Acids, Bases and Salts

By the end of the lesson, you should be able to:

□ Describe the reactions of acids with metals, bases, alkalis, carbonates.□ Describe the reactions of bases with acids and of soluble bases (alkalis) with ammonium salts.□ State the ions present in aqueous solutions of acids and alkalis.□ Explain what happens in neutralization.□ State the effect of acids and alkalis on indicators, including Universal Indicator.□ Describe and explain the importance of controlling the pH of soil.□ Classify oxides as either acidic, basic or amphoteric related to metallic/non-metallic character.□ Describe the preparation, separation and purification of salts using filtration and crystallization, for: - acids with metals - acids with insoluble bases (eg metal oxides) - acids with insoluble carbonates□ Describe the preparation of a salt by a titration method.□ Suggest a method of preparing a given salt from suitable starting materials.□ Know which common salts are soluble and which are insoluble.

Please tick in the box if you can do any of the above.

Acids

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The table below shows the names of some common acids, their formulae and the ions they produce in aqueous solution.

Complete the table below.

Many naturally occurring acids, such as citric acid found in oranges, are sometimes known as organic acids.

Definition of acid: A substance which produces hydrogen ions, H+, when it is dissolved in water.

All acids contain hydrogen but not all compounds that contain hydrogen are acids. For example, both ammonia, (NH3) and methane (CH4) contain hydrogen, but they are not acids because they do not produce hydrogen ions in water.

Properties of acids

1. Acids have a sour taste.

2. Acids dissolve in water to form solutions which conduct electricity.

3. Acids turn blue litmus paper red.

4. Acids react with metals Most dilute acids react with metals to produce a salt and a

hydrogen gas. The general equation for the reaction is:

Metal + Acid Salt + Hydrogen

For example, when magnesium ribbon is added to dilute sulphuric acid, bubbles of hydrogen gas can be seen. The equation for this reaction is:

Magnesium + dilute sulphuric acid magnesium sulphate + hydrogen

Mg(s) + H2SO4(aq) MgSO4(aq) + H2(g)

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What is the salt produced in the above reaction?MgSO4(aq)

What do you call the salts formed from the following acids:Sulphuric acid, sulphatesNitric acid, nitratesHydrochloric acid? Chlorides

If a lighted splint is placed at the mouth of the test tube, what will you observed?

A ‘pop’ sound will be heard.

What does this observation tell you? Hydrogen is confirmed produce.

Quick Questions: Complete the word equations and then write a balanced

chemical equation for each.a) hydrochloric acid + zinc metal _______________________________________________________

b) sulphuric acid + magnesium metal _______________________________________________________

c) nitric acid + lithium metal _______________________________________________________

d) hydrochloric acid + magnesium metal _______________________________________________________

e) phosphoric acid + aluminium metal _______________________________________________________ 5. Acids react with carbonates

Most dilute acids react with carbonates to produce a salt, carbon dioxide and water.

The general equation for the reaction is:

Carbonate + acid salt + water + carbon dioxide

In the reaction between sodium carbonate and dilute hydrochloric acid, what will be the products?

Sodium chloride, water and carbon dioxide.

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Write a word equation and a balanced chemical equation for the reaction between sodium carbonate and dilute hydrochloric acid.

Sodium carbonate + hydrochloric acid sodium chloride + water + CO2

_______________________________________________________

Using the set-up shown above , what would you expect your observation of the limewater? Explain your observation.

Observation:The limewater will turn chalky.Explanation: This shows that carbon dioxide is produced in the reaction.

Quick Questions: Complete the word equations and then write a balanced

chemical equation for each.

a) hydrochloric acid + zinc carbonate _______________________________________________________

b) sulphuric acid + magnesium carbonate _______________________________________________________

c) nitric acid + potassium carbonate _______________________________________________________

d) hydrochloric acid + copper(II) carbonate _______________________________________________________

e) phosphoric acid + aluminium carbonate _______________________________________________________

6. Acids react with metal oxides and hydroxides Most metal oxides and hydroxides react slowly with warm

dilute acids forming a salt and water. The general equation for the reaction are:

Metal oxide + acid salt + waterMetal hydroxide + acid salt + water

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Write a word equation and a balanced chemical equation for the reaction between zinc oxide and dilute sulphuric acid.

