2010 preparation of salts

Chapter 12: Salts

Understanding Goals•Students will understand that salt is an ionic compound formed when metallic ion replaces one or more hydrogen ion of an acid.

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What are salts?

A salt is a compound formed when a metallic ion or an ammonium ion (NH4

+) replaces one or more hydrogen ions of an acid.

Example: Zinc sulphate is the salt formed when the zinc ion replaces the hydrogen ion of sulphuric acid.


ZnO (s) + 2HCl (aq) ZnCl2(aq) + H2O(l)

Examples of salts

zinc oxide + hydrochloric acid zinc chloride + water

Zn(CO3) (s) + 2HNO3 (aq) Zn(NO3)2 (aq) + CO2 (g) + H2O(l)

zinc carbonate + nitric acid zinc nitrate + carbon dioxide + water

Zn(OH)2(s) + H2SO4(aq) ZnSO4(aq) + 2H2O(l)

zinc hydroxide + sulfuric acid zinc sulfate + water


Examples of salts and reactants used to make them


Water of Crystallisation

Many salts combine with water molecules to form crystals. These water molecules in the crystals are known as water of crystallisation.

Salts that contain water of crystallisation are called hydrated salts.

Salts that do not contain water of crystallisation are called anhydrous salts. Anhydrous salts are often powders.


Water of Crystallisation

The dot ‘.’ in a formula means that if the substance is heated, everything after the dot will be given off first.

For hydrated salts, water will be given off first on heating.

ZnSO4.7H2O


When a hydrated salt is heated, water of crystallisation is given off. For copper(II) sulphate, there is a colour change.

Chapter 12: Salts

Understanding Goals•Students will understand the general rules of solubility for common salts which include

• nitrates, • chlorides, • sulfates, • carbonates, • hydroxides, • ammonium and

• Group I Alkali Metals

Solubility Rules for Salts in water at room temperature

(It is essential to remember the solubility rules and apply them to solve problems.)

Soluble Salts Insoluble Salts

Ammonium salts All soluble

Alkali metal(Group I) All soluble

Nitrate salts All soluble

Chloride salts All soluble , except for… Silver chloride, lead(II) chloride, mercury

chloride

Sulfate salts All soluble , except for… Barium sulfate,Calcium sulfate, lead(II) sulfate

Carbonate salts Alkali metal carbonates, ammonium carbonate

All insoluble, except for…

Hydroxides Alkali metal, barium hydroxide, strontium hydroxide, ammonium

hydroxide, calcium hydroxide (sparingly soluble)

All insoluble, except for…


Chapter 12: Salts

Understanding Goals•Students will understand that techniques used in preparation, separation and purification of salts include filtration, evaporation and crystallization.

•Students will understand that soluble salts can be prepared by reactions of acids with

• Insoluble substances or• Soluble substances.

•Students will understand that Alkali Metal salts and ammonium salts can be prepared by titration.

Methods of preparing salts

Focusing Questions:

• What do I know about the solubility of the given salt?

• What starting materials (reagents) do I need to prepare the salt?

• What do I know about the solubility of the starting materials in water?

• What approach or strategy would I use to prepare the given salt?


Preparation of Zinc Sulfate



• Zinc sulfate is a soluble salt.

• Since zinc sulfate is soluble, we can be prepare it by reacting dilute sulfuric acid with any of the following:• Zinc metal, • Zinc carbonate or• Zinc oxide / Zinc

hydroxide


Preparation of Zinc Sulfate



• All the starting materials for making the salt are INSOLUBLE in water.

Method: Reaction of acids with excess

metal/base/metal carbonate



Making zinc sulphate crystals


Why is this method NOT suitable for some metals?

This method is suitable for moderately reactive metals such as zinc, magnesium, aluminium and iron.

It is not suitable for reactive metals such as potassium, sodium and calcium. They react violently with acids

It is not suitable for unreactive metals such as copper and silver. These metals do not react with dilute acids.

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MAZIL

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What are the two differences between reacting acid with metal carbonates and

bases/metals?

2. There is effervescence, so we can easily tell when all the acid has been used up.

1. No heating is required.

2 major differences compared with reacting with bases:


Focusing Questions:






Preparation of Sodium Nitrate



• Sodium nitrate is a soluble salt.

• Since sodium nitrate is soluble, we can be prepare it by reacting dilute nitric acid with any of the following:• Sodium carbonate, • Sodium hydroxide

“I observe that both all the reactants are soluble / aqueous solution.”


Preparation of Sodium Nitrate



• All the starting materials for making the salt are SOLUBLE in water.

“How can I tell that the neutralization reaction is completed?”

• Method: Titration



When do we use titration?

Sodium, potassium and ammonium salts are prepared using the titration method.

Examples: NaCl, K2SO4, NH4Cl


Preparation of Sodium Nitrate by Titration

The preparation of sodium nitrate

involves

a) titration to determine the volumes

of reactants required,

b) actual preparation of sodium

nitrate.


How can we tell a reaction is complete in titration?

