transition elements year 2 Flashcards
d-block elements
-located between group 2 and 13 of the periodic table.
- all metallic, displaying the physical properties of metals. Have high melting and boiling points, shiny and conduct electricity and heat
electron configuration of chromium and copper
chromium- fill every orbital half full to make it more stable- 3d and 4s
copper- fill every orbital in the d orbital fully full so 3d10 - give additional stability
transition elements
- defined as d-block elements that form at least one ion with a partially filled d-orbital. Although Sc and Zn are d-block elements they aren’t classified as transition elements
- form compounds in which the transition element has different oxidation states
- form coloured compounds
- elements and their compounds can act as catalysts
Transition metals as catalysts
- catalyst is a substance that increases the rate of a chemical reaction without itself changing. A catalyst works by providing an alternative reaction pathway with a lower activation energy
- the harber process uses an iron catalyst- this is the manufacture of ammonia
- the contact process uses vanadium oxide V205- manufacture of sulfur trioxide
- the hydrogenation of vegetable fats in the manufacture of margarine uses nickle catalyst
- decomposition of H202 uses MnO2
complex ions
- d-block elements are able to form complex irons eg hydrated copper (11) sulfate
- metal iron surrounded by co-ordinately bonded ligands
- in the formula the complex ion is enclosed inside square brackets with the overall charge of the complex shown outside the square brackets. The overall charge on a complex ion is the sum of the charges on the central metal ion and any ligands present eg [Cr(H2O)6]3+
Ligand
An iron or molecule that forms a co-ordinate bond with a transition metal. It does this by donating a pair of electrons to the central metal
- the coordination number indicates the number of coordinate bonds attached to the central metal ion
monodentate/ unidentate ligands
- ligand that is able to donate one pair of electrons to a central metal ion, so can only form one coordinate bond
- eg water, ammonia, chloride ion, cyanide ion and hydroxide ion
- water has two lone pairs but they are so close together they can only form one coordinate bond at a time
bidentate ligands
- ligands that can form two coordinate bonds as can donate two lone pairs of electrons to the central metal ion eg 1-2,diaminoethane and the ethanedioate (oxalate) ion
- in 1,2-diaminoethane each nitrogen atom donates a pair of electrons to the central metal ion
- in an oxalate ion each negatively- charged oxygen atom donates a lone pair of electrons to the central metal ion
multidentate ligands
- can form three or more coordinate bonds by donating at least three lone pairs of electrons to the metal ion
- extremely stable as reaction results in an increase in entropy
- EDTA is a common example- forms 6 coordinate bonds as 6 lone pairs
- complex ions with multidentate ligands are called chelates
- EDTA can be used to remove metal ions from solution- six water ligands replaced by one EDTA ligand. This results in an increase in the number of particles in a solution
oxidation state of metal ion + sum of oxidation states of ligands= total oxidation state of complex
Shapes of complex ions
- shape of a complex ion depends on its coordinate number.
- many complex ions have a coordination number of 6, giving it an octahedral shape. eg when MnSO4 is dissolved in water, the complex ion forms. This has an octahedral shape with bond angles around the manganese of 90
- complexes with coordination numbers of four form tetrahedral or square planar shapes. Tetrahedral shape has bond angles of 109.5 around the central metal ion eg [CoCl4]2- and [CuCl4]2-
- square planar shapes occur in complex ions of transition metals with eight d-electrons in the highest energy d-subshell. This is platinum(II), palladium(II) and gold(III). These form complexes with bond angles of 90 degrees and the ligands are arranged in the corners of a square
variable oxidation states
Transition elements form compounds with more than one oxidation state. eg iron forms two chlorides- iron(II) chloride and iron(III) chloride
- the number of oxidation states increases across the transition elements series to manganese, and then decreases. All of the transition elements form compounds with an oxidation number of +2
- eg Cr +2 is blue and Cr +3 is green
copper+2 is blue
-Mn+2 is pink and Mn +4 is purple
The transition elements therefore form a range of coloured compounds eg potassium dichromate (VI) is bright orange and hydrated copper(II) sulfate is blue
steriosimers
Have the same structural formula but a different arrangement of the atoms in space. Complex ions can display cis-trans isomerism and optical isomerism.
