Autoionization of water | Water, acids, and bases | Biology | Khan Academy

Summary

The video explains the concept of water's autoionization, where water molecules interact to form hydrogen bonds and occasionally ionize. It describes how one molecule may 'nab' a proton from another, resulting in the formation of hydronium (H3O+) and hydroxide (OH-) ions. Despite the rarity of this process, it highlights that even in pure water, a small concentration of hydronium ions exists. The video also explores the concept of concentration in terms of moles, emphasizing the balance between hydronium ions and water molecules, providing insight into the dynamics of water at a molecular level.

Takeaways

• 😀 Water molecules interact through hydrogen bonds, which are formed due to the polarity of the molecules.
• 😀 The hydrogen bonds give water its unique properties, but chemistry is messier than diagrams show, with many complex interactions occurring.
• 😀 Water molecules and their electrons are constantly moving, and their interactions are probabilistic, meaning the bonds constantly change.
• 😀 A water molecule's electrons may, under the right conditions, be 'nabbed' by another molecule, forming ions.
• 😀 Hydrogen atoms in water are typically just protons and electrons, but their behavior changes during these interactions.
• 😀 When one water molecule 'nabs' a hydrogen proton from another, it forms a hydronium ion (H₃O⁺) and a hydroxide ion (OH⁻).
• 😀 The process of one water molecule ionizing another is called the autoionization of water, a critical concept in chemistry.
• 😀 The concentration of hydronium ions (H₃O⁺) in pure water is approximately 1 × 10⁻⁷ M (molar).
• 😀 A 'mole' is a quantity of 6.02 × 10²³ molecules, and it is used to express quantities of substances in chemistry.
• 😀 Despite the large number of hydronium ions in a liter of water, the concentration is still small compared to the total number of water molecules present.
• 😀 The ratio of hydronium ions to water molecules in a liter is about 1 to 560 million, highlighting the rarity of ionization events in pure water.

Q & A

What causes hydrogen bonds to form between water molecules?

Hydrogen bonds form due to the polarity of the water molecules. The oxygen atom is partially negative, while the hydrogen atoms are partially positive, creating an attraction between them.

Why is the behavior of water molecules described as probabilistic?

The behavior of water molecules is probabilistic because their electrons are constantly moving and can spend more time around the oxygen atom, forming partial charges. The exact position of these electrons cannot be predicted precisely at any given moment.

What happens when one water molecule 'nabs' a hydrogen proton from another molecule?

When one water molecule grabs a hydrogen proton from another, it forms a hydronium ion (H3O+), while the other molecule becomes a hydroxide ion (OH-). This results in the autoionization of water.

What is the typical structure of a hydrogen atom in a water molecule?

A typical hydrogen atom consists of a proton in the nucleus and an electron orbiting around it. In the context of water, this electron is partially pulled toward the oxygen atom, giving the hydrogen a partial positive charge.

What is the net charge of a hydronium ion (H3O+)?

A hydronium ion (H3O+) has a net positive charge due to the addition of a proton from another water molecule, while the oxygen atom retains its electrons.

How does the ionization of water lead to the formation of hydroxide ions?

When one water molecule loses a proton (H+), it becomes a hydroxide ion (OH-) with a negative charge. This happens alongside the formation of a hydronium ion.

What is the concentration of hydronium ions (H3O+) in typical water?

The concentration of hydronium ions in typical water is 1 × 10^(-7) M (molar), which means there are approximately 1 × 10^(-7) moles of hydronium ions per liter of water.

What does the term 'mole' mean in chemistry?

A mole is a unit of measurement in chemistry that refers to approximately 6.022 × 10^23 of a given substance, such as molecules or atoms. It is used to count large quantities of small entities like molecules.

How many molecules of hydronium (H3O+) are there in a liter of water?

There are roughly 6 × 10^16 molecules of hydronium ions in a liter of water, based on the concentration of 1 × 10^(-7) molar hydronium.

Why is the concentration of hydronium ions so low in water?

The concentration of hydronium ions in water is low because for every hydronium ion, there are approximately 560 million water molecules, making the ionization of water a rare occurrence.