Self Ionization of water

 Hello fam! We are here again for another lesson. I believe that we have been learning a lot from the discussions that we have been having on this page. If you have been benefitting from our lessons here, kindly like the page and share to help us reach a wider audience. It has been my privilege to lead us on this voyage of learning chemistry with simplicity and ease.


Yesterday, we were able to see that the products of an acid base reaction were in themselves conjugate acids or bases. We can then write a general reaction in the form;

HA(aq) + :BH (aq)-->BH2^+(aq) + A^-(aq)

Acid    Base     

BH2^+ may become :BH by loss of a proton hence it is the conjugate acid of :BH.

A^- could become HA by accepting a proton hence it is the conjugate base of HA.


Is this fact clear? I believe that repetition is the key to assimilation. That’s why I have reiterated this fact many times over and I have taken my time to emphasize and reemphasize on key concepts that I want you to internalize.


I told you a few days ago that water could act both as an acid and as a base. Now I want to introduce you to a concept that is called the self ionization of water. This concept would prove to be very useful soon hence you need it in your tool kit.


Let us look at the following reaction;

H2O(l) + H2O (l)-->H3O^+(aq) + OH^-(aq)



Clearly, this reaction involves two water molecules. One water molecule is acting as an acid (by donating a proton) while the other water molecule is acting as a base (by accepting a proton). We can write the equilibrium expression for this reaction as follows;

Kw = [H3O^+] [OH^-]/[H2O]^2

Remember that liquids do not appear in the equilibrium expression (Refer to the introductory lesson on equilibrium on 19/06/2023 on the page timeline).

Thus we have that;

Kw =  [H3O^+] [OH^-]


The process just described is called the self ionization of water or AUTOPROTOLYSIS and this self ionization only occur to a small extent in water. This reaction above justifies the fact that water is AMPHIPROTIC, that is, water could act as an acid or base depending on the unique circumstances of the reaction(This was demonstrated in the lesson on 3/7/2023 on the page timeline). 


Experimentally at 25 degrees Celsius(room temperature), it has been determined that in pure water;

[H3O^+] = 1 *10^-7 M

[OH^-] = 1 * 10^-7 M

Then it follows that;

Kw = (1 *10^-7) (1 *10^-7)

Kw = 1 * 10^-14

The value 1 * 10^-14 is otherwise known as the ion product of water.


One important thing that we should take away from this lesson is that the concentrations of oxonium ion and hydroxide ions are equal in water. This is our definition of neutrality.


Salient point:

Water is neutral because;

[H3O^+] = [OH^-] = 1 * 10^-7 M


Hope you enjoyed our lesson today?

Follow  our page  and share!

Do you have assignments, term papers, projects and difficult homework questions? Contact us for help now! We also offer home and remote tutoring services to students at all levels in science subjects.



Comments

Post a Comment

Popular posts from this blog

Strong and weak Bases

Image
 Hello! Welcome to today’s lesson. Today, we would consider the idea of strong and weak bases. We have previously considered the idea of strong and weak acids(see the page timeline). We said that a strong acid is an acid that dissociates completely in solution while a weak acid is an acid that does not dissociate completely in solution. We said that the percentage dissociation of a weak acid may be as low as 0.3% at about 1 M concentration of the solution. On the other hand, a strong acid may attain a 100% dissociation in solution. A similar scenario applies to bases also. A strong base is a base that dissociates completely in solution. A weak base is a base that does not dissociate completely in solution. When we talk about the dissociation of a specie, we are talking about the break up of the specie into its constituent ions. Now consider the hypothetical base HB. The base would be dissociated as follows; HB(aq)  --> H^+(aq) + B^-(aq) This dissociation may also be called ...
Read more

pH range of color change for indicators

Image
 The pH range of color change for an indicator is the range of pH values over which the indicator changes color. Different indicators have different pH ranges of color change. Let me quickly remind you of something that we established in the last lesson. We saw that the pH at which the color change occurs must be the point at which pH = pKin (check the page timeline for details of the previous lesson). At this point, the color change is observed in the system and the color change usually spans through two pH units. It then follows that if we know the pKin of the indicator, we can be able to establish the pH range by counting two pH units around the value of pKin of the indicator. Let me also state that the pKin is the same as the pKa of the indicator. Let us look at phenolphthalein or instance; the pKa (pKin) is 9, the pH range is 8 – 10. For methyl orange; the pKa(pKin) is 3.5 while the pH range is 3 – 5. Finally let me say this; the pH range of color change for an indicator is de...
Read more

Lewis Acids

Image
 Hello, welcome to today’s lesson. I believe that this series is making a lot of sense to you. Yesterday, we showed that in order to have a substance behave as a Lewis acid, then the substance would have to possess a central atom that has an empty orbital into which it can be able to receive an electron pair. We can never stop repeating this fact because it is the most fundamental fact that you need to know about Lewis acids. We also showed that BF3 and AlCl3 could all act as Lewis acids because we established the fact that there is a free 2pz and 3pz orbital on the central boron and aluminum atoms respectively. This is the orbital into which the electron pair is received when the BF3 or AlCl3 acts as a Lewis acid. Here we have shown the reaction of ammonia with BF3 and AlCl3. In either case, we can see that what is going on actually the formation of a dative or coordinate covalent bond (Refer to the earlier lesson on the formation of H3O^+ ion). The ammonia molecule has a lone pa...
Read more