Layne Morsch's Organicers New Practical User Case Study on the use of the
ChemWiki in an Organic Course is now live. Check it out.
The formation of hydrogen ions (hydroxonium ions) and hydroxide ions from water is an endothermic process. Using the simpler version of the equilibrium:
\[ H_2O_{(l)} \rightleftharpoons H^+_{(aq)} + OH^-_{(aq)}\]
Hence, the forward reaction, as written, absorbs heat.
According to Le Châtelier's Principle, if you make a change to the conditions of a reaction in dynamic equilibrium, the position of equilibrium moves to counter the change you have made. Hence, if you increase the temperature of the water, the equilibrium will move to lower the temperature again. It will do that by absorbing the extra heat. That means that the forward reaction will be favored, and more hydrogen ions and hydroxide ions will be formed. The effect of that is to increase the value of \(K_w\) as temperature increases.
The table below shows the effect of temperature on \(K_w\). For each value of \(K_w\), a new pH has been calculated. It might be useful if you were to check these pH values yourself.
| T (°C) | Kw (mol2 dm-6) | pH |
|---|---|---|
| 0 | 0.114 x 10-14 | 7.47 |
| 10 | 0.293 x 10-14 | 7.27 |
| 20 | 0.681 x 10-14 | 7.08 |
| 25 | 1.008 x 10-14 | 7.00 |
| 30 | 1.471 x 10-14 | 6.92 |
| 40 | 2.916 x 10-14 | 6.77 |
| 50 | 5.476 x 10-14 | 6.63 |
| 100 | 51.3 x 10-14 | 6.14 |
You can see that the pH of pure water decreases as the temperature increases. Interestingly, the pOH also decreases.
If the pH falls as temperature increases, this does not mean that water becomes more acidic at higher temperatures. A solution is acidic if there is an excess of hydrogen ions over hydroxide ions (i.e., pH < pOH). In the case of pure water, there are always the same concentration of hydrogen ions and hydroxide ions and hence, the water is still neutral (pH = pOH) - even if its pH changes.
The problem is that we are all familiar with 7 being the pH of pure water, that anything else feels really strange. Remember that to calculate the neutral value of pH from \(K_w\). If that changes, then the neutral value for pH changes as well.
Jim Clark (Chemguide.co.uk)
Analytical Chemistry
Biological Chemistry
Inorganic Chemistry
Organic Chemistry
Physical Chemistry
Theoretical Chemistry
Diablo Valley College
Hope College
Howard University
Purdue University
Sacramento City College
United Arab Emirates U
UC Davis
UC Irvine
General Chemistry
Analytical Chemistry
Organic Chemistry
Physical Chemistry
Theoretical Chemistry
This material is based upon work supported by the National Science Foundation under Grant Number 1246120
Unless otherwise noted, content in the UC Davis ChemWiki is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License. Permissions beyond the scope of this license may be available at copyright@ucdavis.edu. Questions and concerns can be directed toward Prof. Delmar Larsen (dlarsen@ucdavis.edu), Founder and Director. Terms of Use