The key difference between half equivalence point and equivalence point is that half equivalence point is the midpoint between the starting point and the equivalence point of a particular titration whereas equivalence point is where the chemical reaction ends.
Titrations are analytical techniques in chemistry that are important in determining the unknown concentrations of given samples.
CONTENTS
1. Overview and Key Difference
2. What is Equivalence Point
3. What is Half Equivalence Point
4. Side by Side Comparison – Half Equivalence Point vs Equivalence Point in Tabular Form
5. Summary
What is Half Equivalence Point?
The half equivalence point of a titration is the halfway between the equivalence point and the starting point (origin). The importance of this point is that at this point, the pH of the analyte solution is equal to the dissociation constant or pKa of the acid used in the titration. The half equivalence point occurs at the one-half volume of the first equivalence point of the titration. If there are multiple equivalence points in the titration, there are several half equivalence points that are equal to the number of equivalence points. For example, a second-half equivalence point occurs at the midpoint between first and second equivalence points.
What is Equivalence Point?
Equivalence point in a titration is the actual point where the desired chemical reaction in the reaction mixture ends. We usually do titrations to determine the concentration of a substance in a liquid. If we know the substance, a titrant (a solution used to determine the concentration of a component in a liquid mixture) having a known concentration can be used to react with the analyte. Here, we call the titrant as a standard solution because its exact molarity is known.
Figure 01: Titration Curve indicating the Equivalence Point
In the reaction between NaOH and HCl, which is an acid-base reaction, we can use either NaOH or HCl as the titrant having the known concentration. Here, the titrant is placed in the burette, and slowly we can add it to the titrand/analyte until a colour change occurs in the reaction mixture. We need to use an indicator because NaOH and HCl are not self-indicators. The point where a colour change occurs is the endpoint of the titration, which is not the equivalence point of the reaction.
In this titration, the equivalence point is the point where all HCl molecules have reacted with NaOH (or the point where all NaOH molecules have reacted with HCl). Then the moles of titrant should be equal to the moles of the unknown analyte since the stoichiometry between HCl and NAOH is 1:1. There are different methods to determine the equivalence point of a titration.
- Colour change of self-indicators – In reactions involving self-indicators as reactants, the colour change reveals the equivalence point of the titration since they do not use indicators.
- Endpoint– Sometimes, equivalence point can be taken as the endpoint since they are approximately equal.
- Conductance– In this method, conductance is measured throughout the titration, and the equivalence point is where a rapid change of conductance occurs. This is a somewhat difficult method.
- Spectroscopy– This is a method suitable for colourful reaction mixtures. The equivalence point is determined according to the rapid change in wavelengths that are absorbed by the sample.
What is the Difference Between Half Equivalence Point and Equivalence Point?
Titrations are analytical techniques in chemistry that are important in determining the unknown concentrations of given samples. The key difference between half equivalence point and equivalence point is that half equivalence point is the midpoint between the starting point and equivalence point of a particular titration whereas equivalence point is where the chemical reaction ends.
Below tabulation summarizes the difference between half equivalence point and equivalence point.
Summary – Half Equivalence Point vs Equivalence Point
Titrations are analytical techniques in chemistry that are important in determining the unknown concentrations of given samples. The key difference between half equivalence point and equivalence point is that half equivalence point is the midpoint between the starting point and equivalence point of a particular titration whereas equivalence point is where the chemical reaction ends.
Reference:
1. “Titration Curves & Equivalence Point (Article).” Khan Academy, Available here.
Image Courtesy:
1. “Titration of weak acid with strong base” By Quantumkinetics – Own work (CC BY 3.0) via Commons Wikimedia