Acids and bases play a significant role in every aspect of life. Their chemistry is integral to good health. Their effect in the environment impacts our quality of life and the health of our planet. A good foundation in acid-base chemistry is essential as you progress toward your career goal whether it is in a medically related field, environmental field, or in an industry setting.
In this presentation, we will confine our acid-base chemistry consideration to aqueous solutions. In organic and higher level courses, acid-base chemistry in other solvents will be discussed.
The preceding section Water equilibrium established that pure water is an equilibrium reaction between water molecules and H+ and OH– ions.
The presence of very low concentrations of ions can be explained by the observation there are hydrogen bonds between water molecules. See the video Intermolecular Forces for a review of hydrogen bonds. The water molecules are constantly moving so it is possible that between two neighboring water molecules a H+ could momentarily associate with its neighbor to form an ion of H2O.H+ (hydronium ion). The water molecule that looses the H+ would have a charge of OH–. In a very short time, the H2O.H+ ion and the OH– ion would attract each other and would form water molecules again.
Because there is a very large number of water molecules, at any instant some water molecules would be forming H3O+ ions and OH– ions and at the same time some of these ions are combining to form water. The water molecules and the H3O+ ions and OH– ions are in equilibrium.
Water in equilibrium with the H3O+ ions and OH– ions is depicted in the The Youtube animation Self ionization of water. This is a slow motion animation of the hydronium ion and hydroxide ion formation in the water equilibrium
The video at hydronium and water is a faster animation of the water equilibrium. The hydronium ion is the brown and white model.
The reaction that describes this process is:
The equilibrium concentrations of the participants are: [H2O] = 55.5 M, [H3O+] = 1 x 10-7 M, and [OH–] = 1 x 10-7 M. Substitution into the equilibrium constant expression gives:
The molarity of water remains essentially constant when a solute is added to make a solution. Therefore, [H2O]2 in the denominator on the right side can be considered as a constant that can be multiplied by the Kc value of 3.2 x 10-18 to yield the equilbrium equation:
This equilibrium equation is called the “Water Constant Equation” and is written as:
This describes the equilibrium of “pure” water. If a solute is added to water that increases the H3O+, the water reaction will be taken out of equilibrium and will shift to the reactants until equilibrium is achieved again. When equilibrium is re-established, the [H3O+] will be greater than 1 x 10-7 M and the [OH–] will be less than 1 x 10-7 M. The product of these two concentrations will still equal the equilibrium constant of 1 x 10-14. A reminder. When [ ] is used around a chemical specie, it indicates the molar concentration of that substance.
If a solute is added which increases the [OH–], when equilibrium is re-established, the concentration of this ion would be greater than 1 x 10-7 M and the [H3O+] would be less than 1 x 10-7 M.
As an example, a solute is added which after equilibrium is re-established results in a [H3O+] = 1 x 10-4 M. The [OH–] would equal 1 x 10-10 M
People doing research in this area realized they were working with very small numbers, so they devised the p scale to provide more convenient numbers to use. The p scale is defined by the following definition: pX = – log X.
If X is the concentration of [H3O+], then the p value is pH. If X is the concentration of [OH–], then the p value is pOH. In fact X could be any chemical specie in solution. If X is [Cu+2], then the p value is pCu+2 . If you take enough chemistry you will encounter pK values. This is the equilibrium constant expressed as a p value. The Kw for water is 1 x 10-14. Hence p scale value would be pKw =14.
The Water constant equation can be re-written in pValues as:
pKw = pH + pOH or 14 = 7 + 7
The p scale is inversely related to the actual value. This means an increase in [H3O+] results in a lower pH. An increase in [OH–] results in a higher pH.
pH is a measure of how acidic or basic an aqueous solution is. pOH is rarely mentioned because it is directly related to pH and once the pH value is known, the pOH is also known ( 14 = pH + pOH).
An acidic solution is one which has a greater [H3O+] than [OH–] . A basic solution is one that has a greater [OH–] than [H3O+].
pH values are used to easily determine if a solution is acidic or basic and how acidic or basic it is. The range of 0 to 7 is an acidic solution. The lower the pH number is from 7, the more acidic the solution is. The range of 7 to 14 is a basic solution. The higher from 7 the pH is, the more basic the solution is. A solution with a pH of 7 is a neutral solution because the [H3O+] is equal to the [OH–] . The solution is neither acidic or basic.
See Logarithms and pH to get a brief discussion of logarithms and also how to calculate pH.
To finish this section, it needs to be mentioned that the hydrogen ion can be shown as H+, H+(aq) or H3O+. They all mean the same thing. So don’t be concerned when you see any of the three representations.