Question

Methyl red, HMR is a common acid-base indicator. In solution it ionizes according to the equation: $$ \begin{array}{c} \mathrm{HMR}(\mathrm{aq}) \rightleftarrows \mathrm{H}^{+}(\mathrm{aq})+\mathrm{MR}^{-}(\mathrm{aq}) \\ \mathrm{red} \end{array} $$ If methyl red is added to distilled water, the solution turns yellow. If a drop or two of \(6 \mathrm{M}\) HCl is added to the yellow solution, it turns red. If to that solution one adds a few drops of \(6 \mathrm{M} \mathrm{NaOH}\) the color reverts to yellow. a. Why does adding \(6 \mathrm{M}\) HCl to the yellow solution of methyl red 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 \(6\quad \mathrm{M}\quad \mathrm{NaOH}\) 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 HMR?)

Short answer

a) Adding $6 M$ HCl to the yellow solution of methyl red increases the concentration of \( H^+ \) ions in the solution. According to Le Chatelier's Principle, the equilibrium of the ionization reaction of methyl red shifts towards the reactant side (HMR), causing a color change from yellow to red. b) Adding $6 M$ NaOH to the red solution produces \( OH^- \) ions, which react with \( H^+ \) ions to form water, reducing the concentration of \( H^+ \) ions. The system responds by shifting the equilibrium in the ionization reaction of methyl red towards the product side (MR ion), reverting the solution back to yellow.

Step-by-step solution

Understanding the ionization of methyl red

Methyl Red (HMR) is an acid-base indicator, which changes color depending on the pH of the solution it's in. In its ionized form, it breaks down into \( H^+ \) ions (H) and \( MR^- \) ions (MR) according to the reaction \( HMR \rightleftarrows H^+ + MR^- \). When HMR is in an equilibrium in solution, it exhibits a red color, but if the equilibrium is shifted towards the MR ion, the color changes to yellow.

Effect of adding HCl

When HCl is added to the solution, it ionizes almost completely into \( H^+ \) and \( Cl^- \) ions. This increases the concentration of \( H^+ \) ions in the solution. According to Le Chatelier’s Principle, the system responds by moving towards the direction that can handle or reduce the change. In this case, the equilibrium of the ionization reaction of methyl red shifts towards the reactant side (HMR), bringing about a color change from yellow to red.

Effect of adding NaOH

When a solution of NaOH (a base) is added to the red solution, it ionizes to produce \( Na^+ \) and \( OH^- \) ions. The \( OH^- \) ions react with \( H^+ \) ions to form water. This reduces the concentration of \( H^+ \) ions in the solution. Again, according to Le Chatelier’s Principle, the system tends to minimize this disturbance by shifting the equilibrium in the ionization reaction of methyl red towards the product side (MR ion), reverting the solution back to yellow.

Key concepts

Le Chatelier’s Principle

Le Chatelier's Principle is a fundamental concept that predicts how a chemical system at equilibrium will respond to changes in concentration, temperature, volume, or pressure. To simplify, imagine a seesaw balanced delicately in the middle. If you add weight to one side, the seesaw will tip in that direction to accommodate the added weight. Similarly, in chemistry when a change occurs in a system at equilibrium, the reaction will shift to counterbalance the change and attempt to restore balance, or equilibrium.

For instance, in the exercise involving methyl red, adding strong acid like HCl increases the concentration of hydrogen ions ( H^+ ) in solution. According to Le Chatelier’s Principle, the system reacts by shifting the equilibrium to the left, towards the HMR, the reactant side, which effectively reduces the excess hydrogen ions by associating them with MR^- ions to form non-ionized HMR. This shift results in a visible color change from yellow to red because the equilibrium concentration of HMR rises relative to that of MR^- ions.

Understanding Le Chatelier’s Principle helps students to anticipate the direction in which a reaction will shift when subjected to various changes. This understanding is crucial not just in interpreting color changes of indicators but also in broader applications such as chemical manufacturing, where achieving optimal yields can depend on controlling equilibrium shifts.

Chemical Equilibrium

Chemical equilibrium refers to the state in which both reactants and products are present in concentrations that have no further tendency to change with time. This situation occurs when the rate at which the reactants produce products is equal to the rate at which the products revert back to reactants. It's a dynamic balance rather than a static one, akin to two teams playing tug-of-war with equal strength, neither side winning nor losing ground.

Using the example of the indicator methyl red, we have a balance between the unionized form (HMR) and the ionized forms ( H^+ and MR^- ). When this equilibrium is disturbed by adding an acid or a base, the ratio of the concentrations of the reactants and products changes, but eventually, a new equilibrium will be established. It’s important to highlight that even though the color change may seem like a reaction has been pushed to completion, it actually signifies a new point of equilibrium has been reached. Furthermore, maintaining chemical equilibrium is a crucial aspect often leveraged in industries such as pharmaceuticals, where precise conditions are required to ensure the right concentrations of active ingredients are achieved.

Ionization Reactions

Ionization reactions are processes by which molecules break apart into ions when dissolved in water or other solvents. These reactions are essential in the study of acid-base chemistry because they explain the behavior of acid and base indicators like methyl red. In solution, methyl red can ionize, separating into H^+ and MR^- . The extent of this ionization reaction depends on the pH of the solution.

Indicators are designed to 'report' the ionization state through color changes. For methyl red, the red color indicates a higher concentration of the non-ionized form, prevalent in acidic solutions, whereas the yellow color corresponds to a higher concentration of ionized MR^- , indicative of more basic conditions. Adding an acid (HCl) increases H^+ concentration, driving the ionization reaction towards the formation of non-ionized form, thus showing red. Conversely, adding a base (NaOH) increases OH^- concentration, consuming H^+ ions to form water and shifting the reaction towards ionization, resulting in a yellow solution.

By studying ionization reactions, students can gain insights into the behavior of substances in different pH environments, which is indispensable for fields like environmental science, medicine, and any application involving pH-sensitive processes.