C5: Acids, bases and salts

C5: Acids, bases and salts

By: Bart Grootswagers

Learning objectives

• Common acids, their characteristics, where and how they occur

• the pH scale and indicators

• ​the colour changes of useful indicators

• the treatment of acid soils and waste water treatment

• the characteristic properties of bases and salts​

• the ions present in acid and alkali solutions

• acids and alkalis in the analysis of salts

• the preperation of soluble salts by various methods, including titration​

C5.01 What is an acid?

• Is a sour tasting substance

• Can be corrosive, which means they can eat through things

• can be identified by using an indicator, such as

  • litmus

  • methyl orange

  • Universal indicator

C5.01 What is an acid?

pH scale

Multiple Choice

Select the correct answer about the pH scale:

• Acid = pH 8 -14

  Neutral = pH 7

  Alkaline = pH 1 - 6

• Neutral = pH 8 - 16

  Acid = pH 7

  Base = pH 1 -6

• Base = pH 8 -14

  Neutral = pH 7

  Acid = pH 1-6

• Base = pH 8 - 14

  Acid = pH 7

  Neutral = pH 1 - 6

Multiple Choice

Which of the following can be used as an Indicator?

• Water

• methyl orange

• Soap

• Hydrochloric acid

C5.02 Acid and alkali solutions

The importance of water

Some acid are 'acids-in-waiting'. When such a substance (like gaseous hydrochloric acid) is dissolved in an organic solvent it doesn't gain the properties of an acid, but when dissolved in water they create a strong acidic solution.

​

The most useful definition of acid is when it's dissolved in water, this is when they show their acidic properties the best. The same goes for alkaline aqueous solutions​

​

Dilute solution = ​acid or alkaline mixed with a lot of water

Concentrated ​solution = acid or alkaline mixed with little water

C5.04 Acid reactions in everyday life

​Soil pH and plant growth

The pH of a soil usually varies between pH 4 and pH 8.3.

This is affected by:

• peat content

• mineral content

• rotting vegetation

• amount of oxygen

• use of fertilisers​

• pH of rainfall​

  Many plants have a specific pH they prefer​

C5.04 Acid reactions in everyday life

​Soil pH and plant growth

When a soil is too acidic it can be limed to neutralize it. The following can be added to the soil:

• calcium oxide

• calcium hydroxide

• powdered chalk

• limestone (calcium carbonate)

  If the soil is too alkaline the following can be used to neutralize it​

• add peat

• add decaying organic matter ​(compost or manure)

Multiple Choice

When does an acid really show it's an acid?

• When dissolved in an organic solvent

• When dissolved in an alkaline

• When dissolved in water

• When in a gaseous form

Multiple Choice

Which of the following can be added to lower the pH of soil?

• Peat

• Limestone

• Powdered chalk

• Calcium oxide

C5.04 Acid reactions in everyday life

​Effluent and waste water treatment

Liquid waste from factories is often acidic.

​

Often slaked lime is added to neutralize it.

​

To reduce sulfur dioxide factories often spray acidic waste with jets of slaked lime in a flue-gas desulfuriser ('scrubber').​

Some text here about the topic of discussion

Multiple Choice

What is often used to neutralize acidic waste water from factories?

• hydrogen peroxide

• sodium hydroxide

• slaked lime

• dead organic matter

C5.06 Characteristic reactions of acids

The reactions of acids

Three major types of reaction in which acids are involved:

1. reactions with reactive metal

2. reactions with a base

3. reactions with a metal carbonate

   ​

​One product is is present in all these reactions, which is a metal compound which is called a salt in chemistry. In all reactions the hydrogen is replaced by a metal to create a salt. Beside sodium chloride (table salt) there are many others.

C5.06 Characteristic reactions of acids

The reaction of acids with metals

Reactive metals can replace the hydrogen from an acid safely. When this happens hydrogen gas is produced. The salt that is made is depends on the combination of metal and acid used:

metal + acid -> salt + hydrogen

​

For example:

​

magnesium + nitric acid -> magnesium nitrate + hydrogen

Mg(s) + 2HNO₃(aq) -> Mg(NO₃)₂(aq) + H₂(g)

​

zinc + hydrochloric acid -> zinc chloride + hydrogen

Zn(s) + 2HCl(aq) -> ZnCl₂(aq) +H₂(g)​

​

Remember​: s = solid, aq = disolved in liquid, g = gas

C5.06 Characteristic reactions of acids

The reaction of acids with bases and alkalis

When acid reacts with a base salt and water is produced:

​

acid + base -> salt + water

​

The salt ​will depend on the combination of reactants used. For example:

​

sodium hydroxide + hydrochloric acid -> sodium chloride + water

NaOH(aq) + HCl(aq) -> NaCl(aq) + H₂O(l)

​

Remember: l = liquid​

C5.06 Characteristic reactions of acids

The reaction of acids with bases and alkalis

When acid reacts with a carbonate (any substance ending with CO₃) carbon dioxide is produced. The reaction is as follows:

​

acid + metal carbonate -> salt + water + carbon dioxide

​

For example:

​

hydrochloric acid + calcium carbonate -> calcium chloride + water + carbon dioxide

2HCl(aq) + CaCO₃(s) -> CaCl₂(aq) + H₂O(l) + CO₂(g)​

Multiple Choice

Which of the following is a reaction of an acid with a metal?

