Brønsted–Lowry acids and bases | Chemical reactions | AP Chemistry | Khan Academy

Summary

This video script delves into the formal definition of acids, focusing on the Bronsted-Lowry theory from the 1920s. It explains that an acid is a proton or hydrogen ion donor, while a base is a proton acceptor. Using hydrochloric acid as an example, the script illustrates the acid-base reaction in an aqueous solution, highlighting the formation of hydronium ions from protons and water, and the resulting chloride anions. The explanation clarifies the dual role of water as both an acid and a base, and the concept of conjugate acids and bases.

Takeaways

• 🔬 The video discusses the formal definition of an acid, focusing on the Bronsted-Lowry definition.
• 🌟 An acid, according to Bronsted-Lowry, is a proton (H+) or hydrogen ion donor.
• 💧 A base is defined as a proton (H+) or hydrogen ion acceptor.
• 🧲 The video explains that a proton and a hydrogen ion are essentially the same, especially in the context of the most common hydrogen isotope.
• 🌊 Hydrochloric acid (HCl) is used as an example of a strong acid that readily donates protons in an aqueous solution.
• 💧 The process of HCl donating a proton to a water molecule is detailed, resulting in the formation of hydronium (H3O+) and chloride (Cl-) ions.
• 🔁 Water acts as a base in this reaction, accepting a proton from the acid, and can also act as an acid or base depending on the circumstances.
• 🔄 The reaction of HCl in water is a conjugate reaction, producing the conjugate base (Cl-) and the conjugate acid (H3O+).
• 📚 The video script clarifies that while sometimes hydrogen ions are depicted as free in aqueous solutions, they actually form hydronium ions when combined with water molecules.
• 📖 The script emphasizes the importance of understanding the Bronsted-Lowry definition of acids and bases for a foundational understanding of chemistry.

Q & A

What is the main focus of the video script?

The video script focuses on providing a formal definition of an acid, specifically using the Bronsted-Lowry definition of acids and bases.

Who are Bronsted and Lowry, and what did they contribute to the field of chemistry?

Bronsted and Lowry are chemists who came up with the Bronsted-Lowry definition of acids and bases in the 1920s.

According to the Bronsted-Lowry definition, what is considered an acid?

An acid, according to the Bronsted-Lowry definition, is a proton or hydrogen ion donor.

Why are a proton and a hydrogen ion considered the same thing?

A proton and a hydrogen ion are considered the same because the most common isotope of hydrogen consists of just a proton in its nucleus with no neutron, and when ionized, it loses its electron, leaving only the proton.

What is the role of a base in the Bronsted-Lowry definition?

A base, in the Bronsted-Lowry definition, is a proton or hydrogen ion acceptor.

Can you provide an example of a strong acid mentioned in the script?

Hydrochloric acid (HCl) is an example of a strong acid mentioned in the script.

What happens when hydrochloric acid is placed in an aqueous solution?

In an aqueous solution, hydrochloric acid donates a proton to a water molecule, forming a chloride anion and a hydronium ion.

What is the role of water in the reaction involving hydrochloric acid in an aqueous solution?

In the reaction, water acts as a base by accepting a proton from the hydrochloric acid, forming a hydronium ion.

What are the products of the reaction when hydrochloric acid is dissolved in water?

The products of the reaction are chloride anions and hydronium ions.

What are conjugate acid-base pairs in the context of the reaction described in the script?

The conjugate acid-base pairs in the reaction are the chloride anion (conjugate base of hydrochloric acid) and the hydronium ion (conjugate acid of water).

Why do the protons in an aqueous solution not exist by themselves?

Protons in an aqueous solution do not exist by themselves because they are immediately grabbed by a water molecule to form hydronium ions.

How does the script clarify the dual nature of water as both an acid and a base?

The script clarifies that water can act as both an acid and a base by accepting a proton from hydrochloric acid (acting as a base) and also being able to donate protons under the right circumstances (acting as an acid).

