Arrhenius definition of acids and bases | Biology | Khan Academy

Summary

This script delves into the modern concept of acids and bases, introduced by Svante Arrhenius, a Nobel laureate in Chemistry. It explains that acids increase hydrogen ion concentration in aqueous solutions, exemplified by hydrochloric acid forming hydronium ions. Bases, like sodium hydroxide, increase hydroxide concentration, dissociating into sodium ions and hydroxide anions in water. The explanation invites comparison with other definitions of acids and bases, such as the Bronsted-Lowry and Lewis theories.

Takeaways

• 🔬 Svante Arrhenius was the third recipient of the Nobel Prize in Chemistry in 1903 and developed the modern concept of acids and bases.
• 🧪 According to Arrhenius, an acid is a substance that increases the concentration of hydrogen protons (H+) when dissolved in an aqueous solution.
• 💧 A base, under the Arrhenius definition, increases the concentration of hydroxide ions (OH-) when dissolved in an aqueous solution.
• 📈 Hydrochloric acid (HCl) is an example of a strong Arrhenius acid, as it disassociates completely in water to release hydrogen ions and chloride ions.
• 💥 The hydrogen proton released by HCl doesn't exist alone in water; it bonds with a water molecule to form hydronium ions (H3O+).
• ⚡ The strong disassociation of hydrochloric acid in water is what makes it a strong acid by the Arrhenius definition.
• 🧱 Sodium hydroxide (NaOH) is an example of a strong Arrhenius base, as it disassociates completely in water to release sodium ions and hydroxide ions.
• 🔗 Sodium hydroxide has an ionic bond between the sodium ion (Na+) and the hydroxide ion (OH-), which disassociates in water.
• 🌊 When sodium hydroxide is dissolved in water, it increases the hydroxide ion concentration, making it a strong base according to Arrhenius.
• 🧠 The video encourages comparing the Arrhenius definition of acids and bases with the Bronsted-Lowry and Lewis definitions to better understand the categorization of substances.

Q & A

Who introduced the modern conception of acids and bases?

The modern conception of acids and bases was introduced by Svante Arrhenius, who was the third recipient of the Nobel Prize in Chemistry in 1903.

According to Arrhenius, what defines an acid?

According to Arrhenius, an acid is something that increases the concentration of hydrogen protons (H+) when put in an aqueous solution.

How does Arrhenius define a base?

Arrhenius defines a base as something that increases the concentration of hydroxide ions (OH-) when put in an aqueous solution.

What happens when hydrochloric acid (HCl) is put in an aqueous solution?

When hydrochloric acid (HCl) is put in an aqueous solution, it dissociates into hydrogen protons (H+) and chloride ions (Cl-), increasing the concentration of hydrogen ions in the solution.

Why is hydrochloric acid considered a strong acid by the Arrhenius definition?

Hydrochloric acid is considered a strong acid by the Arrhenius definition because it significantly increases the concentration of hydrogen protons (H+) in an aqueous solution.

What happens to hydrogen protons (H+) in water?

In water, hydrogen protons (H+) bond with water molecules to form hydronium ions (H3O+), rather than existing as free protons.

What is the chemical reaction that occurs when hydrochloric acid is dissolved in water?

When hydrochloric acid is dissolved in water, the reaction can be written as: HCl + H2O → H3O+ + Cl-, where HCl dissociates to form hydronium ions (H3O+) and chloride ions (Cl-).

What defines a strong Arrhenius base?

A strong Arrhenius base is defined as a substance that significantly increases the concentration of hydroxide ions (OH-) when dissolved in water.

What happens when sodium hydroxide (NaOH) is dissolved in water?

When sodium hydroxide (NaOH) is dissolved in water, it dissociates into sodium ions (Na+) and hydroxide ions (OH-), increasing the hydroxide ion concentration in the solution.

How does sodium hydroxide (NaOH) bond at the molecular level?

Sodium hydroxide (NaOH) consists of a sodium ion (Na+) that has lost an electron, giving it a positive charge, and a hydroxide ion (OH-) that has a negative charge. These ions are attracted to each other, forming an ionic bond.

