Chemistry · Class 11 · Chemical Equilibrium and Acids · Term 2

Hydrolysis of Salts

Students will predict the pH of salt solutions based on the hydrolysis of their constituent ions.

• CBSE Learning Outcomes
• NCERT: Equilibrium - Class 11

About This Topic

Hydrolysis of salts occurs when ions from the salt react with water, producing acidic, basic, or neutral solutions based on the strengths of the parent acid and base. Class 11 students classify salts such as sodium chloride from strong acid-strong base pairs as neutral, ammonium chloride from strong acid-weak base as acidic due to NH4+ hydrolysis, and sodium acetate from weak acid-strong base as basic from CH3COO- hydrolysis. They write balanced equations like NH4+ + H2O ⇌ NH3 + H3O+ and use hydrolysis constants Kh = Kw/Ka or Kw/Kb to predict pH trends.

This topic aligns with the CBSE Chemistry curriculum unit on Chemical Equilibrium, reinforcing ionic equilibrium concepts, conjugate acid-base pairs, and Le Chatelier's principle. Students calculate approximate pH values for dilute solutions, developing skills in applying equilibrium expressions to real-world scenarios like buffer systems and industrial processes.

Active learning suits this topic well because students can test pH of prepared salt solutions using indicators or meters, compare predictions with observations, and discuss discrepancies in groups. Such approaches make ion behaviour tangible, improve prediction accuracy, and build confidence in handling equilibrium calculations.

Key Questions

1. Predict whether a salt solution will be acidic, basic, or neutral based on the strengths of its parent acid and base.
2. Explain the process of salt hydrolysis and how it affects the pH of a solution.
3. Construct the hydrolysis reactions for various salt ions.

Learning Objectives

• Classify salts into four categories based on the strength of their parent acid and base.
• Explain the mechanism of hydrolysis for cations and anions, relating it to pH changes.
• Construct balanced chemical equations for the hydrolysis reactions of specific salt ions.
• Calculate the approximate pH of a salt solution using the hydrolysis constant (Kh) and Kw.
• Compare the predicted pH of a salt solution with experimental results and analyze discrepancies.

Before You Start

Acids and Bases (Definitions and Strength)

Why: Students need to understand the concepts of strong and weak acids and bases, and their dissociation behaviour, to predict salt solution pH.

Ionic Equilibrium and Equilibrium Constant (Kc, Kp)

Why: Understanding equilibrium principles is fundamental to grasping how salt ions react with water to establish a new equilibrium.

Water Autoionization (Kw)

Why: Knowledge of Kw and its temperature dependence is essential for calculating hydrolysis constants and predicting pH.

Key Vocabulary

Salt Hydrolysis	The reaction of ions of a salt with water molecules, altering the pH of the solution.
Hydrolysis Constant (Kh)	An equilibrium constant that describes the extent of salt hydrolysis, related to Kw and the dissociation constants of the parent acid or base.
Parent Acid/Base Strength	Refers to whether the acid or base from which a salt is derived is strong (completely dissociates) or weak (partially dissociates).
Conjugate Acid/Base	An acid or base that differs from another acid or base by a single proton (H+); for example, NH4+ is the conjugate acid of NH3.

Watch Out for These Misconceptions

Common Misconception

All salts dissolved in water give neutral pH of 7.

What to Teach Instead

Salts hydrolyse based on parent acid-base strengths; for example, NaCl is neutral but CH3COONa is basic. Group testing of salt solutions reveals patterns, helping students revise ideas through evidence and peer explanations.

Common Misconception

Hydrolysis involves breaking the salt into acid and base.

What to Teach Instead

Hydrolysis is ion-water reaction producing H3O+ or OH-; salts do not revert to parent acid-base. Writing and discussing equations in pairs clarifies partial reactions, with pH tests confirming outcomes.

Common Misconception

pH of salt solutions depends only on cation strength.

What to Teach Instead

Both cation and anion can hydrolyse, though usually one dominates. Prediction activities followed by measurements show this, as students compare results for salts like NH4NO3.

