Acids & Bases (Part I): Preparation, Properties & Uses of Acids

Introduction

Growing up, we used to think that any substance that burns is an acid, until we learnt about acids, bases and salts in our foundational chemistry class. It was then clear to us that there is more to acids than corrosivity, and not every substance that is corrosive, is an acid.

Definition

The definitions of an acid is based on three different concepts of acid-base reactions, namely the Lewis, Brønsted-Lowry and Arrhenius concepts.

Lewis Acids
According to G. N. Lewis, in acid-base reactions, the reactants undergo co-ordinate covalent bonding, in which one reacting species has the ability to accept a lone pair of electrons, while the other can readily donate a lone pair of electrons. A Lewis acid is an electron pair acceptor, because it has an empty orbital. Examples are H+, H3O+, Cu2+, Fe3+.

Brønsted-Lowry Acids
According to J. H. Brønsted and M. Lowry, an acid-base reaction involves the transfer of proton from one of the reactants to the other. A Brønsted-Lowry acid is a proton donor. For instance,

            HA ----> H+ + A-
            HNO3 ----> H+ + NO3-

Our focus on this topic will be the Arrhenius concept of acid-base reactions, which gives the basic definition of acids.

Arrhenius Acids
According to Arrhenius, an acid is any substance that produces hydrogen ion, H+ or hydroxonium ion, H3O+ as the only positive ion, when dissolved in water.

Example, hydrogen chloride gas dissolves in water to form hydroxonium and chloride ions.

HCl(g) + H2O(l) ----> H3O+(aq) + Cl-(aq)

Other examples include:

H2SO4(l) + 2H2O(l) ----> 2H3O+(aq) + SO4--(aq)

HNO3(l) + H2O(l) ----> H3O+(aq) + NO3-(aq)

H3PO4(l) + 3H2O(l) ----> 3H3O+(aq) + PO4---(aq)

The above equations can be rewritten in the abbreviated form as follows:

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

H2SO4(aq) ----> 2H+(aq) + SO4--(aq)

HNO3(aq) ----> H+(aq) + NO3-(aq)

H3PO4(aq) <----> 3H+(aq) + PO4---(aq)

The presence of the H3O+ or the H+ in the above equations is what accounts for the acidity of the substances.

Examples of some acids are:

Formula       |        IUPAC Name
HCl               |     Hydrochloric acid
HNO3           |     Trioxonitrate (V) acid
HCN             |     Hydrocyanic acid
CH3COOH   |     Ethanoic acid
HClO3          |     Trioxochlorate (V) acid
H2SO3         |     Trioxosulphate (IV) acid
H2SO4         |     Tetraoxosulphate (VI) acid
H2CO3         |     Trioxocarbonate (IV) acid
H2C2O4       |     Ethanedioic acid
H3PO4         |     Tetraoxophosphate (V) acid

Basicity of Acids

The basicity of an acid is the number of replaceable hydrogen or hydroxonium ions present in one molecule of the acid when it dissociates in water.

The following equations show the dissociation of some acids in solution, and the accompanying table shows a summary of their basicity.

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

HCN(aq) ----> H+(aq) + CN-(aq)

HNO3(aq) ----> H+(aq) + NO3-(aq)

HClO3(aq) ----> H+(aq) + ClO3-(aq)

CH3COOH(aq) <----> CH3COO-(aq) + H+(aq)

H2SO4(aq) ----> 2H+(aq) + SO4--(aq)

H2SO3(aq) <----> 2H+(aq) + SO3--(aq)

H2CO3(aq) <----> 2H+(aq) + CO3--(aq)

H2C2O4(aq) <----> 2H+(aq) + C2O4--(aq)

H3PO4(aq) <----> 3H+(aq) + PO4---(aq)

Basicity (No of H+/molecule) |                Acids
            1 (monobasic)               |  HCl, HCN, HNO3, CH3COOH
            2 (dibasic)                      |  H2SO4, H2SO3, H2CO3, H2C2O4
            3 (tribasic)                      |  H3PO4

Types of Acids

Acids can be classified based on two major categories - their strength and nature.

Classification by Strength

There are two types of acids based on their strength. These are strong acids and weak acids.

Strong Acids: These are acids that dissociate or ionize completely when dissolved in water. Some examples of strong acids are hydrochloric acid, trioxonitrate (V) acid and tetraoxosulphate (VI) acid

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

HNO3(aq) ----> H+(aq) + NO3-(aq)

H2SO4(aq) ----> 2H+(aq) + SO4--(aq)

Weak Acids: These are acids that undergo partial or incomplete ionization in water. Examples include H2CO3, H2SO3, CH3COOH, H3PO4.