Outlines

00:00

🔬 Introduction to Arrhenius Theory of Acids and Bases

This paragraph introduces the modern concept of acids and bases as defined by Svante Arrhenius, the 1903 Nobel Prize winner in Chemistry. It explains that under Arrhenius' definition, an acid is a substance that increases the concentration of hydrogen ions (protons) in an aqueous solution, while a base is one that increases the concentration of hydroxide ions. The paragraph uses hydrochloric acid as an example of a strong Arrhenius acid, detailing its dissociation in water to form hydrogen ions and chloride ions. It also explains the formation of hydronium ions as a result of hydrogen ions bonding with water molecules. The paragraph concludes with the definition of a strong Arrhenius base, exemplified by sodium hydroxide, setting the stage for further exploration of acid-base theories.

05:00

🧪 Dissection of Sodium Hydroxide as a Strong Base

The second paragraph delves deeper into the concept of a strong base according to Arrhenius' definition, using sodium hydroxide as an example. It describes the molecular structure of sodium hydroxide, highlighting the ionic bond between the positively charged sodium ion and the negatively charged hydroxide ion. The paragraph explains the dissociation process of sodium hydroxide in an aqueous solution, resulting in the separation of sodium ions and hydroxide ions. It emphasizes the increase in hydroxide ion concentration in the solution, which qualifies sodium hydroxide as a strong Arrhenius base. The summary encourages further exploration and comparison with other definitions of acids and bases, such as the Bronsted-Lowry and Lewis definitions, to gain a comprehensive understanding of acid-base chemistry.

Keywords

💡Svante Arrhenius

Svante Arrhenius was a Swedish scientist who is known for being the third recipient of the Nobel Prize in Chemistry in 1903. He introduced one of the earliest modern concepts of acids and bases, defining an acid as a substance that increases the concentration of hydrogen ions (protons) in an aqueous solution. His work laid the foundation for our current understanding of acid-base chemistry.

💡Arrhenius Acid

An Arrhenius acid is a substance that, when dissolved in water, increases the concentration of hydrogen ions (H+). This concept is central to the video, which uses hydrochloric acid (HCl) as an example. In an aqueous solution, HCl dissociates into hydrogen ions and chloride ions, thereby increasing the hydrogen ion concentration and making the solution acidic.

💡Arrhenius Base

An Arrhenius base is defined as a substance that increases the concentration of hydroxide ions (OH-) when dissolved in water. The video uses sodium hydroxide (NaOH) as an example, explaining how it dissociates into sodium ions and hydroxide ions in an aqueous solution, thereby increasing the hydroxide ion concentration and making the solution basic.

💡Aqueous Solution

An aqueous solution is a solution in which water is the solvent. In the video, it is explained that both acids and bases dissociate into ions when dissolved in water, forming an aqueous solution. For example, hydrochloric acid dissociates in water to form hydrogen ions and chloride ions, while sodium hydroxide dissociates to form sodium ions and hydroxide ions.

💡Hydrogen Proton

A hydrogen proton is essentially a hydrogen ion (H+), which is a hydrogen atom that has lost its electron, leaving only the nucleus, which is a single proton. The video highlights that Arrhenius acids increase the concentration of hydrogen protons in an aqueous solution. For instance, hydrochloric acid releases hydrogen protons when dissolved in water.

💡Hydroxide Ion

A hydroxide ion (OH-) is a negatively charged ion made up of one oxygen and one hydrogen atom. It is produced when bases, such as sodium hydroxide, dissociate in water. The video explains that an Arrhenius base increases the concentration of hydroxide ions in an aqueous solution, which is crucial for determining the basicity of the solution.

💡Dissociation

Dissociation refers to the process by which a compound breaks down into its component ions when dissolved in a solvent, such as water. The video discusses how both acids and bases dissociate in aqueous solutions. For example, hydrochloric acid dissociates into hydrogen and chloride ions, and sodium hydroxide dissociates into sodium and hydroxide ions.

💡Hydronium Ion

A hydronium ion (H3O+) is formed when a hydrogen ion (H+) bonds with a water molecule (H2O). The video describes how, in an aqueous solution, free hydrogen protons do not exist in isolation but quickly combine with water to form hydronium ions. This process is essential to understanding the acidic nature of solutions.

