Chapter 22, Problem 1ASA

Methyl orange, HMO, is a common acid-base indicator. In solution it ionizes according to the equation:

$\begin{array}{ccc}\mathrm{HMO}\left(\mathrm{aq}\right)\leftrightharpoons & {\mathrm{H}}^{+}\left(\mathrm{aq}\right)+& {\mathrm{MO}}^{-}\left(\mathrm{aq}\right)\\ \mathrm{red}& & \mathrm{yellow}\end{array}$

If methyl orange is added to distilled water, the solution turns yellow. If 1 drop or two of 6 M HCl is added to the yellow solution, it turns red. If to that solution one adds a few drops of 6 M NaOH, the color reverts to yellow.

a. Why does adding 6 M HCl to the yellow solution of methyl orange tend to cause the color to change to red? (Note that in solution HCl exists as ${\mathrm{H}}^{+}$ and ${\mathrm{Cl}}^{-}$ ions.)

b. Why does adding 6 M NaOH to the red solution tend to make it turn back to yellow? (Note that in solution NaOH exists as ${\mathrm{Na}}^{+}$ and ${\mathrm{OH}}^{-}$ ions.) How does increasing $\left[{\mathrm{OH}}^{-}\right]$ shift Reaction 3 in the discussion section? How would the resulting change in $\left[{\mathrm{H}}^{+}\right]$ affect the dissociation reaction of HMO?

Interpretation Introduction

Interpretation:

The reason for the appearance of red color by addition of 6 M HCl solution should be predicted.

Concept Introduction :

The substances which represent a change in color on the addition of an acid or an alkali is termed as an indicator. Different types of indicators are known. For example; methyl orange, phenolphthalein which are an acid-base indicator.

Answer to Problem 1ASA

The reason for the appearance of red color by the addition of 6 M HCl is that it leads to shifting the direction of the reaction in the left direction due to an increase in the concentration of hydrogen ions.

Explanation of Solution

The chemical equation for the ionization of methyl orange is represented as follows:

  $\text{HMO}\left(aq\right)\rightleftharpoons {\text{H}}^{+}\left(aq\right)+{\text{MO}}^{-}\left(aq\right)$

The color of the solution consisting of an indicator depends on the concentration of an indicator and its conjugate base ${\text{MO}}^{-}\left(aq\right)$ . If the concentration of HCl is increased then there will be the formation of HMO causing the reaction to shift in the left direction according to the Le-Chatelier principle. Therefore, the color changes from yellow to red on the addition of 6 M HCl.

Interpretation Introduction

Interpretation:

The reason for the appearance of yellow color back by addition of 6 M NaOH solution should be predicted.

Concept Introduction :

The substances which represent a change in color on the addition of an acid or an alkali is termed as an indicator. Different types of indicators are known. For example; methyl orange, phenolphthalein which isan acid-base indicator.

Answer to Problem 1ASA

The reason for the appearance of yellow color back by addition of 6 M NaOH solution is that it leads to increase in the concentration of hydroxide ions which leads to shifting the direction of a reaction in the forward direction.

Explanation of Solution

The addition of 6 M NaOH causes an increase in the concentration of hydroxide ions which reacts with ${\text{H}}^{+}$ ions leading to the formation of a conjugate base that is ${\text{MO}}^{-}$ thus, the direction of the reaction goes in the forward direction leading to a change in the color of a solution to yellow.

  $\begin{array}{l}\text{HMO}\left(aq\right)\rightleftharpoons {\text{H}}^{+}\left(aq\right)+{\text{MO}}^{-}\left(aq\right)\\ \text{Red}\text{Yellow}\\ {\text{OH}}^{-}\left(aq\right)+{\text{H}}^{+}\left(aq\right)\to {\text{H}}_{\text{2}}\text{O}\left(l\right)+{\text{MO}}^{-}\left(aq\right)\end{array}$