Produto Iônico da Água (Kw): Autoionização da Água | Equilíbrio Químico | Aula 14

Name: Produto Iônico da Água (Kw): Autoionização da Água | Equilíbrio Químico | Aula 14
Uploaded: 2020-06-23T22:10:59.000Z
Duration: 40 min 13 s

Introduction to Acids and Bases

In this section, the speaker introduces the concept of acids and bases in our daily lives. They explain that substances like oranges, lemons, and pineapples taste sour because they are acidic, while green bananas and cashews taste astringent due to their basic properties.

Understanding Acid-Base Reactions

The speaker discusses two reactions involving water to help understand how to calculate concentrations of H+ (hydrogen ions) and OH- (hydroxide ions) in dilute aqueous solutions.

The first reaction involves water reacting with ammonia (NH3), resulting in the formation of ammonium ions (NH4+). This shows that water can act as an acid by donating hydrogen ions.

The second reaction involves water interacting with acetic acid (CH3COOH), found in vinegar, forming acetate ions (CH3COO-) and hydronium ions (H3O+). This demonstrates that water can also act as a base by accepting hydrogen ions.

Amphiprotic Substances

The speaker explains that substances exhibiting both acidic and basic characteristics are called amphiprotic or amphoteric substances. Water is one such substance because it can react with both acids and bases.

Examples of other amphiprotic substances include amino acids, which can react with both acids and bases, as well as certain oxides like aluminum oxide.

Autoionization of Water

The speaker introduces the concept of autoionization of water, which refers to the reaction that occurs when pure water interacts with itself. This process leads to the formation of hydronium ions (H3O+) and hydroxide ions (OH-).

Autoionization involves the breaking of covalent bonds in water molecules, resulting in the generation of ions. This ionization is specific to covalent bonds, where electrons are shared between non-metal atoms.

Ionization and Formation of Ions

The speaker explains that during autoionization, two water molecules can combine to form hydronium ions (H3O+) and hydroxide ions (OH-). Each water molecule can act as either an acid or a base.

When one water molecule acts as an acid, it donates a hydrogen ion (H+), which is accepted by another water molecule acting as a base. This results in the formation of hydronium ions.

The geometry and polarity of water molecules allow them to form a covalent coordinate bond with hydrogen ions. This stabilizes the incoming hydrogen ion and forms hydronium ions.

The speaker emphasizes that hydrogen prefers to have two electrons for stability, similar to helium. Therefore, each pair of electrons from oxygen's lone pairs can be shared with an incoming hydrogen ion through a coordinate bond.

Formation of Hydroxide Ions

The speaker explains how one water molecule can act as a base by accepting a hydrogen ion from another water molecule acting as an acid. This results in the formation of hydroxide ions (OH-).

The hydrogen ion forms a coordinate bond with the other hydrogen atom in the water molecule, resulting in the formation of a covalent coordinate bond. This leaves a residual positive charge on the oxygen atom, forming hydronium ions.

The speaker clarifies that even though there is a positive charge on hydronium ions, it does not donate electrons because covalent bonds involve electron sharing rather than donation.

When one water molecule accepts a hydrogen ion, it forms hydroxide ions with a negative charge. This process is known as ionization and occurs within water molecules themselves.

Conclusion

The section provides an introduction to acids and bases, explaining their presence in everyday substances. It discusses acid-base reactions involving water and highlights how water can act as both an acid and a base. The concept of amphiprotic substances is introduced, along with examples such as amino acids and certain oxides. The autoionization of water is explained, leading to the formation of hydronium and hydroxide ions. The process of ionization and the formation of hydronium and hydroxide ions are also discussed.

Timestamps may vary slightly depending on the version of the transcript used.

Auto-Ionization of Water and Chemical Equilibrium

In this section, the process of auto-ionization of water and its relation to chemical equilibrium is discussed. The equilibrium constant (Kw) is introduced as a measure of the concentration of products in the reaction.

Auto-Ionization of Water and Equilibrium Constant

The auto-ionization of water refers to the self-breaking of water molecules, resulting in the formation of hydronium ions (H+) and hydroxide ions (OH-).

This process can be represented as a chemical equilibrium, which can be expressed using an equilibrium constant (K).

