How Does Water Self-Ionise? Reaction, Diagram & Significance of Kw
The ionic product of water is a central concept in JEE Main Chemistry, particularly in ionic equilibrium. It explains why pure water is electrically neutral and underpins calculations for pH, pOH, and acid-base strengths. Understanding the formula, derivation, and temperature dependence of the ionic product (Kw), as well as its applications, is critical for mastering equilibrium and solving related numerical questions in JEE exams.
Water undergoes a process called self-ionisation or autoionisation, in which two water molecules react to produce hydronium (H3O+) and hydroxide (OH−) ions. The chemical equation is written as: H2O + H2O ⇌ H3O+ + OH−. For simplicity, it is often shown as H2O ⇌ H+ + OH−, where H+ ions are assumed to exist as H3O+ in solution.
The extent of water's self-ionisation is very small. In 1 litre of pure water at 25°C, only about 1 in 107 water molecules dissociate at a given time. Both H+ and OH− ions are present in equal concentration, making water electrically neutral under standard conditions.
Ionic Product of Water: Definition, Formula, and Value
Ionic product of water (Kw) is defined as the product of the molar concentrations of hydrogen ions and hydroxide ions in pure water at a given temperature. Mathematically, Kw = [H+][OH−]. It is a constant at a particular temperature due to equilibrium. The ionic product increases with temperature, as the dissociation of water is an endothermic process.
| Temperature (°C) | Value of Kw (mol2 L−2) |
|---|---|
| 0 | 0.11 × 10−14 |
| 25 | 1.00 × 10−14 |
| 50 | 5.48 × 10−14 |
| 100 | 7.50 × 10−14 |
At 25°C (standard lab temperature), Kw = 1.0 × 10−14 mol2 L−2. Thus, in neutral pure water, [H+] = [OH−] = √Kw = 1.0 × 10−7 mol L−1.
To derive the ionic product of water, use the law of chemical equilibrium for the reaction:
H2O (l) ⇌ H+ (aq) + OH− (aq)
The equilibrium constant expression is K = [H+][OH−]/[H2O]. Because water only dissociates slightly, [H2O] is almost constant. Multiplying K by [H2O], we get: Kw = K × [H2O] = [H+][OH−].
Self-Ionisation and Units of Ionic Product of Water
The units of the ionic product of water are mol2 L−2. This comes from multiplying the units of [H+] and [OH−], each of which are in mol L−1. Whenever a question asks for units or the standard value of Kw at 25°C, remember to provide both.
- Self-ionisation of water is also called autoionisation or autoprotolysis.
- Kw depends strongly on temperature (increases with higher T).
- At 25°C, [H+] = [OH−] in neutral water.
- Kw is used to relate pH and pOH: pH + pOH = 14.
- In acid solutions, [H+] > [OH−]; in base, [OH−] > [H+].
Any addition of acid or base changes the concentrations of H+ and OH−, but their product remains fixed at Kw for that temperature. This concept is crucial in analyzing titrations, buffer solutions, and salt hydrolysis in the JEE syllabus.
Applications: Calculations, pH, and JEE Numericals
In JEE Main, questions often require using the ionic product of water to solve for unknown ion concentrations, pH or pOH. Recall that pKw = −log10Kw. At 25°C, pKw = 14, simplifying many calculations.
- For a solution with [H+] = 1 × 10−4 mol L−1, [OH−] = Kw / [H+] = 1 × 10−14 / 1 × 10−4 = 1 × 10−10 mol L−1.
- If [OH−] increases, [H+] decreases, keeping Kw constant.
- pH + pOH = pKw = 14 at 25°C.
- For strongly acidic or basic solutions, check units and significant digits as per JEE standards.
Common JEE problems involve using Kw to classify solutions as acidic, basic, or neutral, and calculating unknown [H+] or [OH−] given one ion concentration.
| [H+] (mol L−1) | [OH−] (mol L−1) | Nature of Solution |
|---|---|---|
| > 1 × 10−7 | < 1 × 10−7 | Acidic |
| 1 × 10−7 | 1 × 10−7 | Neutral |
| < 1 × 10−7 | > 1 × 10−7 | Basic (Alkaline) |
A classic mistake is forgetting that Kw increases with temperature. So, pH of neutral water drops below 7 at higher temperatures, but the solution is still neutral because [H+] equals [OH−]. This subtlety often appears in higher-order JEE questions.
Significance, Pitfalls, and Revision
The ionic product of water is essential for predicting the direction and result of acid-base reactions as well as for problems involving buffer action and hydrolysis. In salt hydrolysis or buffer calculations, incorrect use of units or overlooking temperature effects leads to common calculation errors in JEE Main.
- Always use SI units: mol L−1 for concentrations, mol2 L−2 for Kw.
- For non-25°C conditions, refer to correct Kw value from tables.