The type of stereoisomerism depends on the number and type of ligands that are attached to the central metal ion, and the shape of the complex
- some four-coordinate (always square planar*) and six-coordinate complex ions containing two different monodentate ligands show cis-trans isomerism
- some six-coordinate complex ions containing monodentate and bidentate ligands can show both cis-trans and optical isomerism
cis-trans isomerism in complex ions
No double bond is involved and the shape of the complex holds groups in different orientations about the central metal ion
- in square planar complexes the ligands are arranged in the same plane at the corners of a square. In the cis-isomer, the two identical groups are adjacent to each other, whereas in the trans isomer the two identical groups are opposite each other. In the cis isomer the coordinate bonds between the identical ligands are 90 degrees apart and in the trans isomer the coordinate bonds between the identical ligands are 180 degrees apart
In octahedral complexes:
The two different monodentate ligands will be opposite vertically for trans, and will be 90 degrees apart/next to each other for cis
optical isomerism
occurs ONLY in octahedral complexes containing two or more bidentate ligands
Optical isomers are non-superimposable mirror images of each other
can be seen when using bidentate ligands for [Co(NH2CH2CH2NH2)2Cl2]+, but only for the cis isomer in this example
Can also be seen in complexes containing three bidentate ligands
role of cis-platin in medicine
Cis platin is [PtCl2(NH3)2]
Cis platin is used in medicine to attack tumours and shrink them in size. Used extensively in the treatment of cancer. It works by forming a platinum complex inside of a cell which binds to DNA and prevents the DNA of the cell from replicating. Activation of the cells own repair mechanism eventually leads to apoptosis, or systematic cell death
naming complex ions
name contains in this order:
- number of ligands
- name of ligands
- transition metal ion and its oxidation state (in roman numerals)
water- aqua
chloride- chloro
ammonia- amino
cyanide- cyano
hydroxile- hydroxo
If more than one ligand is present you name them alphabetically
- when complex is an anion (negatively charged ion), metal name ends in ate
eg cobaltate, chromate, cuprate, ferrate
ligand substituion
A ligand substitution reaction is one in which one ligand in a complex ion is replaced by another ligand
- often water molecules in an aqueous solution of a complex ion are replaced by another ligand
- when copper(II) sulfate is dissolved in water, the pale blue complex ion [Cu(H20)6]2+ is formed in aqueous solution
ligand substitution with ammonia
When an excess of aqueous ammonia is added to a solution containing hexaquacopper(11), the pale blue solution changes colour to form a dark blue solution
[Cu(H2O)6]2+ + 4NH3 –>[Cu(NH3)4(H2O)2]+2 + 4H20
four ammonia ligands have replaced four of the wager ligands
-NH3 ligands are held more tightly as coordinate bonds are shorter, distorting the octahedral shape
When adding ammonia dropwise a blue precipitate is originally formed. The blue precipitate then dissolves in excess ammonia to form a dark blue solution
The blue precipitate is [Cu(H20)4(OH)2]
ligand substitution with chloride ions
Concentrated HCl can be used as a source of chloride ions. When an excess of concentrated HCl is added to a solution containing hexaquacopper, the pale blue solution changes colour to form a yellow solution. Six water ligands have been replaced with four chloride ligands. If water is added to the yellow solution, a blue solution will be formed although it will be more dilute and paler in colour than the original blue solution
- an intermediate green solution is also formed. This is a result of the yellow solution mixing with the blue solution to give a green colour
The equation below is in equilibrium:
[Cu(H20)6]2+ + 4Cl- <—–> [CuCl4]2- + 6H2O
This reaction involves a change in coordination number, a change in colour, and a change in shape. However, the oxidation state of copper remains as +2, a chloride ligands are larger in size than the water ligands so fewer chloride ligands can fit around the central Cu2+ ion
reactions of aqueous chromium(III) ions
when chromium(III) potassium sulfate, KCr(SO4)2.12H2O
is dissolved in water the complex ion Cr(H2O)6]3+ is formed. This is a pale purple solution.
When chromium(III) sulfate is dissolved in water a green solution containing chromium(III) is formed. this is: [Cr(H2O)5SO4]+ where one of the water ligands has been replaced by the sulfate ion. Both the purple and green solutions contain chromium ions in oxidation state+3
Reactions with ammonia:
[Cr(H2O6}3+ takes part in a ligand substitution reaction with an excess of aqueous ammonia forming the complex ion [Cr(NH3)6]3+. When the ammonia is added drop-wise to the chromium solution,
initially Cr(OH)3 is formed which is a grey precipitate. Then dissolves in excess ammonia to form [Cr(NH3)6]3+
ligand substitution and haemoglobin
Blood carries oxygen around the body due to presence of haemoglobin, the iron-containing protein present in all red blood cells. Contains four protein chains held together by weak intermolecular forces. Each protein chain has a haem molecule within its structure. The central metal ion in a haem group if Fe2+ which can bind to oxygen gas
As blood passes through the lungs, the haemoglobin bonds to oxygen because of the increased oxygen pressure in the capillaries of the lungs. This forms oxyhaemoglobin, which releases oxygen to body cells where required. Can also bond to CO2, which is carried back to the lungs. CO2 is then released from the red blood cells and CO2 is exhaled
Carbon monoxide can also bind to the Fe2+ ion in haemoglobin. The complex formed is known as carboxyhaemoglobin. If CO is breathed in, a ligand substitution reaction takes place where the oxygen in haemoglobin is replaced by CO.
