Conjugate acid–base pairs | Chemical reactions | AP Chemistry | Khan Academy

Khan Academy
24 Jun 201608:26
EducationalLearning
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TLDRThis video explores conjugate acid-base pairs, using the example of hydrogen fluoride (HF) in water. It explains how HF, a weak acid, partially dissociates to form fluoride ions and hydronium ions, highlighting the dynamic equilibrium between these species. The video defines conjugate pairs as related species differing by one proton, with examples including HF and fluoride ions, and water and hydronium. It also clarifies misconceptions about pairs like hydronium and hydroxide, which differ by two protons.

Takeaways
  • 🌟 Conjugate acid-base pairs are introduced through the example of hydrogen fluoride (HF) reacting with water.
  • 🔬 Hydrogen fluoride is a weak acid that partially dissociates in water, forming fluoride ions and hydronium ions.
  • ⚖️ The process involves a dynamic equilibrium where the forward and reverse reactions occur at equal rates.
  • 🔄 Hydrofluoric acid (HF) loses a proton to become fluoride ion (F-), and fluoride ion can gain a proton to revert to HF.
  • 💧 Water acts as a base by accepting a proton from HF, forming hydronium ion (H3O+), and can lose a proton to revert to water.
  • 🔊 The relationship between HF and F-, and between water and H3O+, is defined as conjugate acid-base pairs based on their ability to exchange a proton.
  • 📚 The official definition of a conjugate acid-base pair is two species related by one H+ ion, where one is an acid and the other is a base.
  • 🔍 The Bronsted-Lowry definition of acids and bases is used to determine which species is the acid and which is the base in a conjugate pair.
  • 🔬 Examples of conjugate acid-base pairs include HF and F-, and water and H3O+, where the acid donates a proton and the base accepts it.
  • 🚫 Not all related species are conjugate pairs; for instance, H3O+ and OH- are not because they differ by two protons, not one.
  • 🚫 Sodium fluoride and fluoride ion are not a conjugate pair because they are related by a sodium ion, not a proton.
Q & A

  • What is the main topic of the video?

    -The main topic of the video is conjugate acid-base pairs, focusing on their definition, relationship, and examples.

  • What is an example reaction used to introduce the concept of conjugate acid-base pairs?

    -The example reaction used is between hydrogen fluoride (HF) and water, where HF partially dissociates in water to form fluoride ions and hydronium ions.

  • How does hydrogen fluoride behave when it is placed in water?

    -Hydrogen fluoride is a weak acid that partially dissociates in water, releasing fluoride ions and donating a proton to water, forming hydronium ions.

  • What is the significance of the dynamic equilibrium in the reaction between HF and water?

    -The dynamic equilibrium signifies that the forward and reverse reactions are occurring at the same rate, allowing the system to reach a stable state where both HF and water can exist in various forms.

  • What is the relationship between HF and F- in terms of acid-base chemistry?

    -HF and F- are related as a conjugate acid-base pair, where HF can lose a proton to form F-, and F- can gain a proton to reform HF.

  • What is the relationship between water and hydronium ions?

    -Water and hydronium ions are also a conjugate acid-base pair. Water can accept a proton to form hydronium ions, and hydronium ions can lose a proton to revert to water.

  • According to the Bronsted-Lowry definition, what is an acid and what is a base?

    -According to the Bronsted-Lowry definition, an acid is a substance that can donate a proton (H+), and a base is a substance that can accept a proton.

  • How can the relationship between conjugate acid-base pairs be generalized?

    -The relationship can be generalized as any generic acid HA losing a proton to form its conjugate base A-, and the conjugate base A- gaining a proton to reform the acid HA.

  • What is the conjugate base of a strong acid like hydrochloric acid (HCl)?

    -The conjugate base of hydrochloric acid (HCl) is the chloride ion (Cl-), which is formed when HCl completely dissociates in water.

  • Why are hydronium ions (H3O+) and hydroxide ions (OH-) not considered a conjugate acid-base pair?

