Salts

Salts

All salts are ionic compounds. A salt is formed when a metallic

ion or an ammonium ion (NH4+)

replaces one or more hydrogen ions of an acid.

HCl NaCl

HNO3 NH4NO3

replaced by

replaced by

Find out what is an acid salt!

Salts

Where have you heard of salts? Salts are essential to animal life in

small quantities, but in large excess will be very harmful.

Are all salts salty? Group I ions similar in size to sodium

tend to give salty taste. Which ions do you think give a salty taste?

Example

How do you form CaSO4? Acid + Base Salt + water

CaSO4

Comes from base

Comes from acid

One base that can be used is Calcium hydroxide.

One acid that can be used is Sulfuric acid. Ca(OH)2 + H2SO4 CaSO4 + 2H2O Can you use calcium carbonate or

calcium?

Example

How do you form NaNO3? Acid + Base Salt + Water

NaNO3

Comes from base

Comes from acid

One base that can be used is Sodium hydroxide.

One acid that can be used is Nitric acid. NaOH + HNO3 NaNO3 + H2O

Negative ion (anion)

Negative ion Acid used

SO42- (sulfate ion)

NO3- (nitrate ion)

Cl- (chloride ion)

Sulfuric acid (H2SO4)

Nitric acid (HNO3)

Hydrochloric acid (HCl)

Quickcheck

What acids do you use to get the following salts?

Sodium nitrate Potassium phosphate Ammonium ethanoate Copper(II) iodide Sodium citrate Aluminium sulfate

Water of crystallization

Many salts combine with water molecules to form crystals.

These water molecules are called water of crystallization.

Salts that contain water of crystallization are called hydrated salts.

Salts that do not contain water of crystallization are called anhydrous salts.

Water of crystallization

Name of salt Formula of anhydrous salt

Formula of hydrated salt

copper(II) sulfate

CuSO4 CuSO4.5H2O

magnesium sulfate

MgSO4 MgSO4.7H2O

sodium carbonate

Na2CO3 Na2CO3.10H2

Ozinc sulfate ZnSO4 ZnSO4.7H2OWhat does the ‘dot’ mean?

Removing water of crystallization Heating a hydrated salt removes water of

crystallization.

CuSO4.5H2OHeat

CuSO4 + 5H2O

hydrated copper(II) sulfate

anhydrous copper(II) sulfate + water

Heat

Removing water of crystallization

Cobalt(II) chloride

CoCl2.6H2OHeat

CoCl2 + 6H2O

hydrated cobalt(II) chloride

anhydrous cobalt(II) chloride + water

Heat

Find out the use of cobalt(II) chloride paper in the lab.

Soluble and insoluble saltsChlorides/ bromides/

iodides

Sulfates Carbonates Nitrates

All are soluble except:

• lead(II) chloride/bromide/iodide

• silver chloride/bromide/iodide

All are soluble except:

• barium sulfate

• calcium sulfate

• lead(II) sulfate

All are NOT soluble except:

• Sodium carbonate

• Potassium carbonate

• Ammonium carbonate

All are soluble.

Sodium salts

Potassium salts

Ammonium salts

All are soluble.

Salt Preparation

There are 3 ways of preparing salts. But to choose which way depends on:

1. Whether the salt is soluble in water?

2. Whether the starting materials are soluble in water?

Methods of preparing saltsIs the salt soluble?

No

Method 3: Precipitation

Yes

Reaction with acids

• Acid + metal• Acid + base

• Acid + carbonateAre the starting materials soluble?

YesNo

Method 1: Reaction of acids with insoluble substances

• Acid + metal• Acid + base

• Acid + carbonate

Method 2: Titration• Acid + alkali

• Acid + carbonate solution

1) Filter the mixture

2) Collect filtrate

Salt solution

Salt crystals (dry with filter paper)

1) Concentrate2) Crystallize3) Filter

Method 1

Recap: Acid + Metal oxide/hydroxide Salt +

Water Acid + Carbonate Salt + Water +

Carbon dioxide Acid + Metal Salt + Hydrogen gas In Method 1, all the substances in

red are insoluble.

Method 1

The acid is reacted with an excess of the substances (metal, carbonate or base).

Why?

AcidInsoluble base

Method 1a

Acid + Metal Salt + Hydrogen gas

ZnSO4

Comes from metal

Comes from acid

Zinc is insoluble in water and reacts with sulfuric acid. We can use zinc here.

Method 1a: Using metal

sulfuric acid zinc sulfate solution +

unreacted zinc

Keep adding zinc until no more effervescence is observed.

zinc sulfate solution

1) Filter the mixture

2) Collect filtrate

zinc sulfate crystals 1) Concentrat

e2) Crystallize3) Filter

Metals that can be used

Only moderately reactive metals like zinc, aluminum, magnesium and iron can be used.

Not suitable for 1. Very reactive metals such as sodium,

potassium and calcium. Why?2. Unreactive metals like copper and

silver. Why?

Method 1b

Acid + Metal oxide/ hydroxide Salt + Water

CuSO4

Comes from metal oxide

Comes from acid

Copper(II) oxide is insoluble in water and reacts with sulfuric acid. We can use Copper(II) oxide here. Why can’t we use copper metal?

Method 1b: Using insoluble base

sulfuric acidCopper(II) sulfate

solution + unreacted

Copper(II) oxide

Keep adding copper(II) oxide until no more effervescence is observed.

Copper(II) sulfate solution

1) Filter the mixture

2) Collect filtrate

Copper(II) sulfate crystals 1) Concentrat

e2) Crystallize3) Filter

Method 1c

Acid + Carbonate Salt + Carbon dioxide + Water

MgCl2

Comes from carbonate

Comes from acid

Magnesium carbonate is insoluble in water and reacts with hydrochloric acid.

