Transition Metals

Question 1

A coordinate bond is formed when a transition metal ion reacts with a ligand. Explain how this coordinate bond is formed

Answer 1

An electron pair on the ligand is donated from the ligand to the central metal ion.

Question 2

In terms of bonding, explain the meaning of the term complex.

Answer 2

atom or ion or transition metal bonded to / surrounded by
one or more ligands
by co–ordinate bonding

Question 3

describe what bidentate is

Answer 3

Two atoms or two points of attachment (1)

Each donating a lone electron pair

Question 4

The complex ion formed in aqueous solution between cobalt(II) ions and chloride
ions is a different colour from the [Co(H2O)6]
2+ ion.
Explain why these complex ions have different colours

Answer 4

In different complexes the d orbital splitting will be different

Light energy is absorbed causing an electron to be excited

Different wavelength light will be absorbed or transmitted

Question 5

Suggest why it is important that the container for each sample has the same
dimensions.
in a colorimeter

Answer 5

Absorption depends on (proportional to) path length / distance travelled through solution

Question 6

Suggest one reason why a colorimetric method might be chosen in preference
to titration.

Answer 6

Quicker to analyse extracted samples than by titration / uses smaller
volumes of solution

Question 7

Give the equation that relates the energy change ΔE to the Planck constant h and
the frequency of the visible light v.

Answer 7

ΔE = hv

Question 8

Explain why this electron transition causes a solution containing the transition metal
ion to be coloured.

Answer 8

light is absorbed (to excite the electron)
The remaining colour / frequency / wavelength / energy is transmitted (through
the solution)

Question 9

The energy change shown in the diagram represents the energy of red light and
leads to a solution that appears blue.
Blue light has a higher frequency than red light.
Suggest whether the energy change ΔE will be bigger, smaller or the same for a
transition metal ion that forms a red solution. Explain your answer.
Energy change …………………………………………………………………………………
Explanation ..

Answer 9

higher frequency of blue light absorbed, this means v is more, leads toa higher delta e

Question 10

State 5 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

Answer 10

(Identity of the) metal
• Charge (on the metal) / oxidation state / charge on complex
• (Identity of the) ligands
• Co-ordination number / number of ligands
• Shape

Question 11

When an aqueous solution containing [Cr(H2O)6]
3+ ions is warmed in the presence of
Cl– ions, [Cr(H2O)5Cl]2+ ions are formed and the colour of the solution changes.
Name this type of reaction.

Suggest, in terms of electrons, why the colours of the complex ions are different.

Answer 11

(ligand) substitution / replacement / exchange
Allow nucleophilic substitution

The energy levels/gaps of the d electrons are different

A different wavelength/frequency/colour/energy of light is absorbed (when d electrons are excited)

Question 12

if fe3+ is purple why may it look brown

Answer 12

[Fe(H2O)6]3+(aq)
purple solution

may look yellow brown due to some
[Fe(H2O)5
(OH)]2+(aq)
due to rust

so fe3+ = yellow brown or purple

Question 13

when something is warmed in the prescence of cl- ions another complex is formed and the colour of the solution changes, name this reac and suggest in terms of electrons why the colours of the complex ions differ

Answer 13

ligand subs
the energy gap of electrons are diff so a diff wavelength of light will be absorbed and a diff wavelength of light will be reflcted or transmitted

Question 14

why may well water be more beneficial than pure water

Answer 14

contains iron, needed for haemoglobin for red blood cells to carry oxygen around the body

Question 15

give the meaning of the term autocatalysed

Answer 15

the catalyst is a reaction product

Question 16

example of a bidentate ligand

Answer 16

write them out like molecular so

c2o4- or h2nch2ch2nh2

Question 17

By considering the properties of the reactants and products, state why it is possible
to use a spectrometer to measure the concentration of the manganate(VII) ions in
this reaction mixture.

Answer 17

As manganate VII ions are coloured (purple)

Question 18

By reference to aqueous copper(II) ions, state the meaning of each of the
three terms in the equation ΔE = hv.

Answer 18

ΔE is the energy gained by the (excited) electrons (of Cu2+)
allow:

1
h (Planck’s) constant
1
v frequency of light (absorbed by Cu2+(aq)

Question 19

Calcium fluoride crystals absorb ultra-violet light. Some of the energy gained is
given out as visible light. The name of this process, fluorescence, comes from the
name of the mineral, fluorite.
Use your knowledge of the equation ΔE = hν to suggest what happens to the
electrons in fluorite when ultra-violet light is absorbed and when visible light is given
out.

Answer 19

u. v. absorbed: electrons/they move to higher energy
(levels) /electrons excited

visible light given out: electrons/they fall back down/move to
lower energy (levels)

Question 20

Explain the meaning of the term complex ion.

Answer 20

ligands linked by covalent bonds

Question 21

how is the light absorbed anyway

Answer 21

absorbed due to the electrons becoming excited. thats why, its bc they move to higher en levels

Question 22

With reference to haemoglobin, explain why carbon monoxide is toxic.

