What is the Difference Between Stoichiometric Point and Endpoint

In chemistry, the terms stoichiometric point and endpoint are often useful in the context of titration, a technique for determining the concentration of a solution. However, they represent different stages in the titration process.

What is the difference between stoichiometric point and endpoint? Stoichiometric point is the theoretical point in a chemical reaction where the reactants have been completely consumed according to the balanced chemical equation whereas endpoint is the observable point at which we determine the reaction is complete.

Key Areas Covered

1. What is a Stoichiometric Point 
      – Definition, Features 
2. What is an Endpoint 
      – Definition, Features 
3. Similarities Between Stoichiometric Point and Endpoint 
      – Outline of Common Features
4. Difference Between Stoichiometric Point and Endpoint
      – Comparison of Key Differences
5. FAQ: Stoichiometric Point and Endpoint
      – Answers to Frequently Asked Questions

Key Terms

Stoichiometric Point, Endpoint, Equivalence Point

What is a Stoichiometric Point

A stoichiometric point or equivalence point marks the exact moment in a reaction where the moles of reactants have been perfectly consumed according to the balanced chemical equation. Stoichiometric point is a theoretical concept and one cannot observe it directly. Hence, indicators, substances that change colour based on the solution’s properties, help to determine the endpoint. Ideally, the stoichiometric point coincides with the endpoint in most cases. A well-chosen indicator changes color at a pH very close to the theoretical equivalence point, minimizing the difference between the two.

A stoichiometric point helps to calculate the unknown concentration of the analyte. We can do this by knowing the volume and the concentration of the titrant used up in the endpoint and the stoichiometry of the reaction, and by doing a reverse calculation using these data.

This stoichiometric point is essential in demining volumes or concentrations of solutions in titrations. Here, chemists slowly add a known solution (titrant) to an unknown solution (analyte) until the reaction between them completes. Furthermore, the stoichiometric point indicates that enough titrant has been added to fully react with the analyte.

What is an Endpoint

The endpoint in a titration is the observable point at which a chemist can determine that the reaction is complete, serving as a practical marker. A color change provided by the indicator signals the completion of the reaction.

For accurate measurements, the endpoint should ideally occur as close as possible to the stoichiometric point (equivalence point). This is the point where the exact amount of titrant has been added to react with all the analytes according to the balanced chemical equation.

By identifying the endpoint, the chemist can stop adding the titrant. He can then use the volume of titrant used up to this point to calculate the unknown concentration of the analyte in the solution being titrated.

Similarities Between Stoichiometric Point and Endpoint

1. They signify the point where the reaction between the titrant (known solution) and the analyte (unknown solution) is considered complete.
2. Moreover, both points are essential for calculating the unknown concentration of the analyte in the solution being titrated.

Difference Between Stoichiometric Point and Endpoint

Definition

• A stoichiometric point is a theoretical concept based on the balanced chemical equation, while an endpoint is an observable sign, typically a color change, indicating the reaction is likely to complete.

Nature

• Stoichiometric point cannot be directly observed during the experiment, while endpoint is a usual or instrumental observation that signifies the likely completion of the reaction.

Precise or Not

• The stoichiometric point represents the ideal scenario where the reaction goes to completion exactly as per the equation, but the endpoint can be slightly imprecise due to factors like indicator sensitivity or reaction speed.

Order

• The stoichiometric point occurs at a specific point in the reaction when all reactants have completed their consumption. Meanwhile, the endpoint occurs slightly after the stoichiometric point due to the time it takes for the indicator to change.

Conclusion

In conclusion, the stoichiometric point, also known as the equivalence point, is a theoretical moment in a chemical reaction where reactants are completely consumed based on the balanced equation. The endpoint, on the other hand, is the observable moment, often indicated by a color change, when the reaction is practically considered complete. Thus, this is the basic difference between stoichiometric point and endpoint.

FAQ: Stoichiometric Point and Endpoint

1. What is the difference between an equivalence point and an endpoint?

The equivalence point is the theoretical point in a chemical reaction where the reactants have been completely consumed according to the balanced chemical equation, whereas the endpoint is the observable point at which we determine the reaction is complete.

2. What is an example of an endpoint in chemistry?

When a solution of sodium chloride and hydrochloric acid becomes pink in the presence of phenolphthalein, the solution has reached its endpoint. This is an example of an endpoint.

3. How is titration used to measure endpoints?

In titration, special dyes change color when the reaction is complete, signaling chemists that they’ve reached the endpoint. This color change is a clear indicator of the end of the titration.

4. Why is a white tile used in titration?

In titrations, a white tile offers a neutral background against which chemists can easily observe any color change during the titration process. Moreover, this ensures better visibility of subtle color changes.

5. At what pH is phenolphthalein pink?

Phenolphthalein is a pH indicator that undergoes a color change in response to changes in the acidity or alkalinity of a solution. In acidic conditions (pH below 8.2), it remains colorless. However, as the pH increases and becomes more basic, phenolphthalein transitions to a pink hue, with the color becoming fully pronounced above a pH of 9.0.

Reference:

1. “Equivalence Point.” Wikipedia. Wikipedia Foundation.

Image Courtesy:

1. “Titration of weak acid with strong base” By Quantumkinetics – Own work (CC BY 3.0) via Commons Wikimedia
2. “Person with Facemask and Goggles Looking at a Test Tube” (CC0) via Pexels