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Acids

Module by: Hatimu Muyanja co-author: Luutu Suleiman

Summary: In everyday life we consume fruits or fruit juice. These fruits have some constituents known as acids as according to chemists. For example, oranges, lemons and grape fruits contain citric acid and ascorbic acid (more commonly known as Vitamin C). There are many other fruits or plants which contains these acids and other acids as acetic and boric acids.

Introduction

In everyday life we consume fruits or fruit juice. These fruits have some constituents known as acids as according to chemists. For example, oranges, lemons and grape fruits contain citric acid and ascorbic acid (more commonly known as Vitamin C). There are many other fruits or plants which contains these acids and other acids as acetic and boric acids.

Acids such as hydrochloric, sulfuric and nitric acids are industrially prepared from naturally occurring compounds called minerals. Acids obtained from minerals are generally stronger than those that are contained in fruits and greater care due to the fact that they can be corrosive to skin and burn clothing, wood, fibre and so on.

By the end of the study, the learners should be able to:

1. Define and explain the meaning of acids, their types and reactions.
2. Give and explain the properties of acids
3. Explain the various way of preparing acids
4. Give the various industrial uses of acids

Acids

What is an acid?

An acid is a substance that has atleast an ionaisable hydrogen atom and which when dissolved in water, produces hydrogen ions as the only positively charged ions in solution.

An acid is a proton in solution.

Acids exist broadly as     (i) Mineral acids     and     (ii) Organic acids.

Examples of mineral acids include:

Examples of Organic acids include:

Note that not all hydrogen atoms contained in a substance are ionaisable, meaning that, not all hydrogen containing substances are acids, for example ammonia (NH3) is not an acid since none of its hydrogen atoms is ionaisable.

Ionization equations below will help us view the ionaisable hydrogen atoms of substances and their hydrogen ions in solution.

Hydrogen atoms and ions for acids.

Mineral Acids

Organic Acids

The “H” represents the ionaisable hydrogen atom and the “H+” represents the ionized hydrogen ion.
Note that organic acids have only one ionaisable hydrogen atom.

What is Basicity?

These are the number of moles of hydrogen ions produced per mole of an acid ionizing.
If one hydrogen ion is produced per mole of an acid ionizing the acid is Monobasic.
If two hydrogen ions are produced per mole of an acid ionizing the acid is Dibasic.
If three hydrogen ions are produced per mole of an acid ionizing the acid is Tribasic.

For Example:

HCl (aq)  → H+(aq) + Cl-(aq)
H2SO4 (aq) → 2H+ (aq) + SO42- (aq)

All organic acids are monobasic.  For example;
CH3COOH (aq) ⇌ CH3COO-(aq) +   H+ (aq)

What is strength of an acid?

The terms weak and strong as applied to acids refers to the extent to which the acid is ionised.
A strong acid is one that is almost completely ionised in solution. Mineral acids are strong acids.

A weak acid is an acid that partially ionises to give few hydrogen ions.
Organic acids are weak acids for example.
    HCOOH (aq) ⇌ HCOO - (aq) +   H+ (aq)

Properties of acids

1. aste: Acids have a sour sharp taste. In many unripe fruits the sourness is due to presence of organic acids like citric acid in lemons.
2. Indicators:
   - Acids turn blue litmus to red.
   - Acids turn methyl orange red
   - Acids turn phenolphthalein colourless 
3. Reaction with carbonates and Hydrogen carbonates: Acids liberate carbondioxide when they react with carbonates and hydrogen. Bubbles of carbondioxide are evolved.
   
                2H+ (aq) + CO32-(aq) → CO2 (g) + H2O (l)  
                H+ (aq) + HCO3-(aq) → CO2 (g) + H2O (l)  
4. Reaction with oxides and hydroxides: Acids react with oxides and hydroxides of metals to form salts and water only.
   