_______________________________________________________

Write a word equation and a balanced chemical equation for the reaction between zinc hydroxide and dilute nitric acid.

_______________________________________________________

Quick Questions: Complete the word equations and then write a balanced

chemical equation for each.a) hydrochloric acid + lithium oxide _______________________________________________________

b) sulphuric acid + barium hydroxide _______________________________________________________

c) nitric acid + sodium oxide _______________________________________________________

d) hydrochloric acid + magnesium hydroxide _______________________________________________________

e) Ethanoic acid + Aluminium hydroxide _______________________________________________________

The Importance of Water for Acids

Citric acid is a solid acid. It has no effect on a piece of dry blue litmus paper. Also, the solid does not react with a piece of magnesium ribbon.

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A solution of citric acid in water behaves differently. It changes the colour of blue litmus paper to red and produces bubbles of hydrogen gas with magnesium.

Similar results occur with all acids. For example, hydrogen chloride gas, HCl(g) behaves differently when dissolved in alcohol and in water.

- Hydrogen chloride in alcohol There is no colour change with litmus and no reaction with magnesium.

- Hydrogen chloride in waterIt turns blue litmus red and reacts with magnesium.

Again, we find that water is needed for an acid to show acidic properties. How can we explain this?

The explanation can be given from the diagram shown below:

The diagram above shows how hydrogen chloride dissociates in water to form hydrogen and chloride ions.

Acids can only show the properties of acids when they are dissolved in water. This is because acids dissociate in water to

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produce hydrogen ions which are responsible for the acidic properties.

Hydrogen chloride can exist as two different types of particles.

Hydrogen chloride exists as covalent molecules. In the absence of water, for example, in organic solvents (alcohol), they do not behave as acids.

Quick Questions:

1. Explain why solid citric acid does not react with magnesium metal and has no effect on litmus paper.Because solid citric acid cannot dissociate to form hydrogen ions in the absence of water.Basicity of an Acid

Some acids form just one hydrogen ion when dissolved in water whereas others form more than one.

The maximum number of hydrogen ions produced by a molecule of an acid is called the basicity of the acid.

Complete the table below:

Quick questions:

1. How many hydrogen atoms does ethanoic acid contain? 4

2. How many of these hydrogen atoms form hydrogen ions? 1

Strong and Weak Acids

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In a solution of strong acid, all the acid molecules become ions in the water.

For example, a solution of hydrochloric acid only contains H+(aq) and Cl-(aq) ions and no HCl molecules. The HCl molecules have all ionized in water.

HCl(aq) H+(aq) + Cl-(aq)

Give examples of strong acid._______________________________________________________

A strong acid is one that completely ionizes (dissociates) in water.

In a solution of a weak acid, most of the acid molecules remain unchanged in water.

Few acid molecules are ionized to become hydrogen ions. An example is a solution of ethanoic acid, CH3COOH.

In 1.0 mol/dm3 solution of ethanoic acid, only about 4 out of every 1000 acid molecules become ions in water. The other 996 molecules remain unchanged.

A weak acid is one that partially ionizes (dissociates) in water.

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Another weak acid, sometimes called carbonic acid, is formed when carbon dioxide gas dissolves in water:

A solution of carbonic acid contains many H2CO3 molecules but only a few of them ionize to form H+(aq) ions and CO3

2-(aq) ions.

Weak acids are often found in foods and drinks. Vinegar is a weak acid; it contains ethanoic acid. Soft drinks contain carbonic acid.

Quick Questions:1. The equation for the dissociation of ethanoic acid contains a double arrow . What do you think it means?It means the reaction is a reversible reaction.

2. Draw a diagram to show the kinds of particles in carbonic acid.

Reactivity of Strong and Weak Acids

Between strong acids and weak acids, which one will react more vigorously? Strong acids

In the diagram below, label which reaction belongs to strong or weak acids.

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Comparing Strong and Weak Acids with Concentrated and Dilute Acids

The term ‘strong’ DOES NOT have the same meaning as ‘concentrated’.

Similarly, the term ‘weak’ DOES NOT have the same meaning as ‘dilute’.

Strong and weak refer to the extent of ionisation.

Concentrated and dilute refer to the amount of solute in the solution.

Quick Questions:1. For each of the following bottles of acids, label them as either ‘concentrated solution’ or ‘dilute solution’ and also as either ‘strong acid’ or ‘weak acid’.

