S3 CHEM CHAPT 11 Acid and Bases part 2

S3 CHEM CHAPT 11 Acid and Bases part 2

By Mr Choo Shi Qian

Lesson Objectives

At the end of the lesson you should be able to:

1. describe the meanings of the terms alkali in terms of the ions it produce in aqueous solution

2. ​describe the reaction between hydrogen ions and hydroxide ions to produce water H⁺ + OH^– → H₂O, as neutralisation

3. ​describe the characteristic properties of bases in reactions with acids and with ammonium salts

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Recap

In Chapter 6 – Ionic Bonding, we learnt about the type of particles (cations and anions) present in ionic compounds. We learnt to use structure to explain the physical properties of ionic compounds for e.g. ionic compounds are good conductors of electricity in aqueous solution and when molten as they have freely moving ions (cations and anions).

Subject | Subject

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Introduction

In the second part of this chapter – Bases and Alkalis (Chapter 11), we are going to learn about physical properties of bases & alkalis. You will also learn about chemical properties of bases and alkalis. By chemical properties, we meant chemical reactions of bases and alkalis. You have learnt some of these reactions in lower Secondary Science and in the previous set of notes on Properties of Acids. Can you recall any of these reactions?

Subject | Subject

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Open Ended

Can you state a chemical reaction that involved bases or alkali?

2.1 Definition of Base

What is a base?

Any substance which reacts with acid to produce salt and water.

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Open Ended

Recall and try to list down substance that will react with acid to produce salt and water only.

2.2 Classification of Bases

Based on the definition of bases, the following substances are classified as bases as they can react with acids and neutralise them:

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1. metal oxides, such as copper oxide

2. metal hydroxides, such as sodium hydroxide

3. ammonia

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Open Ended

List four examples of bases (apply the definition of bases)

 (Hint : Bases are ionic compounds – think cations and anions)

Open Ended

Complete the table by stating two more example of base (metal oxides). Hint: Many possible answers – to arrive at the answer : think of the list of metal ions (cations) and combined with oxide ion to deduce various examples of metal oxides and metal hydroxides.

Open Ended

Complete the table by stating two more example of base (metal hydroxides). Hint: Many possible answers – to arrive at the answer : think of the list of metal ions (cations) and combined with hydroxide ion to deduce various examples of metal oxides and metal hydroxides.

2.3 Physical Properties of Bases

1. Bases have high melting and boiling points (non-volatile substances). The natural physical state of bases at r.t.p is solid state. Metal hydroxides which are soluble in water can also be in aqueous state.

2. ​Only some bases are soluble in water.

3. Bases are good conductor of electricity when molten and in aqueous solutions​

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Open Ended

Recall and apply the concept of bonding and structure to explain why bases which are ionic compounds have high melting and boiling points. (Chapter 6 – Ionic Bonding)

How do we know whether a base is soluble in water?

Follow the following guidelines:

1. metal oxides and hydroxides of all Group I elements;​

2. ​ metal oxides and hydroxides of last four Group II elements;

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Open Ended

List down all the group I metal oxides

Open Ended

List down all the group I metal hydroxides

Open Ended

List down all the group II metal oxides

Open Ended

List down all the group II metal hydroxides

What happens when a water-soluble base dissolves in water?

1. Metal oxides react with water to form aqueous solution of corresponding metal hydroxides.

2. Solid metal hydroxides dissolve in water to form aqueous solution of metal hydroxides.

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Metal oxides react with water to form aqueous solution of corresponding metal hydroxides.

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Metal oxides react with water to form aqueous solution of corresponding metal hydroxides.

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Example 1: Potassium oxide dissolves in water and react to form aqueous potassium hydroxide (aka potassium hydroxide solution).

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Word Equation​

 Potassium oxide(s)  +  water(l) -->  potassium hydroxide solution(aq)

 

Balanced Chemical Equation

  K₂O(s)  +  H₂O(l)  --> 2KOH(aq)​

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Metal oxides react with water to form aqueous solution of corresponding metal hydroxides.

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Example 2: Dissolving of calcium oxide, a water-soluble base, in water.

 

Word Equation​

calcium oxide(s)  +  water(l)  -->  calcium hydroxide solution(aq)

 

Balanced Chemical Equation

​CaO(s)  +  H₂O(l)  -->  Ca(OH)₂(aq)​

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Solid metal hydroxides dissolve in water to form aqueous solution of metal hydroxides.

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Example 1: Sodium hydroxide pellets dissolve in water to aqueous sodium 

                   hydroxide (aka sodium hydroxide solution).