💡Chloride Ion

A chloride ion (Cl-) is a negatively charged ion formed when chlorine gains an electron. In the video, when hydrochloric acid (HCl) dissociates in water, it produces chloride ions and hydrogen protons. The chloride ion remains in the aqueous solution and is essential in maintaining the overall charge balance.

💡Ionic Bond

An ionic bond is a type of chemical bond formed between two ions with opposite charges. The video explains how sodium hydroxide (NaOH) consists of sodium ions (Na+) and hydroxide ions (OH-) that are held together by ionic bonds. In an aqueous solution, these bonds break, leading to the dissociation of NaOH into its constituent ions.

Highlights

Svante Arrhenius was the third recipient of the Nobel Prize in Chemistry in 1903 and introduced the modern conception of acids and bases.

According to Arrhenius' definition, an acid is something that increases the concentration of hydrogen protons when put in an aqueous solution.

A base, under Arrhenius' definition, increases the hydroxide concentration when put in an aqueous solution.

Hydrochloric acid (HCl) is a strong acid by Arrhenius' definition, as it increases the amount of hydrogen ions (protons) in an aqueous solution.

In an aqueous solution, hydrochloric acid dissociates into hydrogen protons and chloride ions, increasing the concentration of protons.

Hydrogen protons in water bond with water molecules to form hydronium ions (H3O+).

Arrhenius' definition of acids also includes the increase of hydronium ion concentration in an aqueous solution.

Hydrochloric acid is considered a strong acid because it significantly increases the concentration of hydronium ions in water.

Sodium hydroxide (NaOH) is an example of a strong base by Arrhenius' definition.

Sodium hydroxide dissociates in water into sodium ions (Na+) and hydroxide ions (OH-), increasing the hydroxide concentration in the solution.

The sodium ion (Na+) has a positive charge, while the hydroxide ion (OH-) has a negative charge, forming an ionic bond in sodium hydroxide.

When sodium hydroxide is dissolved in water, it dissociates completely, making it a strong base according to the Arrhenius definition.

Arrhenius' definition of acids and bases focuses on the increase in concentration of hydrogen ions (for acids) and hydroxide ions (for bases) in aqueous solutions.

Hydrogen chloride in water increases hydronium ions, demonstrating the behavior of a strong acid.

The transcript encourages comparing Arrhenius' definition with the Bronsted-Lowry and Lewis definitions of acids and bases for a deeper understanding.

Transcripts

- [Voiceover] The first, I guess you could say,

modern conception of an acid and base

comes from this gentleman right over here,

Svante Arrhenius, and he was actually

the third recipient of the Nobel Prize in Chemistry in 1903.

And his definition of acids, under his definition

of acids and bases, an acid is something that increases

the concentration, increases

the concentration, concentration

of Hydrogen protons,

and we can say protons

when put in an aqueous solution,

when in aqueous, aqueous solution,

and that's just a water solution.

And then you can imagine what a base would be.

You could think, oh maybe a base is something

that decreases the protons and that's

one way to think about it.

Or you could say, it decreases,

or actually let me write this, it increases

the hydroxide concentration.

It increases the hydroxide concentration.

when put in aqueous solution.

When in aqueous, aqueous solution.

So let's make that concrete.

Let's look at some examples.

So a strong Arrhenius acid, and actually,

this would be a strong acid by other definitions as well,

would be hydrochloric acid.

Hydrochloric acid,

you put it in a an aqueous solution.

So that's the hydrogen.

You have the chlorine.

You put it in an aqueous solution.

You put it in an aqueous solution,

it will readily disassociate.

This is a, this reaction occurs,

strongly favors moving from the left to right.

You're going to have the chlorine strip off

the two electrons in the covalent bond with the hydrogen,

leaving the hydrogen with no electrons,

so the hydrogen is just going

to be left as a hydrogen proton.

And then the chlorine,

the chlorine has just nabbed that electron.

It had the electrons it had before, and then it just nabbed

an electron from the hydrogen, and so it now has

a negative charge, and these are

both in aqueous solution still.

It's still, they're still both dissolved in the water.

