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Chemistry - Acids and Alkalis
May 13, 2015
This presentation was tailored to meet the GCE 'O' Level Chemistry (Pure) syllabus.
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This presentation was produced by the Science department of Temasek Secondary School. Redistribution or reproduction of this resource is prohibited by copyright regulations.
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
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