Definition of Acid 1

 Today is yet another interesting study as we continue to talk about acids. I hope we all have been learning something and I anticipate your feedback via the comments section, messenger, WhatsApp and text messages/calls.

In one of the previous lessons, I mentioned something about the Arrhenius definition of acids. I did write that the Arrhenius definition of acids is only one of the definitions of acids that exist. Can you remember the Arrhenius definition of acids? I would remind you in the course of this lesson in case you have forgotten. You may also want to go over the earlier lessons to have a quick recap of that concept.

So let’s begin! How many definitions of acid are there? We shall find out in a jiffy! 

1) Arrhenius definition: Let me take you back to something we talked about sometime ago. The Arrhenius definition looks at an acid as a substance that produces hydrogen ion as its only positive ion when dissolved in water. So far we have only looked at acids from the Arrhenius perspective. We have always written equations like;

HA(aq) --> H^+(aq) +A^-(aq). This implies that the acid MUST contain hydrogen (a proton) in order to be classified as an acid. The reference medium where acidity is measured must also be water because this definition can not be applied to nonaqueous media (media that does not contain water). This last sentence is the limitation of this definition of acids. Can’t we define acids outside aqueous media? Let’s continue the journey. Maybe we will soon find the answer to our question.

2) Brønsted– Lowry Definition: Remember that I told you that the definition proposed by Arrhenius is limited in the sense that it does not explain the behavior of acids in nonaqueous media such as liquid ammonia, liquid sulfur dioxide and liquid BF3. Let us retain the Arrhenius idea that acids must contain hydrogen but in this case, the medium could be aqueous or non aqueous. Let us assume that dry HCl gas was dissolved in liquid ammonia. Would dry HCl gas show acid properties in that medium? Well, it depends on the kind of reaction that occurs between the two substances when they interact with each other.

Ammonia has a lone pair of electrons which can be donated into an empty orbital of the hydrogen ion from HCl(Refer to the lesson on the formation of H3O^+ for details on this). Thus, we can see that the HCl donates a proton to ammonia because ammonia does have a non bonding pair of electrons localized on the nitrogen atom which is able to accept the proton and donate electrons into its empty orbital leading to the formation of a dative covalent bond between the two (refer to the image attached  for more clarification).

Thus when we write the reaction equation;

NH3(l) + HCl(g) --> NH4^+(l) + Cl^-(l) we also have HCl acting as an acid since it can donate a hydrogen ion (proton).

So how does Brønsted & Lowry define an acid? 

AN ACID IS A SUBSTNACE WHICH IS ABLE TO DONATE A PROTON. Can you see this definition playing out in the reaction above?

Exercise: Now determine if there would be acid property displayed in the Brønsted– Lowry sense when considering the following reactions;

1) Reaction of methanol and HNO3

2) Reaction of H2S and NH3

Remember to write down the reaction equation as well as draw an arrow showing the proton transfer where applicable.

Let me know your answers to these questions before the lesson tomorrow morning (7 am WAT). We would then look at the questions together and then proceed to discuss the Lewis definition of acids.

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