    -Hydronium ions and hydroxide ions are not a conjugate acid-base pair because they differ by two protons instead of one, violating the definition of conjugate pairs.

  • Why are sodium fluoride and fluoride ions not considered a conjugate acid-base pair?

    -Sodium fluoride and fluoride ions are not a conjugate acid-base pair because they are related by a sodium ion, not by the gain or loss of a proton.

Outlines
00:00
🌐 Introduction to Conjugate Acid-Base Pairs

This paragraph introduces the concept of conjugate acid-base pairs using the example of hydrogen fluoride (HF) reacting with water. Hydrogen fluoride is a weak acid that partially dissociates in water, forming fluoride ions (F-) and hydronium ions (H3O+). The process is in dynamic equilibrium, meaning the forward and reverse reactions occur at equal rates. The paragraph explains that HF and F-, as well as water and H3O+, are related through the exchange of a proton (H+), making them conjugate acid-base pairs. The Bronsted-Lowry definition of acids and bases is used to clarify that an acid donates a proton and a base accepts one. The paragraph concludes by defining conjugate acid-base pairs as two species related by one H+.

05:01
🔬 Examples and Clarifications of Conjugate Acid-Base Pairs

This paragraph expands on the concept of conjugate acid-base pairs by providing additional examples and clarifying misconceptions. It explains that when an acid donates a proton, it forms its conjugate base, and vice versa. Examples include hydrofluoric acid (HF) and its conjugate base fluoride (F-), and water acting as a base to form hydronium (H3O+). The paragraph also discusses the conjugate base of a strong acid like hydrochloric acid (HCl), which is chloride (Cl-). It then addresses cases where species might seem to be conjugate pairs but are not, such as hydronium (H3O+) and hydroxide (OH-), which differ by two protons, and sodium fluoride (NaF) and fluoride (F-), which are related by a sodium ion rather than a proton. The main takeaway is that a conjugate acid-base pair must differ by exactly one proton.

Mindmap
Keywords
💡Conjugate acid-base pairs
Conjugate acid-base pairs are a fundamental concept in chemistry that refers to two species that are related by the loss or gain of a proton (H+). In the context of the video, the theme revolves around understanding how these pairs interact in aqueous solutions. For example, hydrofluoric acid (HF) and fluoride ion (F-) are a conjugate pair because HF can lose a proton to become F-, and vice versa, F- can gain a proton to reform HF.
💡Hydrogen fluoride (HF)
Hydrogen fluoride, commonly known as HF, is a weak acid that partially dissociates in water to form fluoride ions (F-) and hydronium ions (H3O+). The video uses HF as an example to illustrate the concept of a conjugate acid-base pair, showing how it can donate a proton to become a fluoride ion, which is its conjugate base.
💡Dissociation
Dissociation is the process by which a compound separates into its constituent ions in a solution. In the video, the dissociation of HF in water is highlighted to demonstrate the formation of fluoride ions and hydronium ions, which is a key step in understanding the dynamics of acid-base reactions.
💡Dynamic equilibrium
Dynamic equilibrium is a state in a reversible reaction where the rates of the forward and reverse reactions are equal, resulting in no net change in the concentrations of the reactants and products over time. The video explains that the dissociation of HF in water is an example of a dynamic equilibrium, as the process of HF losing a proton and water gaining a proton occurs simultaneously at equal rates.
💡Hydride ion (H+)
The hydride ion, commonly denoted as H+, is a proton that can be donated or accepted in acid-base reactions. In the video, the transfer of the H+ ion is central to the concept of conjugate acid-base pairs, as it is the ion that is exchanged between the acid and its conjugate base.
💡Water (H2O)
Water is a unique molecule that can act both as an acid and a base, depending on the reaction context. In the video, water is shown to accept a proton from HF, becoming hydronium (H3O+), and thus serving as a base in the reaction. Conversely, in the reverse reaction, water can act as an acid when it donates a proton to fluoride ions.
💡Hydonium ion (H3O+)
The hydronium ion, symbolized as H3O+, is the species formed when water accepts a proton. In the video, it is presented as the conjugate acid of water, illustrating how water can participate in acid-base reactions by accepting or donating protons.
💡Bronsted-Lowry definition
The Bronsted-Lowry definition is a fundamental concept in chemistry that defines acids as substances that can donate a proton (H+) and bases as substances that can accept a proton. The video uses this definition to explain which species in a conjugate pair is the acid and which is the base, based on their ability to donate or accept protons.
💡Conjugate base
A conjugate base is the species formed when an acid donates a proton. In the video, the fluoride ion (F-) is the conjugate base of hydrofluoric acid (HF) because it is the result of HF losing a proton. The concept is crucial for understanding how the properties of acids and bases are related through the exchange of protons.
💡Generic acid (HA)
In the video, a generic acid is represented by HA to illustrate the general behavior of acids in conjugate acid-base pairs. When HA donates a proton, it forms its conjugate base, A-. This representation helps to generalize the concept of conjugate pairs and shows the relationship between acids and their corresponding bases.
💡Strong acid
A strong acid is an acid that completely dissociates in water, donating all of its protons. In the video, hydrochloric acid (HCl) is used as an example of a strong acid, which, when it dissociates, leaves behind its conjugate base, the chloride ion (Cl-), even though Cl- is not particularly basic.
💡Chloride ion (Cl-)
The chloride ion, denoted as Cl-, is the conjugate base of the strong acid hydrochloric acid (HCl). Although it is not a strong base, the video explains that it is still considered the conjugate base because it is the species formed after HCl donates its proton.
Highlights