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Example 2: Potassium hydroxide pellets dissolve in water to aqueous potassium 

                   hydroxide (aka potassium hydroxide solution).

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Physical states of water-soluble bases (alkalis) and bases which are insoluble in water.​

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Bases are good conductor of electricity when molten and in aqueous solutions

Recap

Bases are ionic compounds. (Recall and Apply the concept of why ionic compounds are good conductors of electricity in Chapter 6 – Ionic Bonding)

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Bases are good conductor of electricity when molten and in aqueous solutions

Recap

Example 1

Explain why aqueous barium hydroxide is a good conductor of electricity at r.t.p. [1]

      

The barium ions and hydroxide ions (or Ba²⁺ and OH^- ions) in aqueous barium hydroxide are freely moving (or mobile) hence these ions can move about throughout the substance and act as charge carriers.

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Bases are good conductor of electricity when molten and in aqueous solutions

Recap

Example 2

Explain why white solid of barium oxide cannot conduct electricity but once the white solid dissolved in water, the colourless aqueous solution can conduct electricity. [2]

Draw

Lithium oxide is a solid at r.t.p.

When lithium oxide dissolves in water, it reacts with water to produce an aqueous alkaline solution. Write a balanced chemical equation for the reaction between lithium oxide and water.

Draw

Lithium oxide is a solid at r.t.p.

When lithium oxide dissolves in water, it reacts with water to produce an aqueous alkaline solution. What is the strength of the alkaline solution obtained?

                    Write an equation to show the ionisation of the alkaline solution in water. Include state symbols.

Draw

Draw the electronic structure of lithium oxide. 

        You only need to show outer shell electrons.

Open Ended

Explain, in terms of bonding and structure, why lithium oxide is a solid at r.t.p.

Open Ended

Compare the electrical conductivity of lithium oxide and the alkaline 

 solution  at r.t.p. Explain your reasoning.

2.4 Chemical Property of Bases

Also known as chemical reactions of bases​

General word equation for the reaction of bases:

BAWS​

Base + Acid --> Water + Salt ​

This reaction is also known as neutralisation reaction.​

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2.4 Chemical Property of Bases

Also known as chemical reactions of bases​

Recap : From the earlier slides, we have learned that base can be either a metal oxide or metal hydroxide.

BAWS​: Base + Acid --> Water + Salt ​

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​metal oxide  +  dilute acid  --->   salt  +  water

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metal hydroxide  +  dilute acid ---> salt  +  water

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Multiple Select

Which of the following are the products of the following reaction:

lead(II) oxide + dilute nitric acid -->

• lead nitrate

• lead nitric

• water

• hydrogen gas

Draw

Construct a balanced chemical equation for the following reaction: lead(II) oxide + dilute nitric acid --->  lead(II) nitrate  +  water

Multiple Select

Which of the following are the products of the following reaction:

aluminium hydroxide + dilute sulfuric acid -->

• aluminium nitrate

• aluminium sulfate

• water

• hydrogen gas

Draw

Construct a balanced chemical equation for the following reaction: lead(II) oxide + Al(OH)₃  +  3H₂SO₄ →  Al₂(SO₄)₃ +  3H₂O

2.5 Definition of Alkalis

What is an alkali? (Define alkali)

A substance which produces hydroxide ions in aqueous solution*.

Recall aqueous solution is a mixture. This mixture is obtained when alkali (solute) dissolves in water (solvent.)

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Singular Noun: Alkali             Plural Noun: Alkalis​

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When sodium hydroxide dissolves in water, an aqueous solution of sodium hydroxide is formed.

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2.5 Definition of Alkalis​

In aqueous solution of sodium hydroxide, each formula unit of sodium hydroxide ionises to produce 1 sodium ion and 1 hydroxide ion.

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2.5 Definition of Alkalis​

These sodium and hydroxide ions are freely moving, hence aqueous solutions of alkalis are good conductors of electricity (physical property).

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2.5 Definition of Alkalis​

The hydroxide ions are also responsible for the chemical properties of alkalis i.e. chemical reactions of alkalis with acids, ammonium salts, aqueous solution of metal salts and for changing the colour of indicators.

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2.5 Definition of Alkalis​

Open Ended

State the physical state of alkalis and explain the importance of being in this physical state

Not all alkalis are metal oxides and metal hydroxides.

An example of an alkali that is not a metal oxide and metal hydroxide is aqueous ammonia. 

When ammonia gas dissolves in water, an aqueous solution of ammonia is formed. This solution is known as aqueous ammonia or ammonia solution.

