IB Chemistry R3.1.12 the Ph of a Salt Solution Notes

Hydrolysis of Ions in a Salt

When a salt such as ammonium chloride (NH$_4$Cl) dissolves in water, it dissociates into ions: NH$_4^+$ and Cl$^−$.

While it may seem that these ions simply exist in solution without further interaction, the ammonium ion (NH$_4^+$) actually reacts with water, releasing hydrogen ions (H$^+$) and slightly lowering the pH.

This process, called hydrolysis, explains why some salt solutions are acidic, basic, or neutral.

Understanding Hydrolysis: The Basics

Definition

Hydrolysis of a salt

When a salt dissolves in water, its ions may undergo hydrolysis, reacting with water to form either H$^+$ (acidic) or OH$^−$ (basic) ions.

The pH of the resulting solution depends on the nature of the ions involved:

Cations (positive ions): Certain cations, like NH$_4^+$, act as weak acids by donating protons to water.
Anions (negative ions): Certain anions, like CH$_3$COO$^−$, act as weak bases by accepting protons from water.

Key Rule:

Only ions from weak acids or weak bases undergo hydrolysis.

Ions derived from strong acids (like Cl$^-$) or from strong bases (like Na$^+$) do not hydrolyze, so on their own they leave the solution neutral.

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IB Chemistry R3.1.12 the Ph of a Salt Solution Notes

Hydrolysis of Ions in a Salt

Understanding Hydrolysis: The Basics

Key Rule:

Predicting the pH of Salt Solutions

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Hydrolysis of Ions in a Salt

Understanding Hydrolysis: The Basics

Key Rule:

Predicting the pH of Salt Solutions

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Structure 1. Models of the particulate nature of matter5 subtopics

Structure 2. Models of bonding and structure4 subtopics

Structure 3. Classification of matter2 subtopics

Reactivity 1. What drives chemical reactions?4 subtopics

Reactivity 2. How much, how fast and how far?3 subtopics

Reactivity 3. What are the mechanisms of chemical change?4 subtopics

IB Chemistry R3.1.12 the Ph of a Salt Solution Notes

Structure 1. Models of the particulate nature of matter5 subtopics

Structure 2. Models of bonding and structure4 subtopics

Structure 3. Classification of matter2 subtopics

Reactivity 1. What drives chemical reactions?4 subtopics

Reactivity 2. How much, how fast and how far?3 subtopics

Reactivity 3. What are the mechanisms of chemical change?4 subtopics

Hydrolysis of Ions in a Salt

Understanding Hydrolysis: The Basics

Key Rule:

Predicting the pH of Salt Solutions

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Hydrolysis of Ions in a Salt

Understanding Hydrolysis: The Basics

Key Rule:

Predicting the pH of Salt Solutions

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