Endpoint vs Stoichiometric Point
Difference between endpoint and stoichiometric point must be of interest to you if you are interested in learning chemical reactions. An acid-base titration involves a neutralization reaction, which occurs at the point where an acid reacts with an equal amount of base as appears in the balanced chemical reaction. The theoretical point where the reaction exactly ends and the point where we detect practically, have small differences. This article discuses about those two points, the equivalence point and endpoint, in a titration. We use the name equivalence point to name the stoichiometric point. There are several methods to determine the equivalence point of a reaction. However, practically these methods do not give the exact equivalence point, they determine the point just after the equivalence point instead. That is called endpoint of a chemical reaction.
Acid base titrations
Titrimetric methods used to determine the unknown concentration of an acid or base, by reacting an acid or base with an acid or a base with a known concentration. It is a neutralization reaction. In most of the titrations, indicators are used to determine the point where the given amount of acid reacts completely with the given base. Depending on the type of the acid-base titration (such as strong acid-strong base, strong acid-weak base, weak acid-strong base, etc.) , various indicators are used.
What is Stoichiometric Point?
Equivalence point is the name used for stoichiometric point. It is the point where the acid or base completes its neutralization reaction.
HCl(aq) + NaOH(aq) —> H2O + NaCl(aq)
(0.1 M, 100 ml) (0.5 M, v ml)
According to the stoichiometry in the reaction,
HCl :NaOH = 1 : 1
=> nHCl = nNaOH
0.1 M * 100 ml = 0.5 M * V
VNaOH = 0.1 * 100/ 0.5
= 200 ml
Theoretically, reaction completes at the point which 100 ml of HCl(0.1M) reacts with 200 ml of NaOH (0.5 M). This point is called the stoichiometric point or the equivalent point.
What is Endpoint?
The point at which the reaction is observed to be completed is called the endpoint. We can experimentally determine this point. Let us consider the previous example to understand this practically.
Example: 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; shows a pink colour in the basic medium.
Initially, only acid (HCl 0.1 M/100 ml) in the titration flask; pH of the solution is equals to 2 (pH=-log10[0.1*100*10-3]). As we add NaOH, pH of the solution increases due to the neutralization of some amount of acid in the medium. We add the base continuously drop by drop till it reaches the completion. The pH of the reaction equals to 7 when the reaction completes. 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 the difference between Endpoint and Stoichiometric Point?
It is better if we can determine when the equivalent point is reached. However, we observe the reaction completion at the end point.
According to the above example,
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 salt (HCl and NaCl). At the equivalence point, we only have salt in the medium. At the end point, we have salt and the base ( NaCl and NaOH) in the medium.
- Equivalence point is the most accurate point at which neutralization completed. Endpoint comes just after the neutralization is reached.
- Both of these measures consider as nearly same such that the endpoint coincides with or is very close to the equivalence point.
The use of a pH meter to determine the stoichiometric point can be considered as an accurate method (A plot of pH vs milliliters of titrant).
Endpoint vs Stoichiometric Point – Summary
Titrimetric method is one of the most widely used quantitative analytical methods used to determine the unknown concentration of an acid or a base. This involves the determination of the point at which neutralization is completed. There is a slight difference between the theoretical point at which real completion occurs and the point at which reaction completion is observed. Those two points are called stoichiometric point and endpoint respectively. Those two points are considered as nearly similar. Thus endpoint is considered as the point where the acid-base reaction completion occurs.