2.5.3

The Ionic Product of Water (A2 Only)

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The Ionic Product of Water

Water is very slightly dissociated into its ions.

Dissociation of water

Dissociation of water

  • Water very slightly dissociates into its ions according to the following equation:
    • 2H2O(l) ⇋ H3O+(aq) + OH-(aq)
Equilibrium constant

Equilibrium constant

  • The equilibrium constant for the dissociation of water is:
    • Kc = [H3O+][OH−][H2O]\frac{[H_3O^+][OH^-]}{[H_2O]}
Ionic product of water

Ionic product of water

  • The equilibrium constant can be simplified because the concentration of water is pretty much constant.
    • Water only dissociates slightly, so the concentration doesn’t really change.
    • We write a new equation: Kw = [H3O+(aq)][OH-(aq)]
    • It may also be written as: Kw = [H+(aq)][OH-(aq)]
Temperature dependence of K<sub>w</sub>

Temperature dependence of Kw

  • Kw is just an equilibrium constant.
    • Because of this, it behaves the same as any other equilibrium constant, Kc.
  • Importantly, this means it has a temperature dependence. The hotter the water is, the more dissociated it is and the larger its Kw value.

Use of Kw

Kw can be used to calculate the concentration of hydroxide ions given the pH.

The value of K<sub>w</sub>

The value of Kw

  • At room temperature, it just so happens that Kw is around 1 ×10-14.
    • As the concentration of hydroxide ions must equal the concentration of hydrogen ions for pure water, we know the concentration of both must be 1 × 10-7.
    • If the [H+] is 1 ×10-7, then the pH is 7.
    • This is why we say a neutral solution has a pH of 7.
    • A neutral solution is one with the same concentration of hydroxide ions and hydrogen ions.
Use of K<sub>w</sub>

Use of Kw

  • The Kw is a constant at a specific temperature.
    • This means that at a specific temperature, we can relate the [H+] to the concentration of [OH–].
  • If we know the pH of a solution, we can calculate the concentration of [OH–].
Example

Example

  • Say we dissolve one mole of HCl into a litre of water:
    • The concentration of H+ ions is 1moldm-3.
    • We can rearrange the ion product equation from:
    • Kw = 1 × 10-14 = [H3O+(aq)][OH-(aq)]
  • To:
    • [OH-(aq)] = 1 × 10-14 ÷ 1moldm-3
      So we can calculate that [OH-(aq)] = 1 × 10-14.
    • The [OH-(aq)] is much lower in an acidic solution than in a neutral solution.
Jump to other topics
1

Physical Chemistry

1.1

Atomic Structure

1.1.1Fundamental Particles1.1.2Isotopes & Mass Number1.1.3Mass Spectrometry1.1.4Electron Shells, Sub-Shells & Orbitals1.1.5Electron Configuration1.1.6Ionisation Energy1.1.7Factors Affecting Ionisation Energies1.1.8Trends of Ionisation1.1.9Specific Impacts on Ionisation Energies1.1.10End of Topic Test - Atomic Structure1.1.11A-A* (AO3/4) - Atomic Structure
1.2

Amount of Substance

1.2.1Relative Masses1.2.2The Mole1.2.3The Ideal Gas Equation1.2.4Empirical & Molecular Formulae1.2.5Balanced Equations1.2.6Percentage Yield1.2.7A-A* (AO3/4) - Percentage Yield1.2.8Atom Economy1.2.9End of Topic Test - Amount of Substance1.2.10A-A* (AO3/4) - Substances & Yield1.2.11Diagnostic Misconceptions - Moles
1.3

