Chemistry of Elements of 1st Transition Series B.Sc. 2nd Year Notes

Chemistry of Elements of 1st Transition Series

General characteristics of d- block elements

1. The elements of the periodic table in which the last electron enter in the d-orbital, called d-block elements
2. The d-block elements are placed in the groups named I B to VII B and VIII (does not have sub group)
3. In d-block elements the electron gets filled up in the d-orbital of the penultimate shell
4. d-block elements lie between s & p block elements. (Transition Elements)
5. The general formula of these elements is ns^1-2(n-1)d^1-10. Where n = 4 to 7
6. All of these elements are metals(Transition Metals)
7. Out of all the d-block elements, mercury is the only liquid element.
8. They are all metals with high melting and boiling points
9. The compounds of the elements are generally paramagnetic in nature
10. They mostly form coloured ions, exhibit variable valence (oxidation states)
11. They are used as catalysts

Binary compounds of the first transition series

The element with only one other element combines to form binary compound. These are the simplest type of compound formed by transition element with a number of non-metals like oxygen nitrogen phosphorous halogens and carbon to form binary compounds.

Oxides

Oxides of the elements of first transition series shown by general formula as MO, M₂O₃, MO₂, M₂O₅ and MO₃. Their stability increases from scandium to zinc.
Oxides in lower oxidation states are ionic and basic, amphoteric in the intermediate oxidation states and in higher oxidation states, ionic nature decreases and covalent nature increases thereby increasing the acidic character of the oxides. Oxides of first transition series elements may be acidic, basic or amphoteric.
Acidic oxides are of weak acidic nature and are formed by the elements in higher oxidation states. These are soluble in bases. For example, V₂O₅, CrO₃, MnO₃, Mn₂O₇ etc.
Basic oxides are those which are formed by the metals in the lower oxidation states. These are ionic in nature, soluble in non-oxidising acids, e.g. HCl. TiO, CrO, MnO, FeO, Cu₂O, CoO, NiO, etc. are examples of basic oxide
Amphoteric oxides are the oxides containing the metals in the intermediate oxidation states. These oxides are also soluble in non-oxidising acids, e.g., HCl. TiO₂, VO₂, Cr₂O₃, Mn₃O₄, MnO₂, CuO, ZnO, etc are examples of amphoteric oxide.
The lower oxidation state metal oxides are electron donor and acts as a reducing agent. As atomic number increases, the reducing property in the lower oxidation state also increases, e.g., TiO < VO < CrO. The higher oxidation state metal oxides are electron acceptor and acts as a oxidizing agent. e.g., CrO₃, Mn₂O₇ etc.

Halides

The elements of 1st transition series react with halogens at high temperature to give halides. Fluorides are ionic, other halides are ionic as well as covalent nature. Halides are formed by many of these elements in different oxidation states, e.g. TiCl₃ ,TiCl₄,VCl₃,VCl₅ etc.

Sulphides

Metal sulphides may either be prepared by direct heating the mixture of metal and sulphur or by treating metal salt solution with H₂S or Na₂S.
Metal + S ---heat---> metal sulphide
Or metal salt solution + H₂S/Na₂S ------> metal sulphide
Metals in low oxidation state form sulphides which are insoluble in water.

Carbides

The metals interact with carbon at high temperature forming carbides. Metal carbides are generally prepared by the following two methods-
Metal + carbon ---heat---> metal carbide Or metal oxide + carbon ---heat---> metal carbide

Volumetric estimation of Fe⁺²

Fe⁺² can be titrated against by K₂Cr₂O₇ in the presence of dilute sulphuric acid using an internal or an external indicator. The redox reaction involved in this reaction is-
6Fe⁺² + Cr₂O₇⁻ + 14H⁺ → 2Cr⁺³ + 7H₂O.
6ml        1ml
The stoichiometric ratio = No. of moles of K₂Cr₂O₇/No. of moles of Fe⁺ salt = 1/6
The internal indicator is diphenyl amine which is blue in color.
For the titration, K₂Cr₂O₇ solution is gradually added to a known volume of the solution of Fe⁺ salt to which 3 to 4 drops of the indicator along woith suitable amounts of sulphuric acid have been added. The solution which is initially green in color develops, at the end point, a permanent intense bluish voilet color.
Suppose 20ml of Fe⁺² salt solution requires 12.5ml of 0.05M K₂Cr₂O₇ for titration. Since the stoichiometric ratio, as mentioned above is 1/6.
The concentration of Fe⁺² salt solution = (12.5 X 0.05 X 6)/20 = 0.187M
Thus, the amount of Fe⁺² salt in the solution is 0.187M X molecular weight of Fe⁺² salt
Thus , knowing the concentration of Fe⁺² salt solution, we can easily find out the concentration of K₂Cr₂O₇.

Volumetric estimation of Cu⁺²

Copper in Cu (II) salt (e.g. CuSO₄.5H₂0) can be estimated volumetrically by iodometric process. In this process, 3 gm copper sulphate crystals is dissolved in distilled water in presence of dil. H₂SO₄ in a 250 ml measuring flame and the volume is made upto the mark.
The 25 ml of this solution in conical flask is then treated with NH₄OH until a faint permanent ppt. is formed. The ppt. is dissolved in acetic acid and 3 gm of KI is added in it. The flask is kept in dark for few minutes and then rinsed the covering watch glass and the inner walls of the conical flask. It is then titrated with N/10 Na₂S₂O₃ solution to a straw colour. Starch indicator is then added and the titration is continued till the complete disappearance of blue colour.
2CuSO₄ + 4KI → Cu₂I₂ + I₂ + 2K₂SO₄
I₂ + 2Na₂S₂O₃ = Na₂S₄O₆ + 2Nal
Na₂S₂O₃ ≡ CuSO₄.5H₂O ≡ Cu
∵ 25 ml of CuSO₄ solution ≡ xml N-Na₂S₂O₃ solution
∴ 250 ml of CuSO₄ solution ≡ 10xml N-Na₂S₂O₃ solution
∵ 1000ml of N-Na₂S₂O₃ solution ≡ 63.57 gm of Cu
∴ 10ml of N-Na₂S₂O₃ solution contains ≡ 63.57 gm of Cu X xgm of Cu
∵ 3 gm of CuSO₄ contains 0.6357 xgm of Cu
∴ 100 gm of CuSO₄ contains (0.6357 X 100)/3 gm of Cu = 21.19 xgm of Cu