Overview

Prerequisite knowledge:1. Students are able to define acids and bases in terms of the ions they

produce in aqueous solution.2. Students are able to describe the reactions of acids with metals,

carbonates and bases.3. Students are able to describe the reaction between hydrogen ions and

hydroxide ions to form water as neutralization.4. Students are able to differentiate between concentration and strength of

acids and bases.

Specific Instructional Objectives (SIOs):1. By the end of the lesson, students should be able to:2. Describe what pH means and explain how the pH scale is used. 3. Describe the effects of acids and alkalis on Universal Indicator.4. Describe how to test hydrogen ion concentration and relative acidity

using Universal Indicator.

pH and Oxides

Overview of today’s lesson

1. pH2. pH Scale3. Indicators

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 Very alkaline

Neutral

Shows whether a solution is acidic, neutral or alkaline.

pH < 7pH = 7pH > 7

Guess the pH of the following!

Black CoffeeOrange JuiceBleachCoca ColaSea water

53

133.48

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 pHHigh 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

Demonstration

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 Colour in acidic solution

pH 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

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 incidentPhenolphthalein 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.Advantageso more reliable and accurateo portableo 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 + waterSO2(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 hydroxideCaO(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.

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.

Amphoteric Oxides

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

Acid

Does the oxide dissolve?

Unknown oxide

Yes

No

Basic or amphoteric oxide

Acidic or neutral oxide

Alkali

Alkali

Soluble in alkali?

Soluble

Insoluble

Amphoteric oxide

Basic oxide

Soluble in alkali?

Soluble

Insoluble

Acidic oxide

Neutral oxide

Is the oxide soluble in water?Yes

Litmus test

No

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?