CHEMISTRY FORM 3 | TOPIC 3 | ACID,BASES & SALTS

The Natural Sources of Acids and Bases

In everyday life, we deal with many substances that chemists classify as acids. For example, orange juice and grapefruit juice contain citric acid. These juices, and others of the like, contain ascorbic acid, a substance more commonly known as vitamin C. Examples of natural sources of acids and the type of acids they contain are shown in table below.

Some natural sources acids

Source	Type of acid present
Mineral acids (HCl, H2SO4, HNO3, etc.)	Minerals
Tobacco	Salicylic acid
Tea	Tannic acid
Coffee	Chlorogenic acid
Sugar beet	Glutaric and adipic acids
Blackberry	Isocitric acid
Spinach, tomato	Oxalic acid
Sour (fermented milk)	Lactic acid
Bee, ant and nettle stings	Methanoic acid (formic acid)
Grapes, bananas, tamarinds	Tartaric acid
Citrus fruits	Citric acid ( lemons and limes have particularly high concentrations of citric acid; it can constitute as much as 8% of the dry weight of these fruits)

Acids have a sour taste. Vinegar, lemon juice, grapefruit juice and spoilt or fermented milk are all sour tasting because of the presence of acids. The acids present in animal and plant materials are known as organic acids.

Salads are often flavoured with vinegar, which contains dilute acetic acid. Boric acid is a substance that is sometimes used to wash the eyes.

In any chemistry laboratory, we find acids such as hydrochloric acid (HCl), sulphuric acid (H2SO4), and nitric acid (HNO3). These acids are called mineral acids because they can be prepared from naturally occurring compounds called minerals. Mineral acids are generally stronger and should be handled with great care, especially the concentrated acids, for they are very corrosive. They can eat away metals, skin and clothing. Nevertheless, some acids are not corrosive even when they are concentrated. They are called weak acids. Ethanoic acid is one example. It is found in vinegar. In general, organic acids are weaker than natural acids.

You can tell if a substance is acid or not by its effect on litmus. Litmus is a purple dye. It can be used as a solution, or on paper, called litmus paper.Litmus solution is purple. Litmus paper for testing acids is blue while that for testing bases is red in colour. Acids will turn litmus solution red. They will also turn blue litmus paper red.

Bases do not usually occur naturally. So they are not normally obtained from natural sources. However, they are prepared in the laboratory or in industry. Bases can be classified into oxides, hydroxides or carbonates. Therefore, bases can be defined as the oxides, hydroxides or carbonates of metals. Bases taste bitter. A bitter taste is a characteristic of all bases.

Most bases are insoluble in water. The bases which dissolve in water are known as alkalis. The most common alkalis are potassium hydroxide (KOH), sodium hydroxide (NaOH), calcium hydroxide, Ca(OH)2, and ammonium hydroxide (NH4OH), also known as ammonia solution.

Alkalis turn litmus solution blue and red litmus paper blue.A substance, such as litmus, which changes from one colour to another when mixed with an acid or base, is called an indicator. Table 3.2 shows how acids and bases (alkalis) affect the colours of different indicators. We can use this clue of colour changes to tell whether an unknown substance is an acid or base (alkali).

Some common indicator colour changes

Indicator	Colour in acid	Colour in alkali (base)
Methyl orange	orange	yellow
Phenolphthalein	colourless	Pink
Litmus	red	blue
Bromothymol blue	yellow	blue

The Reactions of Acids with Various Materials

Acids react with different substances to produce different products. These reactions are best carried out by using dilute acid solutions. The following are some reactions of dilute acids with various substances.

Reaction with metals

Acids react with quite reactive metals (not very reactive ones) to produce salt and liberate a hydrogen gas.

metal + acid → salt + hydrogen

It is unsafe to try this reaction with very reactive metals such as sodium or calcium. The reaction with such metals is so violent. Metals less reactive than lead, such as silver and gold have no reaction with dilute acids. Even with lead, it is difficult to see any reaction in a short time.