Bonding

1.3.1Ionic Bonding1.3.2Covalent & Dative Bonding1.3.3Carbon Structures1.3.4Metallic Bonding1.3.5Physical Properties1.3.6Shapes of Molecules1.3.7Polarity1.3.8Intermolecular Forces1.3.9Intermolecular Forces 21.3.10End of Topic Test - Bonding1.3.11Exam-Style Question - Shape of Molecules1.3.12A-A* (AO3/4) - Bonding1.3.13Diagnostic Misconceptions - Ions1.3.14Diagnostic Misconceptions - Ionic & Covalent1.3.15Diagnostic Misconceptions - Phase Change1.3.16Diagnostic Misconceptions - Boiling1.3.17Diagnostic Misconceptions - Polar Bonds
1.4

Energetics

1.4.1Enthalpy Changes1.4.2Bond Breaking and Making1.4.3Enthalpy Changes: Standard Conditions1.4.4Calorimetry1.4.5Hess' Law1.4.6Bond Enthalpies1.4.7End of Topic Test - Energetics1.4.8Diagnostic Misconceptions - Calorimetry
1.5

Kinetics

1.5.1Collision Theory1.5.2Maxwell-Boltzmann Distribution1.5.3Increasing Rates1.5.4End of Topic Test - Kinetics
1.6

Equilibria

1.6.1Le Chatelier1.6.2Equilibrium Constants1.6.3A-A* (AO3/4) - Energetics, Kinetics, Equilibria1.6.4End of Topic Test - Equilibria1.6.5Diagnostic Misconceptions - Solids
1.7

Redox

1.7.1Redox1.7.2End of Topic Test - Redox
2

Physical Chemistry 2 (A2 Only)

2.1

Thermodynamics (A2 Only)

2.1.1Born-Haber Cycles (A2 Only)2.1.2Born-Haber Cycle Calculations (A2 Only)2.1.3Entropy (A2 Only)2.1.4Free Energy (A2 Only)2.1.5End of Topic Test - Thermodynamics2.1.6A-A* (AO3/4) - Thermodynamics
2.2

Rate Equations (A2 Only)

2.2.1Rate Equations (A2 Only)2.2.2Rate Equations 2 (A2 Only)2.2.3The Arrhenius Equation (A2 Only)2.2.4Experimental Methods for Rates (A2 Only)2.2.5End of Topic Test - Rate Equations2.2.6Exam-Style Question - Rates2.2.7A-A* (AO3/4) - Rate Equations
2.3

The Equilibrium Constant Kp (A2 Only)

2.3.1Properties of Kp (A2 Only)
2.4

Electrochemical Cells (A2 Only)

2.4.1Electrochemical Cells (A2 Only)2.4.2Electrochemical Series (A2 Only)2.4.3Commercial Application (A2 Only)2.4.4End of Topic Test - Kp & Electrochemistry2.4.5A-A* (AO3/4) - Electrochemical Cells2.4.6Diagnostic Misconceptions - Salt Bridges2.4.7Diagnostic Misconceptions - Electrode, Electrolyte
2.5

Acids & Bases (A2 Only)

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3

Inorganic Chemistry

3.1

Periodicity & Trends

3.1.1The Periodic Table3.1.2Trends in the Periodic Table3.1.3End of Topic Test - Periodicity & Trends3.1.4Diagnostic Misconceptions - Atomic Radius
3.2

Group 2

3.2.1Group 2 Chemistry3.2.2End of Topic Test - Group 2
3.3

Group 7

3.3.1Introduction to Halogens3.3.2Reactions of Halogens3.3.3Chlorine & Chlorate(I)3.3.4End of Topic Test - Group 73.3.5Exam-Style Question - Group 73.3.6A-A* (AO3/4) - Periodicity, Group 2 & 7
4

Inorganic Chemistry 2 (A2 Only)

4.1

Period 3 (A2 Only)

4.1.1Period 3 Elements & Oxides (A2 Only)4.1.2Period 3 Oxides (A2 Only)4.1.3End of Topic Test - Period 3
4.2

Transition Metals (A2 Only)

