Acid and Base Definitions

One of the most challenging aspects of this chapter will be understanding and differentiating between different definitions that all sound kind of similar. We’re gonna move slowly through this, step-by-step.

The Lewis Definition

Concept: The Lewis definition of acids and bases.

Video Transcript

So the next few videos are going to be dedicated completely to acid-base chemistry. And what I’m going to do is I’m just going to guide you through the easiest stuff first which are just the definitions. Remember that in Gen Chem there are a lot of different definitions of acids and bases so we have to remember what those were. And then I’m going to take you through the complex stuff like basically on chemical equilibrium, pKa’s and eventually what you're going to learn to do is predict the direction of a reaction in acid-base equilibrium. And that’s kind of cool and that’s also going to be kind of the ultimate goal of the section: is to teach you how to predict that equilibrium. So let’s get started with the easy stuff, okay?
So first of all, before we get started you need to know your strong six. This is something from Gen Chem that—I usually tell you you don’t need to learn everything from Gen Chem as we go to Orgo but this is one thing you’re never really allowed to forget. If I tell you that you’re reacting with HCl and you don’t know that that’s a strong acid, like come on, you’re slacking real hard. Alright? You should remember your strong acids. Okay? Other than that, everything else I’m going to teach you here today.
So what I want to do is I’m going to start off with the most general definitions of acids and bases and then go to more specific ones. Okay? And it turns out that the most general definition is going to be the Lewis definition. Okay? Now I do want to make one adjustment here. Notice that, in your PDF it looks fine, but in my PDF it got messed up. So I’m just going to write “Base”, “Base”. We’re comparing acids and bases, right? Cool. So I hope that didn’t confuse you too much. Alright.
So the general definition, the most general definition of acids and bases is just going to be the same as what we already learned in electrophiles and nucleophiles. Okay? So basically what that means is remember that I said that nucleophiles have extra electrons—it’s a negative, and then electrophiles are missing electrons. So it’s the same exact thing. What we’re going to say is that an acid is going to be an electron pair acceptor. Okay? So if one of these were to be an electron pair acceptor, which one would that be? Would it be the nucleophile or the electrophile? Well, think about it. The name electrophile means electron-lover. So this one would be the electrophile. Does that make sense? So every time I say electrophile that also means I’m talking about a Lewis acid. Okay? The same exact thing, just a different way to say it. Then a Lewis base is going to be an electron pair donor. Which of these is the donor of electrons? The nucleophile because the nucleophile has extra electrons. So any time I say nucleophile now you know that I’m talking about a Lewis base. Okay?
And now that we’ve linked these two things together now you can even predict what the reactions are going to look like based on what I taught you guys about electrophiles and nucleophiles reacting together.

The Lewis definition is the most general of the definitions that applies to all cases. It’s actually just another name for what we learned earlier as Nucleophiles and Electrophiles.

  • A Lewis Acid is an electron pair ACCEPTOR
  • A Lewis Base is an electron pair DONOR

Brønsted-Lowry Definition

Concept: The Brønsted-Lowry definition of acids and bases.

Video Transcript

Now what I want to talk about is the more specific one which is Brønsted Lowry which only has to do with protons. Okay? So the Brønsted Lowry definition only has to do with protons and what it says is that—maybe this is the one you might remember from Gen Chem—what it says is that Brønsted Lowry acid is going to be a proton donor. You’re going to give away protons. And then a Brønsted Lowry base is a proton acceptor. Okay? Now many at a time, most of the time, most of the time these are going to be the same. Most of the time your Lewis Acid is also going to be a Brønsted Lowry acid. Okay? So I would say more than 90% of the time they’re the same. But sometimes they’re different. And what that means is that sometimes one of these things is going to be true. Maybe it’s an electron-pair acceptor but the other one is not going to be true, that which is a proton donor. Okay. So that’s what we have to learn today.

The Brønsted-Lowry definition is a more specific definition that only applies to some acids and bases. In this definition, electrons don’t matter- we only look at protons.

  • A Brønsted-Lowry Acid is a proton DONOR
  • A Brønsted-Lowry Acid is a proton ACCEPTOR

Concept: Understanding the difference between Lewis and Brønsted -Lowry using 6 examples.


Let’s watch a few examples so you can get the hang of the differences between these definitions:

Heads up: By definition, any Brønsted-Lowry acid is also a Lewis acid. This is because if you are able to donate a proton, you are always able to accept an electron in return!

