How pH would affect the dative bonding

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Discussion Overview

The discussion centers on how pH affects the stability of the complex formed between aminoethanoic acid and calcium ions in solution, particularly in the context of a titration experiment. The scope includes theoretical reasoning related to coordination chemistry and the behavior of ligands at different pH levels.

Discussion Character

  • Homework-related
  • Exploratory
  • Technical explanation

Main Points Raised

  • One participant expresses uncertainty about how pH influences dative bonding.
  • Another participant suggests that the presence of hydroxide ions at higher pH may interfere with bonding.
  • A different participant explains that at pH 4, aminoethanoic acid exists primarily as an ammonium ion and a carboxylic acid, which may affect its ability to act as a ligand.
  • It is noted that at pH 4, the ammonium compound is not considered electron-rich due to having accepted a proton, while at pH 10, the carboxylic acid exists as COO-, allowing for better coordination with Ca2+.
  • One participant confirms the reasoning presented regarding the stability of the complex at different pH levels.

Areas of Agreement / Disagreement

Participants generally agree on the reasoning that the stability of the complex is influenced by the protonation state of aminoethanoic acid at different pH levels, although some uncertainty remains about the specifics of how hydroxide ions may interfere with bonding.

Contextual Notes

The discussion does not resolve the exact mechanisms by which pH affects dative bonding, nor does it clarify all assumptions regarding the behavior of ligands in different pH environments.

Kushal
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Homework Statement



The concentration of Ca2+ in hard water can be determined by titration with a reagent X which forms a complex with Ca2+ giving a change in colour. Since the colour change is sensitive to pH, the titration has to be carried out in an alkaline buffer.

Th structure of X is similar to that of an amino acid. The way X acts as a ligand can be understood by considering how aminoethanoic acid forms co-ordinate bonds with a calcium ion.

Explain why the complex between aminoethanoic acid and aqueous calcium ions is more stable at pH 10 than at pH 4.

Homework Equations





The Attempt at a Solution



I really have no idea how pH would affect the dative bonding.
 
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Well, as pH is based on how many hydrogen ions (and thus how few hydroxide ions) there are in the solution, I'd guess that hydroxide interferes in the bonding.
 
Remember that aminoethanoic acid is an amino acid. At pH 4 the amino functionality exists as an ammonium ion and the carboxylic acid exists largely as an acid (R-CO-OH).

Ligands are electron rich species and calcium in solution exists as a cation. Is an ammonium compound electron-rich? Is a carboxylic acid?
 
bah no...

the ammonium compound is not electron rich because it has already accepted a proton... right?? (the nitrogen lone pair is used in dative bonding)

at pH 4 the dissociation of the organic acid is repressed. it cannot donate its proton and therefore is not electron rich.

but at pH 10, the carboxylic acid exists as COO-. and the lone pair on the nitrogen can coordinate with Ca2+.

is this correct?
 
Yep.
 
nice, thnks!
 

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