Module 5 (chapter 24) - transition elements Flashcards
background information about d-block elements
- are all metallic, displaying the typical physical properties of metals
- they have high melting and boiling points, shiny in appearance and conduct electricity and heat
uses of transition metals
- copper, silver, nickel and zinc have been used in coinage for many years
- iron is used in construction and the production of tools
- copper is used for electrical cables snd water pipes
- titanium is known for its great strength and use in aerospace and joint replacement
electron configuration of d-block elements
-the electron configuration of an atom or iron shows the arrangement of electrons in shells and sub shells
-electron occupy orbitals in order of increasing energy
-the electron configuration of chromium and copper don’t follow the expected principle for placing electron singly in orbitals before pairing
-this is due to stability
-a half filled d5 sub-shell and a fully filled d10 sub shell give additional stability to atoms
chromium (1s2,2s2,2p6,3s2,3p6,3d5,4s1)
copper (1s2,2s2,2p6,3s2,3p6,3d10,4s1)
rules when gaining and losing electrons
- when forming an atom, the 4s orbital fills before the 3d orbital
- when forming an ion, the 4s orbital empties before the 3d orbital
transition elements
they are d-block elements that form at least one ion with a partially filled d-orbital
-you can be a d-block element but not a transition element
why is scandium not a transition element?
scandium only forms Sc3+ ions by loss of two 4s electrons and one 3d electron
- it only has one electron in the 3d orbital and therefore its electron configuration becomes 1s2,2s2,2p6,3s2,3p6
- does not have a partially filled d orbital and so isn’t a transition element
why is zinc not a transition element?
- zinc only forms the Zn2+ ion by the loss of two 4s electrons
- the electron configuration of Zinc means that when these two electrons are lost it still has a full 3d shell snd so it is not half full.
properties of transition metals
- they form compounds in which the transition element has different oxidation states
- they form coloured compounds
- the elements and their compounds can act as catalysts
variable oxidation states
- transition elements form compounds with more than one oxidation state.
- iron forms two chlorides for example (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, resulting from the loss of two electrons
formation of coloured compounds
- compound and ions of transition elements are frequently coloured
- potassium dichromate is bright orange
- cobalt (II)chloride is pink/purple
- nickel sulphate is green
- hydrated copper sulfate is blue
- the colour of a solution is linked the partially filled d-orbitals of the transition metal ion. the colour can vary with different oxidation states (e.g. iron)
catalysts
- iron is used to catalyse the hater process which manufactures ammonia
- vanadium oxide is used to catalyse the contact process and the production of surfer trioxide from surfer dioxide
- nickel is used yo catalyse the hydrogenation of vegetable fats when making margarine
- manganese oxide is used to catalyse the decomposition of hydrogen peroxide to form oxygen
what type of catalysts are the named transition metals
- heterogeneous catalysts
- some are homogeneous
- e.g. reaction between iodide ions and peroxodisulfate ions is catalysed by Fe2+ ions, with reactants and catalyst all in aqueous solution
- when the reaction is carried out with a trace of starch, a blue-black colour forms showing the formation of iodine. when catalyst added this precipitate forms much quicker
complex ions
d-block elements form complex ions
-other elements like aluminium can also form complex ions
how do complex ions form?
- when one of more molecules of negatively charged ions bond to a central metal ion. these molecules or ions are known as ligands
- the coordination number indicates the number of coordinate bonds attached to the central metal ion
ligand
a molecule of ion that donates a paid of electrons to a central metal ion to form a coordinate bond or date covalent bond
how are complex ions represented
- complex ion is enclosed inside square brackets with the overall change of the complex shown outside the square brackets
- the overall charge is the sum of the charges on the central metal ion and any ligand present.
monodentate ligands
-a ligand that is able to donate one pair of electrons to a central metal ion water (neutral) ammonia (neutral) chloride (-1) cyanide (-1) hydroxide (-1)
bidentate ligands
- ligands that can donate two lone pairs of electrons to the central metal ion, forming two coordinate bonds
- the most common are:
- 1,2 -diaminoethane (each nitrogen atom donates a paid of electrons to the central metal ion forming a coordinate bond)
- ethandioate (each negatively charged oxygen atom donates a lone pair of electrons to the central metal ion)
shapes of complex ions
- depends upon its coordination number. the commonest coordination numbers six and four
- many complex ions have a coordination number of six, given an octahedral shape (bond angles of 90 degrees)
four coordinate complexes
- tetrahedral complexes are the most common with bond angles of 109.5 degrees around the central metal ion (e.g. CoCl4 and CuCl4) both 2-
- square planar complexes occur in complex ions of transition metals with eight d electrons in the highest energy d sub shell (platinum, palladium and gold)
- Im this structure, ligands are arranged at the corners of a square
stereoisomers
complex ions can display two types of steriosiomerism
- cis/trans isomerism
- optical isomerism
- the type of stereoisomerism depends on the number and type of ligands that are attached to the central atom plus the shape of the complex
- some four coordinate and six coordinate complex ions containing two different types of 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 C=C double bond is required and the shape of the complex holds groups in different orientations about the central metal ion
- occurs in square planar and octahedral complexes
cis-trans isomerism in quake planar complexes
- ligands are arranged in the same planes with 90 degree bond angles