Carbon monoxide binds to haemoglobin more strongly that oxygen, so a small concentration of carbon monoxide in the lungs can prevent a large proportion of the haemoglobin molecules from carrying oxygen. The bond is so strong that this process is irreversible. If the concentration of carboxyhaemoglobin becomes too high, oxygen transport is prevented, leading to death
precipitation reactions
a precipitation reaction occurs when two aqueous solutions containing ions react together to form an insoluble ionic solid, called a precipitate
- transition metal ions in aqueous solution react with aqueous sodium hydroxide and aqueous ammonia to form precipitates. However, some of these precipitates will dissolve in excess to form complex ions
precipitation reactions with sodium hydroxide
- Cu2+ Blue solution of Cu2+ reacts to form a blue precipitate of copper(II) hydroxide. This is insoluble in excess
- Fe2+. Pale green solution forms a green precipitate of iron(II) hydroxide
Insoluble in excess NaOH but turns brown at its surface on standing in air as iron(II) is oxidised to iron(II). Fe(OH)2(s) —> Fe(OH)3(s) - Fe3+. Pale yellow solution reacts to form an orange brown precipitate of iron(III) hydroxide. Precipitate is insoluble in excess NaOH
- Mn2+. Pale pink solution reacts to form a light brown precipitate of manganese(II) hydroxide which darkens on standing in air
Precipitate insoluble in excess NaOH - Cr3+. Violet solution reacts to form a grey-green precipitate of chromium(III) hydroxide. The precipitate is soluble in excess NaOH forming a dark green solution of hexaqua chromate
precipitation reactions with ammonia
- ligand substitution reactions of hexaquacopper and hexaquachromium react with an excess of aqueous ammonia
Cu2+ + 2OH- –> Cu(OH)2
copper hydroxide is a blue precipitate that dissolves in excess ammonia to form a deep blue solution
Cr(OH)3 is a green precipitate which dissolves in excess ammonia to form [Cr(NH3)6]3+ which is a purple solution
redox reactions involving Fe2+ and Fe3+
- when reacted with manganate ions Fe2+ is oxidised and MnO4- is reduced to Mn2+, this occurs in acidic conditions
purple—> colourless
When a solution of Fe3+ reacts with iodide ions the orange-brown Fe3+ are reduced to pale green Fe2+. The colour change is obscured by the oxidation of iodide ions to form iodine, which has a brown colour
redox reactions of dichromate(VI) and chromium(III)
- Aqueous dichromate ions Cr2O72- have an orange colour and Cr3+ have a green colour
Acidified Cr2O72- can be reduced to Cr3+ by the addition of zinc
With an excess of zinc, chromium(III) ions are reduced further to chromium(II) which is a pale blue colour
Reactions of Cu2+ and Cu+
When aqueous copper(II) ions react with excess iodide ions a redox reactions occurs
2Cu2+ + 41- —> 2CuI + I2
pale blue white precipitate brown
oxidation of Cr3+ to CrO42-
- hot alkaline H2O2 is a powerful oxidising agent and can be used to oxidise chromium(III) in Cr3+ to chromium(VI) in CrO4-2
3H202 + 2Cr3+ + 10OH- —> 2CrO42-+ 8H20
chromium is oxidised from +3 to +6
oxygen is reduced from -1 to -2
disproportionation of Cu+ ions
- when solid copper(I) oxide, reacts with hot dilute sulfuric acid, a brown precipitate of copper is formed together with a blue solution of copper(II) sulfate. In this reaction copper(I) ions, Cu+ have been simultaneously oxidised and reduced. As the same element has been reduced and oxidised, this reaction is disproportionation
ammonium test
- when heated with hydroxide ions, NH4+, reacts to produce ammonia and water.
To test for the ammonium ion, aqueous sodium hydroxide, is heated gently with the solution being analysed. If ammonia is evolved, damp red pH indicator paper will turn blue, confirming the presence of NH4+ ions