Uses of Acids List the uses of the following acids.

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Bases

The table below shows the names of some common bases and their formulae.

Complete the table below.

Bases are a group of compounds that act differently from acids.

As we shall see, when we mix an acid and a base, each destroys the properties of the other.

“Neutralisation” For this reason, we often think of acids and bases as chemical

opposites.

What are bases?Bases are the oxides or hydroxides of metals.

Give two examples of bases.Copper(II) oxide (insoluble), Sodium hydroxide (soluble)

Some bases are insoluble in water. Others are soluble. For example, copper(II) oxide is insoluble in water, so it is a base but not an alkali.

What are alkalis?Alkalis are bases that are soluble in water.

All alkalis are bases, but not all bases are alkalis.

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The relationship between bases and alkalis can be shown as:

Properties of Alkalis

1. Alkalis have a bitter taste and soapy feel.

2. Alkalis turn red litmus paper blue.

3. All alkalis produce hydroxide ions when dissolved in water.

For example,

Sodium hydroxide sodium ion + hydroxide ion_______________________________________________________

When ammonia gas dissolves in water, what ions will be formed?

Ammonia + water ammonium ions + hydroxide ion_______________________________________________________

4. All alkalis can react with acids to form a salt and water.This reaction is called neutralisation.

In this reaction, the hydrogen ions from the acid react with the hydroxide ions of the alkali. The ionic equation for this reaction is:

H+(aq) + OH-(aq) H2O(l)

The neutralisation reaction is exothermic. This means that heat is given out in the reaction.

When an alkali is slowly added to an acid from a burette, the temperature steadily increases until all the acid has reacted (is neutralised)

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If more alkali is then added, the temperature does not increase as there is no more acid to react.

5. Alkalis heated with ammonium salts to give off ammonia gas.

The general equation for the reaction is:

Alkali + Ammonium salt ammonia + water + salt

Quick questions: Complete the word equations and then write a balanced

chemical equation for each.a) calcium hydroxide + ammonium chloride_______________________________________________________

b) sodium hydroxide + ammonium nitrate _______________________________________________________

c) barium hydroxide + ammonium sulphate _______________________________________________________

How do we test that ammonia is being evolved by the end of the reaction between an alkali and an ammonium salt?

Characteristic pungent smell and turns red litmus paper blue.

6. Alkalis can react with a solution of one metal salt to give metal hydroxide and another metal salt.

The general equation for this reaction:

Alkali + salt(of metal A) metal hydroxide + salt( of metal B)

The metal hydroxide appears as a precipitate if it is insoluble in water.

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For example, sodium hydroxide reacts with a solution of iron(II) sulphate to give iron(III) hydroxide and sodium sulphate.

Construct a balanced chemical equation for the reaction mentioned above.

_______________________________________________________

Quick questions: Complete the word equations and then write a balanced

chemical equation for each.a) sodium hydroxide + copper sulphate _______________________________________________________

b) calcium hydroxide + silver nitrate _______________________________________________________

c) barium hydroxide + iron chloride _______________________________________________________Uses of Bases and Alkalis

List the uses of the following bases and alkalis.

The pH Scale

Most solutions of acids and alkalis are colourless. Therefore, we cannot identify them just by appearance.

To test for acids and alkalis, we use indicators.

What is an indicator?An indicator is a substance that has different colours in acidic and alkaline solutions.

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Give three examples of indicators.Phenolphthalein, methyl orange and litmus.

Using the bench reagents, use the following indicators and write down your observation of the colour when the indicator is in acid and in alkali in the table below.Indicator Colour in acids Colour in alkalis

PhenolphthaleinMethyl Orange

Screened methyl orange

Simple indicators, such as litmus, tell whether a solution is an acid or an alkali.

However, other indicators not only indicate an acid or an alkali, but also how acidic or alkaline a solution is.

A measure of the acidity or alkalinity of a solution is known as pH. The pH scale is a set of numbers used to indicate whether a

solution is acidic, neutral or alkaline.

What will be the pH for the following:a) acidic solution, less than 7b) neutral solution, exactly 7c) alkaline solution, greater than 7

The pH of a solution is calculated based on the number of hydrogen ions or hydroxide ions present in a solution.

Acids with a smaller pH value have a higher concentration of hydrogen ions.

Alkaline solutions with a larger pH value have a higher concentration of hydroxide ions.

