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A2 Chemistry Unit 1 > 2.3 Transition Elements > Flashcards

2.3 Transition Elements Flashcards

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0
Q

What are d-block elements?

A

Elements with their outer electrons in the d sub-shell

  • The highest energy sub-shell is a d sub-shell
  • Found between group 2 and group 3 in the periodic table
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1
Q

Define a transition element

A

A d-block element at forms an ion with an incomplete d sub-shell

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2
Q

Which d-block elements aren’t transition elements?

A

The first and last columns in the row - i.e. Scandium and zinc, since their d-orbitals are empty and full respectively

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3
Q

How is the electron configuration for chromium and copper different and why is this?

A

The 4s orbitals only contain 1 electron:

  • Chromium puts the “2nd” electron into the 3d orbital to make it half full, which creates a more stable molecule
  • Copper does the same, making the 3d orbital full
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4
Q

Which electrons are lost first: 3d or 4s? Why?

A

4s, since these have a higher energy once the electrons occupy the orbitals

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5
Q

List some physical properties of transition metals

A
  • Shiny in appearance
  • High densities
  • High melting points
  • High boiling points
  • When solid, exist as giant metallic lattices containing delocalised electrons, which move to conduct electricity
  • Form compounds in which the transition metal has different oxidation states, which form coloured solutions when dissolved in water
  • They all form 2+ ions
  • Their highest oxidation state is often found in a strong oxidising agent
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6
Q

Why do transition elements form coloured aqueous compounds?

A
  • When light passes through a solution containing transition metal ions, some of the wavelengths of visible light are absorbed
  • The colour that we observe is a mixture of the wavelengths of light that have not been absorbed
  • The colour is linked to the partially filled d-orbitals
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7
Q

What are the 2 main ways in which transition metal catalysis can take place?

A
  • They provide a surface on which a reaction can take place. Reactants are adsorbed onto the surface of the metal and held in place while a reaction occurs. After the reaction, the products are desorbed and the metal remains unchanged
  • They have the ability to change oxidation states by gaining or losing electrons. They then bind to reactants, forming intermediates as part of a chemical pathway with a lower activation energy
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8
Q

Give examples of a chemical reaction that uses a transition metal compound as a catalyst

A
  • The Haber process: makes ammonia, iron metal catalyst (increases R.o.R, lowers temp at which the reaction takes place)
  • Contact process: converts sulfur dioxide into sulfur trioxide, in the manufacture of sulfuric acid, vanadium(V) oxide catalyst
  • Hydrogenation of alkenes: nickel metal catalyst, used to lower the temp and pressure needed to carry out the reaction
  • Decomposition of hydrogen peroxide: manganese(IV) oxide (MnO2) catalyst, used to increase R.o.R
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9
Q

What is a precipitation reaction?

A

One in which soluble ions, in separate solutions, are mixed together to produce an insoluble compound (which settles out of solution as a solid)

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10
Q

What happens when transition metal ions react with aqueous NaOH?

A

They form coloured precipitates

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11
Q

What is a complex ion?

A

A transition metal ion bonded to 1 or more ligands by coordinate bonds (dative covalent bonds)

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12
Q

What is a ligand?

A

A molecule or ion that can donate a pair of electrons with the transition metal ion to form a coordinate bond

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13
Q

What is the coordination number of a complex ion?

A

The total number of coordinate bonds formed between a central metal ion and its ligands

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14
Q

How do you work out the overall charge of a complex ion?

A

Work out the sum of the individual charges of the transition metal ion and those of the ligands present in the complex

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15
Q

What is a monodentate ligand?

A

A ligand that donates just 1 pair of electrons to the central metal ion to form 1 coordinate bond

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16
Q

What is the charge on the water ligand?

A

None: it is a neutral ligand

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17
Q

What is the charge on the ammonia ligand? (:NH3)

A

None: it is a neutral ligand

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18
Q

What is the charge on all ligands other than water or ammonia?

A

-1

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19
Q

What is the most common shape of a complex ion?

A

The octahedral shape

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20
Q

How many coordinate bonds do octahedral complexes have attached to the central ion? What bond angle do they have?

A

6 (4 ligands in the same plane, 1 above the plane, 1 below the plane)
Bond angle = 90°

21
Q

When can cis-trans isomerism occur in complex ions?

A

When they are an octahedral shape
- Contains 2 different ligands, 4 of one ligand and 2 of another
Or when they are in a square planar shape
- The (4) ligands are arranged at the corners of a square
- Must contain 2 different ligands

A cis isomer forms when the 2 same ligands are together
A trans isomer forms when the 2 same ligands are at opposite corners to each other

22
Q

What are bidentate ligands?

A

A ligand that can donate 2 lone pairs of electrons to the central metal ion to form 2 coordinate bond

23
Q

What is the most common bidentate ligand?

A

Ethane-1,2-diamine, NH2CH2CH2NH2

  • Each nitrogen atom donates its lone pair to the metal ion
  • Often shortened to ‘en’
24
Q

What shape complexes can bidentate ligands form?

A

Octahedral

  • Each ligand forms 2 coordinate bonds to the central metal ion
  • There are 3 ligands, so there will be 6 coordinate bonds
25
Q

Can bidentate ligands show cis-trans isomerism?

A

Yes: as long as they are an octahedral complex with 2 different ligands

26
Q

What is a hexadentate ligand?

A

A ligand that has 6 lone pairs of electrons, each forming a coordinate bond to a metal ion in a complex ion

27
Q

What is an example of a hexadentate ligand?

A

EDTA^4-

28
Q

What can EDTA^4- be used for?