CH3COOH(aq) <----> CH3COO-(aq) + H+(aq)

H2CO3(aq) <----> 2H+(aq) + CO3--(aq)

H2SO3(aq) <----> 2H+(aq) + SO3--(aq)

H3PO4(aq) <----> 3H+(aq) + PO4---(aq)

The backward and forward arrows in the equations indicate an incomplete ionization. Actually, only 4 out of every 1000 molecules of ethanoic acid in solution are ionized. Generally, all organic acids like ethanoic acid and ethanedioic acids are weak acids.

Classification by Nature

There are also two types of acids by nature. These are the organic and inorganic acids.

Organic Acids: These are acids, which occur naturally in plant and animal matters. Some examples of organic acids are ethanoic (acetic) acid from vinegar, amino acids from protein, lactic acid from milk, ascorbic acid (Vit. C) from oranges, citric acid from lime and lemon, palmitic acid from palm oil, ethanedioic (oxalic) acid, etc.

Inorganic Acids: These are acids synthesized from mineral elements. They are also known as mineral acids. Examples include hydrochloric acid, trioxocarbonate (IV) acid, tetraoxosulphate (VI) acid, trioxonitrate (V) acid etc.
 
Concentration of Acids

The concentration of an acid is the amount of the acid in moles present in a given volume of water. If a little volume of water is added to a large amount of acid, then the solution is said to be a concentrated acid solution. Conversely, when a large volume of water is added to a small amount of acid, a dilute acid solution will be obtained. 

For instance, given two solutions A and B of H2SO4, of concentrations 10 mol dm^-3 and 0.1 mol dm^-3 respectively; the former contains 10 moles or 980g of the acid in 1dm^3 (1000 cm^3) of distilled water, while the latter contains 0.1 mole or 9.8g of the acid in 1 dm^3 of distilled water. Hence, solution A is a concentrated solution, while solution B is a dilute solution, because the amount of H2SO4 present in 1 dm^3 of water is greater in A than in B.

Physical Properties of Acids

1. They turn blue litmus paper red.
2. The dilute acids possess a sour taste. The presence of acids accounts for the sour taste of unripe fruits, vinegar and rancid (stale) milk.
3. The strong acids are good electrolytes, while the weak acids are weak electrolytes.
4. The concentrated forms of strong acids are corrosive.

Precaution: You are NEVER expected to add water to a concentrated acid, as it may cause severe acid burns. If you must dilute a concentrated acid, ALWAYS ADD THE ACID TO A LARGER VOLUME OF WATER.

Remember, when cooking you DO NOT add water to hot oil, rather the reverse is done. The same principle is applicable when dealing with concentrated acids and water.

Chemical Properties of Acids

There are three major properties that all acids exhibit, irrespective of their nature, and these include:

(a) Action on active metals: Active metals displace the hydrogen atoms in acids to form salts with the liberation of hydrogen gas.

acid + active metal ----> salt + hydrogen

Examples:
2HCl(aq) + Zn(s) ----> ZnCl2(aq) + H2(g)

H2SO4(aq) + 2Na(s) ----> Na2SO4(aq) + H2(g)

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

It is important to note that unlike other acids, trioxonitrate (V) acid does not undergo this reaction with the metals, except in the action of very dilute trioxonitrate (V) acid (about 1%) on calcium, magnesium or manganese.

2HNO3(aq) + Ca(s) ----> Ca(NO3)2(aq) + H2(g)

This is due to its highly oxidizing nature, which causes the hydrogen to be oxidized to water, while the acid, itself, is reduced to nitrogen (IV) oxide or nitrogen (II) oxide gas, depending on the reaction conditions, as shown in its reaction with copper.

4HNO3(aq) + Cu(s) ----> Cu(NO3)2(aq) + 2NO2(g) + 2H2O(l) (hot concentrated HNO3)

8HNO3(aq) + 3Cu(s) ----> 3Cu(NO3)2(aq) + 2NO(g) +4H2O(l) (cold concentrated HNO3)

(b) Action on bases: All acids react with a base to form salt and water. This is known as a neutralization reaction.

acid + base ----> salt + water

Examples:
HCl(aq) + NaOH(aq) ----> NaCl(aq) + H2O(l)

H2SO4(aq) + CaO(s) ----> CaSO4(aq) + H2O(l)

HNO3(aq) + KOH(aq) ----> KNO3(aq) + H2O(l)

CH3COOH(aq) + NaOH(aq) ----> CH3COONa(aq) + H2O(l)

The products are formed by the exchange of negative ions and radicals between the acids and the bases.