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2.5 Definition of Alkalis​

In aqueous solution of ammonia, some of the ammonia molecules react with water molecules to form ammonium ions and hydroxide ions. One ammonia molecule reacts with one water molecule to produce 1 ammonium ion and 1 hydroxide ion. Please note that aqueous ammonia is not known as ammonium hydroxide although it contains ammonium ion and hydroxide ion.

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2.5 Definition of Alkalis​

Balanced chemical equation showing the ionisation of ammonia molecules to produce hydroxide ion

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NH₃(aq) + H₂O(l) --> NH₄⁺(aq) + OH^-(aq)

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2.5 Definition of Alkalis​

List of Alkalis (In O Level Syllabus)

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• aqueous ammonia, NH₃(aq)

• Water soluble bases: (refer to earlier slides)

1. metal oxides and hydroxides of all Group I elements;

2. metal oxides and hydroxides of last four Group II elements;

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2.5 Definition of Alkalis​

2.6 Classification of Alkalis - Strength of Alkalis

Alkalis can be classified according to their strength. What is strength of alkali? (Define strength of alkali)

The extent of ionisation of alkali to produce hydroxide ions in aqueous solutions.

Alkalis can be classified into two groups according to their strength:

1. Strong alkalis

2. Weak alkalis

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2.6 Classification of Alkalis - Strength of Alkalis

Define strong alkali – derived from the definition of alkalis.

A substance which ionise completely to produce hydroxide ions in aqueous solution.

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Define weak alkali – derived from the definition of alkalis.

A substance which ionise partially to produce hydroxide ions in aqueous solution.

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Draw

Construct a balanced chemical equation to show the ionisation of potassium hydroxide in aqueous solution (when alkali dissolves in water) Include state symbols.

Draw

Construct a balanced chemical equation to show the ionisation of rubidium hydroxide in aqueous solution (when alkali dissolves in water) Include state symbols.

Multiple Choice

Which of the following shows the chemical formulae of calcium hydroxide

• CaOH

• CaOH₂

• Ca(OH)₂

• CaO₂H₂

Draw

Construct a balanced chemical equation to show the ionisation of calcium hydroxide in aqueous solution (when alkali dissolves in water) Include state symbols.

Multiple Choice

Which of the following shows the chemical formulae of barium hydroxide

• BaOH

• BaOH₂

• Ba(OH)₂

• BaO₂H₂

Draw

Construct a balanced chemical equation to show the ionisation of barium hydroxide in aqueous solution (when alkali dissolves in water) Include state symbols.

Multiple Choice

Which of the following shows the chemical formulae of aqueous ammonia

• NH_³

• NH₄

• NH₄⁺

• NH₃⁺

Draw

Construct a balanced chemical equation to show the ionisation of aqueous ammonia in aqueous solution (when alkali dissolves in water) Include state symbols.

2.7 Physical Properties of Alkalis

The following are the physical properties of alkalis:

1. Alkalis have a bitter taste.

2. Alkalis feel soapy.

3. Aqueous solutions of alkalis are good conductor of electricity

4. All alkalis are soluble in water.

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2.8 Chemical Properties of Alkalis

Also known as chemical reactions of alkalis​

General word equations for the reactions of alkalis:

1. alkali  +  dilute acid -->  salt  +  water

2. alkali  +  ammonium salt --> salt  +  ammonia gas  +  water

3. alkali    +  aqueous solution of metal B salt -->  metal B hydroxide  +  salt

(containing cation       (containing cation of metal B)                (contain cation of metal A)                

                                                                                                                                                                               

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2.8 Chemical Properties of Alkalis

a) Reaction between alkalis and dilute acids

This reaction is also known as neutralisation reaction.​

 

General word equation:

 

 alkali + dilute acid ---> salt  +  water

 

       

    

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2.8 Chemical Properties of Alkalis

a) Reaction between alkalis and dilute acids

​Ionic equation of neutralisation reaction between acid and alkali:

Include state symbols.

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H⁺(aq)  +  OH^-(aq)  ---> H₂O(l)

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2.8 Chemical Properties of Alkalis

Understanding the ionic equation of neutralisation reaction between acid and alkali

​An ionic equation is a chemical equation thst shows the participating ions which took part in the reaction to form the products. Recall that hydrogen ions of acids and hydroxide ions of alkalis are responsible for the chemical properties of acids and alkalis. Thus in the neutralisation reaction, hydrogen ions react with hydroxide ions to form water, a neutral compound with a pH value of 7.

