Protonic acids are compounds with protons that can ionize in water to give a proton, H+. The compounds themselves should be known as acid hydrides. For example the acid anhydride hydrogen chloride when dissolve in water would give the hydrochloric acid.
 J.N. Bronsted |
This protonated water ion is known as the hydronium ion (or hydroxonium ion, or hydrated proton). Another name for protonic acid is Bronsted acid.
pH SCALE
Chemists had devised a parameter pH to indicate the concentration of hydronium ion (and so the strength of the acid) in an aqueous solution. The the pH is defined as;
where a, known as activity, is defined as the mole of hydronium ion per kilogram of solvent, per 1 mole of solvent per kilogram of solvent.
By dividing the molal concentration with 1 mole per kilogram of the solvent a is than a ratio, making the logarithm of a more rational. It is meaningless to log(concentration term).
The pH is taken as a measurement of the acidity of an aqueous solution. As a is a ratio, pH has no unit.
Example: For a solution of {H3O}+ with a molarity concentration of 0.01 mole per litre, the molality is taken to be the same the molarity since the density of water is 1 g ml‾� (assuming that the hydronium is not high enough to change the density). The activity will then be 0.01 M/(1 mole per litre) = 0.01. So;
Tutorial 1
Can a protonic acid compound show acidity in the absence of water? Answer
Some compounds may have more than one displaceable proton. An example will be hydrogen sulphate, H2SO4.
Protonic acids with two displaceable protons are known as dibasic acids. Similar those with three displaceable protons will be known as tribasic acids.
Bronsted Equilibrium
If HB represents a protonic acid the Bronsted equilibrium is represented by
| a{H3O}+) X a(B‾) | where | K | = | ----------------------- | a(HB) X a(H2O) |
For fairly dilute solutions a(H2O) = 1, we define an acidity constant as;
| a{H3O}+) X a(B‾) | Acidity constant, Ka | ≈ | ----------------------- | a(HB) |
Since a has no unit Ka do not carry a unit.
STRONG ACIDS
Acids with Ka > 1, is considered a strong acid.
Let us work out an example of a 0.100M sulphuric acid (with Ka = 100). Let the fraction of hydrogen sulphate that is present as {H3O}+ be "f", then the fraction of sulphuric acid present will be (1-f). So;
| a({H3O}+) = fx0.100 M/M | |
| and | a(sulphuric acid) = (1-f) X 0.100 M/M |
The acidity constant will then be;
The value of f computed was 0.999, or a{H3O}+] = 0.0999, and
Alternatively for a strong acids we can assume that it is fully dissociated.
WEAK ACIDS
Acids with Ka < 1, is considered a weak acid.
Let us repeated the above computation with a 0.100M of acetic acid (Ka = 1.8 x 10 ‾5). Accordingly a{H3O}+ = f x 0.100 M/M and a(acetic acid) = (1-f) x 0.100 M/M.
a({H3O}+) is computed to be 0.00133. So pH = - log10{0.00133} = 2.88
Alternatively since it is a weak acid we can assume that there is no dissociation. Then
Giving, a(H3O}+) = 1.34x10‾�, and pH = - log10{0.00134} = 2.87. This is close enough for industrial applications. Acetic acid is commonly known as vinegar.
Tutorial 2
- Compute the pH for 0.100M aqueous solution of the following acids
- Boric acid, once used as food preservative but is now prohibited by law.
- Carbonic acid, a compound found in carbonated (fizzy) drinks
- Lactic acid, a compound found in sour milk
- Phenol. Hospital usually smells of phenol related compounds. One of its members, chloroxylenol, is used as an antiseptic in Dettol (4.8 w/v %)
- What is acid rain? Answer
Tutorial 3
What will be the pH of a solution containing 2.5x10 ‾� mole of sodium hydroxide (NaOH) and 2.5x10‾� of hypochlorous acid (HClO)?
Ka(HClO) = 3.5x10 ‾8M. AnswerTutorial 4
In class today we talked about Molar solution. I'm wondering why does a 1 Molar solution of sulfuric acid have a lower pH than a 1 Molar solution of acetic acid? Amanda Petersen (12th grader, America). Answer
AMPHOTERIC COMPOUNDS
Oxides and hydroxides very often show very distinct behaviour towards an acid or a base. They will behave as a base towards acid, or as an acid towards a base. But the oxides and hydroxides of some elements in the middle of the Periodic Table have "amphi" (in Greek it means both) behaviour. When added to an acid it behaves as a base, but when added to a base it behaves as an acid. Example.
Part 2: Indicators, & Buffers