(c) Action on trioxocarbonate (IV), (CO3--): All acids react with trioxocarbonate (IV) salts to produce salt and water, and liberate carbon (IV) oxide.

acid + trioxocarbonate (IV) ----> salt + water + CO2

Examples:
2HCl(aq) + Na2CO3(aq) ----> 2NaCl(aq) + H2O(l) + CO2(g)

H2SO4(aq) + CaCO3(s) ----> CaSO4(aq) + H2O(l) + CO2(g)

2HNO3(aq) + K2CO3(aq) ----> 2KNO3(aq) + H2O(l) + CO2(g)

CH3COOH(aq) + PbCO3(s) ----> (CH3COO)2Pb(s) + H2O(l) + CO2(g)

Methods of Acids Preparation

The following are some of the methods employed in preparing acids in the laboratory:

1. Dissolving an acid anhydride in water. Acid anhydrides are nonmetallic oxides, mostly gases, which dissolve in water to form acids. Examples include CO2, SO2, P4O10 etc

Examples:
(a) Carbon (IV) oxide dissolves in water to form trioxocarbonate (IV) acid.

CO2(g) + H2O(l) ----> H2CO3(aq)

(b) Sulphur (IV) oxide dissolves in water to form trioxosulphate (IV) acid.

SO2(g) + H2O(l) ----> H2SO3(aq)

(c) Sulphur (VI) oxide dissolves in water to produce tetraoxosulphate (VI) acid.

SO3(g) + H2O(l) ----> H2SO4(aq)

(d) Phosphorus (V) oxide dissolves in water to produce tetraoxophosphate (V) acid.

P4O10(s) + 6H2O(l) ----> 4H3PO4(aq)

(e) Nitrogen (V) oxide and nitrogen (III) oxide dissolve in water to form trioxonitrate (V) acid and dioxonitrate (III) acids respectively.

N2O5(g) + H2O(l) ----> 2HNO3(aq)

N2O3(g) + H2O(l) ----> 2HNO2(aq)

2. Using a strong acid to displace a volatile or weak acid from its salts.

(a) Concentrated tetraoxosulphate (VI) acid is used to displace a volatile gas like hydrogen chloride from a chloride salt.

H2SO4(aq) + NaCl(s) ----> Na2SO4 + HCl(g)

The hydrogen chloride produced, dissolves readily in water to form hydrochloric acid.

(b) Concentrated tetraoxosulphate (VI) acid can also be used to displace weak trioxocarbonate (IV) acid from a trioxocarbonate (IV) salt.

H2SO4(aq) + CaCO3(s) ----> CaSO4(aq) + H2CO3(aq)

The trioxocarbonate (IV) acid, which is not stable, readily decomposes to water and carbon (IV) oxide.

H2CO3(aq) ----> H2O(l) + CO2(g)

(c) Concentrated tetraoxosulphate (VI) and hydrochloric acids are used to displace trioxonitrate (V) acid from excess trioxonitrate (V) salts.

H2SO4(aq) + 2KNO3(s) ----> K2SO4(aq) + 2HNO3(g)

HCl(aq) + NaNO3(s) ----> NaCl(aq) + HNO3(g)

3. Direct combination of elements. Most acids that are binary compounds, such as the hydrogen halides can be prepared by combining the constituent elements at appropriate conditions.

Examples:
(a) Hydrogen burns rapidly in chlorine in the presence of activated charcoal as a catalyst to form hydrogen chlorine gas, which then dissolves in water to produce hydrochloric acid.

H2(g) + Cl2(g) + activated charcoal + heat ----> 2HCl(g)

HCl(g) + H2O(l) ----> H3O+(aq) + Cl-(aq)

(b) Hydrogen combines with bromine vapour in the presence of platinum and heat to form hydrogen bromide gas, which then dissolves readily in water to form hydrobromic acid.

H2(g) + Br2(g) + platinum + heat ----> 2HBr(g)

HBr(g) + H2O(l) ----> H3O+(aq) + Br-(aq)

The hydrogen bromide gas produced, dissolves readily in water to form hydrobromic acid.

Uses of Acids

Some uses of acids include:

1. Manufacture of textiles, paints, fertilizers, drugs and other chemicals. E.g. H2SO4, HCl, HNO3.

2. As drying and dehydrating agents. E.g. H2SO4

3. In food preservation and dyeing of textiles. E.g. CH3COOH

4. As solvents in qualitative analysis. E.g. Dilute HCl, dilute HNO3

5. As an oxidizing agent. E.g. Conc. HNO3

6. In oil refineries for refining of some petroleum products. E.g. H2SO4

7. In the manufacture of baking soda, soft drinks and health salts. E.g. H2CO3.

Do These:

Question 1
Define an acid based on the three concepts of acid-base reactions, and give one example each.

Question 2
Give the natural sources of the following organic acids:
(a) Ascorbic acid
(b) Acetic acid
(c) Palmitic acid
(d) Citric acid

Question 3
(a) How would you prove that a given colourless liquid is an acid?
(b) State three chemical properties of acids.

Question 4
(a) Differentiate between a strong acid and a concentrated acid.
(b) Carbon (IV) oxide is described as an acid anhydride. Explain.
(c) State four uses of acids.

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