Hydrolysis of Salts Mark as Favorite (7 Favorites)

LAB in Net Ionic Equation, Indicators, Strong vs Weak, Salts. Last updated August 17, 2019.

Summary

In this lab, students will observe the hydrolysis of several salt samples. They will first predict which solutions are acidic, basic or neutral, and then discover the pH of each through the use of indicators. Students will share and compile their experimental results, as well as have an opportunity to determine the net-ionic equations for each reaction.

Grade Level

High School

AP Chemistry Curriculum Framework

This lab supports the following units, topics and learning objectives:

Unit 4: Chemical Reactions
- Topic 4.2: Net Ionic Equations
  - TRA -1.B: Represent changes in matter with a balanced chemical or net ionic equation: a. For physical changes. b. For given information about the identity of the reactants and/or product. c. For ions in a given chemical reaction.
- Topic 4.8: Introduction to Acid-Base Reactions
  - TRA-2.B: Identify species as Brønsted-Lowry acids, bases, and/or conjugate acid-base pairs, based on proton-transfer involving those species.
Unit 7: Equilibrium
- Topic 7.13: pH and Solubility
  - SPQ-5.C: Identify the qualitative effect of changes in pH on the solubility of a salt.
Unit 8: Acids and Bases
- Topic 8.2: pH and pOH of Strong Acids and Bases
  - SAP-9.B: Calculate pH and pOH based on concentrations of all species in a solution of a strong acid or a strong base.
- Topic 8.5: Acid-Base Titrations
  - SAP-9.E: Explain results from the titration of a mono- or polyprotic acid or base solution, in relation to the properties of the solution and its components.
- Topic 8.7: pH and pKa
  - SAP-10.A: Explain the relationship between the predominant form of a weak acid or base in solution at a given pH and the pKa of the conjugate acid or the pKb of the conjugate base.

Objectives

By the end of this lab, students should be able to

Explain that the pH of a solution containing a dissolved salt may be acidic, basic or neutral.
Use pH paper and universal indicator to determine if a solution is acidic, basic or neutral.
Describe the meaning of the term hydrolysis.
Write a net-ionic equation for a dissolved salt in water.

Chemistry Topics

This lab supports students’ understanding of

Acids & Bases
Salts
pH
Indicators
Strong vs. Weak Acids/Bases
Solutions
Net Ionic Equations

Time

Teacher Preparation: 30-45 minutes

Lesson: 60 minutes

Materials

100ml beakers (3 per group)
Spatulas
Glass stir rods
pH Hydrion paper
Distilled water
Salts (pea-sized scoop of each)
- Sodium Chloride, NaCl
- Sodium Acetate, NaC₂H₃O₂
- Aluminum Chloride, AlCl₃
- Sodium Oxalate, Na₂C₂O₄
- Sodium Carbonate, Na₂CO₃
- Potassium Nitrate, KNO₃
- Ammonium Chloride, NH₄Cl
- Sodium Bicarbonate, NaHCO₃
- Zinc Sulfate, ZnSO₄
- Copper (II) Sulfate, CuSO₄
- Sodium Phosphate, Na₃PO₄
- Potassium Chlorate, KClO₃
Labeling tape and markers
Universal Indicator
Parafilm
Large poster paper or white board

Safety

Always wear safety goggles and apron when handling chemicals in this lab. All salts can be irritants to skin and eyes.
Students should wash their hands thoroughly before leaving the lab.
When students complete the lab, instruct them how to clean up their materials and dispose of any chemicals.
Potassium chlorate is a strong oxidizing agent; do not heat or poke with the spatula.
Sodium oxalate is a poison. Be sure to wash your hands and wipe down the counters well after the lab.

Teacher Notes

In this lab students will observe and record the pH of various salt solutions. I then coach them to write net ionic equations for each solution in an attempt to explain what is happening to the salts in water.
For the lab, students are divided into four research groups, each of which is assigned three salts, for a total of twelve salts (none which are used in prior examples). Students predict whether each of their assigned salts will be acidic, neutral, or basic in water, then measure the pH of each with Hydrion paper and universal indicator.

Group 1	Group 2	Group 3	Group 4
Sodium Chloride, NaCl Sodium Acetate, NaC ₂H₃O₂ Aluminum Chloride, AlCl₃	Sodium Oxalate, Na₂C₂O₄ Sodium Carbonate, Na₂CO₃ Potassium Nitrate, KNO₃	Ammonium Chloride, NH₄Cl Sodium Bicarbonate, NaHCO ₃ Zinc Sulfate, ZnSO₄	Copper (II) Sulfate, CuSO₄ Sodium Phosphate, Na₃PO₄ Potassium Chlorate, KClO₃

Each group then presents their findings to the class on large poster paper or a white board, showing the net ionic equations and pH levels they recorded. These presentations are key, as each group must explain both their predictions for each assigned salt and whether or not the prediction was supported by the data. Also, students must attempt to explain any data that does not match their predictions or otherwise does not make sense.
The net ionic equations on the posters serve as the models for all students to examine as part of their inquiry experience. As their fellow students present, students ask clarifying questions, and record their findings.
During this lab, the students usually observe pH readings shifted to the acidic end of the scale. As the presentations continue, they gradually realize that this is true across all of the groups, and eventually someone realizes that everyone assumed that the deionized water has a pH of 7. This usually leads to a student excitedly jumping up and testing the pH of the water, which is always slightly acidic due to dissolved carbon dioxide from the air. This allows students to experience first-hand the value of communicating with other groups; in this case, they discover an unexpected variable affecting all of their data.
Engagement in this lab is enhanced by the fact that they are responsible for the predictions, data collection, presentations and, most importantly, discussion of discrepant events.

