Overview
 Prerequisite knowledge:
1.
2.
3.
4.
Students are able to define acids and bases in terms of the ions they produce in
aqueous solution.
Students are able to describe the reactions of acids with metals, carbonates and bases.
Students are able to describe the reaction between hydrogen ions and hydroxide ions
to form water as neutralization.
Students are able to differentiate between concentration and strength of acids and
bases.
 Specific Instructional Objectives (SIOs):
1.
2.
3.
4.
By the end of the lesson, students should be able to:
Describe what pH means and explain how the pH scale is used.
Describe the effects of acids and alkalis on Universal Indicator.
Describe how to test hydrogen ion concentration and relative acidity using Universal
Indicator.
pH and Oxides
Overview of today’s lesson
pH
2. pH Scale
3. Indicators
1.



Universal Indicator
Other indicators
pH probe
Acids and Bases
 How do you measure the strength or concentration
of acids or bases?
 pH Scale.
pH Scale
Very acidic
Neutral
Very alkaline
 Shows whether a solution is acidic, neutral or
alkaline.
 pH < 7
 pH = 7
 pH > 7
Guess the pH of the following!
 Black Coffee
 Orange Juice
 Bleach
 Coca Cola
 Sea water
5
3
13
3.4
8
What exactly does pH mean?
 pH is a measure of the concentration of H+ and OH-
ions present in a solution.
 pH = log10[H+]
High and low pH
 High concentration of hydrogen ions – low pH
 High concentration of hydroxide ions – high pH
Hydrogen ion
Hydroxide ion
pH scale
 One unit of change in pH represents a change of
concentration of H+ ions or OH- by ten times!
Quickcheck!
Hydrogen ion
Hydroxide ion
Match the solutions to the correct pH values.
pH = 7
pH = 2
pH = 11
Determination of pH
 How do we determine the pH of a solution?
 One way is by using an indicator.
 A pH indicator is a chemical compound that
changes colour in solutions of different pH.
 Why is it called an indicator?
Universal Indicator
 The Universal Indicator is one of the most useful
indicators.
 It gives different colours in solutions of different pH.
Universal Indicator
pH and strength of acids/alkalis
 pH can be used to compare the strength of acids or
alkalis of the same concentration.
 For instance, from the demonstration, you can tell
HCl is a stronger acid than ethanoic acid by the pH
value.
Other kinds of indicators
Indicator
methyl orange
screened methyl
orange
litmus
bromothymol blue
phenolphthalein
Colour in acidic
solution
pH range at
which indicator
changes colour
Colour in
alkaline
solution
red
3–5
yellow
violet
3–5
green
red
5–8
blue
yellow
6–8
blue
colourless
8 – 10
pink
What exactly are indicators?
 They themselves are weak acids or bases.
 Indicators normally have two or more forms at
different pH.
 The colour of the different forms result in the colour
of the solutions turning into different colours.
 Why must indicator be added in small amounts?
What happened?
 Indicators used in demonstration at the start of
lesson are:


Phenolphthalein
Methyl Orange
 In the second demonstration, when C (HCl with
methyl orange, hence it is red) is mixed with B
(excess NaOH), HCl is neutralized and since there is
excess NaOH, the solution turns yellow in the
presence of methyl orange.
 What about the first demonstration?
Phenolphthalein in the news
 Po Chai Pills incident
 Phenolphthalein is possibly carcinogenic!
Po Chai Pills in capsule form
Po Chai Pills in pill form
Dove commercial
pH meter
 In the laboratory, we usually use a pH probe
attached to a data logger, or a pH meter.
 Inserting the probe into the solution will give you the
pH accurately.
 Need to be calibrated.
 Advantages
o
more reliable and accurate
o
portable
o
can be used in data logging to record changes in pH
By now you should know…
 What pH represents.
 How the pH scale is used.
 What indicators are.
 The effects of acids and alkalis on the Universal
Indicator.
Just for fun
Jonathan got stung
by a bee while
walking towards the
canteen at HCI. You
are
given
two
substances
–
Colgate and Vinegar.
Which will you apply
on Jonathan?
Homework
 The pH of our blood is about 7.4.
 Find out what happens to our body system if the pH
of blood drops too low, or rises too high.
pH and Soil
 Best pH for most plants is about 6 to 7.
 Too much fertiliser can make the soil too acidic.
 Why is it that Singapore soil is mostly acidic?
Control of Soil pH
 If soil is too acidic, we can add quicklime (calcium




