Name Section Experiment 2 1. Advance Study Assignment: Properties of Systems in Chemical Equilibrium Common acid-base indicator: In solution, it ionizes according to the equation: 1. Methyl orange: HMO HMO(aq) M(aq) MO (aq) red Yellow 1. Methyl orange is added to distilled water, the solution turns yellow. 2. If a drop or two of 6 M HCl is added to the yellow solution, it turns red. 3. If to that solution one adds a few drops of 6 M NaOH, the color reverts to yellow. 4. Why does adding HCl to the yellow solution of methyl orange tend to cause the color to change to red? (Note that in solution, HCl exists as H+ and Cl- ions) 5. Why does adding NaOH to the red solution tend to make it turn back to yellow? (Note that in solution, NaOH exists as Na+ and OH- ions) 6. How does increasing [OH-] affect the dissociation reaction of methyl orange? How would the resulting change in [H+] affect the reaction in the discussion section?
The Correct Answer and Explanation is:
Correct Answers:
4. Adding HCl increases the concentration of H⁺ ions, which shifts the equilibrium of the methyl orange dissociation reaction to the left (toward HMO), changing the color to red.
5. Adding NaOH increases the concentration of OH⁻ ions, which react with H⁺ ions to form water. This decreases [H⁺], shifting the equilibrium to the right (toward MO⁻), changing the color to yellow.
6. Increasing [OH⁻] reduces [H⁺] via neutralization (H⁺ + OH⁻ → H₂O). This shift in equilibrium toward the right favors the dissociation of HMO into H⁺ and MO⁻. In the context of the discussion, this means that the acidic form (red) converts back to the basic form (yellow).
Explanation:
Methyl orange is a pH-sensitive acid-base indicator that exists in equilibrium between two forms: the acidic form (HMO), which appears red, and the basic form (MO⁻), which appears yellow. The equilibrium can be represented as:
HMO ⇌ H⁺ + MO⁻
When methyl orange is added to distilled water, which is neutral (pH ≈ 7), the equilibrium favors the dissociated form, MO⁻, resulting in a yellow color.
Upon addition of 6 M HCl, a strong acid, the concentration of H⁺ ions increases. According to Le Chatelier’s Principle, the system responds by shifting the equilibrium to the left, favoring the formation of HMO. This causes the solution to turn red, indicating a more acidic environment.
If 6 M NaOH, a strong base, is then added, it introduces a high concentration of OH⁻ ions. These OH⁻ ions neutralize H⁺ ions to form water (H⁺ + OH⁻ → H₂O), reducing the [H⁺] in solution. The reduction in [H⁺] shifts the equilibrium to the right, favoring the formation of MO⁻, and the solution returns to a yellow color.
In question 6, increasing [OH⁻] directly reduces [H⁺] through neutralization. This shift encourages further dissociation of HMO to restore equilibrium, increasing the concentration of MO⁻. In the context of the reaction, this change in [H⁺] drives the indicator toward its basic form, explaining the observed color change. This dynamic behavior illustrates how indicators like methyl orange can visually signal changes in chemical equilibrium in response to varying pH.
