Transition Elements

Question 1

What is a transition element?

Answer 1

Transition metals are elements with an incomplete d-subshell that can form at least one stable ion with an incomplete d-subshell

Question 2

Why is scandium not a transition element even though it’s in the d-block?

Answer 2

Scandium does not form at least one stable ion with an incomplete d-subshell.
Scandium only forms the ion Sc3+, configuration [Ar] 3d0

Question 3

Why is zinc not a transition element even though it’s in the d-block?

Answer 3

Zinc does not form at least one stable ion with an incomplete d-subshell.
Zinc only forms the ion Zn2+, configuration [Ar] 3d10

Question 4

What is the Aufbau Principle?

Answer 4

Electrons occupy the lowest energy subshells first

Question 5

Why are chromium and copper different from the other transition metals?

Answer 5

In both cases an electron is promoted from the 4s to the 3d to achieve a half full d-subshell for chromium and full d-subshell for copper.
Cr is [Ar] 3d5 4s1.
Cu is [Ar] 3d10 4s1.
This is because the [Ar] 3d5 4s1 and [Ar] 3d10 4s1 configurations are energetically more stable.

Question 6

What properties do transition metals have that are unlike other metals on the periodic table?

Answer 6

• Variable oxidation states
• Form complex ions
• Form coloured compounds
• Behave as catalysts

Question 7

Why are transition metals said to have variable oxidation states?

Answer 7

The transition elements will lose electrons to form positively charged ions.
However, unlike other metals, transition elements can form more than one positive ion.
They are said to have variable oxidation states.

Question 8

Why can transition metals form complex ions?

Answer 8

Transition metals’ ability to form complex ions is due to their variable oxidation states.
A complex ion is a molecule or ion, consisting of a central metal atom or ion, with a number of molecules or ions surrounding it.

A molecule or ion surrounding the central metal atom or ion is called a ligand.

Due to the different oxidation states of the central metal ions, a different number and wide variety of ligands can form bonds with the transition element.
For example, the chromium(III) ion can form [Cr(NH3)6]3+, [Cr(OH)6]3- and [Cr(H2O)6]3+ complex ions.

Question 9

Why do transition elements make good catalysts?

Answer 9

Since transition elements can have variable oxidation states, they make excellent catalysts.

During catalysis, the transition element can change to various oxidation states by gaining electrons or donating electrons from reagents within the reaction.
Substances can also be adsorbed onto their surface and activated in the process.

Question 10

What is a heterogenous catalyst?

Answer 10

A heterogeneous catalyst is in a different physical state (phase) from the reactants.
The reaction occurs at active sites on the surface of the catalyst.
An example is the use of iron, Fe, in the Haber process for making ammonia.

Question 11

What is a homogenous catalyst?

Answer 11

A homogeneous catalyst is in the same physical state (phase) as the reactants.
The decomposition of hydrogen peroxide is a common reaction in the study of chemical kinetics and uses manganese(IV) oxide as the catalyst.

Question 12

What is a ligand?

Answer 12

A ligand is a molecule or ion that forms a co-ordinate bond with a transition metal by donating a pair of electrons to the bond.

Question 13

How is a ligand the same as a nucleophile?

Answer 13

Ligands have a negative charge or a lone pair of electrons capable of being donated

Question 14

What is the co-ordination number?

Answer 14

Co-ordination number is number of co-ordinate bonds to the central metal atom or ion.

Question 15

What is a monodentate ligand?

Answer 15

Monodentate ligands can form only one dative bond to the central metal ion

Question 16

Outline examples of ligands

Answer 16

Water (H2O) molecules
Ammonia (NH3) molecules
Chloride (Cl–) ions
Cyanide (CN–) ions

Question 17

What are bidentate ligands?

Answer 17

Bidentate ligands can each form two dative bonds to the central metal ion.
This is because each ligand contains two atoms with lone pairs of electrons

Question 18

Describe the structure of a ethanedioate ion

Answer 18

C2O42-

Question 19

What does a 1,2-diaminoethane ion look like?

Answer 19

Question 20

What are octahedral complexes?

