Question

Compare the structure of xylenol orange to phenolphthalein. Do some research to determine why phenolphthalein changes color under acidic or basic conditions. Based on the chemistry of phenolphthalein, explain why xylenol orange is a different color in the presence and absence of zinc ions. It is useful to note that the xylenol orange-zinc complex that is red must abosorb light around 500-550 nm while the free xylenol orange molecule is yellow, so must absorb light around 400-450 nm. This general chemistry II. Please answer well!

Answer 1

3) Xylenol is a big molecule with four –COOH GROUPS, if we consider it to be RCOOH then ,it has the following equilibrium in acidic and basic solution,

RCOOH +H2O↔RCOO- +H3O+

RCOOH is yellow (acidic) and RCOO- is red (basic).

In the presence of Zinc ion,(RCOO)4- and Zn2+ bonds to form complex like Zn2(RCOO) so the equilibrium is pushed to right and solution is yellow.

4)Each colour has its own unique wavelength.Red colour(650-750 nm) is reflected when the conjugate of red(the mixture of other 6 colours of white light) is absorbed ,that is around 500-550nm.

Same with yellow, it reflects its unique wavelength of 550 -600nm, and absorbs the rest.

But which light will be absorbed depends entirely on the chemical composition of the molecule.Every molecule absorb energy basis its electronic transition energy levels of the chromophore.(atom or group responsible for absorbing light).