Chemistry - Acids and Alkalis

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Acids and AlkalisPhysical and Chemical

Properties

Today’s Lesson

What is an acid?

Examples of acids

What is an alkali?

Examples of alkalis

Acids around us

Lactic acid

Carbonic acid

Formic acid

Ethanoic acid

Acids around us

Malic acid Tartaric acidCitric acid

What is common among all these

acids?

Properties of Acids

1. Acids have sour taste.

2. Acids are corrosive.

3. Acids dissolve in water to form

solutions which conduct electricity.

Acids in the Laboratory

Hydrochloric acid, HCl

Sulfuric acid, H2SO4

Nitric acid, HNO3

STRONG acids

NEVER EVER TASTE ACIDS IN THE LAB!

Acids in Food

o Citric acid (in lemons)o Ethanoic acid (in vinegar)o Carbonic acid (in soft drinks)o Tartaric acid (in grapes)o Malic acid (in apples)o Lactic acid (in yoghurt)

Weak organic acids found in food

These acids are safe to consume!

An acid is a substance which produces hydrogen ions when it is dissolved in

water.

The hydrogen ions give an acid its acidic properties.

Acids only behave as acids when they are dissolved in water.

What is an Acid?

What is an Acid?Video showing dissociation of hydrogen chloride, HCl in water:

http://www.youtube.com/watch?v=N3hPh02_EWs&feature=related (play until about 26s)

How is an Acid Formed?

Molecules of hydrogen chloride gas

Before dissolving in water…

H Cl

How is an Acid Formed?

Ions of dilute hydrochloric acid

After dissolving in water…

This process is called ionisation.

Examples of Ionisation

H2SO4 (aq) 2H+ (aq) +

SO42- (aq)Sulfuric acid Hydrogen

ionsSulfate

ions

HCl (aq) H+ (aq) +

Cl- (aq)Hydrochloric acid

Hydrogen ions

Chloride ions

HNO3(aq) H+ (aq) +

NO3- (aq)Nitric acid Hydrogen

ionsNitrate

ions

Do you notice a difference between the ionisation of hydrochloric acid and sulfuric

acid?

Examples of Ionisation

H2SO4 (aq) 2H+ (aq) +

SO42- (aq)

HCl (aq) H+ (aq) +

Cl- (aq)

HNO3(aq) H+ (aq) +

NO3- (aq)

The properties and reactions of acids are due to the presence of these hydrogen

ions, H+ (aq).

TAKE NOTE

NOT all substances that contain hydrogen are acids.

For example, •NH3 (ammonia) contains hydrogen but it is not an acid. •H2 (hydrogen) is also not an acid.

Alkalis around usAlkalis around

us

Detergent

Shampoo Soap

Antacid

Toothpaste

What is common

among all these alkalis?

Properties of Alkalis

1. Alkalis have bitter taste.

2. Alkalis have soapy feel.

3. Alkalis dissolve in water to form

solutions which conduct electricity.

Alkalis in the Laboratory

Sodium hydroxide, NaOH

Potassium hydroxide,

KOH

Alkalis in the Laboratory

Aqueous ammonia, NH3

(aq)

Calcium hydroxide, Ca(OH)2

NEVER EVER TASTE ALKALIS IN THE LAB!

• An alkali is a substance which produces hydroxide ions when it is dissolved in water.

• The hydroxide ions, OH-(aq) give an alkali its alkaline properties.

• Alkalis only behave as alkalis when they are dissolved in water.

What is an Alkali?

An acid is a substance which produces hydrogen ions when it is dissolved in water.

Summary

What is an acid?

Hydrochloric acid, sulfuric acid, nitric acid, ethanoic acid, citric acid …

Examples of acids?

An alkali is a substance which produces hydroxide ions when it is dissolved in water.

Summary

What is an alkali?

Sodium hydroxide, potassium hydroxide, calcium hydroxide, aqueous ammonia

Examples of alkalis?

Today’s Lesson

Effect of acids and alkalis on litmus

paper

Reactions of acids and alkalis using

word equations

Indicatorso An indicator is a substance which changes

colour depending on whether the solution tested is acidic or alkaline.

o There are many types of indicators, including the universal indicator and litmus indicator.

Litmus Indicator

o Litmus paper exists in two colours, red and blue.

Litmus Indicator

Acids turn blue litmus paper red.

Red litmus paper remains red.

What about red litmus paper in acid?

Litmus Indicator

Alkalis turn red litmus paper blue.

Blue litmus paper remains blue.

What about blue litmus paper in alkali?

Litmus Indicator

A solution is tested with a piece of red litmus paper and the paper stays red.

Sue says that the solution is acidic.

Marcus says he is not sure and that the solution should also be tested with a piece of blue litmus paper.

Who is correct?

Marcus!

Reactions of Acids and Alkalis

3 important reactions

1.ACIDS + METALS (SOME)

2.ACIDS + CARBONATES

3.ACIDS + ALKALIS

Acids + Metals

Acids react with some metals to form a salt and hydrogen gas.

hydrochloric acid magnesiu

m

magnesium chloride

hydrogen gas+ +

sulfuric acid magnesium

magnesium sulfate

hydrogen gas+ +

1

Acid + Metal Salt + Hydrogen gas

Acids + Metals

The type and name of salt depends on the acid used.

Reaction with

• Hydrochloric acid

• Sulfuric acid

• Nitric acid

Chloride salts

Sulfate salts

Nitrate salts

Acids + Metals

Acids react with some metals to form a salt and hydrogen gas.

Some metals which can react with acids are magnesium, aluminium, zinc and iron.

Explosive reactions occur when using reactive metals like lithium, sodium and potassium.

Exception:

Unreactive metals such as copper and silver do not react with dilute acids.

Acids + Metals

How to test for hydrogen gas? Place a lighted splint in the presence of the gas.

If hydrogen is present, the lighted splint will be extinguished with a ‘pop’ sound.

Acids + MetalsVideo: http://www.youtube.com/watch?v=uprJfeemxoU

Acids + Carbonates

Acids react with carbonates to form a salt, carbon dioxide gas and water.

2

hydrochloric acid

sodium carbonate

sodium chloride

carbon dioxide

gas+ +

nitric acid calcium carbonate

calcium nitrate

+

water+

carbon dioxide

gas+ water+

Acid + Carbonate Salt + Carbon dioxide gas + Water

Acids + Carbonates

How to test for carbon dioxide gas? Bubble the gas through limewater.

Carbon dioxide forms a white precipitate with limewater.

Acids + Carbonates

How to test for carbon dioxide gas? Bubble the gas through limewater.

Carbon dioxide forms a white precipitate with limewater.

Limewater

Bubble carbon

dioxide gasWhite

precipitate formed

Acids + Alkalis

Acids react with alkalis to form a salt and water only.

The reaction between acids and alkalis is called

neutralisation.

3

Acid + Alkali Salt + Water

Acids + Alkalis

Examples:

hydrochloric acid

sodium hydroxide

sodium chloride

Water+ +

sulfuric acidpotassium hydroxide

potassium sulfate

Water+ +

Summary

3 important reactions

1.Acid + Metal

2.Acid + Carbonate

3.Acid + Alkali

Salt + Hydrogen gas

Salt + Water + Carbon dioxide gas

Salt + Water