Outlines

00:00

🔬 Introduction to Bronsted-Lowry Definition of Acids

This paragraph introduces the concept of acids with a focus on the Bronsted-Lowry definition, which is the most commonly used. The video explains that an acid is a proton (hydrogen ion) donor, and a base is a proton (hydrogen ion) acceptor. The concept is illustrated using hydrochloric acid (HCl) as an example of a strong acid. When HCl is placed in an aqueous solution, it donates a proton to a water molecule, forming a chloride anion and a hydronium ion, which is the conjugate acid of water. The paragraph also clarifies that in an aqueous solution, protons do not exist freely but are instead immediately bonded with water molecules to form hydronium ions.

05:00

💧 Hydrochloric Acid Reaction in Aqueous Solution

This paragraph delves deeper into the reaction of hydrochloric acid in water. It describes how the hydrogen from HCl forms a covalent bond with the oxygen in a water molecule, resulting in the formation of a hydronium ion (H3O+) and a chloride anion (Cl-). The video emphasizes that water can act as both an acid and a base, depending on the context. The reaction is depicted as favoring the formation of hydronium and chloride ions, indicating the strength of hydrochloric acid as a proton donor. The paragraph concludes by reinforcing the importance of understanding the formation of hydronium ions in aqueous solutions rather than just considering free protons.

Keywords

💡Acid

An acid is defined by the Bronsted-Lowry definition as a substance that donates a proton, which is equivalent to a hydrogen ion (H+). In the context of the video, the focus is on the Bronsted-Lowry definition, which is central to understanding the chemical behavior of acids. For example, hydrochloric acid (HCl) is highlighted as a strong acid that donates a proton in an aqueous solution, leading to the formation of chloride anions and hydronium ions.

💡Base

A base, according to the Bronsted-Lowry definition, is a substance that can accept a proton. The video explains that a base is a proton acceptor and uses water as an example, illustrating how it can act as a base by accepting a proton from hydrochloric acid, forming a hydronium ion (H3O+).

💡Proton

A proton is the nucleus of a hydrogen atom, which consists of one proton and no neutrons. In the script, the term 'proton' is used to describe the hydrogen ion (H+) that is donated by an acid. The video clarifies that when a hydrogen atom loses its electron, it becomes a proton, which is the entity that acids donate.

💡Hydrogen Ion

A hydrogen ion is essentially a proton, which is the nucleus of a hydrogen atom without its electron. The video script uses 'hydrogen ion' and 'proton' interchangeably to describe what acids donate in chemical reactions, emphasizing that the loss of an electron from a hydrogen atom results in a hydrogen ion.

💡Bronsted-Lowry Definition

The Bronsted-Lowry definition is a concept in chemistry that classifies substances as acids or bases based on their ability to donate or accept protons. The video script focuses on this definition to explain the behavior of acids and bases, particularly highlighting how hydrochloric acid acts as a proton donor in an aqueous solution.

💡Hydrochloric Acid

Hydrochloric acid (HCl) is a strong acid that is used in the video as an example to illustrate the concept of acid-base reactions. The script describes how HCl donates a proton in an aqueous solution, forming chloride anions and demonstrating the behavior of a strong acid.

💡Aqueous Solution

An aqueous solution is a solution in which the solvent is water. The video script mentions an aqueous solution to describe the medium in which the acid-base reaction takes place, emphasizing that the hydrogen ions from hydrochloric acid react with water molecules to form hydronium ions.

💡Chloride Anion

A chloride anion is a negatively charged ion formed when chlorine gains an extra electron. In the script, the chloride anion is the result of hydrochloric acid donating a proton to a chlorine atom, which then has a negative charge, illustrating the formation of the conjugate base of the acid.

💡Conjugate Base

A conjugate base is the species formed after an acid donates a proton. The video script explains that when hydrochloric acid donates a proton, the resulting chloride ion is the conjugate base, highlighting the relationship between the acid and its corresponding base form.

💡Conjugate Acid

A conjugate acid is the species that can donate a proton to become the original acid. The script introduces the concept of a conjugate acid by describing the hydronium ion (H3O+) as the conjugate acid of water, which can potentially donate a proton in other reactions.

💡Hydonium Ion

A hydronium ion is a product of the reaction between a hydrogen ion and a water molecule. The video script uses hydronium ions to illustrate the actual species present in an aqueous solution when acids like hydrochloric acid donate protons, emphasizing that these ions are more representative of the solution's composition than free protons.

