The key difference between endpoint and stoichiometric point is that endpoint comes just after the stoichiometric point, whereas stoichiometric point is the most accurate point at which neutralization completes.
An acid-base titration involves a neutralization reaction, which occurs at the point where an acid reacts with a chemically equal amount of base. However, there is a slight difference between the theoretical point where the reaction exactly ends and the point where we detect it practically. Moreover, you should also note that the term equivalence point is a more commonly used name for stoichiometric point.
CONTENTS
1. Overview and Key Difference
2. What is Endpoint
3. What is Stoichiometric Point
4. Side by Side Comparison – Endpoint vs Stoichiometric Point in Tabular Form
5. Summary
What is Endpoint?
The point at which a reaction seems to be completed is the endpoint of the titration. We can experimentally determine this point. Let us consider an example to practically understand this. Assume we titrate 100 ml of 0.1 M hydrochloric acid (HCl) with 0.5 M sodium hydroxide.
HCl(aq) + NaOH(aq) ⟶ H2O + NaCl(aq)
We keep acid in the titration flask and titrating against NaOH in the presence of methyl orange as an indicator. In acidic medium, the indicator is colourless and it shows a pink colour in the basic medium. Initially, there is only acid (HCl 0.1 M/100 ml) in the titration flask; the pH of the solution is equal to 2. As we add NaOH, the pH of the solution increases due to the neutralization of some amount of acid in the medium. We have to add the base continuously drop by drop till it reaches the completion. The pH of the reaction becomes equal to 7 when the reaction is complete. Even at this point, the indicator shows no colour in the medium since it changes the colour in the basic medium.
To observe the colour change, we need to add one more drop of NaOH, even after completion of the neutralization. The pH of the solution drastically changes at this point. This is the point where we observe as the reaction is completed.
What is Stoichiometric Point?
The equivalence point is the common name for stoichiometric point. It is the point at which acid or base completes its neutralization reaction. A reaction is completed theoretically at this point, but practically we cannot observe the exact point. It is better if we can determine when the equivalent point is reached because it is the exact point where neutralization has taken place. However, we can observe the reaction completion at the endpoint.
Figure 01: Graph for a Titration showing the Equivalence Point
If we consider the same example as above, at the beginning of the reaction, we have only the acid in the medium (HCl). Before it reaches the equivalence point, with the addition of NaOH, we have unreacted acid and formed a salt (HCl and NaCl). At the equivalence point, we only have salt in the medium. At the endpoint, we have salt and the base ( NaCl and NaOH) in the medium.
What is the Difference Between Endpoint and Stoichiometric Point?
Endpoint and stoichiometric point (in common, equivalence point) are always different from each other. The key difference between endpoint and stoichiometric point is that endpoint comes just after the stoichiometric point, whereas stoichiometric point is the most accurate point at which the neutralization completes. Furthermore, we can observe endpoint but cannot observe the stoichiometric point practically.
Summary – Endpoint vs Stoichiometric Point
Endpoint and stoichiometric point (in common, equivalence point) are always different from each other. The key difference between endpoint and stoichiometric point is that endpoint comes just after the stoichiometric point, whereas stoichiometric point is the most accurate point at which the neutralization completes.
Reference:
1. Helmenstine, Anne Marie. “Equivalence Point Definition.” ThoughtCo, May. 7, 2019, Available here.
Image Courtesy:
1.”Titration of weak acid with strong base” By Quantumkinetics – Own work (CC BY 3.0) via Commons Wikimedia