Explain the concept of equivalence point.

The equivalence point is the stage in a titration where the amount of titrant added is enough to completely react with the analyte.

In a titration, the equivalence point is a crucial stage that signifies the exact moment when the solution being analysed (the analyte) has completely reacted with the added solution (the titrant). This point is often indicated by a sudden change in the colour of the solution, which is brought about by an indicator. The indicator is a substance that changes colour at or near the equivalence point, thus providing a visual cue that the reaction is complete.

The equivalence point is not to be confused with the endpoint, although they are often assumed to be the same. The endpoint is the point at which the indicator changes colour, which ideally should coincide with the equivalence point. However, due to the limitations of indicators, the endpoint may not always exactly match the equivalence point. This discrepancy can lead to a titration error, which is the difference between the actual equivalence point and the observed endpoint.

The concept of the equivalence point is fundamental in quantitative chemical analysis, particularly in acid-base titrations. In an acid-base titration, the equivalence point is reached when the moles of acid equal the moles of base in solution. This is the point at which the pH of the solution is 7, indicating that the solution is neutral. However, the pH at the equivalence point can vary depending on the strength of the acid and base involved.

In redox titrations, the equivalence point is reached when the moles of the oxidising agent equal the moles of the reducing agent. In complexometric titrations, it is when the moles of metal ions equal the moles of the ligand.

Understanding the equivalence point is crucial for determining the concentration of a solution in a titration. By knowing the volume of the titrant needed to reach the equivalence point and the concentration of the titrant, one can calculate the concentration of the analyte using the stoichiometry of the reaction.