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Multiple Select

Complete the following word equation by choosing the products produce in this reaction:

potassium hydroxide + dilute sulfuric acid --->

• potassium sulfate

• potassium sulfuric

• water

• hydrogen gas

Draw

Construct a balanced chemical equation for the following reaction:

potassium hydroxide + dilute sulfuric acid --->  potassium sulfate  + water

Draw

Construct an ionic equation (with state symbols) for the following reaction:

potassium hydroxide + dilute sulfuric acid --->  potassium sulfate  + water

Multiple Choice

Complete the following word equation by choosing the products produce in this reaction:

aqueous ammonia + dilute hydrochloric acid --->

• Ammonium chloride

• Ammonia chloride

• water

• hydrogen gas

NH₃  +  H₂O  --> NH₄⁺ + OH^-

Hence, when aqueous ammonia react with hydrochloric acid: NH₄⁺  +  OH^- + HCl -->  NH₄Cl  +  H₂O

which is equivalent to NH₃ + H₂O + HCl  -->  NH₄Cl  +  H₂O

On cancelling out water which appears on both sides of the equation: NH₃  +  H₂O + HCl   NH₄Cl  +  H₂O

The final equation is NH₃  + HCl   NH₄Cl

Recall the ionisation equation of aqueous ammonia

aqueous ammonia + dilute hydrochloric acid -->  ammonium chloride

Word equation

2.8 Chemical Properties of Alkalis

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2.8 Chemical Properties of Alkalis

b) Reaction between alkalis (except aqueous ammonia) and ammonium salts In order for this reaction to take place, the reaction mixture must be warmed.

General word equation:

 

 alkali + ammonium salt --> salt  +  ammonia gas  +  water

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Multiple Select

Complete the following word equation by choosing the products produce in this reaction:

sodium hydroxide + ammonium carbonate --->

• sodium carbonate

• ammonia

• water

• carbon d

Draw

Construct a balanced chemical equation for the following reaction:

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

Multiple Select

Complete the following word equation by choosing the products produce in this reaction:

Rubidium hydroxide + ammonium sulfate --->

• Rubidium sulfate

• ammonia

• water

• hydrogen

Draw

Construct a balanced chemical equation for the following reaction:

rubidium hydroxide + ammonium sulfate ---> rubidium sulfate + ammonia + water

Multiple Select

Complete the following word equation by choosing the products produce in this reaction:

Calcium hydroxide + ammonium chloride --->

• Calcium chloride

• ammonia

• water

• carbon dioxide

Draw

Construct a balanced chemical equation for the following reaction:

Calcium hydroxide + ammonium chloride ---> calcium chloride + ammonia + water

The setup on the right is used to test the gas produced when an alkali is heated with an ammonium salt.

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2.8 Chemical Properties of Alkalis

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Open Ended

What will be observed* when an alkali is heated with an ammonium salt? *Observations – made by seeing (colour change, change in physical state effervescence, vigorous bubbling etc), hearing (sound produced) & smelling (gas with a distinct smell is produced)

Open Ended

Effervescence is not observed when this reaction takes place. Explain why.

Multiple Choice

Reactions of alkalis with ammonium salts produce ammonia gas.

     How do we prove that ammonia gas is produced?

• Apply a moist red litmus paper to the gas

• Apply a moist blue litmus paper to the gas

• Apply a red litmus paper to the gas

• Apply a blue litmus paper to the gas

Multiple Select

Reactions of alkalis with ammonium salts produce ammonia gas.

     Which of the following is a possible observation when ammonia gas is produced in the reaction?

• moist red litmus paper turn blue

• moist blue litmus paper turn red

• moist red litmus paper turn white

• moist blue litmus paper remain blue

Test yourself 3

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Multiple Select

Complete the word equation and write the chemical equation for each of the reactions. State symbols are not required.

zinc oxide + dilute hydrochloric acid -->

• Zinc chloride

• Zinc chloric

• water

• hydrogen

Multiple Select

Complete the word equation and write the chemical equation for each of the reactions. State symbols are not required.

barium hydroxide + dilute ethanoic acid -->

• Barium ethanoate

• Barium ethanoic

• water

• hydrogen

Multiple Select

Complete the word equation and write the chemical equation for each of the reactions. State symbols are not required.

ammonium nitrate + strontium hydroxide -->

• Strontium nitraate

• ammonia gas

• water

• hydrogen

Multiple Select

Complete the word equation and write the chemical equation for each of the reactions. State symbols are not required.

ammonium sulfide + potassium hydroxide -->

• Potassium sulfide

• ammonia gas

• water

• hydrogen