- Dissociation
- Ammonia
- Calculation example
- K b
- pK b
- pH
- Percentage of ionization
- Properties
- Examples
- Amines
- Nitrogen bases
- Conjugate bases
- References
The weak bases are species with little tendency to donate electrons dissociate in aqueous solutions, or accepting protons. The prism with which its characteristics are analyzed is governed by the definition arising from the studies of several famous scientists.
For example, according to the Bronsted-Lowry definition, a weak base is one that accepts a hydrogen ion H + in a very reversible (or null) way. In water, its H 2 O molecule is the one that donates an H + to the surrounding base. If instead of water it were a weak acid HA, then the weak base could hardly neutralize it.
Source: Midnightcomm, from Wikimedia Commons
A strong base would not only neutralize all acids in the environment, but could also participate in other chemical reactions with adverse (and fatal) consequences.
It is for this reason that some weak bases, such as milk magnesia, or phosphate salts tablets or sodium bicarbonate, are used as antacids (top image).
All weak bases have in common the presence of an electron pair or a stabilized negative charge on the molecule or ion. Thus, CO 3 - is a weak base compared to OH -; and the base that produces the least OH - in its dissociation (Arrenhius definition) will be the weakest base.
Dissociation
A weak base can be written as BOH or B. It is said to undergo dissociation when the following reactions occur with both bases in the liquid phase (although it can occur in gases or even solids):
BOH <=> B + + OH -
B + H 2 O <=> HB + + OH -
Note that although both reactions may appear different, they have the production of OH - in common. Furthermore, the two dissociations establish an equilibrium, so they are incomplete; that is, only a percentage of the base actually dissociates (which does not happen with strong bases such as NaOH or KOH).
The first reaction "fits" more closely with the Arrenhius definition for bases: dissociation in water to give ionic species, especially the hydroxyl anion OH -.
Whereas the second reaction, obeys the Bronsted-Lowry definition, since B is being protonated or accepts H + from water.
However, the two reactions, when they establish an equilibrium, are considered weak base dissociations.
Ammonia
Ammonia is perhaps the most common weak base of all. Its dissociation in water can be outlined as follows:
NH 3 (aq) + H 2 O (l) <=> NH 4 + (aq) + OH - (aq)
Therefore, NH 3 falls into the category of bases represented by 'B'.
The ammonia dissociation constant, K b, is given by the following expression:
K b = /
Which at 25 ° C in water is about 1.8 x 10 -5. Calculating then its pK b we have:
pK b = - log K b
= 4.74
In the dissociation of NH 3, it receives a proton from water, so water can be considered as an acid according to Bronsted-Lowry.
The salt formed on the right hand side of the equation is ammonium hydroxide, NH 4 OH, which is dissolved in water and is nothing more than aqueous ammonia. It is for this reason that the Arrenhius definition for a base is fulfilled with ammonia: its dissolution in water produces the ions NH 4 + and OH -.
NH 3 is capable of donating a pair of unshared electrons located on the nitrogen atom; This is where the Lewis definition for a base comes in,.
Calculation example
The concentration of the aqueous solution of the weak base methylamine (CH 3 NH 2) is as follows: before dissociation = 0.010 M; after dissociation = 0.008 M.
Calculate K b, pK b, pH, and percent ionization.
K b
First the equation of its dissociation in water must be written:
CH 3 NH 2 (aq) + H 2 O (l) <=> CH 3 NH 3 + (aq) + OH - (aq)
Following the mathematical expression of K b
K b = /
In equilibrium, =. These ions come from the dissociation of CH 3 NH 2, so the concentration of these ions is given by the difference between the concentration of CH 3 NH 2 before and after dissociating.
dissociated = initial - equilibrium
dissociated = 0.01 M - 0.008 M
= 0.002 M
So, = = 2 ∙ 10 -3 M
K b = (2 ∙ 10 -3) 2 M / (8 ∙ 10 -2) M
= 5 ∙ 10 -4
pK b
Calculated K b, it is very easy to determine pK b
pK b = - log Kb
pK b = - log 5 ∙ 10 -4
= 3,301
pH
To calculate the pH, since it is an aqueous solution, the pOH must first be calculated and subtracted from 14:
pH = 14 - pOH
pOH = - log
And since the OH - concentration is already known, the calculation is straightforward
pOH = -log 2 ∙ 10 -3
= 2.70
pH = 14 - 2.7
= 11.3
Percentage of ionization
To calculate it, it must be determined how much of the base has been dissociated. As this was already done in the previous points, the following equation applies:
(/ °) x 100%
Where ° is the initial concentration of the base, and the concentration of its conjugated acid. Calculating then:
Percent ionization = (2 ∙ 10 -3 / 1 ∙ 10 -2) x 100%
= 20%
Properties
-The weak amine bases have a characteristic bitter taste, present in fish and which is neutralized with the use of lemon.
-They have a low dissociation constant, which is why they cause a low concentration of ions in aqueous solution. Not being, for this reason, good conductors of electricity.
-In aqueous solution they originate a moderate alkaline pH, which is why they change the color of litmus paper from red to blue.
-Mostly they are amines (weak organic bases).
-Some are the conjugate bases of strong acids.
-The weak molecular bases contain structures capable of reacting with H +.
Examples
Amines
-Methylamine, CH 3 NH 2, Kb = 5.0 ∙ 10 -4, pKb = 3.30
-Dimethylamine, (CH 3) 2 NH, Kb = 7.4 ∙ 10 -4, pKb = 3.13
-Trimethylamine, (CH 3) 3 N, Kb = 7.4 ∙ 10 -5, pKb = 4.13
-Pyridine, C 5 H 5 N, Kb = 1.5 ∙ 10 -9, pKb = 8.82
-Aniline, C 6 H 5 NH 2, Kb = 4.2 * 10 -10, pKb = 9.32.
Nitrogen bases
The nitrogenous bases adenine, guanine, thymine, cytosine and uracil are weak bases with amino groups, which are part of the nucleotides of nucleic acids (DNA and RNA), where the information for hereditary transmission resides.
Adenine, for example, is part of molecules such as ATP, the main energy reservoir of living beings. Furthermore, adenine is present in coenzymes such as flavin adenyl dinucleotide (FAD) and nicotin adenyl dinucleotide (NAD), which are involved in numerous oxidation-reduction reactions.
Conjugate bases
The following weak bases, or that can fulfill a function as such, are arranged in decreasing order of basicity: NH 2 > OH - > NH 3 > CN - > CH 3 COO - > F - > NO 3 - > Cl - > Br - > I - > ClO 4 -.
The location of the conjugate bases of the hydracids in the given sequence indicates that the greater the strength of the acid, the lower the strength of its conjugate base.
For example, the anion I - is an extremely weak base, while NH 2 is the strongest in the series.
On the other hand, to finish, the basicity of some common organic bases can be arranged in the following way: alkoxide> aliphatic amines ≈ phenoxides> carboxylates = aromatic amines ≈ heterocyclic amines.
References
- Whitten, Davis, Peck & Stanley. (2008). Chemistry. (8th ed.). CENGAGE Learning.
- Lleane Nieves M. (March 24, 2014). Acids and bases.. Recovered from: uprh.edu
- Wikipedia. (2018). Weak base. Recovered from: en.wikipedia.org
- Editorial Team. (2018). Base force and basic dissociation constant. chemical. Recovered from: iquimicas.com
- Chung P. (March 22, 2018). Weak acids & bases. Chemistry Libretexts. Recovered from: chem.libretexts.org