Difference Between Stepwise and Overall Stability Constants

Key Difference – StepwisevsOverall Stability Constants

术语稳定性常数是指平衡常数for the formation of a complex compound in a solution. It is a way of measuring the stability of these complexes oftransition metalions. As or all other equilibrium constants, stability constants are also temperature dependent. The definition for stability constant can be given as “the constant for an equilibrium existing between a transition metal ion surrounded by water ligands and, the complex formed when some transition metal ions undergo ligand substitution reactions”. the symbol for stability constant isK_stab。Usually, the ligands are substituted one by one as a stepwise process. These steps are given stepwise stability constants. However, the stability constant for the overall process can also be given. It is the overall stability constant. Thekey differencebetween stepwise and overall stability constants is that逐步稳定性常数的值低于相同反应的总稳定性常数，而总体稳定性常数始终具有高于每个逐步稳定性常数的值。

内容

1.Overview and Key Difference
2.What are Stepwise Stability Constants
3.What are Overall Stability Constants
4.Relationship Between Stepwise and Overall Stability Constants
5。并排比较 - 逐步与总体稳定性常数为表格形式
6。概括

什么是逐步稳定性常数？

逐步稳定性常数是配体取代过程的每个步骤给出的平衡常数。When a transition metal ion complex has water ligands surrounding the metal ion, the ligand substitution takes place as a stepwise process. There, only one water molecule is replaced by the ligand that is involved in the substitution. Let us consider an example to understand this process.

Example

以氨配体的替代为例。

Hexaaquacopper（II）离子的化学公式为[Cu（H₂o）₆]²⁺。可以用氨配体代替六个水配体（NH₃).One water ligand is replaced by one ammonia ligand at a time.

[Cu(H₂o）₆]²⁺+NH₃↔ [Cu(NH₃)(H₂o）₅]²⁺K1

[CU（NH₃)(H₂o）₅]²⁺+ NH₃↔ [Cu(NH₃)₂(H₂o）₄]²⁺K2

[CU（NH₃)₂(H₂o）₄]²⁺+ NH₃↔ [Cu(NH₃)₃(H₂o）₃]²⁺K3

[CU（NH₃)₃(H₂o）₃]²⁺+ NH₃↔ [Cu(NH₃)₄(H₂o）₂]²⁺K4

[CU（NH₃)₄(H₂o）₂]²⁺+ NH₃↔ [Cu(NH₃)₅(H₂o）]²⁺K5

[CU（NH₃)₅(H₂o）]²⁺+ NH₃↔ [Cu(NH₃)₆]²⁺K6

The stability constant for the first substitution is given as K1. For the second substitution, it is k2 and vice versa. For each equilibrium above, expressions can be derived as below.

对于第一个替换，k1 = {[cu（nh）₃)(H₂o）₅]²⁺} / {[Cu(H₂o）₆]²⁺}* {NH₃}

In which, {[Cu(NH₃)(H₂o）₅]²⁺}，{[Cu（H₂o）₆]²⁺}一nd {NH₃}一re concentrations of each chemical species inside the brackets.Expressions like above can be written for other stepwise stability constants as well (K2, K3, K4, K5 and K6).

What are Overall Stability Constants?

总体稳定性常数是总体反应的平衡常数。对于上述反应，可以按以下方式给出总体稳定常数。

[Cu(H₂o）₆]²⁺+ NH₃↔ [Cu(NH₃)₆]²⁺

Hence, the overall stability constant is the equilibrium constant for the equilibrium between transition metal ion surrounded by water ligands and the transition metal ion surrounded by substituted ligands. Then the expression for overall stability constant can be derived as below.

总体k_stab= {[cu（nh₃)₆]²⁺} / {[Cu(H₂o）₆]²⁺} * {NH₃}

What is the Relationship Between Stepwise and Overall Stability Constants?

The overall stability constant can be obtained by multiplying all the stepwise stability constants together. For above example,

总体k_stab= K1 * K2 * K3 * K4 * K5 * K6

What is the Difference Between Stepwise and Overall Stability Constants?

逐步vsOverall Stability Constants
逐步稳定性常数是配体取代过程的每个步骤给出的平衡常数。	总体稳定性常数是总体反应的平衡常数。
Nature
给出了在过渡金属离子复合物中发生的替代反应的步骤。	在过渡金属离子配合物中进行的整个替代反应给出了总体稳定性常数。
价值
The values of stepwise stability constants are lower than the overall stability constant of the same reaction.	总体稳定性常数始终具有高于每个逐步稳定性常数的值。

概括–逐步vsOverall Stability Constants

The stepwise stability constant and overall stability constant are equilibrium constants given for transition metal complexes in solutions. The difference between stepwise and overall stability constants is that the values of stepwise stability constants are lower than the overall stability constant of the same reaction whereas the overall stability constant always have a higher value than each stepwise stability constant.

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Reference:

1.Substitution in complex ions – stability constants.Available here
2.“Stability constants of complexes.” Wikipedia, Wikimedia Foundation, 11 Feb. 2018.Available here

Image Courtesy:

1.’Hexaaquacopper(II)-3D-balls’By Ben Mills – Own work, (Public Domain) viaCommons Wikimedia