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The diagram below shows the pH scale and colour chart for Universal Indicator. On the diagram, indicate the position of the following substances in the pH scale:

Hydrochloric acid Nitric acid Sulphuric Acid Ethanoic AcidPure water Aqueous ammonia Sodium hydroxide

Measuring the pH of a solution

1. Universal Indicator It contains a mixture of dyes. It gives different colours in

solutions of different pH.

2. pH meter Universal Indicator gives only approximate values. We can use a

pH meter to obtain accurate values. The probe is dipped into the solution and the meter shows the pH

either on a scale or digitally.

3. pH sensor and computer A pH sensor connected to a computer through an interface can

be used to measure the pH of a solution. The pH reading is displayed on the computer screen.

The Importance of pH

1. pH and the body Substances in the body have different pH values. Acidic conditions in the stomach (pH ~ 1.5) and alkaline

conditions in the small intestine (pH ~ 8.4)

2. pH and food preservation Many fresh foods quickly go bad. This is mainly due to

microorganisms, such as bacteria, which are present in the food. Microorganisms do not grow well in solutions of low pH.

Therefore, acids can be used to preserve foods.

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Give three acids that are commonly used in food preservation.Ethanoic acid(vinegar), benzoic acid, citric acid.

3. pH in the garden The pH of soil is important for good plant growth. Here are

examples of the pH of soil in which plants grow best:- Orchids – more acidic soil (pH 4-5)- Azaleas, strawberries, apples, potatoes – weakly acidic soil (pH 5-6)- Common vegetables, e.g. beans, peas, lettuce, onion – about neutral soil (pH 6-8)

4. pH and hair Normal hair is weakly acidic with a pH of about 5. At this pH, the

hair is smooth, strong and healthy. Alkaline solutions make hair curly. What do you think the pH of shampoo?Alkaline (pH more than

7) Why do you think we use conditioners after we shampoo our

hair?To restore the pH of hair to its normal value because most hair conditioners contain weak natural acids.Types of Oxides Many acids and alkalis are formed by dissolving oxides in water. Most oxides can be grouped into four types: acidic oxides, basic

oxides, amphoteric oxides and neutral oxides.

Acidic oxides Oxides of non-metals are acidic oxides. Most acidic oxides dissolve in water to form an acid.

Give examples of acidic oxides.Sulphur trioxide, carbon dioxide, sulphur trioxide and nitrogen dioxide. Acidic oxides do not react with acids. However, they react with

alkalis to form a salt and water. Construct a balance chemical equation for the reaction

between carbon dioxide and sodium hydroxide._______________________________________________________

Basic oxides Oxides of metals are basic oxides.

Give examples of basic oxides.Magnesium oxide, copper (II) oxide and calcium oxide Basic oxides react with acids to form a salt and water.

Construct a balance chemical equation for the reaction between calcium oxide and nitric acid.

_______________________________________________________

Amphoteric oxides

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Amphoteric oxides are metallic oxides that react with both acids and bases to form salts and water. Give examples of amphoteric oxides.

Aluminium oxide, zinc oxide and lead(II) oxide

Construct a balance chemical equation for the reaction between zinc oxide and hydrochloric acid.

_______________________________________________________

Construct a balance chemical equation for the reaction between zinc oxide and sodium hydroxide.

Na2ZnO2 + H2O

Salts A salt is obtained from an acid by replacing the acid hydrogen

atoms with metal ions (or with ammonium ions)e.g. sodium chloride, NaCl, is obtained by replacing the H of HCl with a sodium ion, Na+.Ammonium sulphate, (NH4)2SO4, is obtained by replacing the two H atoms of H2SO4 with two ammonium ions, NH4

+.

Acid salts only have some (but not all) of the acid hydrogen atoms replaced by metal atoms.e.g. Sodium hydrogensulphate, NaHSO4, is an acid salt from sulphuric acid (only one H atom of H2SO4 is replaced; one H remains).

Reactions producing salts:

ACID + METAL SALT + HYDROGEN

ACID + ALKALI SALT + WATER

ACID + METAL OXIDE SALT + WATER

ACID + METAL CARBONATE SALT + WATER + CARBON DIOXIDE

Although salts are ionic compounds, not all salts are soluble in water. Figure below summarises the solubilities of the common salts in water at room temperature.

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Preparation of Salts

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