Acids, bases and salts - (CCEA)

• Indicators are chemicals that can change colour to identify acidic, alkaline and neutral solutions. Universal indicator can determine the strength of an acidic or alkaline solution.

• Acids and alkalis can be strong or weak depending on how much they ionise in water.

• Acids can react with metals, metal carbonates, metal hydrogencarbonates, bases (metal oxides/hydroxides) and ammonia.

• Practical methods can be used to prepare pure salts from acids.

An indicator is a chemical that changes colour in acidic, alkaline and neutral solutions.

Some common indicators include red litmus paper, blue litmus paper, phenolphthalein, and methyl orange.

These indicators allow you to easily work out if a substance is an acid or an alkali.

However, none of these indicators tell you about the strength of an acid or alkali.

The pH scale gives a measure of the strength of an acidic or alkaline solution.

The range for the pH scale is 0 (strong acid) to 14 (strong alkali).

• pH 0 – 2: strong acid
• pH 3 – 6: weak acid
• pH 7: neutral
• pH 8 – 11: weak alkali
• pH 12 – 14: strong alkali

Universal indicator solution or paper turns a range of colours, depending on the strength of the acid or alkali.

The most accurate method of measuring the pH of a solution is by using a pH meter.

This an electronic device that can measure the pH of a solution to at least one decimal place.

An acid is a substance that dissolves in water to produce hydrogen ions, H⁺(aq).

The higher the concentration of hydrogen ions in the solution, the lower the pH. (Higher tier only).

An alkali is a substance that dissolves in water to produce hydroxide ions, OH^-(aq).

What are strong and weak acids/alkalis?? (Higher tier only)

The difference between strong and weak acids is how much they are able to ionise in water.

Ionising involves the acid molecule splitting apart to form separate ions (one of which is an H⁺ ion).

Strong acids completely ionise in water.

They break up completely to produce a high concentration of hydrogen ions in the solution.

For example, hydrochloric acid ionises completely into hydrogen ions and chloride ions:

HCl(aq) → H⁺(aq) + Cl^-(aq)

Strong acids include:

• hydrochloric acid (HCl)
• sulfuric acid (H₂SO₄)
• nitric acid (HNO₃)

Weak acids only partially ionise in water.

Only a small fraction of their molecules break into hydrogen ions when added to water.

For example, ethanoic acid (CH₃COOH) is a weak acid, it only partially ionises in water.

CH₃COOH(aq) ⇌ H⁺(aq) + CH₃COO^-(aq)

The ‘reversible arrow’ (⇌) indicates a reversible reaction.

What are the strong and what are the weak alkalis? (Higher tier only)

Strong alkalis completely ionise in water.

They break up completely to produce a high concentration of hydroxide ions in the solution.

For example, sodium hydroxide ionises completely into sodium ions and hydroxide ions:

NaOH(aq) → Na⁺(aq) + OH^-(aq)

Strong alkalis include:

• sodium hydroxide (NaOH)
• potassium hydroxide (KOH)

Weak alkalis only partially ionise in water.

Only a small fraction of their molecules break into hydroxide ions when added to water.

For example, ammonia (NH₃) is a weak alkali, it only partially ionises in water.

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

The ‘reversible arrow’ (⇌) indicates a reversible reaction.

What is the difference between an alkali and a base?

A base is a metal oxide or metal hydroxide that neutralises an acid to produce a salt and water.

An alkali is a soluble base.

For example, sodium hydroxide and potassium hydroxide both neutralise acids so they are bases.

They both also dissolve in water, so can be described as alkalis as well.

What is the difference between dilute and concentrated solutions? (Higher tier only)

Concentration is a measurement of the amount of an acid or alkali that has been dissolved in a solution. It is measured in mol/dm³ (moles per decimetre cubed).

• A concentrated acid/alkali contains a large number of acid/alkali particles dissolved per unit volume.
• A dilute acid/alkali contains a small number of acid/alkali particles dissolved per unit volume.

What is a neutralisation reaction?

A neutralisation reaction is a reaction between an acid and a base producing a salt and water.

In this reaction, the hydrogen ions in an acid and the hydroxide ions in an alkali join together to make water.

The ionic equation for neutralisation is (higher tier):

H⁺(aq) + OH^-(aq) → H₂O(l)

Neutralisation is an exothermic reaction – one in which heat is given out.

The maximum temperature is reached when the acid solution is completely neutralised by the alkali.

Please use the link below to access the article on: Prescribed practical (C2) - Investigate the reactions of acids

(https://truemac.vn/bitesize/articles/z6m8s82)

How to work out the name of the salt produced during a reaction

When an acid reacts it is converted into a salt.

You need to be able to work out the name (and formula) of a salt produced in a reaction.

Salts are ionic compounds composed of two parts.