               H2SO4 (aq) + CuO (s) → CuSO4 (aq) + H2O (l).
       That is;     2H+ (aq) + O2-(s) → H2O (l)  
   
           HCl (aq) + NaOH (aq) → NaCl (aq) + H2O (l).
   That is;      H+ (aq) + OH-(s) → H2O (l)
5. Acids are electrolytes when dissolved in water, because they conduct ions which will be left free.
6. Acids react with electropositive metals above hydrogen in the reactivity series to give hydrogen gas.
   
   Zn(s) + H2SO4 (aq) → ZnSO4 (aq) + H2 (g).
   
   Note:    - Zinc, magnesium and aluminum require hot concentrated hydrochloric acid.
   - The less electropositive metals like lead require hot concentrated hydrochloric acid.
   - The reaction of lead with Sulphuric acid produces insoluble lead (II) sulphate which coats it and prevents it from further reaction.
   - Nitric acid does not liberate hydrogen from metals, because it is a strong oxidizing agent, instead oxides of nitrogen are produced.
   - Concentrated Sulphuric acid is an oxidizing agent and does not liberate hydrogen with metals, but rather liberates sulphurdioxide. 
7. Acids react with bases to give a salt and water only.
   
             H2SO4 (aq) + 2NaOH (aq) → Na2SO4 (aq) + H2O (l).
              2HCl (aq) + CuO (l) → CuCl2 (aq) + H2O (l).

Preparations of Acids

1. Reaction with gases related to acids: Acids may be prepared when certain gases, related to acids (acid anhydrides), are dissolved in water. For example, carbondioxide, sulphurdioxide, nitrogen dioxide and phosphorus oxide dissolve.

Dissolving carbondioxide:    CO2 (g) + H2O (l) → H2CO3 (aq)     (carbonic acid)
Dissolving sulphurdioxide:    SO2 (g) + H2O (l) → H2SO3 (g).    (Sulphurous acid)
    This method is not suitable for the preparation of Sulphuric acid from sulphurtrioxide, because the reaction is highly exothermic and the heat generated breaks down sulphurtrioxide.

2. By displacing a weak acid (more volatile) from a salt by a strong acid (less volatile).

NaCl (s) + H2SO4 (l) → NaHSO4 (s) + HCl (g).
KNO3 (s) + H2SO4 (l) → KHSO4 (s) + HNO3 (l).

3. By direct synthesis.

H2 (g) + Cl (g) → 2HCl (g).

Uses of Acids

1. Sulphuric acid is one of the most commonly used chemicals in industry.
2. Sulphuric acid is used to prepare other acids such as hydrochloric and nitric acid. This is due to the fact that the boiling point of Sulphuric acid is higher than that of other acids, thus, allowing the acid being produced to be distilled and collected separately from the original material.
3. Sulphuric acid is used to remove the surface oxide layers from metals (also known as pickling), before the metals are coated with materials that prevent rusting. For example, before iron is coated with chromium (in chromium plating), the iron is dipped into dilute Sulphuric acid to remove the iron oxide normally present on the surface of the iron.
4. Sulphuric acid is used in a storage cell. In a lead storage cell, dilute sulphuric acid serves as the electrolyte through which ions move between the lead plates, acting as the cathode, and the spongy lead dioxide, acting as the anode. Several such cells connected together make up the type of storage battery used in automobiles.
5. Nitric acid, another important industrial acid, is used in the manufacture of fertilizers, plastics, photographic film and dyes.
6. Nitric acid is also used in the preparation of such explosives as dynamite and TNT.
7. Hydrochloric acid, like Sulphuric acid, is used to clean metals.
8. Hydrochloric acid is used to clean bricks and tiles.
9. Hydrochloric acid is used in the manufacture of sugar and glue.
10. Hydrochloric acid is produced in small quantities in the stomach where the acid aids digestion.

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This work is licensed by Hatimu Muyanja under a Creative Commons Attribution License (CC-BY 2.0), and is an Open Educational Resource.

Last edited by Hatimu Muyanja on August 17, 2009 23:19