Basis for another definition of Ka - Introduction



 Hello fam, today’s lesson is going to focus on an introduction to another way that we can define the acid dissociation constant Ka. We would recall that yesterday, we showed that if we have a hypothetical weak acid whose formula can be given as HA, then the dissociation of the weak acid would give rise to an equation that looks thus;

HA(aq) <----> H+(aq) + A-(aq)
In that case we can write the equilibrium constant that shows the dissociation of the weak acid as;
Ka = [H+] [A-]/[HA]
Now let’s take the matter a bit further. In an earlier lesson I mentioned that an acid can only show acidic properties when it is dissolved in water. I mentioned in that lesson that gaseous HCl is not an acid until the gas interacts with water. Why would gaseous HCl not show the properties of an acid? That is one of the points that we would clarify in this lesson.
Let us go back to our hypothetical acid HA. One of the popular definitions of an acid was given by Arrhenius. In his definition, he said that an acid is any substance that can produce hydrogen ions in solution. This is not the only definition of acids but this particular definition fits well into the discussion that we are having today. We shall look at other definitions of acids subsequently. The hydrogen ion that I am talking about here is sometimes referred to as a proton and has the symbol H+.
The hydrogen ion is called the proton because it has a charge of +1 just like the positive charge that is found in the nucleus of an atom. You would recall that one of the fundamental subatomic particles in the nucleus of an atom is called the proton and it has a charge of +1.
Now let us look at the basis that would lead us to another possible mathematical definition of the acid dissociation constant Ka. If we again consider our hypothetical acid HA, this acid can act as an acid only when it is dissolved in water. The equation that shows the dissolution of the acid in water is;
HA(aq) + H2O (l)--> H3O^+(aq) + A-(aq).
Aha! We can see that the hydrogen ion (proton) from the acid can interact with water to form a particular specie that we haven’t been familiar with until this time, H3O+. This specie is named the hydroxonium ion, hydronium ion or simply the oxonium ion.
This ion is formed by dative or coordinate covalent bonding between water and the proton from the acid. Recall that there are two kinds of covalent bonding;
• Ordinary covalent bonding where each of the bonding species contributes one electron each to the shared pair.
• Coordinate covalent or dative bonding where only one of the bonding species contributes the electrons that are shared in the bond between the two atoms. I have attached images that show ordinary covalent bonds (in CH4 and HCl) and dative bonding (in H3O+) to this post.
Let us remember that oxygen has two lone pairs of electrons. A coordinate covalent bond occurs when a specie that has a lone pair of electrons can share that lone pair with a specie that is lacking in electrons so that the both species participating in the bond can fulfill the requirement of the octet rule.
According to the octet rule, an atom must have two or eight electrons in its outermost shell in order to be stable. This is why helium and neon belong to the noble gas group (group 18) and do not easily participate in a chemical reaction. Neon and helium are believed, along with other noble gases to have fulfilled the requirement for stability of atoms hence they do not need to participate in reaction. The octet rule that I have described here is believed to be the driving force for all chemical reactions.
A lone pair of electrons is an electron pair in an atom that is not involved in bonding. In other words, it is a pair of electrons that is attracted to the nucleus of only one atom instead of being attracted to the nucleus of two atoms as we have in a shared pair of electrons.
If we look at the image that is attached showing the coordinate bonding in H3O+, we would notice that there are two electron pairs in oxygen that do not participate in the formation of water. The lone pair is attracted only to the nucleus of oxygen unlike the shared pair that is attracted to the nucleus of hydrogen and oxygen alike. One of these lone pairs can now donate an electron pair to hydrogen ion (H+) which has no electron (note that hydrogen has only one electron and it is lost when H+ is formed so H+ has no electron at all). In that case, the specie H3O+ would now consist of three atoms of hydrogen and one atom of oxygen that all obey the octet rule as we can see from the images attached.
Let me state that the formation of H3O+ in solution is the reason for acid properties. Hence an acid cannot display the properties of an acid until it is dissolved in water and H3O+ is formed. This is why HCl in gaseous state do not display acidic properties. In solution, the acid, HA do not exist as H+ and A- but as H3O+ and A- as shown above. Showing the positive ion in acid as H+ is just an oversimplification to aid easy comprehension at the beginning of our discussion.
We have laid the most important foundation that would lead us to an alternative but related definition of Ka and we would continue from there tomorrow. See you then!
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