And so you see very clearly here,

you put this in an aqueous solution,

you're going to increase the amount of,

you're going to increase the amount of hydrogen ions,

the amount of protons in the solution.

And we've talked about this before,

you'll often see a reaction written like this,

but the hydrogen protons, they just don't sit there

by themselves in the water.

They are going to bond with the water

molecules to actually form hydronium.

So another way that you'll often see this is like this.

You have the hydrochloric acid,

hydrochloric acid.

It's in an aqueous solution,

just a fancy way of saying it's dissolved in the water,

and then you have the H2O.

You have the water molecules,

H2O, and you'll sometimes see written,

okay, it's in its liquid form,

and it's going to yield.

Instead of just saying that you have a hydrogen ion

right over here, you'll say, "Okay, that thing,

"the hydrogen is actually gonna get bonded

"to a water molecule."

And so what you're gonna be left with

is actually H3O.

Now this thing, this was a water molecule,

and all it got was a hydrogen ion.

All that is is a proton.

It didn't come with any electrons,

so now this is going to have a positive charge.

It's going to have a positive charge,

and we could now say that this is going to be

in an aqueous solution, hydronium

is going to be in an aqueous solution,

and you're going to have plus,

now you're going to still have the chloride ion,

or it's a negative ion, so we call it an anion.

Chloride, chloride anion,

and this is still in an aqueous solution.

It is dissolved in water,

and remember all that happened here

is that the chlorine here took all of the electrons,

leaving hydrogen with none.

Then that hydrogen proton gets nabbed by a water molecule

and becomes hydronium.

So even by this definition you might say

it increases the concentration of hydrogen protons.

You could say it increase the concentration of hydronium,

of hydronium right over here.

Hydronium ions.

So that makes, by the Arrhenius definition,

that makes hydrochloric acid a strong acid.

That makes it a

strong acid.

Now what would be a strong base by the Arrhenius definition

of acids and bases?

Well one would be sodium hydroxide.

So let me write that down,

so if I have sodium hydroxide,

sodium Na, that's the sodium,

and then I have the hydroxide.

That's an oxygen bonded to a hydrogen.

So that's sodium hydroxide, and actually

if you wanted to see what this molecule looked like

you have a oxygen having a covalent bond

to a hydrogen.

Let me do these in different colors.

Oxygen has a covalent bond

to a hydrogen.

to a hydrogen right over here.

And it actually has three alone pairs.

It actually has three alone pairs right over here.

It's actually nabbed the electron from,

from somebody some place,

and so it's going to have a negative charge.

It is going to have a negative charge.

Actually I could write it both,

let me just write it like that.

It has a negative charge,

and then you have a sodium ion

that has lost its electron somehow.

So you have a sodium ion that has lost

an electron somehow, so it has a positive charge,

and for all we know, it could have lost the electron

to the oxygen right over here, making the oxygen

negative and making the sodium positive,

and so this is now positive, this is negative,

they're going to be attracted to each other,

and they form an ionic bond,

so sodium hydroxide, they have an ionic bond

because the sodium is actually positive,

and the hydroxide part right over here.

That is negative,

and that's what draws them together,

but anyway, you put this in an aqueous solution.

You throw some sodium hydroxide into an aqueous solution,

it will disassociate into,

into sodium with its positive charge, the sodium ions,

and actually you know the sodium ion is still part of this.

That's what makes it attracted to the hydroxide anion,

but it's still going to be

in an aqueous solution,

and then you're going to have the hydroxide.

You're going to have the hydroxide anion,

so essentially this thing just disassociates.

This has a negative charge,

and it's still going to be dissolved in the water,

so aqueous solution.

So you throw sodium hydroxide in water,

it's going to increase the concentration.

It's going to increase the concentration of hydroxide

in the water.

It's going to increase the hydroxide concentration,

and so by the Arrhenius definition of acids and bases,

this would be a strong Arrhenius base.

This would be a strong,

a strong base by the Arrhenius definition.

Now, and I encourage you to look at that

relative to the other definitions,

the Bronsted-Lowry definition of acids and bases

and the Lewis definition of acids and bases,

and see how you would think about categorizing things.