- 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 vs 180 degrees in the trans-isomer
cis-trans isomerism in octahedral complexes
- complexes containing four of one type of ligand and two as another can show this. the cis isomer have identical glands adjacent to each other, the trans at 180 degrees
- octahedral complexes containing bidentate ligands can also shoe cis-trans isomerism
- if the other two atoms (not bidentate) are nest to each other is it cis, if they are opposite it is trans
optical isomerism
- can only occur in octahedral complexes containing two or more bidentate ligands
- optical isomers called enantiomers are non-superimposable mirror images of each other
- trans isomers cannot form optical isomers as a mirror image is exactly the same and cannot be superimposed
- can also be seen in complexes containing three bidentate ligands
ligand substitution
- a reaction where one ligand in a complex ion is replaced by another ligand
- e.g. when copper (II) sulphate is dissolved in water, the pale blue complex ion is formed in aqueous solution
ligand substitution with ammonia
- when an excess of (aq) ammonia is added to a solution containing [Cu(H2O)6]2+ the pale blue solution changes colour to form a dark blue solution
- four ammonia ligands have replaced four water ligands
- add ammonia drop wise so that all observations are seen (two reactions take place)
- a pale blue precipitate of Cu(OH)2 is formed in the first stage of the reaction
- the Cu(OH)2 precipitate then dissolves in excess ammonia to form a dark blue solution
ligand substitution with chloride ions
- concentrated HCl is used as a source of chloride ions
- when an excess of concentrated hydrochloric acid is added to a solution containing [Cu(H20)6]2+, the pale blue solution changes colour to form a yellow solution [CuCl4]2-
- six water ligands have been replaced by four chloride ligands
- if water is added to the yellow solution, a blue solution will be formed, although more dilute and paler in colour than the original blue solution
- an intermediate green solution is formed. this isn’t a new species but is the result of the yellow solution mixing with the blue solution to give a green colour as the reaction proceeds
- the oxidation state of copper remains as +2. chloride ligands are large in size than water ligands, so fewer can fit around the central Cu2+ ion. this explains the change in coordination number
reactions of aqueous chromium ions (chromium sulphate)
- when chromium (III) potassium sulphate is dissolved in water, the complex ion [Cr(H2O)6]3+ is formed (purple solution)
- when chromium sulphate is dissolved in water, a green solution containing chromium is formed [Cr(H2O)5 SO4]+ where one of the water ligands has been replaced by the sulphate ion
- both solutions contain chromium (III) ions in oxidation state 3+
reaction with ammonia
- [Cr(H2O)6]3+ takes part in a ligand substitution reaction with an excess of (aq) ammonia forming [Cr(NH3)6]3+
- when the ammonia is added drop-wise to the chromium(III) solution two steps take place
- grey green precipitate forms of Cr(OH)3
- the Cr(OH)3 precipitate dissolves in excess ammonia to form the complex ion [Cr(NH3)6]3+
- all six ligands are substituted
ligand substitution and haemoglobin
- blood caries oxygen around the body due to the presence of haemoglobin (contains four proteins held together by weak intermolecular forces)
- the central metal ion is Fe2+ which can bind to oxygen gas (O2)
- blood passes through the lungs and the increased oxygen pressure allows haemoglobin to bind to it
- it then binds to carbon dioxide after oxygen is released and carries this back to the lungs
- carbon monoxide can also bind to the Fe2+. if carbon monoxide is breathed in a ligand substitution reaction takes place where the oxygen in haemoglobin is replaced by carbon monoxide
- it bonds more strongly preventing a large proportion of the molecules form carrying oxygen (it is so strong the bond is irreversible)
precipitation reactions
-occurs when two aqueous solution containing ions react together to form an insoluble ionic solid called a precipitate
-transition elements react with aqueous sodium hydroxide and aqueous ammonia to form precipitates
Cu2+, Fe2+, Fe3+ and Mn2+ all form precipitates on reaction with excess sodium hydroxide, none dissolve in excess
oxidation of iron(II) to iron(III)
-acid conditions
-Fe2+ is oxidised to Fe3+
-MnO4- is reduced to Mn2+
-the solution containing MnO4- ions is purple and is decolourised by Fe2+ ions to form a colourless solution containing Mn2+ ions
MnO4- + 8H- + 5Fe2+ –> Mn2+ + 5Fe3+ +4H2O
reduction of Fe3+ to Fe2+
-when reacted with iodide ions, I-, the orange-brown Fe3+ ion are reduced to pale green Fe2+ ions
-colour change is obscured by the oxidation of iodide ions to form iodine which is a brown colour
2Fe3+ + 2I- –> 2Fe2+ + I2
-iodide ions are oxidised
-Fe3+ ions are reduced
electrode potentials in redox reactions
-the more positive the electrode potential value, the equilibrium is more likely to gain electrons shift to the right and undergo reduction
reactions of dichromate and chromium
-aqueous dichromate ions have an orange colour and aqueous chromium ions have a green colour
-acidified dichromate ions can turn can be reduced to chromium 3+ ions by the addition of Zinc
Cr2O7 (2-) +14H+ +3Zn –> 2Cr3+ +7H2O +3Zn2+
-with an excess of zinc, chromium ions are reduced further to chromium (2+) which is pale blue
Zn + 2Cr3+ –> Zn2+ + 2Cr2+
oxidation of chromium ions to CrO4 (2-)
-hot alkaline hydrogen peroxide (H2O2) is a powerful oxidising agent
3H2O2 + 2Cr3+ + 10OH- –> 2CrO4- + 8H2O
-chromium oxidised from 3+ to +6
-oxygen reduced from -1 to -2
reduction of copper (II) to copper (I)
when aqueous copper reacts with excess iodine ions:
-I- is oxidised to brown iodine (I2)
-Cu2+ is reduced to Cu+
-this forms a white precipitate of copper iodide
2Cu2+ +4I- –> 2CuI + I2
disproportionation of Cu+ ions
- when solid copper (I) oxide Cu2O reacts with hot dilute sulphuric acid, a brown precipitate of copper is formed together with a blue solution of copper sulphate
- Cu+ ions have been oxidised and reduced
- reduction (goes from +1 to 0 in Cu)
- oxidation (goes from +1 to +2 in CuSO4)
