Why is k4 Greater than k3 in Cadmium Complex Formation?

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Discussion Overview

The discussion revolves around the formation of cadmium complexes with bromide ions (Br-) and the comparison of successive equilibrium constants (k3 and k4). Participants explore the reasons behind k4 being greater than k3, touching on concepts from chemical equilibrium, symmetry, electronic configuration, and coordination chemistry.

Discussion Character

  • Homework-related
  • Conceptual clarification
  • Exploratory

Main Points Raised

  • One participant notes that larger values of equilibrium constants indicate exothermic reactions and suggests that a strongly negative Gibbs free energy (ΔG) could lead to more products at equilibrium.
  • Another participant proposes analyzing the symmetry and electronic configuration of the complexes to determine stability, although they acknowledge the lack of structural information.
  • A different participant mentions that cadmium ions are initially complexed by water molecules, which are then replaced by bromide ions, adding complexity to the analysis.
  • One participant shares that they are learning about chemical equilibrium and have some background in crystal field theory and coordination compounds, indicating a desire to understand the reaction involving Br- replacing OH- in the complex.
  • A later post references a solution manual that describes the reaction and suggests that the increase in the number of particles leads to greater disorder, which may explain the increase in k4 compared to k3.

Areas of Agreement / Disagreement

Participants express various hypotheses regarding the relationship between k3 and k4, but there is no consensus on a definitive explanation. The discussion remains exploratory with multiple viewpoints presented.

Contextual Notes

Participants note the absence of structural information about the complexes, which may limit the analysis of stability and equilibrium constants. There are also references to specific chemical principles that may not be fully resolved in the discussion.

Who May Find This Useful

This discussion may be useful for students studying chemical equilibrium, coordination chemistry, or those interested in the properties of cadmium complexes and their formation dynamics.

Titan97
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Homework Statement


The formation of cadmium complexes with Br- exhibits successive equilibrium constants such that log(k1)=1.56, log(k2)=0.54, log(k3)=0.05, log(k4)=0.37.
Explain why k4 is greater than k3

Homework Equations


$$K=e^{\frac{-\Delta H}{RT}}\cdot e^{\frac{\Delta S}{R}}$$

The Attempt at a Solution


After reading chemical equilibrium from peter Atkins's chemical principles, I learned that larger values of K means the reactions is exothermic. (This can be said using the above equation).
Larger value of K, could also mean that ##\Delta G## is strongly negative causing more products at equilibrium.
This explains why some equilibrium constants are large. But this does not explain why K3<K4.
 
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I would try to analyze symmetry and electronic configuration of complexes involved to see if there are no indications to which one is more stable.
 
But no information about the structure of complex is given. (I think its tetrahedral)
 
This is even more complicated, as cadmium ion is first complexed by water molecules, they are then replaced by Br-.

In what context was this question asked? I mean - what is the thing you are learning now?
 
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I am learning chemical equilibrium. But I know the basics about crystal field theory and coordinate compounds. So the reaction is about Br- replacing OH- in the complex. Its OK. I would like to learn about this. (I will post the solution given in the solution manual, I don't have it right now).
 
I got the solution manual @Borek , this is the equation given
$$[CdBr_3(OH)_2]+Br^-\to [CdBr_4]^{2-}+3H_2O$$
According to the author, "there is an increase in the number of particles and hence an increase in dissorderness. This is because of formation of a less restricted coordination sphere."
Am I supposed to learn the equation or is there a general rule to be followed?