Answer 22

Because it bonds to the iron/haemoglobin
Allow blocks site
/CO has greater affinity for haemoglobin
/carboxyhaemoglobin more stable than oxyhaemoglobin
1
Displaces oxygen
Or prevents transport of oxygen

Question 23

why is something colourless

Answer 23

Ti(III) has a d electron that can be excited to a higher level
Allow idea that d electrons can be excited to another level (or
move between levels)
1
Absorbs one colour of light from white light
Allow idea that light is absorbed
1
Ti(IV) has no d electron so no electron transition with
energy equal to that of visible light

Question 24

Give two reasons why the use of a spectrometer is the most appropriate
method for measuring the concentration of the coloured ions in this
experiment.

Answer 24

Rapid determination of concentration
Or easy to get many readings
1
Does not use up any of the reagent/does not interfere
with the reaction

Question 25

Sketch a curve to show how you would expect the concentration of
manganate(VII) ions to change with time until the colour has faded because
the concentration has reached a very low value. Explain the shape of the
curve (5)

Answer 25

Curve starts with small gradient (low rate)
1
Because negative ions collide so Ea high
1
Curve gets steeper
1
Because autocatalyst (Mn2+) formed
1
Curve levels out approaching time axis
Can score this mark and next one ONLY with simple curve
(that is curve with gradually decreasing gradient)
1
Because MnO4
–
 ions used up

Question 26

explain why a cobalt compound is able to act as a catalyst for this process- being regenerated

Answer 26

as it acts as an intermediate that lowers the activation energy

Question 27

when talking about oxidation in the question, why , Suggest one reason why the mole ratio of chloride ions to cobalt ions that you have
calculated is different from the expected value

Answer 27

due to incomplete oxidation in the prep step

Question 28

The determination of the concentration of copper(II) ions in a single sample of river
water gives an unreliable value for the copper(II) ion pollution in the river.
Give one reason why this value is unreliable.

Answer 28

concs may vary over time or throughout river

Question 29

how can poisoning be reduced

Answer 29

by purifying the reactants

Question 30

Explain why the pH of a solution of iron(II) chloride is higher than that of a solution
of iron(III) chloride of the same concentration.

Answer 30

The Fe2+ ion has a smaller charge to size ratio
1
Hence less polarising than Fe3+ or less weakening
effect on O-H bonds

always talk about a larger or smaller charge to size ratio

Question 31

Explain the meaning of the term chelate effect.

Answer 31

(Substitution of a monodentate ligand by a) bi or multidentate
ligand (1) giving a more stable complex (1) or with an increase
in entropy / disorder

Question 32

Explain how heterogeneous catalysts work, give one example of a reaction
catalysed in this way and discuss why different catalysts have different activities. (8 marks)

Answer 32

reactants brought together or increased concentration on surface
or increased collision frequency (1)
reactants must be correctly orientated (1)
reaction on the surface (1)
products desorbed (1)
example of a catalysed reaction (not a named process) (1)
a suitable catalyst for this reaction (1)
penalise incorrect second reactions and catalysts
If absorption too weak reactants not brought together (1)
e.g. silver (1)
If adsorption too strong products not desorbed (1)
e.g. tungsten (1)

Question 33

Suggest why V3+(aq) ions will also act as a catalyst for this reaction but Mg2+(aq) ions
will not.

Answer 33

Magnesium only forms Mg2+

, or has only one oxidation state

Question 34

Give two reasons why the use of a spectrometer is the most appropriate
method for measuring the concentration of the coloured ions in this
experiment

Answer 34

Rapid determination of concentration
Or easy to get many readings

Does not use up any of the reagent/does not interfere
with the reaction

Question 35

State the origin of the colour of transition-metal complexes

Answer 35

Electron transitions/electrons excitedin d shell (1) or d-d transition
r
(Energy in) visible range (1)

Question 36

Suggest why the enthalpy change for the reaction in part (c) is approximately zero

Answer 36

Cu–N bonds formed have similar enthalpy / energy to Cu–N bonds broken

And the same number of bonds broken and made

Same number (1), and same type of bonds (1), broken and made

Question 37

State and explain one risk associated with the use of cisplatin as an anticancer
drug.
Risk ……………………………………………………………………………………………….
Explanation

Answer 37

Killing them or causing damage (medical side effects)
Allow any correct side effect (e.g. hair loss)
Allow kills healthy (or normal) cells
1
May attach to DNA in normal cells

Question 38

State the origin of the colour of transition-metal complexes.

Answer 38

Electron transitions/electrons excitedin d shell (1) or d-d transition
Do NOT allow charge transfer
(Energy in) visible range (1)

Question 39

define ligand and coordinate bond

Answer 39

molecule that can donate a pair of electrons to a metal ion.

covalent bond in which both electrons are donated by one atom.

Question 40

define triangle e

Answer 40

energy absorbed by the electron

Question 41

3 factors that can change the frequency of light absorbed

Answer 41

oxidation state
ligand
coordination number