Highlights

Introduction to the concept of acids beyond everyday understanding.

Focus on the Bronsted-Lowry definition of acids and bases.

Historical context: Bronsted-Lowry definition developed in the 1920s.

Acid defined as a proton (hydrogen ion) donor.

Explanation of the equivalence of a proton and a hydrogen ion in the context of common hydrogen isotopes.

Base defined as a proton (hydrogen ion) acceptor.

Use of hydrochloric acid as an example of a strong acid.

Illustration of hydrochloric acid ionization in an aqueous solution.

Role of water molecules in the acid-base reaction.

Conversion of hydrochloric acid to chloride anion and hydronium ion in water.

Identification of the acid and base in the hydrochloric acid and water reaction.

Concept of water acting as both an acid and a base.

Explanation of conjugate acid-base pairs in the reaction.

Different representation of hydronium ion formation in some textbooks.

Clarification on the actual process of protons forming hydronium ions in aqueous solutions.

Emphasis on the importance of understanding the true nature of reactions in aqueous solutions.

Summary of the Bronsted-Lowry definition and its implications for understanding acid-base chemistry.

Transcripts

- [Voiceover] You've probably heard the term acid

used in your everyday life.

But what we want to do in this video

is get a more formal definition of an acid.

And particular, we'll focus on the one

that is most typically used.

Although we'll see future videos that there's

other fairly common definitions of acids used as well

beyond the one that we're going to see here.

But the one that we're going to focus on

is the Bronsted-Lowry definition.

The Bronsted-Lowry definition of acids and bases.

And this is a picture of Bronsted.

This is a picture of Lowry.

And they came up with this acid-base definition

in the 1920s.

So, we're going to do the Bronsted-Lowry,

Bronsted-Lowry definition,

definition of acids and bases.

So, according to them, according to them,

an acid, an acid is a proton,

proton,

or instead of writing proton we could actually write

hydrogen ion donor.

So why is a proton and a hydrogen ion the same thing?

Well, in the most common isotope of hydrogen,

we would, in it's nucleus, we would find

just a proton and no neutron.

And if it's neutral, you would have an electron

buzzing around, jumping around in its orbital.

So, you would have it's electron jumping around

in its orbital.

But if you were to ionize it,

you're getting rid of its electron.

So, if you're getting rid of it's electron,

so, if you're getting rid of this,

all you're going to be left with is a proton.

So that's why a proton, an H plus,

is usually referring to the exact same,

is referring to the exact same thing.

So, that's what an acid is.

So what would a base be?

Well, you could imagine by this definition

A base, a base would be a proton,

would be a proton, or you could say

a hydrogen ion acceptor, acceptor.

So let's make this a little bit more tangible

with some examples.

So one of the stronger acids we know

is hydrochloric acid.

Let me, let me draw.

So, it's a hydrogen having a, having a covalent bond.

Having a covalent bond with chlorine.

With chlorine, with chlorine right over there.

And if we want to,

let's draw actually chlorine's lone pairs.

So outside of the electron that is contributing

to this pair in the covalent bond.

It also has, it also has three other lone pairs.

It also has three other lone pairs, just like that.

So, if you were to take hydrochloric acid,

place it in an aqueous solution,

so it's in an aqueous solution right over here.

And actually an aqueous solution,

you'll see this written like that.

That just means it's in a solution of water.

So you could write like this, you could write hey,

hydrochloric acid in an aqueous solution

if you want to make it a little bit more explicit.

You could say hey, look, this is going to be

around some water molecules in its liquid form.

Aqueous solution just means it's dissolved in liquid water.

So, some water molecules in their liquid form.

So, this is a water molecule.

Whoops, water molecule.

Right over here.

So, an oxygen bonded to two hydrogens.

And sometimes you'll see it written like this,

that it's in its liquid, it's in its liquid form.

Well, what do you think is going to happen?

Well, I already said that this is a strong acid

right over here.

So this is going to really want to donate protons.

It's really going to want to donate this hydrogen,

but not let the hydrogen keep its electrons.

So what's likely to happen here?