For the Student

Lesson

Background

Many chemical reactions are carried out in water solution. Water, a solvent, provides a medium in which the reacting substances may come together. In acidic and basic solutions, we have considered the H⁺ and OH^- ions formed directly from acids and bases by the process called ionization. We will now consider the formation of H⁺and OH^- ions by the reaction of an ion with water. This reaction is called hydrolysis.

Normally salts are produced by acid-base neutralization. If this were entirely true, a dissolved salt would always produce a neutral solution in water. However, the solutions of some salts are not neutral. Pure water ionizes:

2H₂O_(l) ↔ H₃O⁺_(aq) + OH^-_(aq)

But the H₃O⁺ and OH^- concentrations are very low; approximately 1 x 10^-7 ions per mole.

Water molecules can act either as a proton (H⁺) donors or proton acceptors. A salt that is formed from a weak acid and a strong base will generally form water solutions that are basic. The basic anion accepts a proton from a water molecule forming a weak acid and leaving OH^- ions from the water molecules in solution. The solution will be basic due to the increase of OH^- ion concentration.

Salts that are formed from a strong acid and a weak base generally form water solutions that are acidic. The acidic cation donates a proton to the water molecule, forming a weak base and creating H₃O⁺ ions in solution. In addition, water molecules may accept protons from the hydrated metallic cations; imagine the water molecules surrounding the metal ions being replaced by hydroxide ions.

For metal(II) ions: M(H₂O)₆²⁺ + H₂O ⇄ M(H₂O)₅(OH)⁺ + H₃O⁺

For metal (III) ions: M(H₂O)₆³⁺ + H₂O ⇄ M(H₂O)₅(OH)⁺² + H₃O⁺

Objective

The purpose of this experiment is to observe hydrolysis and to discover which salt solutions are neutral, acidic, or basic. Your job is to predict whether each salt will be acidic, basic or neutral in solution, test your predictions, and then report back to the class.

Materials

100ml beakers
Spatulas
Glass stir rods
pH Hydrion paper
Distilled water
Salts (see chart)
Labeling tape and markers
Universal Indicator
Parafilm
Large poster paper or white board

Safety

Always wear safety goggles and apron when handling chemicals in this lab. All salts can be irritants to skin and eyes.
Wash your hands thoroughly before leaving the lab.
Follow the teacher’s instructions for cleanup of materials and disposal of chemicals.
Potassium chlorate is a strong oxidizing agent; do not heat or poke with the spatula.
Sodium oxalate is a poison. Be sure to wash your hands and wipe down the counters well after the lab.

Pre-lab

You are part of a research team. Each team has been assigned three salts to investigate. For each of your assigned salts, predict whether the salt will be acidic, basic or neutral in water.

Write your predictions for each salt, as well as your reasons for your predictions, in complete sentences below.

Procedure

Label a beaker clearly with the name and number of each salt.
Mix a small amount (pea-sized scoop) of each salt in about 30-50 mL of deionized water in a 100ml beaker and stir with the glass stirring rod. Do not stir with the metal spatula.
Add 3-4 drops of universal indicator. Note the color. Add more indicator as needed to see a definite color. Compare the colors with this chart. Record the color in the data table provided.
Determine the approximate pH by using a glass stir rod to place a drop of the solution on pH hydrion paper. Record your data in the chart provided.
Keep your beakers to use as exhibits in your reports. Cover with Parafilm.
Rinse off your stir rods and spatulas.
Throw away used pH paper.

Data

During your experiment complete the rows for your assigned salts. During the group presentations complete the remaining rows in the data table for the rest of the salts.

	Normal salt solution	Color with U.I.	Approximate pH (hydrion)	Parent acid	Parent Acid: strong or weak?	Parent base	Parent Base: strong or weak?
1	NaCl
2	NaC₂H₃O₂
3	AlCl₃
4	Na₂C₂O₄
5	Na₂CO₃
6	KNO₃
7	NH₄Cl
8	NaHCO₃
9	ZnSO₄
10	CuSO₄
11	Na₃PO₄
12	KClO₃

Analysis

For each salt, compare your predictions (pre-lab question) with your results. Write a sentence or two about how your predictions are similar or different from your results for each reaction. If they are different, also explain why you think the pH’s turned out as they did.
Write the net ionic equations for each ion of each of your assigned salts in water. (Equations for the other salts will be completed after the class presentations) Add only one proton to basic anions. Make sure your equations agree with your results (hydronium ions produced if it was acidic, hydroxide ions produced if it was basic, no net ionic equation if it was neutral).

	Salt	Net ionic equations in water
1	NaCl
2	NaC₂H₃O₂
3	AlCl₃
4	Na₂C₂O₄
5	Na₂CO₃
6	KNO₃
7	NH₄Cl
8	NaHCO₃
9	ZnSO₄
10	CuSO₄
11	Na₃PO₄
12	KClO₃

Presentations

You will report your findings to the rest of the class. Decide in advance what information will be reported by each member—each person should report on some section of your research. Use your beakers and the large poster paper/white board provided to aid in your presentation. Be sure to include in your report:
1. Your predictions and explanations
2. Your observations
3. Your revisions to your predictions, with explanations
4. Your equations (written large on poster paper/)
As other groups report, record their findings in the space provided in your data table. Pay attention!
Ask questions of the reporting groups. Challenge them to explain their predictions and final equations.
Write the net ionic equations for both ions of every salt of this lab in water (all twelve) in the table provided. If there is no Net Ionic Equation, write “No NIE”.
Clean-Up: Once everyone has reported, pour your solutions down the sink, remove labels and rinse well with water. Wash your hands and wipe down the counters well.

Conclusion

Write a paragraph about what you learned from this lab. Use your results as well as results from other groups as examples. How could this lab be improved?