Introduction to the concept of conjugate acid-base pairs using an example reaction between hydrogen fluoride and water.

Hydrogen fluoride is a weak acid that partially dissociates in water to form fluoride ions and hydronium ions.

The process of acid dissociation in water is in dynamic equilibrium, with forward and reverse reactions occurring simultaneously.

Hydrofluoric acid (HF) loses a proton to become fluoride ion (F-), demonstrating the concept of a conjugate acid-base pair.

Fluoride ion (F-) can accept a proton to reform hydrofluoric acid (HF), illustrating the reversible nature of acid-base reactions.

Water acts as a base by accepting a proton from HF, forming hydronium ions (H3O+).

Hydronium ions can lose a proton to revert back to water, showing the relationship between water and hydronium as a conjugate acid-base pair.

Official definition of a conjugate acid-base pair as two species related by the transfer of a single proton.

HF and F-, as well as water and H3O+, are identified as conjugate acid-base pairs based on their relationship through proton transfer.

The Bronsted-Lowry definition of acids and bases is used to distinguish which species in a pair is the acid and which is the base.

Hydrofluoric acid is identified as the acid in its conjugate pair due to its ability to donate a proton.

Fluoride acts as the base in the pair, accepting a proton to reform the acid, in accordance with the Bronsted-Lowry definition.

Water is shown to act as a base by accepting a proton, while hydronium acts as an acid by donating a proton.

A general representation of conjugate acid-base pairs is provided, with HA representing the acid and A- representing the base.

Examples of conjugate acid-base pairs are given, including HF and F-, and water and H3O+.

The conjugate base of a strong acid like hydrochloric acid (HCl) is chloride, even though it is not particularly basic.

Clarification that H3O+ and OH- are not a conjugate acid-base pair because they differ by two protons, not one.

Sodium fluoride and fluoride are not a conjugate acid-base pair because they are related by a sodium ion, not a proton.

Conjugate acid-base pairs are defined by having the same formula except for the presence or absence of one proton.

Transcripts

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Related Tags
Conjugate PairsAcid-Base ChemistryHydrofluoric AcidWaterHydronium IonFluoride IonDissociationEquilibriumBronsted-LowryStrong AcidsWeak Acids