Well, the both of these electrons in this pair

are going to be grabbed by this chlorine.

And then this hydrogen ion,

because its electron was grabbed,

well this could be nabbed by some water molecule passing by.

Remember, in a real solution,

it's not like they know what to do.

They're just all bumping past each other.

And based on how badly they want to do things,

these reactions happen.

And so you can imagine this lone pair right over here,

well maybe it's able to form a covalent bond

with this hydrogen.

And so what's going to happen?

What's going to happen?

And I'll draw it with just an arrow

because this reaction favorably goes,

very strongly goes to the right,

because this is such a strong acid.

Well, then you're going to be left with,

you're gonna be left with,

the chlorine is now going to have its three lone pairs

that it had before.

And then it also grabbed these two electrons

right over here.

It also grabbed those two electrons right over there,

so it gained an extra electron.

It now has a negative charge.

It is now the chloride anion.

So it has a negative charge.

And what about this water molecule?

Well this water molecule,

you have your oxygen,

you have your hydrogens, you have your hydrogens,

but now you don't just have two hydrogens,

you grabbed this hydrogen right over here.

And maybe I'll do this hydrogen

in a slightly different color

so that you could keep track of it.

You have this hydrogen right over there.

And this lone pair, this lone pair you can view it

as now forming this covalent bond.

You had your other two covalent bonds

to the other two hydrogens.

And then you still have this lone pair right over here.

You still have that lone pair sitting right over there.

And what just happened?

Well, this water molecule just gained a proton.

This hydrogen did not come with an electron.

So if you just gain a proton, you are now,

if you were neutral before,

you are now going to have a positive charge.

So what just happened?

You put hydrochloric acid in a water solution,

in an aqueous solution,

this thing has donated a proton to a water molecule.

And so, what is the acid and what is the base here?

Well, when we look at the reaction this way,

we see that this is the acid,

the hydrochloric acid,

it's literally called hydrochloric acid.

And here, water is acting as a base.

Water is acting as a base.

And as you could see,

water can actually act as an acid or a base.

So, water is acting as a base.

Now you might be saying, okay,

this reaction goes strongly to the right,

hey, but like you know, I could imagine

in certain circumstances where chloride might accept

a proton because it has this negative charge.

And you would be right.

This reaction goes strongly to the right,

but once an acid has donated its proton,

the thing that is left over,

this is called a conjugate base.

And I'll do the same color.

So, this is the conjugate base of hydrochloric acid.

The chloride anion.

Conjugate, conjugate base

of hydrochloric acid.

And this right over here is the conjugate acid

because you could imagine this hydronium ion,

this could, under the right circumstances,

donate protons to other things.

Donate a hydrogen without donating electron to other things.

And so this is actually the conjugate acid of H2O.

Conjugate acid of water,

of a water molecule.

And as we'll see, water can act as an acid or a base.

But this this gives you a kind of a baseline of at least

the Bronsted-Lowry definition of acids and bases.

And actually, one other thing I want to add.

In some books here, so over here I said, hey,

put this in an aqueous solution you're gonna form

some hydronium, sometimes you'll see it written like this.

And I'll just write it a little bit, a little bit,

sometimes you'll see it like this.

So you have your hydrochloric acid,

and I won't draw the details this time,

in an aqueous solution.

So it's in a solution of water.

And they'll just draw the reaction going like this,

where they say hey, you're gonna be left with,

you're gonna be left with some hydrogen ions,

these protons.

And you're going to be left with,

and actually we could say

it's gonna be in a aqueous solution,

aqueous solution.

And you're gonna be left with some chloride anions.

Some chloride anions and it's in an aqueous solution.

Now this isn't incorrect, but it's important to realize

what they're talking about when they're talking

about these hydrogen ions right over here.

We know that if you have the hydrogen ions

in an aqueous solution they don't just

hang out by themselves.

They get grabbed by a water molecule

and they form hydronium.

So, it's much more, I guess, it's much more close to

the actual of what's happening,

is if you actually talk about hydronium forming.

As opposed to just the protons.

'Cause these protons in an aqueous solution,

in a water solution, they're gonna be grabbed

by a water molecule to form hydronium.

And that's why I did it the way, this way up here.