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Master Salt Hydrolysis: Key to Acid-Base Chemistry
Dive into the world of salt hydrolysis with our engaging video introduction. Learn to identify spectator ions, predict solution pH, and apply your knowledge to real-world chemistry problems.

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Now Playing:Hydrolysis – Example 0a
Intros
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  1. What is hydrolysis?
  2. What is hydrolysis?
    Salts dissolving in water.
  3. What is hydrolysis?
    Which ions react with water?
Examples
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  1. Predict the effect on pH when dissolving the following salts in water.
    Predict the effect on pH when the following salts are added to neutral water at 25oC, explaining your answer:
    1. NH4Cl

    2. KI

    3. Na2CO3

Introduction to acid-base theory
Jump to:NotesConceptFAQsPrerequisites
Notes

In this lesson, we will learn:

  • The meaning of the term spectator ion and to identify them in salt and water interactions.
  • To predict hydrolysis reactions and changes in pH of water when salt solutions are made.
  • How to use Ka and Kb to determine pH changes when amphiprotic species are produced in solution.

Notes:

  • Recall that strong acids and strong bases completely dissociate in water. Salts that are water-soluble are also assumed to completely dissociate in water. This means that interactions between water and a water-soluble salt MX can be put in an equation:

    MX = H2O → M+ + X- + H2O

  • Some but not all aqueous ions react with water and this can have a major effect on pH.
    Strong acids dissociate completely in water, so their conjugate bases are such poor bases that they can’t go back to being the conjugate acid. Once they become the conjugate base, they stop reacting – they are ‘spectator’ ions. The conjugate pair of any strong acid or strong base is a spectator ion.
    There are some general and specific spectator ions that should be remembered when learning about hydrolysis of salts:
    • The M+ metal ions of group 1 (alkali metals) and group 2 (alkali earth metals) are spectator ions.
    • The ions I-, Br-, Cl-, NO3- and ClO4- are spectator ions as they are the conjugate bases of strong acids.
    • Once you have identified any spectator ions formed in solution from a salt and water interaction, they can be ignored in any reaction to water.

  • The ions that are not spectator ions can react with water in a hydrolysis reaction. Hydrolysis is the breaking down of a compound (in a chemical reaction) by water. This can cause changes in pH because salts contain oppositely charged ions that dissociate in solution. If one of the ions is a spectator ion but the other (the counter-ion) is not, then the effect on water will be asymmetrical! See below for two examples:
    • Ammonium chloride, NH4Cl is soluble in water and the dissociation can be written in an equation:

      NH4Cl → NH4+ + Cl-

      The two ions that make ammonium chloride have an asymmetric effect on water; chloride, Cl- is a spectator ion but ammonium, NH4+, isn’t! The result is an unopposed hydrolysis reaction of the NH4+ ions with neutral water:

      NH4+ + H2O ⇌\rightleftharpoons NH3 + H3O+

      As the equation shows, H3O+ is a product (the ammonium ion is a weak acid) so the resultant solution is more acidic. The overall effect of adding ammonium chloride to water is a lowering of pH.

    • Sodium ethanoate, CH3COONa is soluble in water. When it dissociates, the Na+ ion is a spectator but the ethanoate ion is not. A hydrolysis reaction can be shown with the equation:

      CH3COO- + H2O ⇌\rightleftharpoons CH3COON + OH-

      The ethanoic acid formed is a moderately ‘strong’ weak acid, but because all of this acid will have come from the ethanoate originally formed, the net effect is OH- ions being made, so the resultant solution is more basic. The overall effect of adding sodium ethanoate to water is a rise in pH.

  • There will be some instances where the salt dissolved in water gives two spectator ions – in this case, the resultant solution is neutral.

  • In some cases, the ions produced when salts dissolve in water will be amphiprotic; molecules that are capable of accepting and donating protons (all amphiprotic molecules are amphoteric molecules):
    • For example: hydrogen oxalate, HC2O4- has two carboxylic acid groups. One is still protonated and could donate a proton, behaving as an acid. The other has already been deprotonated; it could be re-protonated, behaving as a base if this happened. Amphiprotic molecules can and will take part in reactions to accept and donate a proton, but will do one more than the other. The Ka and Kb values for your amphiprotic molecule will tell you if it is a stronger base or acid (the Ka or Kb value will be larger).
      This greater acid/base behaviour will give you the net effect on the pH of the solution.
Concept

