How the Common Ion Effect Influences pH in Chemistry Experiments
With the addition of soluble compounds in the solution having a common ion with precipitate, the ionic precipitate solubility decreases. This is the concept of the common ion effect. In other terms, we refer to the shift in equilibrium due to the introduction of common ions in a solution is the common ion effect.
This effect is seen with the ionization of weak acid and weak base, which is also reversible. The reactions involve the formation of the precipitate, with the addition of chemicals having the same ions, is common with residue. You’ll understand ph change by common ion in case of weak acids and weak bases experiment with examples.
The pH of Weak Acid and Weak Base
The weak acid and weak base study is experimental-based observation and an important concept that deals with forming a salt precipitate. We will understand this concept in detail with examples:
Let us take one weak acid and one weak base as HA and BOH, respectively.
HA is H+ and A-
BOH is B+ and OH-
CH3COOH + H2O ⇋ CH3COO- + H3O+
CH3COONa → CH3COO- + Na+
In the first reaction above, H+ ions concentration increases. Thus reaction takes a reverse change and shifts equilibrium to the left-hand side. The second reaction states the shift in equilibrium.
CH3COOH is a weak acetic acid, and NH4OH is Ammonium hydroxide, a weak base.
The reaction depicts the shift is equilibrium with a decrease in H3O+ ions, and the pH of the entire solution increases.
Weak bases get partially dissociated. Thus ammonium hydroxide dissociates partially, and when we add Ammonium chloride to it, the concentration of NH4+ ions increases. The shift in equilibrium is noted in the below reaction:
NH4OH ⇋ NH4+ + OH-
NH4Cl → NH4+ + Cl-
Let us learn more about weak acid and weak base reaction.
Experiment on the pH of Weak Acid and Weak Base
The Common Ion effect deals with forming residue with a decrease in the concentration of ions. As studied under Le Chatelier's principle, a reaction’s equilibrium does not change unless you apply any external force. This external force can be in the form of tempurature, pressure, or the addition of some ions in the compound.
If a force is applied, it will help is re-establishing equilibrium. In all, the effect of solubility of solute decreases.
Aim:
To study pH change by common ion in case of weak acids and weak bases experiment.
Things Required:
You’ll require pH paper, test tubes, glass rod, Test tube stand, sodium acetate, acetic acid, ammonium chloride, and ammonium hydroxide for the experiment.
The Change in pH Level of a Solution for Weak Acid with Common Ion Effect:
The first step is to note the pH value of 10 ml acetic acid in a test tube.
To get the exact pH value, compare the shade with the standard pH chart.
With the help of a glass rod, mix 1g of sodium acetate with acetic acid in a test tube.
Now note down the final pH value considering the standard pH chart as a reference.
Now again, add 1g of sodium acetate to the solution and note down the solution’s pH value.
Repeat the experiment and keep noting your observations for the same.
Your Observations
The noticed changes show the solution’s pH value increases with the addition of common ions to weak acid.
The Change in pH Level of a Solution for Weak Base with Common Ion
In a test tube, add 10 ml of ammonium hydroxide, note down its pH value using pH paper, and consider the standard pH chart as a reference.
Now keep adding 1.5g of ammonium chloride in the test tube using a glass rod and keep mixing.
Take pH paper and note down pH value considering the pH chart as a reference.
Again add 1.5g of ammonium chloride to the test tube and mix well.
Note down the pH value of the final solution noting the pH chart as a reference.
Repeat the experiment and note down the final pH value in the observation table.
Your Observations
Change in pH of a weak base with the addition of conjugate acid is noted in the table below:
Experimental Precautions to be Taken:
Be careful while noting your observations after matching them with the standard pH chart.
Keep noting your observations simultaneously.