Answer 20

Octahedral complexes are formed when a central metal atom or ion forms six coordinate bonds

Question 21

How many bidentate ligands does it take to form an octahedral complex?

Answer 21

Six coordinate bonds with three bidentate ligands

Question 22

When there are 4 co-ordinate bonds, what shape do complexes form?

Answer 22

When there are four coordinate bonds, the complexes often have a tetrahedral shape, or a square planar shape.

Question 23

What are the bond angles in tetrahedral complexes?

Answer 23

The bond angles in tetrahedral complexes are 109.5o

Question 24

Why do chloride ions mainly form tetrahedral complexes?

Answer 24

Complexes with four chloride ions most commonly adopt a tetrahedral shape because chloride ligands are large, so only four will fit around the central metal ion.

Question 25

What are the bond angles in a square planar complex?

Answer 25

The bond angles in a square planar complex are 90o

Question 26

What does cisplatin look like?

Answer 26

Square planar.
Bond angles = 90 degrees.

Question 27

Which complex structures exhibit cis-trans isomerism?

Answer 27

Square planar and octahedral complexes with two pairs of different ligands.

Question 28

How are cisplantin and transplantin different?

Answer 28

Cis-plantin has NH3 molecules on the same side.
Trans-plantin has NH3 molecules on different sides.

Question 29

How do octahedral compounds exhibit cis-trans isomerism?

Answer 29

If the two ‘different’ ligands are next to each other then that is the ‘cis’ isomer, and if the two ‘different’ ligands are opposite each other then this is the ‘trans’ isomer.

Question 30

Which complexes can form optical isomers?

Answer 30

Octahedral complexes with bidentate ligands also have optical isomers.
This means that the two forms are non-superimposable mirror images of each other.

Question 31

How do optical isomers differ from each other?

Answer 31

The optical isomers only differ in their ability to rotate the plane of polarised light in opposite directions

Question 32

How does cis-plantin work in cancer treatment?

Answer 32

The cis-platin works by binding to the nitrogen atoms on the bases in DNA.
The cis-platin passes through the cell membrane and undergoes ligand exchange where the chlorines are replaced by water molecules.
The nitrogen is a better ligand than water and forms dative covalent bonds with the cis-platin.
The cis-platin distorts the shape of the DNA and prevents the DNA from replicating.

Question 33

Observation of Cu2+ with NaOH(aq)

Answer 33

Blue solution reacts to form blue precipitate of copper (II) hydroxide.

Precipitate insoluble in excess sodium hydroxide.

Cu2+ (aq) + 2OH- (aq) -> Cu(OH)2 (s)

Question 34

Observation of Fe2+ with NaOH(aq)

Answer 34

Pale green solution reacts to form a green precipitate of iron (II) hydroxide.

Precipitate insoluble in excess sodium hydroxide but turns brown at surface when in air as iron (II) is oxidised to iron (III).

Fe2+ (aq) + 2OH- (aq) -> Fe(OH)2 (s)

In air:
Fe(OH)2 (s) -> Fe(OH)3 (s)

Question 35

Observation of Fe3+ with NaOH (aq)

Answer 35

Pale yellow solution reacts to form an orange-brown precipitate of iron (III) hydroxide.

Precipitate insoluble in excess sodium hydroxide.

Fe3+ (aq) + 3OH- (aq) -> Fe(OH)3 (aq)

Question 36

Observation of Mn2+ with NaOH (aq)

Answer 36

Pale pink solution reacts to form light brown precipitate of manganese (II) hydroxide which darkens on standing on air.

Precipitate insoluble in excess sodium hydroxide.

Mn2+ (aq) + 2OH- (aq) -> Mn(OH)2 (s)

Question 37

Observation of Cr3+ with NaOH (aq)

Answer 37

Violet solution reacts to form a grey-green precipitate of chromium (III) hydroxide.

Precipitate is soluble in excess sodium hydroxide forming a dark green solution.

Cr3+ (aq) -> Cr(OH)3 (s) -> [Cr(OH)6]3- (aq)