Question

Define the equivalence point of an acid-base titration. Describe it for a weak acid-strong base titration (will the pH be >,<, or = 7).

Answer 1

The equivalence point is defined as the point where the moles of strong acid added = initial moles of base B in solution. Graphically, the equivalence point is where the curve is most vertical.

Figure 3. Comparing equivalence point with midpoint. It is apparent by studying a graph that the equivalence point is distinctly different from the midpoint. On the curve, the equivalence point is located where the graph is most steep. There is a fast and abrupt change of pH around this point, which can be observed by the color change the takes place during titration.

At the equivalence point, an ICE table is required to determine volume and acidity. At this point in the titration, however, the reaction is flipped. This is because the base B has been fully titrated, which means adding more titrant will not yield the same products. The reaction goes backwards.

Table 3. ICE table for reaction at equivalence point. The reaction at the equivalence point essentially goes backwards because all the base available to be titrated has been titrated. Think of the titration as an escalator. Once the highest level, or "equivalence point," is reached, the only option is to take a U-turn and go back down the other escalator lane.

At the equivalence point, there is no more of base B. Hence,