Chapter 16: Problem 107
Predict whether the following solutions are acidic, basic, or nearly neutral: (a) \(\mathrm{NaBr},(\mathrm{b}) \mathrm{K}_{2} \mathrm{SO}_{3},(\mathrm{c}) \mathrm{NH}_{4} \mathrm{NO}_{2}\), (d) \(\mathrm{Cr}\left(\mathrm{NO}_{3}\right)_{3}\)
Short Answer
Expert verified
(a) Neutral, (b) Basic, (c) Slightly acidic, (d) Acidic.
Step by step solution
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
pH of Solutions
The pH of a solution is a measure of how acidic or basic it is on a scale from 0 to 14. Acids have a pH less than 7, bases have a pH greater than 7, and neutral solutions have a pH of exactly 7.
Understanding the pH helps us identify the nature of a solution, which is crucial in chemistry as it dictates the chemical reactions that might occur.
When we dissolve salts in water, they can potentially affect the pH of the solution.
- Salts formed from strong acids and strong bases, like NaBr, typically do not change the neutral pH of water, thus, the solution remains neutral.
- Salts from weak acids or bases, however, can impact the pH more significantly, leading to either acidic or basic solutions. This adjustment occurs due to the hydrolysis of ions in the water.
Ion Hydrolysis
Ion hydrolysis is an essential concept in acid-base chemistry as it explains how the ions from dissolved salts interact with water. When salts dissolve, their constituent ions can react with water molecules in a process called hydrolysis.
- Hydrolysis is key in determining the solution's pH when weak acids or bases are present.
- For instance, NH₄⁺ ions from NH₄NO₂ can react with water, generating H⁺ ions and slightly acidifying the solution.
Here's how it works:
1. The ion from a weak acid/base can bond with water, forming either H⁺ or OH⁻ ions.
2. These new ions change the balance of ions in the solution, resulting in an acidic or basic environment.
In simple terms, ion hydrolysis helps to explain why solutions of salts might not always be neutral, even if made from acids and bases. It's this ionic interaction that leads to pH variations.
Salt Solutions
A salt solution is formed when a salt (an ionic compound) dissolves in water. Salts can arise from various combinations of acids and bases, influencing the resultant solution's acidity or basicity.
- In general, when a salt dissolves, it splits into its individual ions. These ions can then interact with water molecules.
Depending on the nature of the parent acid and base:
- Salts from a strong acid and strong base will create neutral solutions. For example, NaBr is neutral because both NaOH and HBr are strong, canceling each other's effects.
- Salts from strong bases and weak acids, like K₂SO₃, form basic solutions as the SO₃²⁻ ion readily accepts H⁺ ions from water, creating OH⁻ ions.
- The opposite is encountered with salts from strong acids and weak bases, making the solution acidic, like Cr(NO₃)₃, since NO₃⁻ doesn’t significantly neutralize any excess H⁺ produced by Cr³⁺.
Strong and Weak Acids and Bases
In acid-base chemistry, understanding the strength of an acid or base is crucial as it influences the behavior of the salt solutions formed by these compounds.
- **Strong Acids and Bases:** Completely dissociate into ions in water. This characteristic means they have a significant, predictable effect on pH. Examples include HBr and NaOH.
- **Weak Acids and Bases:** Only partially dissociate in water, resulting in increased ion availability for reactions, like hydrolysis, affecting the overall pH. NH₄OH and H₂SO₃ are examples, leading to significant pH changes when included in salt formulations.
The concept of dissociation strength explains why the origin of a salt—from what acid and base it was formed—directly impacts whether its solution is acidic, basic, or neutral. The predictable nature of strong compounds and the variability of weak ones guide chemists in determining the likely pH outcome of a mixed solution.