Active Learning Ideas

Stations Rotation: pH Prediction and Testing

Prepare stations with solutions of NaCl, NH4Cl, CH3COONa, and (NH4)2CO3. Groups predict pH based on parent acid-base strengths, test with universal indicator or pH paper, record results, and write one hydrolysis equation per station. Rotate every 10 minutes and share findings.

45 min·Small Groups

Pair Prediction Challenge: Salt Cards

Distribute cards naming salts like KCN or AlCl3. Pairs classify each as acidic, basic, or neutral, justify with hydrolysis reaction, and estimate pH range. Pairs swap cards with neighbours for peer review and discussion.

30 min·Pairs

Whole Class: Hydrolysis Equation Relay

Divide class into teams. Teacher calls a salt; first student writes cation or anion hydrolysis, passes to next for completion and pH prediction. Teams race while ensuring accuracy through quick checks.

25 min·Whole Class

Individual: pH Calculation Worksheet

Provide worksheets with Ka/Kb values for given salts. Students calculate Kh and approximate pH for 0.1 M solutions using formulas. Follow with pair sharing to verify steps.

35 min·Individual

Real-World Connections

• Water treatment plants adjust the pH of drinking water using salts like aluminium sulfate (alum), which undergoes hydrolysis. Understanding this process helps ensure water is safe and palatable.
• The formulation of certain pharmaceuticals and cosmetics involves salts whose solutions can be acidic or basic due to hydrolysis. Pharmacists and cosmetic chemists must control pH for product stability and efficacy.
• In agriculture, soil pH is crucial for nutrient availability to plants. The type of salts present in fertilisers can affect soil pH through hydrolysis, impacting crop growth.

Assessment Ideas

Quick Check

Present students with a list of salts (e.g., KCl, NH4Cl, CH3COONa, NH4CN). Ask them to write down for each salt: (1) the parent acid and base, (2) whether the solution will be acidic, basic, or neutral, and (3) the ion that undergoes hydrolysis.

Exit Ticket

Give each student a card with a salt name (e.g., Ammonium Acetate). Ask them to: (1) Write the hydrolysis reaction for the ion that determines the pH. (2) State whether the solution will be acidic, basic, or neutral. (3) Briefly explain why.

Discussion Prompt

Pose the question: 'Why does a solution of sodium chloride remain neutral, while a solution of ammonium chloride becomes acidic?' Facilitate a class discussion where students explain the role of parent acid/base strength and ion hydrolysis in determining solution pH.

Frequently Asked Questions

How to predict if a salt solution is acidic, basic, or neutral?

Examine parent acid and base: strong-strong gives neutral (NaCl, pH 7); strong acid-weak base gives acidic (NH4Cl); weak acid-strong base gives basic (CH3COONa). Write hydrolysis equation for the weaker component and use Kh to confirm. Practice with common salts builds quick judgement.

What is the hydrolysis reaction for sodium acetate?

CH3COO- + H2O ⇌ CH3COOH + OH-, making solution basic since acetate is conjugate of weak acetic acid. Kh = Kw/Ka ≈ 5.6 × 10^-10. For 0.1 M, pH ≈ 8.7, calculated from [OH-] ≈ sqrt(Kh × C). This shows anion hydrolysis clearly.

How can active learning help students understand salt hydrolysis?

Activities like testing pH of salt solutions in small groups let students predict outcomes first, then observe matches or mismatches, reinforcing ion roles. Pair discussions on equations and calculations address errors instantly. Such hands-on work improves retention of abstract equilibrium concepts over rote memorisation.

Why is ammonium chloride solution acidic?

NH4+ hydrolyses: NH4+ + H2O ⇌ NH3 + H3O+, as ammonium is conjugate acid of weak base NH3. Kh = Kw/Kb ≈ 5.6 × 10^-10. Cl- from strong HCl does not hydrolyse, so solution pH < 7. Experiments confirm this prediction reliably.