catalyst
• Explain why transition metals which adsorb strongly are not usually good catalysts. (1)

  Reactants cannot move on surface or products not desorbed or the Active sites blocked
• Explain why transition metals which adsorb weakly are not usually good catalysts. (1)

  -Reactants not brought together

  -No increase in reactant concentration on catalyst surface

  -Reactants not held long enough for a reaction to occur

  -Reactant bonds not weakened
• Explain why an incorrect value for the number of moles of iron(II) chloride formed would have been obtained if the original solution had been titrated with potassium manganate(VII). (1)

  (KMnO4) will also oxidise (or react with) Cl- (or chloride or HCl)
• Explain why this electron transition causes a solution containing the transition metal ion to be coloured. (2)

  (One colour of) light is absorbed (to excite the electron)

  
  

  The remaining colour / frequency / wavelength / energy is transmitted (through the solution)
• State three different features of transition metal complexes that cause a change in the value of ΔE, the energy change between the ground state and the excited state of the d electrons. (3)

  1

  (d) Any three from:

  • (Identity of the) metal

  • Charge (on the metal) / oxidation state / charge on complex

  • (Identity of the) ligands

  • Co-ordination number / number of ligands

  • Shape
• Suggest why ions from s block elements do not usually act as catalysts.

  Because only exist in one oxidation state
• Due to their electron arrangements, transition metals have characteristic properties including catalytic action and the formation of complexes with different shapes.

  (a) Give two other characteristic properties of transition metals. For each property, illustrate your answer with a transition metal of your choice. (4)

  Variable oxidation state

  eg Fe(II) and Fe (III)

  (Characteristic) colour (of complexes)

  eg Cu2+(aq) / [Cu(H2O)6]2+ is blue
• Other than octahedral, there are several different shapes shown by transition metal complexes. Name three of these shapes and for each one give the formula of a complex with that shape. (6)

  Tetrahedral

  [CuCl4]2- / [CoCl4]2-

  
  

  Square planar

  (NH3)2PtCl2

  
  

  Linear

  [Ag(NH3)2]+
• Write an equation for the overall reaction that occurs, in aqueous solution, between S2O82- ions and I- ions.


  2I- + S2O82- → I2 + 2SO42-
• Give one reason why, in the absence of a catalyst, the activation energy for the reaction between S2O82- ions and I- ions is high.

  Both (ions)have a negative charge
• Write two equations to show how Fe2+ ions can catalyse the reaction between S2O82- ions and I- ions. Suggest one reason why the activation energy for each of these reactions is low.

  2Fe2+ + S2O82- → 2Fe3+ + 2SO42-

  
  

  2Fe3+ + 2I- → 2Fe2+ + I2
• The redox reaction, in aqueous solution, between acidified potassium manganate(VII) and sodium ethanedioate is autocatalysed.

  -- Write an equation for this redox reaction.

  Identify the species that acts as the catalyst.

  Explain how the properties of the species enable it to act as a catalyst in this reaction. (6)

  (a) 2MnO4− + 16H+ + 5C2O42− → 2Mn2+ + 8H2O + 10CO2

  Mn2+ OR Mn3+

  (Possible because) Mn can exist in variable oxidation states

  Ea lowered because oppositely charged ions attract

  Mn3+ (reduced) to Mn2+ by C2O42− / equation

  Mn2+ (oxidised (back)) to Mn3+ by MnO4− / equation
• Sketch a graph to show how the concentration of MnO4− ions varies in the autocatalysed reaction with sodium ethanedioate?

  Explain the shape of the graph.

  Explanation marks

  Slope / rate increases as catalyst (concentration) forms

  
  

  Slope / rate decreases as (concentration) of MnO4− ions / reactant(s) decreases
• with reference to electrons explain why Cu2+ ions are blue (3)

  -Absorbs (certain frequencies of) (white) light / photons

  -D electrons excited / promoted

  -The colour observed is the light not absorbed / light reflected / light transmitted
• suggest one reason other than the colour why excess potassium dichromate is undesirable in drinking water (1)

  it has a foul smell
• Why can't dilute hydrochloric acid be used to acidify potassium manganate in a titration (2)

  -MNO4 would oxidise the chloride ions to CL2

  -because the KMNO4 E cell is more positive than the chloride E cell
• give two reasons why a solution of potassium manganate does not need to be kept away from flammable material (1)

  potassium manganate is very dilute
• give one reason why impurities in the reactants can cause problems in the processes that use heterozygous catalysts (1)

  impurities block the active sites on the catalyst
• in terms of electrons explain how the water molecules form bonds to the iron (2)

  -water molecules contain a lone pair of electrons on their oxygen atoms

  -these lone pairs of electrons on the oxygen atoms are donated to iron to form a shared pair of electrons (coordinate bond)
• explain why Fe3+ ions are just as effective as Fe2+ ions in catalysing this reaction (1)


  equations 1 and 2 involving each iron ion can be in any order
• give two reasons why it is essential to avoid the reaction between potassium manganate and iron during titrations?

  -brown solid formed

  -this brown solid makes it difficult to see a colour change

  -therefore a larger than expected tire is produced
• Cucl42- 


  pale yellow Solution 
• I2


  BROWN solution
• Cr3+


  Green ppt
• Cr(NH3)4(OH)2  3+


  Purple solution