• Zn(s) + 2HCl(aq) -> ZnCl₂(aq) +H₂(g)​

• 2HCl(aq) + CaCO₃(s) -> CaCl₂(aq) + H₂O(l) + CO₂(g)​

• NaOH(aq) + HCl(aq) -> NaCl(aq) + H₂O(l)

Multiple Choice

Which of the following is a reaction of an acid with an alkaline?

• Zn(s) + 2HCl(aq) -> ZnCl₂(aq) +H₂(g)​

• 2HCl(aq) + CaCO₃(s) -> CaCl₂(aq) + H₂O(l) + CO₂(g)​

• NaOH(aq) + HCl(aq) -> NaCl(aq) + H₂O(l)

Multiple Choice

Which of the following is a reaction of an acid with an carbonate?

• Zn(s) + 2HCl(aq) -> ZnCl₂(aq) +H₂(g)​

• 2HCl(aq) + CaCO₃(s) -> CaCl₂(aq) + H₂O(l) + CO₂(g)​

• NaOH(aq) + HCl(aq) -> NaCl(aq) + H₂O(l)

C5.07 Acids and alkalis in chemical analysis

The test for carbonates using acid

To test is a substance is a carbonate or not the following can be done.

In case of a rock, add some vinegar, if it fizzes it could be limestone.

A better test is ​to add dilute hydrochloric acid to the powdered substance. The gas the is given of is passed through limewater (calcium hydroxide solution) it it turns cloudy the substance is a carbonate.

C5.07 Acids and alkalis in chemical analysis

The test for metal ions in salts using alkalis

Salts are always ionic compounds, the have a positive metal ion bonded with a negative non-metal ion.

​

In order to identify which metal is in a salt we can add an alkali.​

C5.07 Acids and alkalis in chemical analysis

The test for metal ions in salts using alkalis

Colour hydroxide precipitates:

​

Some of the hydroxide precipitates are coloured. As a result, the salt can be dissolved and an alkali can be added the colour of the precipitate will identify the metal:

​

Copper(ii) salts = light blue precipitate

Iron(ii) salts = ​ light green precipitate

Iron(iii) salt ​= red-brown precipitate

hydroxides of calcium, zinc and aluminium = white precipitate​

C5.07 Acids and alkalis in chemical analysis

The test for metal ions in salts using alkalis

​To identify the white precipitate of zinc, calcium and aluminium, the following can be done:

​

Add excess of sodium hydroxide, if it's doesn't dissolve it's calcium (hydroxide)

​

If it does dissolve repeat with ammonia solution, if it dissolves it's zinc, if not it's aluminium​

Multiple Choice

What metal was in the salt?

• iron(ii)

• iron(iii)

• copper(ii)

• calcium

Multiple Choice

What metal was in the salt?

• iron(ii)

• iron(iii)

• copper(ii)

• calcium

Multiple Choice

What metal was in the salt?

• iron(ii)

• iron(iii)

• copper(ii)

• calcium

Multiple Choice

After testing for the metal in the salt a white precipitate was found. To identify it further an excess of sodium hydroxide was added and it didn't dissolve, what was the metal?

• iron(ii)

• zinc

• aluminium

• calcium

C5.07 Acids and alkalis in chemical analysis

The test for ammonium calts using alkali

​Ammonia is an important fertiliser. Ammonium nitrate and ammonium sulfate are created by reacting ammonia with nitric acid or sulfuric acid. The salts contain NH4+ ions. These salts react with alkali solutions to produce ammonia gas, which can be detected by using a damp litmus paper, which will turn red to blue.

​

ammonium nitrate + sodium hydroxide -> sodium nitrate + water + ammonia

NH4NO3(s) + NaOH(aq) -> NaNO3(aq) + H2O(l) + Nh3(g)​

C5.08 Salt

The imporance of salts - an introduction

Salts are formed when from an acid which has it's hydrogen replaced by a metal. Salts are ionic compounds.

​

When chosing a methode to create a salt ask the following:

• Is the salt soluble or insoluble in water?

• De crystals of the salt contain water of crystallisation?

​

​The first point influences the preparation method chosen. The second point affects how the crystals are handled at the end of the experiment.

C5.09 Preparing soluble salts

Method A - Acid plus solid metal, base or carbonate

1. Excess of solid is added to the acid.

2. The excess solid is filtered out

3. Filtrate is slightly heated to evaporate and concentrate the salt solution.

4. When crystals form heating is turned off.

5. Concentrated solution is cooled to let crystals form. The crystals are filtered off and washed with a little distilled water. Then the crystals are dried between filters papers.

C5.09 Preparing soluble salts

Method B Acid plus alkali by titration

1. Acid solution in poured into burette to accurately measure volume. A known alkali solution is placed in flask. Indicator is added.

2. Acid is added to flask until indicator changes colour. Volumes of noted down.

3. Salt solution is evaporated and cooled to form crystals (as with method A).