The equilibrium constant for this reaction is given by [H+][OH-]/[H2O].

However, it should be noted that the auto-ionization of water is a relatively small process compared to the overall concentration of water molecules.

Only a small fraction of water molecules undergo auto-ionization, while most remain unchanged.

Therefore, the concentration of water remains constant during this process.

The product [H+][OH-] is denoted as Kw, which represents the ionic product constant or auto-ionization constant.

Kw can also be referred to as the constant for self-ionization or product ionization constant.

Temperature Dependence and Product Ionic Constant

This section discusses how temperature affects the auto-ionization process and its impact on the value of Kw.

Temperature Dependence

The auto-ionization process in water is endothermic, meaning it requires heat energy.

Increasing the temperature facilitates this process by providing more energy for ion formation.

As a result, higher temperatures favor greater ion formation and increase Kw.

Product Ionic Constant (Kw)

Kw serves as a measure of the concentration of H+ and OH- ions resulting from the auto-ionization of water.

At a specific temperature, Kw is constant and can be expressed as 10^-14 M^2.

It is important to note that this value is specific to a temperature of 25 degrees Celsius.

However, as the temperature changes, the equilibrium shifts, and Kw varies accordingly.

For example, at higher temperatures, Kw will increase.

Application of Kw in Calculating Concentrations

This section explains how Kw can be used to calculate concentrations of H+ and OH- ions in different types of aqueous solutions.

Calculation of Concentrations

The value of Kw allows for the calculation of H+ and OH- ion concentrations in chemical equilibrium.

In neutral solutions, where H+ equals OH-, both concentrations are equal and can be determined using the value of Kw (10^-14 M^2).

For example, in a neutral solution at 25 degrees Celsius, [H+] = [OH-] = 10^-7 M.

The transcript does not provide further information on calculating concentrations for acidic or basic solutions.

Acid and Base Concentrations

This section discusses the concept of acids and bases according to Arrhenius. It explains that acids increase the concentration of H+ ions in a solution, while bases increase the concentration of OH- ions.

Acid Concentration

According to Arrhenius, acids are substances that increase the concentration of H+ ions in a solution.

The concentration of H+ can be calculated using the product of H+ concentration and OH- concentration, which is always equal to 10^-14 at 25 degrees Celsius.

For example, if the H+ concentration is 10^-2 mol/L, then the OH- concentration would be 10^-12 mol/L.

Base Concentration

Bases are substances that increase the concentration of OH- ions in a solution.

The concentration of H+ can be calculated using the product of H+ concentration and OH- concentration, which is always equal to 10^-14 at 25 degrees Celsius.

For example, if the OH- concentration is 10^-5 mol/L, then the H+ concentration would be 10^-9 mol/L.

Water Autoionization and Kw

This section explains water autoionization and introduces Kw (the ion product constant for water). It mentions that increasing temperature increases Kw but does not affect neutrality.

Water Autoionization

Water can react with itself through breaking covalent bonds to generate H+ and OH- ions.

This process leads to chemical equilibrium known as water autoionization.

Ion Product Constant for Water (Kw)

Kw represents the equilibrium constant for water autoionization.

At 25 degrees Celsius, Kw equals 10^-14.

Increasing temperature increases the value of Kw due to it being an endothermic process.

Neutrality in a solution is maintained when the concentrations of H+ and OH- ions are equal, regardless of changes in temperature.

Acidic, Basic, and Neutral Solutions

This section discusses the classification of solutions as acidic, basic, or neutral based on the concentrations of H+ and OH- ions.

Acidic Solutions

In an acidic solution, the concentration of H+ ions is greater than the concentration of OH- ions.

For example, if the concentration of H+ is 10^-3.9 mol/L, then the concentration of OH- would be 10^-13.9 mol/L.

Basic Solutions

In a basic solution, the concentration of OH- ions is greater than the concentration of H+ ions.

For example, if the concentration of OH- is 10^-5 mol/L, then the concentration of H+ would be 10^-9 mol/L.

Neutral Solutions

A solution is considered neutral when the concentrations of H+ and OH- ions are equal.

Regardless of changes in temperature or Kw value (e.g., 10^-14), neutrality is maintained when [H+] = [OH-].