The first word in a salt name is always the name of the metal in the equation.

The second word in a salt name comes from the acid in the equation:

For example, the salt produced in the reaction of sodium hydroxide with hydrochloric acid is sodium chloride.

Reaction observations

Some general observations can be made during acid reactions:

• if a gas is produced, the observations are ‘bubbles’ or ‘fizzing’.
• if a solid such as a solid metal, base, carbonate or hydrogencarbonate reacts with the acid then the observation is ‘solid disappears and a solution is produced’.
• most acid reactions are exothermic – the observation is ‘heat released’. Exceptions – copper(II) oxide and sodium hydrogencarbonate with acid are both endothermic reactions.

It is useful to know the colour of some compounds commonly reacted with acids to work out if a colour change has occurred.

Acids react with metals to produce a salt and hydrogen.

acid + metal → salt + hydrogen

Example:

hydrochloric acid + magnesium → magnesium chloride + hydrogen

2HCl(aq) + Mg(s) → MgCl₂(aq) + H₂(g)

Observations: grey solid magnesium disappears, colourless solution produced, heat released, bubbles.

The hydrogen in these reactions can be tested. The test for hydrogen is:

• apply a lighted splint
• a popping sound results

Acids react with bases to form a salt and water.

acid + base → salt + water

Example

sulfuric acid + copper(II) oxide → copper(II) sulfate + water

H₂SO₄(aq) + CuO(s) → CuSO₄(aq) + H₂O(l)

Observations: black solid copper(II) oxide disappears, blue solution produced.

Acids react with metal carbonates and metal hydrogencarbonates in the same way.

These reactions produce a salt, water and carbon dioxide.

acid + metal carbonate → salt + water + carbon dioxide

or

acid + metal hydrogencarbonate → salt + water + carbon dioxide

Example - carbonate

hydrochloric acid + copper(II) carbonate → copper(II) chloride + water + carbon dioxide

2HCl(aq) + CuCO₃(s) → CuCl₂(aq) + H₂O(l) + CO₂(g)

Observations: green solid copper(II) carbonate disappears, blue solution produced, heat released, bubbles.

Example - hydrogencarbonate

hydrochloric acid + sodium hydrogencarbonate → sodium chloride + water + carbon dioxide

HCl(aq) + NaHCO₃ (s) → NaCl(aq) + H₂O(l) + CO₂(g)

Observations: white solid sodium hydrogencarbonate disappears, colourless solution produced, bubbles.

The carbon dioxide gas produced in these reactions can be tested. The test for carbon dioxide is:

• bubble gas into colourless limewater (calcium hydroxide solution)
• the solution will change from colourless to milky if the gas is carbon dioxide

Acids react with ammonia to form a salt. There is only one product in this reaction.

acid + ammonia → ammonium salt

Example

sulfuric acid + ammonia → ammonium sulfate

H₂SO₄(aq) + 2NH₃(g) → (NH₄)2SO₄(aq)

A salt is a compound formed when some or all the hydrogen ions in an acid are replaced by metal ions or ammonium ions.

For example, zinc sulfate is the salt formed when zinc ions replace the hydrogen ions of sulfuric acid.

Most Group 1 (I), Group 2 (II), aluminium and zinc salts are white.

They produce colourless solutions when dissolved in water.

Transition metal salts generally produce coloured solutions when dissolved in water.

A pure, dry sample of a soluble salt can be prepared in two ways.

Method one – adding an insoluble solid such as a base/metal carbonate to an acid

In this method, an excess of the insoluble solid is added to the acid.

This ensures that all of the acid is converted to salt.

The leftover solid can be removed by filtration.

• add excess insoluble substance (e.g. base) to warm acid
• filter off excess solid
• evaporate filtrate to half volume
• cool to allow crystals to form
• filter off the crystals

Method 1 is used for most Group 2, aluminium and transition metal salts.

Method two – adding an alkaline solution to an acid

In this method, an indicator is used to ensure that the acid and alkali are mixed in the correct quantities to neutralise each other, so that only a salt remains in the solution.

• add alkali to acid, or vice versa, in the presence of an indicator until the indicator changes colour to show a neutral solution.
• repeat the process without the indicator, or remove the indicator using charcoal
• evaporate the solution to half volume
• cool to allow crystals to form
• filter off the crystals

You can dry the crystals by placing it in a desiccator, in a low temperature oven, or between two sheets of filter paper.

Method 2 is used for sodium, potassium and ammonium salts.

Please use the link below to access the article on: Prescribed practical (C3) - Investigate the preparation of soluble salts

(https://truemac.vn/bitesize/articles/zdh7b7h)

You should always take care in the laboratory, but strong acids and alkalis are particularly dangerous.

Take care with chemicals carrying international chemical hazard labelling.