Self-ionization of water - Autoionization of water - The ion product of water (kw)

Self-ionization of water - Autoionization of water - The ion product of water (kw)

Self-ionization of water - Autoionization of water - The ion product of water (kw)




A water molecule can act as an acid and as a base according to:


In the above reaction – autoionization of water- one water molecule acts as an acid donating a proton to the other water molecule that acts as a base – proton acceptor. From the equation we see that for every hydronium ion H3O+that is formed a hydroxyl ion OH-  is formed also.

Therefore, at equilibrium: [H3O+] = [OH-]
The equilibrium constant kw for the above reaction is known as the ion product of water. The value of kw at room temperature is 1.007 x 10-14.

kw = [H3O+] * [OH-] / [H2O]2= [H3O+] * [OH-] = 1.007 x 10-14     (2)

In pure water, at 25 ℃, since [H3O+] = [OH-], equation (2) becomes:

kw = [H3O+]2= 1.007 x 10-14  and therefore,   [H3O+] = [OH-] ≈ 10-7 M    (25 ℃)
The pH of pure water at 25 ℃:  pH = -log [H+] = -log (10-7) =  -(-7) = 7  


However, the pH of pure water would slightly decrease over time - to 6.6 – 6.8 -due to the dissolved CO2 from the air.
Carbon dioxide CO2 is slightly soluble in pure water and the solubility decreases with temperature. At pressures up to about 4 atm, the solubility follows Henry’s law:

[CO2] = kH * PCO2 = 0.032 * PCO2

  The equilibrium that takes place is given in Fig. I.1:
 
Fig. I.1: CO2 equilibrium between the gaseous and liquid phase

Fig. I.1: CO2equilibrium between the gaseous and liquid phase

Once it has dissolved, a small portion of the CO2 reacts with water to form carbonic acid H2CO3 that dissociates in water to produce H3O+ decreasing the pH of the solution (Fig. I.2):

Fig. I.2: CO2 reacts with water to form carbonic acid H2CO3 that dissociates in water to produce H3O+




Relevant Posts

Solving Weak Acid and Weak Base pH problems

Weak Acid Weak Base pH calculation solved example

Weak Acids and Bases - Calculate the pH of a weak acid

Polyprotic Acids / pH Calculation

Cubic Equation Calculator for Weak Acid-Base Equilibria

References

  1. J.N. Butler “Ionic Equilibrium – Solubility and pH calculations”, Wiley – Interscience, 1998
  2. J-L. Burgot “Ionic Equilibria in Analytical Chemistry”, Springer Science & Business Media, 2012
  3. D. Harvey, “Modern Analytical Chemistry”, McGraw-Hill Companies Inc., 2000
  4. J.N. Spencer et al., “Chemistry structure and dynamics”, 5th Edition, John Wiley & Sons, Inc., 2012

Key Terms

weak acids and bases, calculate the pH of a weak acid, calculate the pH of a weak base, weak acid base chemistry, weak acid base pH calculator,chemistry app calculator,weak acid base ph app calculator ,weak acid base pH chemistry applet,

 

Comments

Post a Comment

Popular posts from this blog

Carbocations: Factors affecting their Stability

Image
A carbocation is a species where a carbon atom bonds to three carbon atoms and has a positive charge. Carbocations are electron deficient species and therefore very reactive and unstable . Anything which donates electron density to the electron-deficient center will help to stabilize them. Factors that stabilize them are the following:  Neighboring carbon atoms (inductive effect) Neighboring carbon-carbon multiple bonds (resonance effect) Neighboring atoms with lone pairs (resonance effect) How carbocations are stabilized by neighboring carbons atoms? The stability of carbocations decreases as the number of carbons attached to the C+ decreases. That means that tertiary carbocations are more stable than secondary that in turn are more stable than primary (Fig. 1). Fig. 1: Carbocation stability increases as methyl substitution increases around the electron deficient carbon C+. The methyl groups (-CH 3 ) are electron donating and therefore stabilize the positive charge (inductive ...
Read more

Standard Enthalpies of Formation of Organic Compounds

Standard Enthalpies of Formation of selected organic compounds Home > Fundamental Physical Constants - Chemical - Physical - Thermochemical Data > Standard Enthalpies of Formation of Inorganic Compounds > Standard Enthalpies of Formation of Organic Compounds Standard Enthalpies of Formation of Organic Compounds   The standard enthalpy of formation ΔΗ° f of a substance is the change in enthalpy for the reaction that forms 1 mol of the substance from its elements with all reactants and products at 1 atm pressure and usually 298.15 K. For any element in its most stable state at 298 K and 1 atm pressure, ΔΗ° f = 0. Standard enthalpies of formation of inorganic compounds were given in a previous post.   Table 1: Standard Molar Enthalpies of Formation ΔΗ° f at 25 °C (298.15K) Substance Formula ΔΗ° f   Acetylene C 2 ...
Read more

CHEMISTRY NET - INTRODUCTION - LIST OF TOPICS

Chemistry Net - Introduction - Table of Contents CHEMISTRY NET Analytical Chemistry Biological Chemistry Inorganic Chemistry Organic Chemistry Physical & Theoretical Chemistry Chemical News & Interesting / Chem. Data Chemistry is the understanding, the definition and the description of the structure and the behavior of matter, the matter that surrounds us and the matter we are consisting off. It is our history, our life and our future. It is sometimes called the central science because of its role in connecting the other physical sciences (physics, astronomy, earth sciences) with the life sciences (biology, medicine) and engineering. There are many approaches to studying this fascinating science but for convenience and historical reasons has been divided it into five fields: analytical , biological , inorganic , organic and physical chemistry. Chemistry is a challenging subject to teach and even more challenging to learn. This si...
Read more