oxide) or slaked lime (calcium hydroxide).
Different from lime juice!
If the soil is too basic, we can add ammonium
phosphate or iron sulfate.
Take a look at this.
Orchids?
http://www.uvm.edu/pss/ppp/pubs/oh34.htm
Oxides
 What are oxides?
 Oxides are compounds of oxygen and another
element only.
 Are these oxides?
 CO2, CO, HNO3, H2O, CuSO4, CaO, NaOH.
Oxides
 4 kinds of oxides.
 Acidic, Basic, Neutral and Amphoteric.
Acidic Oxides
 Acidic oxides are usually formed from oxides of non-
metals.
Acidic Oxides
 Most dissolve in water – forms acids in water.
 For example, sulfur dioxide.
 sulfur dioxide + water → sulfurous acid
 SO2(g) + H2O(l) → H2SO3(aq)
 (note: not sulfuric acid)
 What do you think gives you sulfuric acid when
dissolved in water?
Acidic Oxides
 carbon dioxide + water → carbonic acid
 CO2(g) + H2O(l) → H2CO3(aq)
 phosphorus pentoxide + water → phosphoric acid
 P4O10(s)+ 6 H2O(l) → 4 H3PO4(aq)
Acidic Oxides
 Do NOT react with acids.
 React with alkalis to form salt and water.
 sulfur dioxide + sodium hydroxide → sodium sulfate
(IV) or sodium sulfite + water
 SO2(g) + 2NaOH(aq) → Na2SO3(aq) + H2O(l)
 sulfur trioxide + sodium hydroxide →??
 carbon dioxide + calcium hydroxide →?? (familiar?)
Some examples of Acidic Oxides
Acidic Oxide
Formula Physical
state
Acid produced in water
Name
Formula
carbon dioxide
CO2
Gas
carbonic acid
H2CO3
sulfur trioxide
SO3
Gas
sulfuric acid
H2SO4
phosphorus(V) oxide
P4O10
Solid
phosphoric acid
H3PO4
silicon dioxide
SiO2
Solid
-
-
What kind of bonding
exists here?
What is the structure of
silicon dioxide?
Silicon dioxide
 Some acidic oxides do not dissolve in water.
 For example silicon(IV) oxide is a solid at room
temperature and does not dissolve in water
 It reacts with NaOH to form sodium silicate (a salt)
and water.
 silicon (IV) oxide + sodium hydroxide → sodium
silicate + water
 SiO2(s) + 2NaOH(aq) → Na2SiO3(aq) + H2O(l)
Acid Rain
 Formed when acidic air pollutants (which are mostly
acidic oxides) such as sulfur dioxide and nitrogen
dioxide. Can go lower than 4! (As acidic as coke!)
 SO2(g) + H2O(l) → H2SO3(aq)
 Sulfurous acid can be oxidized to sulfuric acid (H2SO4) by
oxygen.
 4NO2(g) + 2H2O(l) + O2(g) → 4HNO3(aq)
 Normally, unpolluted rain is slightly acidic (around 6) due
to carbonic acid because of carbon dioxide.
 CO2(g) + H2O(l) → H2CO3(aq)
Effects of acid rain
 Look at this video.
 Acid rain reacts with metals and carbonates in
marble and limestone, thus destroying
buildings, monuments and bridges.
 Kills fish and other aquatic life by lowering pH
of water bodies to below 4.
 Leaches important nutrients from soil and
destroy plants.
 Find out how these acidic pollutants are formed!
Uses of sulfur dioxide
 Sulfur dioxide acts as a bleaching agent by
removing oxygen from them.
 It is a reducing agent.
 When added to coloured wood pulp for making
peper, sulfur dioxide is added to the wood pulp to
remove oxygen from the dye, thus bleaching the pulp
to white.
Uses of sulfur dioxide
 Sulfur dioxide is used as a food preservative.
 It is poisonous to all organisms, especially bacteria.
 Added in small amounts to prevent mould and
bacteria growing.
 Why must the addition of sulfur dioxide be strictly
controlled in the food industry?
Basic Oxides
 Basic oxides are oxides of metals.
 Many are insoluble.
 For those that can dissolve in water, they are called
alkalis.
 E.g. Sodium oxide (Na2O) and potassium oxide
(K2O).
Basic Oxides
 Example – calcium oxide.
 When CaO is added to water, calcium hydroxide is formed,




