Let's review transition metal in A-Level Chemistry by looking at some questions!
1. State the equation and observationin the reaction between CoCl2 in aqueous solution and excess concentrated HCl solution
- [Co(H2O)6]2++ 4Cl- ⇨ [CoCl4]2-+ 6H2O
- Pink solution to blue solution
2. State the equation and observationin the reaction between[Cu(H2O)6]2+ and Na2CO3
- [Cu(H2O)6]2++ CO32- ⇨ CuCO3+ 6H2O
- Blue solution to blue-green precipitate
3. State the equations and observations when NH3 solution is added drop by drop until in excess to [Cu(H2O)6]2+
- [Cu(H2O)6]2++ 2NH3 ⇨ Cu(H2O)4(OH)2+ 2NH4+
- Blue solution to blue precipitate
- Cu(H2O)4(OH)2+ 4NH3 ⇨ [Cu(NH3)4(H2O)2]2++ 2H2O+ 2OH-
- Blue precipitate dissolves in excess NH3 solution to give deep blue solution
4. State the equation and observation in the reaction between[Co(H2O)6]2+andNa2CO3
- [Co(H2O)6]2++ CO32- ⇨ CoCO3+ 6H2O
- Pink solution to pink precipitate
5. State the equations and observations when NH3 solution is added drop by drop until in excess to [Co(H2O)6]2+ then the product is left in air
- [Co(H2O)6]2++ 6NH3 ⇨ [Co(NH3)6]2++ 6H2O
- Pink solution to straw solution
- 2[Co(NH3)6]2++ H2O2 ⇨ 2[Co(NH3)6]3++ 2OH-
- Straw solution to brown solution
- Product darkens in air
- Due to oxidation, due to oxygen in air
6. State the equation and observationsin the reaction between [Cr(H2O)6]3+and Na2CO3
- 2[Cr(H2O)6]3++ 3CO32+ ⇨ 2Cr(H2O)3(OH)3+ 3CO2+ 3H2O
- Green solution to green precipitate
- Effervescence of CO2 released
7. State the equations and observations when excess concentrated NaOH solutionis added to [Cr(H2O)6]3+ then H2O2 solution is added to the product
- [Cr(H2O)6]3++ 6OH- ⇨ [Cr(OH)6]3-+ 6H2O
- No visible change, remains green solution
- 2[Cr(OH)6]3-+ 3H2O2 ⇨ 2CrO42-+ 8H2O + 2OH-
- Green solution to yellow solution
8. Explain why Fe3+ ions are more acidic than Fe2+ ions in aqueous solution
- Fe3+ion has a greater charge, greater charge densitythan Fe2+ion
- Fe3+ion has smaller size than Fe2+ion
- Fe3+ion polarises the coordinated water ligands more, weakens the O-H bonds more, releases more H+ions
- Fe3+solution contains more H+ions, more acidic
9. Explain why Cr3+ ions form Cr(H2O)3(OH)3 whereas Cr2+ ions form CrCO3 when Na2CO3 is added to the ions in aqueous solution
- Cr3+ion has a greater charge, greater charge densitythan Cr2+ion
- Cr3+ion has smaller size than Cr2+ion
- Cr3+ion polarises the coordinated water ligands more, weakens the O-H bonds more, releases more H+ions
- Cr3+solution contains more H+ ions, more acidic
10. State an equation for the reaction between [Cu(H2O)6]2+ and C2O42- and explain why the reaction occurs in terms of the chelate effect
- [Cu(H2O)6]2++ 2C2O42- ⇨ [Cu(C2O4)2(H2O)2]2-+ 4H2O
- Increase in number of molecules, 3 moles of species to 5 moles of species
- Entropy change ΔS is positive
- No net change in bond enthalpies
- Enthalpy change ΔH is approximately zero
- Free-energy change ΔG is negative, product ion more stable
Drafted by Eunice (Chemistry)