4.2.1General Properties (A2 Only)4.2.2Substitution Reactions (A2 Only)4.2.3Shapes of Complex Ions (A2 Only)4.2.4Colours of Ions (A2 Only)4.2.5Variable Oxidation States (A2 Only)4.2.6Titrations (A2 Only)4.2.7Homogeneous Catalysts (A2 Only)4.2.8Heterogeneous Catalysts (A2 Only)4.2.9End of Topic Test - Transition Metals4.2.10A-A* (AO3/4) - Transition Metals
4.3

Reactions of Ions in Aqueous Solutions (A2 Only)

4.3.1Acidity (A2 Only)4.3.2Identification (A2 Only)4.3.3End of Topic Test - Reactions of Ions in Solution
5

Organic Chemistry 1

5.1

Introduction

5.1.1Naming Conventions5.1.2Mechanism5.1.3Isomerism5.1.4End of Topic Test - Introduction to Organic Chem
5.2

Alkanes

5.2.1Fractional Distillation5.2.2Cracking5.2.3Combustion5.2.4Chlorination5.2.5End of Topic Test - Alkanes
5.3

Halogenoalkanes

5.3.1Substitution Reactions5.3.2Elimination Reactions5.3.3Ozone Depletion5.3.4End of Topic Test - Halogenoalkanes
5.4

Alkenes

5.4.1Introduction to Alkenes5.4.2Reactions of Alkenes5.4.3Polymerisation Reactions5.4.4Properties of Polymers5.4.5End of Topic Test - Alkenes
5.5

Alcohols

5.5.1Production of Alcohols5.5.2Oxidation of Alcohols5.5.3Oxidation of Alcohols - 25.5.4Elimination Reactions5.5.5End of Topic Test - Alcohols5.5.6Exam-Style Question - Alcohols
5.6

Organic Analysis

5.6.1Reactions5.6.2Reactions 25.6.3Mass Spectrometry5.6.4Infrared Spectroscopy5.6.5End of Topic Test - Organic Analysis
5.7

A-A* (AO3/4) - Organic 1

5.7.1A-A* (AO3/4) - Organic 1
6

Organic Chemistry 2 (A2 Only)

6.1

Optical Isomerism (A2 Only)

6.1.1Optical Isomerism (A2 Only)6.1.2End of Topic Test - Optical Isomerism
6.2

Aldehydes & Ketones (A2 Only)

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6.3

Carboxylic Acids & Esters (A2 Only)

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6.4

Aromatic Chemistry (A2 Only)

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6.5

Amines (A2 Only)

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6.6

Polymers (A2 Only)

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6.7

Biological Organic (A2 Only)

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6.8

Organic Synthesis (A2 Only)

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6.9

NMR Spectroscopy (A2 Only)

6.9.1Understanding NMR (A2 Only)6.9.2Carbon-13 NMR (A2 Only)6.9.3Hydrogen-1 NMR (A2 Only)6.9.4Hydrogen-1 NMR 2 (A2 Only)6.9.5End of Topic Test - NMR
6.10

Chromatography (A2 Only)

6.10.1Overview of Chromatography (A2 Only)6.10.2Thin-Layer Chromatography (A2 Only)6.10.3Column Chromatography (A2 Only)6.10.4Gas Chromatography (A2 Only)6.10.5End of Topic Test - Chromatography
6.11

A-A* (AO3/4) - Organic 2

6.11.1A-A* (AO3/4) - Organic 2

Practice questions on The Ionic Product of Water (A2 Only)

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  1. 1
    What is the best equation to describe the dissociation of water?Multiple choice
  2. 2
    The endothermic dissociation of water means that as water gets hotter:True / false
  3. 3
    When one mole of HCl is poured into a litre of water:Fill in the list
  4. 4
    A neutral solution at room temperature has:True / false
  5. 5
    At room temperature, it just so happens that K<sub>w</sub> is roughly: Multiple choice
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pH (A2 Only)

Weak Acids & Bases (A2 Only)