NOTE: ALL carbonates are insoluble except potassium, sodium and ammonium carbonate!

Method 1c: Using carbonate

hydrochloric acid

Magnesium chloride solution

+ unreacted magnesium carbonate

Keep adding magnesium carbonate until no more effervescence is observed.

Magnesium chloride solution

1) Filter the mixture

2) Collect filtrate

Magnesium chloride crystals 1) Concentrat

e2) Crystallize3) Filter

Summary of method 1

Filter the crystals and dry them by squeezing them between sheets of filter paper.

Pour the filtrate into an evaporating dish and heat to evaporate most of the water. This produces a hot saturated solution of the

salt. Let the solution cool.

Remove the excess metal/metal carbonate/ base by filtering and collect the filtrate. The filtrate contains the solution of the salt you

want.)

Add the metal/metal carbonate/ base slowly with stirring to hot acid (what acid depends on what salts you want) until no more

dissolves. (This means all the acid is used up)

Method 2: Titration

Notice that all sodium, potassium and ammonium salts are SOLUBLE in water.

So you cannot use Method 1 for any of such salts! Why?

Method 2: Titration

To tell when all the acid has been completely used up, we have to use titration, by using an indicator.

What is an indicator?Indicator Colour in acidic

solutionpH range at which indicator changes colour

Colour in alkaline solution

methyl orange red 3 – 5 yellow

screened methyl orange

violet 3 – 5 green

litmus red 5 – 8 blue

bromothymol blue

yellow 6 – 8 blue

phenolphthalein colourless 8 – 10 pink

Steps to titration (Sodium nitrate)

Fill up a burette with dilute nitric acid and note down the initial burette reading (V1 cm3).

V1 cm3

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

Add one or two drops of methyl orange to the NaOH solution. The solution turns yellow.

Add dilute HNO3 from the burette slowly until the solution turns orange permanently. This is the end-point. The acid is all used up.

V2 cm3

Record the final burette reading (V2 cm3). Hence, the volume of acid required for complete neutralization = (V2 – V1) cm3.

Getting Sodium nitrate

1. Pipette 25.0cm3 of NaOH into a beaker. Then add (V2 – V1) cm3 of dilute nitric acid from the burette. This time do not add indicator. Why?

2. Heat to evaporate the water till it is saturated.

3. Cool the saturated solution so that the salt crystallizes.

4. Filter to collect the crystals.5. Dry the crystals between a few sheets

of filter paper.

Titration

Simulation

Summary of method 2

Record the final burette reading (V2 cm3). Hence, the volume of acid required for complete neutralization = (V2 – V1) cm3.

Add dilute HNO3 from the burette slowly until the solution turns orange permanently. This is the end-point.

Add one or two drops of methyl orange to the alkali solution. The solution turns yellow. (Note, if you have a strong acid and weak

base, you use methyl orange, if you have a strong base and weak acid, use phenolphthalein. If both are strong, you can use either

indicator.

Pipette 25.0 cm3 of dilute alkali (depending on what salt you want) into a conical flask.

Fill up a burette with dilute acid (depending on what salt you want) and note down the initial burette reading (V1 cm3).

Summary of method 2

Dry the crystals between a few sheets of filter paper.

Filter to collect the crystals.

Cool the saturated solution so that the salt crystallizes.

Heat to evaporate the water till it is saturated.

Pipette 25.0cm3 of NaOH into a beaker. Then add (V2 – V1) cm3 of dilute nitric acid from the burette.

Insoluble salts

Easiest to prepare Just need to use precipitation Mix a solution containing the positive

ions of the salt with another solution containing the negative ions of the salt to be prepared.

What salts are insoluble?

Example: Lead(II) sulfate

Using lead(II) nitrate and dilute sulfuric acid

1. First, pour 50 cm3 of lead(II) nitrate solution into a small beaker. Add sulfuric acid (in excess) and stir until no more precipitate forms.

2. Filter and collect precipitate.3. Wash the precipitate with a small

amount of distilled water to remove impurities.

4. Allow the precipitate to dry on filter paper.

Summary of method 3

Allow the precipitate to dry on filter paper.

Wash the precipitate with a small amount of distilled water to remove impurities.

Filter and collect precipitate.

Add another reagent (again depending on what salt you want) and stir until no more precipitate forms.

First, pour 50 cm3 of one reagent (depending on what salt you want) into a small beaker.

Test yourself

How do you get the following salts:1. Magnesium sulfate2. Lead(II) chloride3. Potassium nitrate4. Sodium sulfate5. Zinc nitrate

Questions

Are these salts soluble?1. iron(III) nitrate2. potassium carbonate3. sodium ethanoate4. silver chloride5. lead(II) nitrate6. copper(II) carbonate7. ammonium iodide8. titanium(IV) chloride9. barium sulfate

YesYesYesNoYes No Yes Yes No

Test yourself

Which method will you use to get the following salts:

1. Magnesium sulfate2. Lead(II) chloride3. Potassium nitrate4. Sodium sulfate5. Copper(II) chloride6. Lead(II) carbonate7. Silver chloride8. Zinc chloride

Test Yourself

When an acid Z is added to a solution of lead(II) nitrate, an insoluble precipitate is formed.

When Z is added to a solution of silver nitrate, an insoluble precipitate is formed too.

What acid could Z be?A) hydrochloric acid B) sulfuric acidC) nitric acid

Test Yourself

A metal oxide A dissolves in sulfuric acid, hydrochloric acid and nitric acid and does NOT give any precipitate with any of the acids. Which of the following could be A?

A) Barium oxide B) Calcium oxide

C) Silver oxide D) Sodium oxide