which is sparingly soluble in water.
The reaction is as follows:
calcium oxide + water → calcium hydroxide
CaO(s) + H2O(l) → Ca(OH)2(aq)
Watch video.
Basic Oxides
 Basic oxides react with acids to form salt and water




only. (Are they bases?)
Example:
calcium oxide + nitric acid  calcium nitrate + water
CaO(s) + 2HNO3(aq)  Ca(NO3)2(aq) + H2O(l)
What reaction is this?
Flue Gas Desulfurization
 Removing sulfur dioxide from waste gases when
burning fossil fuels.
 These waste gases are called flue gases and the
process of removing sulfur dioxide from these gases
is called desulfurization.
 Watch this video.
 Another way of desulfurization uses basic oxides like
calcium oxide, or carbonates like calcium
carbonate. Read about this in your textbook.
Rust
 Rust is a complex mixture consisting of Fe2O3·nH2O,
Fe(OH)3, Fe3O4 etc, formed in the presence of
moisture and oxygen, and accelerated by the
presence of electrolytes.
 These are basic oxides/hydroxides.
 How would you suggest removing rust?
 Group activity: Imagine I have an iron nail. I want to
store it in water for ten years without it rusting.
What way can you suggest? Note: I am looking out
for the cheapest and most efficient way.
Amphoteric Oxides
 Can behave as either acidic or basic oxide.
 Example, zinc oxide, aluminium oxide and lead(II)
oxide.
 Metallic oxides that react with both acids and bases
to form salts and water.
Zinc Oxide
hydrochloric acid + zinc oxide → zinc chloride + water
(acid
+ basic oxide → salt
+ water)
2HCl(aq)
+ ZnO (s)
→ ZnCl2 (aq)
+ H2O(l))
zinc oxide
+ sodium hydroxide → sodium zincate + water
(acidic oxide + base
→ salt
+ water)
ZnO (s)
+ 2NaOH(aq)
→ Na2ZnO2(aq)
+ H2O(l))
It behaves as both a basic oxide and an acidic
oxide.
Neutral Oxides
 Mostly non-metallic oxides.
 Do not show any acidic or basic properties.
 They are mostly insoluble in water.
 Examples: H2O, CO, NO.
 What is dihydrogen monoxide?
Dihydrogen Monoxide hoax
Corresponding Hydroxides
 NOTE: Corresponding hydroxides of the metal
oxides will have the same characteristics as their
oxides!
 For example, sodium hydroxide will be basic (same
as sodium oxide), zinc hydroxide will be amphoteric
(same as zinc oxide) etc.
Classification of unknown oxide
 Imagine you are a scientist working in ASTAR. You
are now given a sample containing an oxide. How do
you determine if it is basic, acidic, amphoteric or
neutral?
 Come up with a method to classify an unknown
oxide, preferably using a flow chart.
 Hint: Some reagents you can use are hydrochloric
acid and sodium hydroxide.
Classification of unknown Oxide
Is the oxide soluble in water?
Yes
No
Alkali
Soluble
Litmus test
Amphoteric
oxide
Yes
Acid
Soluble in alkali?
Basic or amphoteric oxide
Alkali
Does the oxide
dissolve?
Unknown oxide
Insoluble
Soluble
Basic oxide
Acidic oxide
No
Soluble in alkali?
Insoluble
Acidic or neutral oxide
Neutral oxide
Quickcheck!
 When dilute hydrochloric acid was added to
unknown oxide Z, it dissolved. When sodium
hydroxide was added to Z, it did not dissolve. What
kind of oxide is Z? Can you give an example of what
Z can be?
 When dilute hydrochloric acid was added to
unknown oxide M, it did not dissolve. When sodium
hydroxide was added to M, it did not dissolve as
well. What kind of oxide is M?