Introduction to Salt Hydrolysis

Welcome to our exploration of salt hydrolysis, a fascinating concept in chemistry that plays a crucial role in understanding acid-base reactions. Salt hydrolysis occurs when a salt dissolves in water, causing a change in the solution's pH. This process is essential in various chemical applications and everyday life. To kick off our learning journey, I've prepared an introduction video that will visually demonstrate the key principles of salt hydrolysis. This video will help you grasp the concept more easily, showing how different ions interact in solution. You'll see firsthand how some ions act as spectator ions, while others actively participate in the hydrolysis process. Understanding salt hydrolysis is vital for predicting the acidity or basicity of salt solutions, which has implications in fields ranging from environmental science to pharmaceuticals. As we delve deeper into this topic, you'll discover how salt hydrolysis connects to broader chemical concepts, enhancing your overall chemistry knowledge.

FAQs

Here are some frequently asked questions about salt hydrolysis and spectator ions:

  1. What is meant by hydrolysis of salt?

    Salt hydrolysis refers to the reaction between a salt and water, where the salt's ions interact with water molecules to produce either an acidic or basic solution. This process occurs when the salt of a weak acid or weak base dissolves in water, affecting the solution's pH.

  2. How do you identify the spectator ion?

    Spectator ions can be identified by comparing the ionic equation of a reaction. Ions that appear unchanged on both sides of the equation are spectator ions. They don't participate in the chemical reaction and remain in their aqueous form throughout the process.

  3. What is an example of a hydrolyzed salt?

    A common example of a hydrolyzed salt is sodium acetate (CH3COONa). When dissolved in water, the acetate ion (CH3COO-) reacts with water to produce acetic acid and hydroxide ions, resulting in a basic solution. The reaction can be represented as: CH3COO- + H2O CH3COOH + OH-

  4. Is OH- a spectator ion?

    OH- (hydroxide ion) is not typically a spectator ion. It often participates actively in reactions, especially in acid-base chemistry. However, its role depends on the specific reaction. In some cases, it might be a product or reactant, while in others, it could be part of a neutralization reaction.

  5. How do you write a hydrolysis equation?

    To write a hydrolysis equation, follow these steps: 1) Identify the salt and its dissociation products. 2) Determine which ion (cation or anion) will react with water. 3) Write the reaction of that ion with water, showing the equilibrium. For example, for NH4Cl: NH4+ + H2O NH3 + H3O+

Prerequisites

Hydrolysis is a fundamental concept in chemistry that involves the breakdown of chemical compounds through their reaction with water. To fully grasp this process, it's crucial to have a solid understanding of several prerequisite topics. These foundational concepts provide the necessary context and knowledge to comprehend the intricacies of hydrolysis reactions.

One of the key prerequisites is ionic equations and formulae. Understanding how to write and interpret ionic equations is essential for representing hydrolysis reactions accurately. These equations show the dissociation of compounds in water and the subsequent interactions between ions, which is at the heart of hydrolysis.

Closely related to ionic equations is the concept of ion formation. In hydrolysis, compounds often dissociate into ions when they interact with water. Knowing how ions form, particularly hydroxide ion formation, is crucial for understanding the products of hydrolysis reactions and their effects on solution properties.

Another critical prerequisite is the understanding of strong and weak acids and bases. Hydrolysis often involves the reaction of salts derived from weak acids or bases with water. The strength of the acid or base determines the extent of hydrolysis and its effect on the solution's pH. This knowledge is essential for predicting the outcome of hydrolysis reactions.

Building on this, familiarity with mixing strong acids and bases provides insight into more complex hydrolysis scenarios. While hydrolysis typically involves weaker electrolytes, understanding the behavior of strong acids and bases in solution helps in comparing and contrasting different types of aqueous reactions.

The concept of buffer solutions is also relevant to hydrolysis. Some hydrolysis reactions can create buffer systems, which resist changes in pH. Understanding how buffers work is crucial for predicting the pH stability of solutions undergoing hydrolysis.

Lastly, knowledge of the acid dissociation constant (and its counterpart, the base dissociation constant) is vital. These constants quantify the strength of acids and bases, which directly influences the extent of hydrolysis. They help in calculating the pH of solutions resulting from hydrolysis reactions and in predicting the direction of equilibrium in these processes.

By mastering these prerequisite topics, students can develop a comprehensive understanding of hydrolysis. This foundation enables them to analyze, predict, and explain hydrolysis reactions with confidence, setting the stage for more advanced studies in chemistry and related fields.