CIE A Level Chemistry復習筆記6.3.6 Stability Constants, Kstab
Define & Write a Stability Constant for a Complex
- When transition element ions are in aqueous solutions, they will automatically become hydrated
- Water molecules will surround the ion and act as?ligands?by forming dative covalent bonds to the central metal ion
- When there are other potential ligands present in solution, there is a?competing equilibrium?in?ligand exchange?and the most stable complex will be formed
- For example, a Co(II) ion in solution will form a [Co(H2O)6]2+?complex
- Adding ammonia results in the stepwise substitution of the water ligands by ammonia ligands until a stable complex of [Co(NH3)4(H2O)2]2+?is formed
[Cu(H2O)6]2+?+ 4NH3?? [Cu(NH3)4(H2O)2]2+?+ 4H2O
- For the substitution reaction above, there are?four?stepwise constants:
[Co(H2O)6]2+?+ NH3?? [Co(NH3)(H2O)5]2+?+ H2O? ? ? ? ? ? ??K1
[Co(NH3)(H2O)5]2+?+ NH3?? [Co(NH3)2(H2O)4]2+?+ H2O ? ? ? ? ? ??K2
[Co(NH3)2(H2O)4]2+?+ NH3?? [Co(NH3)3(H2O)3]2+?+ H2O ? ? ? ? ? ??K3
[Co(NH3)3(H2O)3]2+?+ NH3?? [Co(NH3)4(H2O)2]2+?+ H2O ? ? ? ? ? ??K4
- These stepwise constants are summarised in the?overall stability constant,?Kstab
- The?stability constant?is the?equilibrium constant?for the formation of the complex ion in a solvent from its constituent ions or molecules
Expression of?Kstab
- The expression for?Kstab?can be deduced in a similar way as the expression for the equilibrium constant (Kc)
- For example, the equilibrium expression for the substitution of water ligands by ammonia ligands in the Co(II) complex is:
[Co(H2O)6]2+?+ 4NH3?? [Co(NH3)4(H2O)2]2+?+ 4H2O
- The concentration of?water?is?not?included in the expression as the water is in?excess
- Therefore, any water?produced?in the reaction is?negligible?compared to the water that is already present
- The units of the?Kstab?can be deduced from the expression in a similar way to the units of?Kc
- The stability constants can be used to?compare?the?stability?of ligands relative to the?aqueous metal ion?where the ligand is water
- The?larger?the?Kstab?value, the?more stable?the complex formed is
Calculations Involving Stability Constants
- If the concentrations of the transition element complex and the reacting ligands are known, the expression for the stability constant (Kstab) can be used to determine which complex is?more?stable
- The?greater?the value of?Kstab?the?more stable?the complex is
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