FAQs on Understand the pH of Weak Acids and Bases: The Common Ion Effect
1. What is the common ion effect in the context of weak acids and bases?
The common ion effect describes the decrease in the ionisation of a weak electrolyte (like a weak acid or weak base) when a salt containing an ion common to the electrolyte is added to the solution. This shift suppresses the dissociation of the weak acid or base, thereby affecting its equilibrium and pH.
2. Why is the pH of a weak acid higher than that of a strong acid at the same concentration?
The pH of a weak acid is higher because it dissociates only partially in water, releasing a smaller concentration of hydrogen ions (H⁺) compared to a strong acid of the same molarity, which dissociates completely. Since pH is the negative logarithm of the H⁺ concentration, a lower H⁺ concentration results in a higher pH value, meaning it is less acidic.
3. How does adding a common ion change the pH of a weak acid solution?
When a salt containing the conjugate base (the common ion) is added to a weak acid solution, the equilibrium shifts to the left, according to Le Chatelier's Principle. This shift reduces the dissociation of the weak acid, decreasing the concentration of H⁺ ions. A lower [H⁺] concentration causes the pH of the solution to increase, making it less acidic.
4. Can you explain the common ion effect with an example, like acetic acid?
Certainly. Consider a solution of the weak acid, acetic acid (CH₃COOH), which establishes the equilibrium: CH₃COOH ⇌ H⁺ + CH₃COO⁻. If you add sodium acetate (CH₃COONa), a strong electrolyte, it dissociates completely into Na⁺ and acetate ions (CH₃COO⁻). The increased concentration of the common ion (CH₃COO⁻) pushes the equilibrium of the acetic acid to the left. This suppresses the ionisation of CH₃COOH, reducing the [H⁺] and increasing the solution's pH.
5. What happens to the final pH when a weak acid reacts with a weak base?
The pH of the resulting solution depends on the relative strengths of the acid and base, which are determined by their dissociation constants (Ka and Kb). The outcome can be:
- If Ka > Kb, the solution will be acidic (pH < 7).
- If Kb > Ka, the solution will be basic (pH > 7).
- If Ka ≈ Kb, the solution will be approximately neutral (pH ≈ 7).
6. What is the difference between the dissociation of a weak acid and a weak base?
The primary difference lies in the ions they produce in an aqueous solution. A weak acid, such as acetic acid (CH₃COOH), partially donates a proton (H⁺), forming a hydronium ion (H₃O⁺) and its conjugate base. In contrast, a weak base, such as ammonia (NH₃), partially accepts a proton from water, producing a hydroxide ion (OH⁻) and its conjugate acid (NH₄⁺).
7. How does the common ion effect relate to Le Chatelier's Principle?
The common ion effect is a direct application of Le Chatelier's Principle to ionic equilibria. Le Chatelier's Principle states that if a change is applied to a system in equilibrium, the system will shift to counteract the change. Adding a common ion increases a product's concentration, so the equilibrium shifts to the left (towards the undissociated reactants) to reduce this concentration, thereby decreasing the ionisation of the weak electrolyte.
8. What are some important practical applications of the common ion effect?
The common ion effect is a fundamental principle with several important applications in chemistry, including:
- Buffer Solutions: It is the core mechanism that allows buffer solutions to resist significant pH changes.
- Qualitative Analysis: It is used for the selective precipitation of specific ions from a mixture, such as separating metal sulphides in group analysis.
- Purification of Compounds: It helps in precipitating a pure compound from its saturated solution by adding a substance with a common ion.
9. How does the common ion effect influence the solubility of a sparingly soluble salt like AgCl?
The solubility of a sparingly soluble salt like silver chloride (AgCl) decreases significantly in the presence of a common ion. The equilibrium for AgCl is AgCl(s) ⇌ Ag⁺(aq) + Cl⁻(aq). If a soluble chloride salt like NaCl is added, the concentration of the common ion (Cl⁻) increases. This forces the equilibrium to shift to the left, causing more AgCl to precipitate and thus reducing its overall solubility in the solution.
