Should the equilibrium constant $K$ be calculated at the end of an acid-base titration or can we calculate it in the middle of the titration for example? If so, will the calculated equilibrium constant be equal to the equilibrium constant calculated from the final state of the titration?


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  • $\begingroup$ Depends on what you mean by 'calculate' $\endgroup$ – William R. Ebenezer Jan 26 at 18:03
  • $\begingroup$ @WilliamR.Ebenezer like put concentrations in order to get a value? $\endgroup$ – Djillali SE Jan 26 at 18:08
  • $\begingroup$ do you mean plug in the concentration values in something like [product]/[reactant]? $\endgroup$ – William R. Ebenezer Jan 26 at 18:14
  • $\begingroup$ @WilliamR.Ebenezer Exactly what i meant $\endgroup$ – Djillali SE Jan 26 at 18:15
  • $\begingroup$ then you won't get K, I regret. you will get the reaction quotient(Q), which will be numerically smaller than K till equilibrium is established. At equilibrium, Q=K $\endgroup$ – William R. Ebenezer Jan 26 at 18:18

Because proton transfers are very fast, the equation $K_a=\frac{[\text{conjugate base}][\ce{H+}]}{[\text{acid}]}$ will always hold as long as your solution is well mixed. So if you are able to measure those three concentrations, you can determine $K_a$ using any mixture. As the concentrations change during the titration, the value of $K_a$ will remain constant.

Be aware that the value of $K_a$ will vary with temperature, so you should maintain a constant temperature during the experiments.

  • $\begingroup$ You just have to measure one concentration (or the pH) if you know the composition of the solution (which you should for a titration) and the net reaction and $K_\mathrm{W}$. Then, you can calculate all the other concentrations and the equilibrium constant. For example, the pH vs. V(titrant) graph mentioned by the OP would be sufficient to calculate $K_\mathrm{a}$ and verify that it is constant throughout the titration (for ideal solutions). $\endgroup$ – Karsten Theis Jan 29 at 18:23
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    $\begingroup$ I should have written "determine those three concentrations" rather than "measure. . . ". $\endgroup$ – Andrew Jan 29 at 19:13

Acid-base reactions are quite fast. Once you mix the reactants, you can generally assume equilibrium. However, for a general chemical reaction, you have to wait for the chemical kinetics to adjust the concentrations until equilibrium is reached, where the composition given by $K$-values are valid.

  • $\begingroup$ Yes, but my case i'm talking about my tetration so at different volumes of reactors does equilibrium change? $\endgroup$ – Djillali SE Jan 26 at 17:35

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