Changes in Colour

Changes in Colour

In some reactions there is a change in colour which tells us that the reaction is occuring. The faster the colour change the faster the reaction rate.

For example, when ethanoic acid (acetic acid) is titrated with sodium hydroxide, an indicator such as phenolphthalein is added. The solution is clear in an acidic solution and changes to pink when the reaction is complete. If the concentration of the base were increased, the colour change would happen faster (after a smaller volume of base was added), showing that a higher concentration of base increased the reaction rate.

\(\text{CH}_{3}\text{COOH}(\text{aq}) + \text{NaOH}(\text{aq})\) \(\to\) \(\text{Na}^{+}(\text{aq}) + \text{CH}_{3}\text{COO}^{-}(\text{aq}) + \text{H}_{2}\text{O}(\text{l})\)