2.3 Transition Elements Flashcards
What are d-block elements?
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
Define a transition element
A d-block element at forms an ion with an incomplete d sub-shell
Which d-block elements aren’t transition elements?
The first and last columns in the row - i.e. Scandium and zinc, since their d-orbitals are empty and full respectively
How is the electron configuration for chromium and copper different and why is this?
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
Which electrons are lost first: 3d or 4s? Why?
4s, since these have a higher energy once the electrons occupy the orbitals
List some physical properties of transition metals
- 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
Why do transition elements form coloured aqueous compounds?
- 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
What are the 2 main ways in which transition metal catalysis can take place?
- 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
Give examples of a chemical reaction that uses a transition metal compound as a catalyst
- 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
What is a precipitation reaction?
One in which soluble ions, in separate solutions, are mixed together to produce an insoluble compound (which settles out of solution as a solid)
What happens when transition metal ions react with aqueous NaOH?
They form coloured precipitates
What is a complex ion?
A transition metal ion bonded to 1 or more ligands by coordinate bonds (dative covalent bonds)
What is a ligand?
A molecule or ion that can donate a pair of electrons with the transition metal ion to form a coordinate bond
What is the coordination number of a complex ion?
The total number of coordinate bonds formed between a central metal ion and its ligands
How do you work out the overall charge of a complex ion?
Work out the sum of the individual charges of the transition metal ion and those of the ligands present in the complex
What is a monodentate ligand?
A ligand that donates just 1 pair of electrons to the central metal ion to form 1 coordinate bond
What is the charge on the water ligand?
None: it is a neutral ligand
What is the charge on the ammonia ligand? (:NH3)
None: it is a neutral ligand
What is the charge on all ligands other than water or ammonia?
-1
What is the most common shape of a complex ion?
The octahedral shape
How many coordinate bonds do octahedral complexes have attached to the central ion? What bond angle do they have?
6 (4 ligands in the same plane, 1 above the plane, 1 below the plane)
Bond angle = 90°
When can cis-trans isomerism occur in complex ions?
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
What are bidentate ligands?
A ligand that can donate 2 lone pairs of electrons to the central metal ion to form 2 coordinate bond
What is the most common bidentate ligand?
Ethane-1,2-diamine, NH2CH2CH2NH2
- Each nitrogen atom donates its lone pair to the metal ion
- Often shortened to ‘en’
What shape complexes can bidentate ligands form?
Octahedral
- Each ligand forms 2 coordinate bonds to the central metal ion
- There are 3 ligands, so there will be 6 coordinate bonds
Can bidentate ligands show cis-trans isomerism?
Yes: as long as they are an octahedral complex with 2 different ligands
What is a hexadentate ligand?
A ligand that has 6 lone pairs of electrons, each forming a coordinate bond to a metal ion in a complex ion
What is an example of a hexadentate ligand?
EDTA^4-
What can EDTA^4- be used for?
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)
What are the requirements for optical isomerism in complex ions?
- 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
What is cis-platin?
- 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
How do they think cis-platin works?
- 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
If chloride ions are present in a complex ion, what shape does it tend to make?
A tetrahedral shape
What is a ligand substitution reaction?
A reaction in which one ligand in a complex ion is replaced by another ligand
How does ligand substitution work?
The ligand that will form the more stable complex ion becomes part of the complex ion
In the reaction of aqueous copper(II) ions and ammonia, what is the colour change?
From pale blue, to a pale blue precipitate, to dark blue
What happens in the reaction of aqueous copper(II) ions and ammonia?
- 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
In the reaction of aqueous copper(II) ions and hydrochloric acid, what is the colour change?
Pale blue (to green) to yellow
What happens in the reaction of aqueous copper(II) ions and hydrochloric acid?
- 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
What happens in the reaction of aqueous cobalt(II) ions and HCl?
The 6 water molecules in the complex ion are replaced by 4 chloride ions
In the reaction of aqueous cobalt(II) ions and hydrochloric acid, what is the colour change?
Pale pink to dark blue
What is the stability constant?
K(stab) = [products]/[reactants]
What does a large value of K(stab) indicate?
- The complex ion is more stable
- The position of equilibrium lies to the right
Which is more stable, iron(II) or iron (III)?
Iron(III)
- When in the presence of air, or when in contact with another oxidising agent, iron(II) is readily oxidised to iron (III)
How are redox titrations carried out?
- 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
Give an example of an oxidising agent that is self-indicating
Potassium manganate(VII) MnO4^- (purple) ---------> Mn^2+ (almost colourless)
How can MnO4^- oxidise solutions containing iron(II) ions?
- 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)
How can you calculate the concentration of Fe^2+ in a solution after carrying out a redox titration involving MnO4^- ions?
- 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)
How can the concentration of iodine in a solution be determined?
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
What happens if a solution containing Cu^2+ ions is mixed with aqueous iodide ions, I^-?
- 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