To determine whether a reaction is complete, we use an indicator.

In titration, we often react an acid with a soluble substance (e.g. sodium hydroxide or sodium carbonate).

Sodium, potassium and ammonium salts are prepared using the titration method.


Titration: To determine the exact volumes of reactants needed to prepare NaNO3

3. Add 1 or 2 drops of methyl orange (indicator) to the sodium hydroxide solution. The solution turns yellow.

2. Pipette 25.0 cm3 of dilute sodium hydroxide solution into a conical flask.

1. Fill up a burette with dilute nitric acid. Note the initial burette reading (V1 cm3).


Titration (to prepare NaNO3)

4. Add dilute nitric acid from the burette slowly until the solution just turns orange permanently (no longer yellow). This is the end-point.

5. Record the final burette reading (V2 cm3).

Hence, the volume of acid required for complete neutralisation = (V2 – V1) cm3.

Strong Acid – Strong Base Reaction


• Any acid-base indicator that changes colour between pH 4 and pH 10 is suitable to detect the end-point for a strong acid - strong base titration.

•Methyl orange and phenolphthalein are commonly used indicators for strong acid – strong base reaction.

Practical Experiment: Titration

• Handling Burette (acid)• Please rinse the burette before and after

usage.• Use tap water followed by distilled water.• Clamp the Burette carefully to the retort

stand.• Handling Pipette (alkali)

• Insert the pipette filler at the top of pipette – do not force (break!)

• Ensure that air is pumped out of the filler before carrying out the suction of alkali.

Practical Experiment: Titration

• Using Methyl Orange• Add 2-3 drops of indicator into the conical

flask.• Yellow colour observed.

• Transferring of acid into conical flask• Add drop by drop of the acid into the

conical flask• Swirl to ensure complete mixing of the acid

and alkali.• Observe the change in the colour of the

indicator in the conical flask.

Strong Acid – Strong Base Reaction


pH curve 1 pH curve 2


Preparation of Sodium Nitrate after Titration (without any indicator)

1. Pipette 25.0 cm3 of sodium hydroxide solution into a

beaker.

2. Add (V2 – V1) cm3 of dilute nitric acid from the burette.

3. Heat the solution to evaporate the water until it is saturated.

4. Allow the saturated solution to cool and crystallise.

5. Filter to collect the crystals.

6. Dry the crystals between sheets of filter paper.


Detecting End-point Using a pH Meter

The pH of the solution in the conical flask changes during an acid-base titration as solution is added from the burette.

These changes in the pH values can be measured using a pH meter.

Hence, a pH meter can also be used to determine the end-point of a titration.


Detecting End-point Using a pH Meter

We titrate 1.0 mol/dm3 sodium hydroxide solution with a 1.0 mol/dm3 nitric acid. The pH curve is shown here.

The pH decreases from 14 as more nitric acid is added.

It finally ends near pH 0.

At the end-point, there is sharp drop in pH.


Method 2: Titration

Method 1: We can tell when all the acid is used up. When all the acid is used up, we see a residue of the insoluble metal/ carbonates / bases.

Comparison between Method 1 and Titration

Method 1: uses one insoluble substance

Titration: uses two soluble substances.

Titration: No residue seen when reaction completes.

Chapter 12: Salts

Understanding Goals•Students will understand that insoluble salts are prepared by precipitation method.•Students will understand that techniques used in preparation, separation and purification of salts include filtration, evaporation and crystallization.


Method 3: Precipitation – to produce insoluble salts

All insoluble salts can be prepared by precipitation.


Focusing Questions:






Preparation of lead(II) sulfate



• Lead(II) sulfate is an insoluble salt.

• We need to mix 2 soluble salt solutions which must contain:• Lead(II) as the

positive ions (SaltSolution 1)

• sulfate as the negative ions (Salt Solution 2)


Preparation of Lead(II) Sulfate



• All the starting materials for making the insoluble salt are SOLUBLE .

Note that dilute acid is not a

starting reactant.

• Method: Precipitation



Method 3: Precipitation

To precipitate an insoluble salt, mix 2 solutions together. One

solution contains the positive ions of the salt. The other

solution contains the negative ions of the salt to be prepared.

For example, insoluble lead(II) sulphate

can be prepared by

using a soluble lead(II) salt

(such as lead(II) nitrate)

dilute sulphuric acid or

any soluble sulphate. +


Method 3: Precipitation - Steps

“SALT”

• Equipped with an interview guide, you are to conduct an in-depth interview with a very famous artiste cum scientist better known as “SALT”.

• The artiste is well known for his/her ability to prepare any salts.

• Hence today, you are to get him/her to share the synthesis process.

“SALT”

• Choose TWO salts and invite the artiste to share more about the preparation method of these salts.

• Craft out at least 10 different levels of questions to ask the artiste.

• Record her response accordingly – DO NOT CHANGE any words she/he has said. Record accordingly into your notebook.

• Submit the notes to your chief-editor at the end of the interview.

2010 preparation of salts

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