- Do not assume pH 7 means neutral unless temperature is at 25°C.
- Check for equality: [H+] = [OH−] implies neutrality, regardless of pH value.
- For strong acids/bases, final concentration may deviate due to dilution.
| Key Formulae and Values for JEE Main |
|---|
| Kw = [H+][OH−] (at equilibrium, water only) |
| At 25°C: Kw = 1 × 10−14 mol2 L−2 |
| Neutral pH: [H+] = [OH−] = 1 × 10−7 mol L−1 |
| pKw = −log10Kw; at 25°C, pKw = 14 |
| pH + pOH = pKw |
For more on acid-base concepts, calculations, and advanced ionic equilibrium for JEE Main, explore these relevant topics from Vedantu:
- Ionic Equilibrium: Concepts & Calculations
- How to Calculate pH of Solutions
- Ionisation Constant of Acids and Bases
- Chemical Equilibrium
- Solubility and Solubility Product
- Le Chatelier’s Principle
- Salt Hydrolysis
- pH Scale and Acidity
- Redox Reactions and Electrochemistry
- Practice JEE Main Chemistry Questions
Mastering the ionic product of water, its formula, unit, and application is key for scoring in JEE Main. Practise problems and review key tables before your exam for confident, accurate answers. For more detailed theory and solved examples, visit Vedantu and strengthen your Chemistry fundamentals!
FAQs on Ionic Product of Water: Definition, Formula & Kw Value
1. What is the ionic product of water?
The ionic product of water refers to the product of the concentrations of hydrogen ions (H+) and hydroxide ions (OH-) produced by the self-ionisation of pure water.
- Represented as Kw = [H+][OH-]
- At 25°C, its value is 1 × 10-14 mol2 L-2
- Central to understanding acid-base equilibrium and essential for pH and pOH calculations in Chemistry.
2. Why is the ionic product of water considered 1 × 10⁻¹⁴ at 25°C?
The ionic product of water (Kw) is 1 × 10-14 mol2 L-2 at 25°C because this is the measured value of [H+][OH-] at room temperature.
- Kw changes with temperature: it increases as temperature rises
- The standard value is used for convenience and uniformity in calculations
- Enables calculation of pH, pOH, and neutrality of water under standard conditions
3. What is the formula for the ionic product of water?
The formula for the ionic product of water is:
- Kw = [H+][OH-]
- For pure water at 25°C: Kw = 1 × 10-14 mol2 L-2
- Used to calculate pH, pOH and in acid-base equilibrium problems.
4. What are the ions produced by water?
Pure water produces two types of ions by self-ionisation:
- Hydronium ions (H3O+) (often shown as H+)
- Hydroxide ions (OH-)
This chemical process is represented as:
2 H2O ↔ H3O+ + OH-
5. How does temperature affect the ionic product of water?
The ionic product of water (Kw) increases with rise in temperature because ionisation of water is an endothermic process.
- At higher temperatures, [H+] and [OH-] both increase
- Kw at 25°C: 1 × 10-14
- At 40°C: higher than 1 × 10-14
This impacts pH and pOH values at different temperatures.
6. What is the significance of the ionic product of water in chemistry?
The ionic product of water (Kw) is fundamental for understanding acid-base equilibrium, pH, and pOH.
- Determines whether a solution is acidic, basic, or neutral
- Used in calculations for pH and pOH of solutions
- Crucial for numerical problems in JEE, NEET, and Board exams
- Forms the basis for concepts like buffer solutions and hydrolysis of salts
7. What is meant by self-ionisation of water?
Self-ionisation of water is the process where two water molecules interact to form hydronium ions (H3O+) and hydroxide ions (OH-).
- Chemical equation: 2 H2O ↔ H3O+ + OH-
- Basis of the ionic product (Kw)
- Explains water’s ability to conduct electricity slightly.
8. Is the ionic product of water the same as the equilibrium constant?
The ionic product of water (Kw) is a special type of equilibrium constant for the self-ionisation of water.
- Kw is the equilibrium constant for the reaction: H2O ↔ H+ + OH-
- It is distinct from other equilibrium constants as it involves water only.
- Value depends on temperature and represents the extent of ionisation in pure water.
9. What are common mistakes students make in ionic product of water numericals?
Common mistakes in Kw numericals include:
- Using wrong value of Kw for temperatures other than 25°C
- Confusing Kw with [H+] or [OH-] individually
- Forgetting the units: mol2 L-2
- Incorrectly calculating pH or pOH without considering both ions
- Not applying the Kw = [H+][OH-] relationship accurately in buffers or salt hydrolysis problems
10. Why does pure water still conduct electricity slightly?
Pure water conducts electricity slightly due to its self-ionisation, which produces small amounts of H+ and OH- ions.
- These ions carry electric current
- The degree of conduction is very low because ion concentration is very small
- This illustrates the principle behind the ionic product of water (Kw)