Concept: Using a diagram to understand the difference between Lewis and Brønsted -Lowry definitions.


General Features of Equilibrium

When determining how acids and bases react together, it important to understand the concepts of conjugates and Ke

Concept: Equilibrium constant and conjugates.

Video Transcript

Now what I finally want to do is end off talking about equilibrium. And this is just a simple pattern that we’re going to use throughout all these reactions. And all it says is this: a base will always attack an acid, so notice I’m already telling you what’s going to happen, a base (negative) will attack an acid (positive) to produce conjugates—scary word, remember that from Gen Chem? Conjugates in the following chemical pattern.
So this is the way it works. Basically you’re always going to get your stronger base and your stronger acids, okay, reacting together to make a weaker base and a weaker acid. Okay? So you always go from stronger to weaker. It just makes sense. You’re never going to go from weaker to stronger. That doesn’t make any sense. Okay. And then what we’re going to do is everything that’s on the right side of the arrow, okay, everything that’s after reaction we’re going to call that a conjugate. Okay? The conjugate means that’s what happens after it reacted. Okay? So basically and then everything beforehand, we call a regular base and a regular acid. Okay? So what that means is that my base is always going to turn into a conjugate acid and my acid is always going to turn into a conjugate base. I know that’s a little bit confusing and a lot of people mix that up. Okay. But that’s just something that you have to remember. Your base is going to turn into a weaker acid and then your acid is going to turn into a weaker base—they’re the ones that are on the other side.
And then also just that the Ke has also to do with equilibrium constant and all that’s saying is that it’s always going to go to the right, basically Ke/1 means that it’s going to the right if your conjugates are weaker. Okay? If your conjugates are weaker then it’s going to the right. If your conjugates are stronger then it would to the left because it’s going the other way. Does that make sense?
Now I know that last part was a little bit rushed but I think we’re going to do tons of practice with this so by the end of today you’re not going to have a problem with conjugates. Okay? So if you have any questions, let me know, but if not let’s move on to the next topic.

Drawing Reactions Based on Definitions

Even though we still have no idea what we are doing, we should be able to use patterns of nucleophiles and electrophiles, coupled with what we learned about how to draw arrows for mechanisms, to predict our first acid-base reactions.

Problem: Which of the following compound(s) cannot be characterized as a Lewis acid?


Problem: Which of the following compounds is most likely to have come from the stronger acid?


Acid and Base Definitions Additional Practice Problems

Identify the Lewis acid and Lewis base in each of the following reactions. 


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In the reaction below, identify the Lewis acid and the Lewis base. 

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Identify the Lewis Base:

A. B(OH)3

B. NH4+

C. CCl4

D. N(CH3)3

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Identify the Lewis acid:

A. AlCl4-

B. NH3

C. BCl3

D. CCl4

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How many atoms and electrons are directly involved in the bond-making and bond-breaking for a Brønsted-Lowry acid-base reaction?

(A) four atoms, four electrons

(B) four atoms, three electrons 

(C) four atoms, two electrons 

(D) three atoms, four electrons 

(E) three atoms, three electrons 

(F) three atoms, two electrons

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Fill in the roles (abbreviated in the parentheses) of the reactants in the boxes.

Which one is the Lewis base ( B)? Which one is the Lewis acid ( A)?

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A Lewis base can also be known as:

a) an electrophile

b) a nucleophile

c) an audiophile

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Circle all the Lewis Bases in the group of compounds below and  explain your answer.


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Select all the Lewis Acids in the group of compounds below:


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Give the term that best matches the given definition. 


a. Bronsted­-Lowry Acid

b. Bronsted-­Lowry Base

c. Lewis Acid

d. Lewis Base

e. Electronegativity

f. Ionic bond

g. Covalent bond

h. Polar Covalent bond

i. Hydrophobic

j. Hydrophilic


1.________: Any species that can accept electrons

2. ________: The ability of an atom to attract the shared electrons in a covalent bond.

3. ________: A compound that can accept a proton.


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For the equilibrium


The two substances which are both acids are:

   a. HC≡CNa and HC≡CH

   b. CH3NH2 and CH3NHNa

   c. CH3NH2 and HC≡CH

   d. HC≡CNa and CH3NH2

   e. HC≡CNa and CH3NHNa


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Which of the following statements is true?

a. BH3 is a Lewis acid

b. H3CN(CH2CH3)2 is a Lewis base

c. Pyridine is a Lewis acid

d. Both A and B are true, but not C

e. Both A and C are true, but not B

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