A

To bind metals ions

  • It decreases the concentration of metal ions in solutions by binding them into a complex
  • Can be used in detergents (binds to calcium and magnesium ions to reduce hardness in water)
  • In some foods (as a stabiliser to remove metal ions that might catalyse the oxidation of the product)
  • In medical applications (added to blood samples to prevent clotting, used to treat patients suffering from lead and mercury poisoning)
29
Q

What are the requirements for optical isomerism in complex ions?

A
  • A complex with 3 molecules or ions of a bidentate ligand
  • A complex with 2 molecules or ions of a bidentate ligand and 2 molecules of ions of a monodentate ligand
  • A complex with 1 hexadentate ligand
30
Q

What is cis-platin?

A
  • One of the most effective drugs against many forms of cancer
  • The cis-isomer of a platinum complex, [PtCl2(NH3)2]
  • It has a square planar shape
31
Q

How do they think cis-platin works?

A
  • By binding to the DNA of fast-growing cancer cells
  • This alters the DNA in the cancerous cells
  • It is generally believed that the cells are prevented from reproducing by these changes to the DNA structure
32
Q

If chloride ions are present in a complex ion, what shape does it tend to make?

A

A tetrahedral shape

33
Q

What is a ligand substitution reaction?

A

A reaction in which one ligand in a complex ion is replaced by another ligand

34
Q

How does ligand substitution work?

A

The ligand that will form the more stable complex ion becomes part of the complex ion

35
Q

In the reaction of aqueous copper(II) ions and ammonia, what is the colour change?

A

From pale blue, to a pale blue precipitate, to dark blue

36
Q

What happens in the reaction of aqueous copper(II) ions and ammonia?

A
  • 4 of the water ligands are replaced by 4 ammonia ligands
  • This produces [Cu(NH3)4(H2O)2]^2+
  • The copper-oxygen bonds are longer than the copper-nitrogen bonds, so the shape is strictly described as a distorted octahedral shape
  • A pale blue precipitate of copper(II) hydroxide forms after a small amount of NH3 is added
  • This dissolves once excess NH3 is added
37
Q

In the reaction of aqueous copper(II) ions and hydrochloric acid, what is the colour change?

A

Pale blue (to green) to yellow

38
Q

What happens in the reaction of aqueous copper(II) ions and hydrochloric acid?

A
  • The [Cu(H2O)6]^2+ complex ion has 6 ligands but the [CuCl4]^2- complex ion only has 4 ligands
  • This is because the chloride ligands are larger than the water ligands and have stronger repulsions, so fewer chloride ligands can fit around the central metal ion
39
Q

What happens in the reaction of aqueous cobalt(II) ions and HCl?

A

The 6 water molecules in the complex ion are replaced by 4 chloride ions

40
Q

In the reaction of aqueous cobalt(II) ions and hydrochloric acid, what is the colour change?

A

Pale pink to dark blue

41
Q

What is the stability constant?

A

K(stab) = [products]/[reactants]

42
Q

What does a large value of K(stab) indicate?

A
  • The complex ion is more stable

- The position of equilibrium lies to the right

43
Q

Which is more stable, iron(II) or iron (III)?

A

Iron(III)
- When in the presence of air, or when in contact with another oxidising agent, iron(II) is readily oxidised to iron (III)

44
Q

How are redox titrations carried out?

A
  • In a similar way to acid-base titrations
  • Involves the transfer of electrons from one species to another
  • An oxidising agent is titrated against a reducing agent
  • The end point can sometimes be identified without using an indicator
45
Q

Give an example of an oxidising agent that is self-indicating

A 
Potassium manganate(VII)
MnO4^- (purple) ---------> Mn^2+ (almost colourless)
46
Q

How can MnO4^- oxidise solutions containing iron(II) ions?

A
  • The solution is acidified with sulfuric acid (HCl cannot be used because it reacts with MnO4^-)
  • Potassium manganate solution is in the burette and the iron(II) solution in the conical flask
  • As the manganate solution is added to the iron(II) solution, it is decolourised
  • The end point of the titration is when the excess MnO^4- ions are present (the first hint of a permanent pink colour in the solution in the conical flask)
47
Q

How can you calculate the concentration of Fe^2+ in a solution after carrying out a redox titration involving MnO4^- ions?

A
  • Calculate the amount, in moles, of MnO4^- used in the titration (n = c x v)
  • Deduce the amount, in moles, of iron(II) in the solution that was titrated (ratio from equation)
  • Calculate the concentration, in mol dm^-3, of Fe^2+ in the solution (c = n/v)
48
Q

How can the concentration of iodine in a solution be determined?

A

By titration with a solution of sodium thiosulfate of known concentration
This titration can be used to determine the concentration of a solution of an oxidising agent that reacts with iodide ions to produce iodine
- Iodide ions are added to the oxidising agent under investigation (a redox reaction takes place, producing iodine)
- The iodine is titrated against a sodium thiosulfate solution of known concentration
- From the results, the amount of iodine can be calculated and hence the concentration of the oxidising agent can be calculated

49
Q

What happens if a solution containing Cu^2+ ions is mixed with aqueous iodide ions, I^-?

A
  • Iodide ions are oxidised to iodine
  • Copper(II) ions are reduced to copper(I) ions
  • This produces a light brown/yellow solution (due to the iodine) and a white precipitate of copper(I) iodide
  • This mixture is then titrated against sodium thiosulfate of known concentration
  • As the iodine reacts, the iodine colour gets paler during the titration
  • a When the colour is a pale straw, a small amount of starch is added to help with the identification of the end point
  • A blue/black colour forms
  • This disappears sharply at the end point because all the iodine has reacted

A2 Chemistry Unit 1 (3 decks)

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