The salt produced when a dilute acid reacts with a metal depends on the acid and a metal used:

    Mg(s) + 2HNO3(aq) → Mg(NO3)2(aq) + H2(g)

    Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)

Reaction with carbonates

Acids react with carbonates to give salt, water and carbon dioxide. In general, all carbonates give off carbon dioxide when they react with acids.

acid + metal carbonate → salt + water + carbon dioxide

The normal methods of preparing carbon dioxide in the laboratory are based on this reaction. Dilute hydrochloric acid is reacted with marble chips (calcium carbonates):

2HCl(aq) + CaCO3(s) → CaCl2(aq) + H2O(l) + CO2(g)

Reaction with oxides and hydroxides (alkalis)

Hydroxides: acids react with alkalis, forming salt and water:

NaOH(aq) + HNO3(aq) → CaCl(aq) + H2O(l)

Oxides: They also react with metals oxides, forming salt and water:

ZnO(s) + 2HCl(aq) → ZnCl2(aq) + H2O(l)

The bases (oxides, hydroxides) all react in the same way with acids, and in the process, salts are formed. This type of reaction is known as neutralization reaction. It can be summarized up in a general equation:

acid + base → salt + water

Reaction with hydrogencarbonates (bicarbonates)

Acids react with hydrogencarbonates, forming salt, water and liberating carbon dioxide gas:

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

The Reactions of Alkalis with Various Materials

Alkalis react with acids to produce salt and water. All alkalis, except ammonia solution, will react with ammonium compounds liberating ammonia gas. Aqueous solutions of alkalis will precipitate the insoluble hydroxides of other metals from the solutions of metal salts. Caustic alkalis attack aluminium, zinc and lead to form salts. They react with carbon dioxide to form carbonates.

The Reactions of Bases with Various Substances

Characteristic reactions of bases

Dissolution in water

Most bases are insoluble in water. Some are soluble in water. Soluble bases are known as alkalis. The commonest alkalis are sodium hydroxide, (NaOH), calcium hydroxide, Ca(OH)2, potassium hydroxide, (KOH), and ammonium hydroxide or ammonia solution, (NH4OH). All alkaline solutions contain hydroxyl ions, OH-. In sodium hydroxide solution, the ions are produced like this:

NaOH(aq)→ Na+(aq)+ OH-(aq)

Like acids, alkalis can also be classified as strong or weak. Ammonia solution is a weak base because it ionizes just partially:

The rest of the bases are strong bases because they ionize fully into ions in solution.

Reaction with acids

Bases react with acids to produce salt and water. Refer to the reactions of acids with oxides and hydroxides discussed early.

Reaction with ammonium compounds

Alkalis turn litmus solution blue and red litmus paper blue.A substance, such as litmus, which changes from one colour to another when mixed with an acid or base, is called an indicator. Table 3.2 shows how acids and bases (alkalis) affect the colours of different indicators. We can use this clue of colour changes to tell whether an unknown substance is an acid or base (alkali).

Reaction with aqueous salts of metals

Aqueous solutions of alkalis will precipitate the insoluble hydroxides of other metals from the solutions of metal salts. Only NH4OH, KOH and NaOH are soluble enough in water.

All other hydroxides are insoluble and can be precipitated from aqueous solution by these three alkalis.

    When sodium hydroxide solution is added to copper (II) sulphate solution, a pale blue precipitate of copper (II) hydroxide is formed. CuSO4(aq) + 2NaOH(aq) → Cu(OH)2(s) + Na2 SO4(aq)

    Another example is the reaction between potassium hydroxide and iron (II) chloride, which precipitates iron (II) hydroxide. FeCl3(aq) + 3KOH(aq) → Fe(OH)3(s) + 3KCl(aq)

Reaction with metals

Caustic alkalis attack very few metals. The metals known to be attacked by the alkalis are aluminum, zinc and lead, where the aluminate, zincate and plumbate (II) are formed respectively. The aluminum will react thus:2Al(s) + 6NaOH(aq) + 6H2O(l) → 2Na3Al(OH)6(aq) + 3H2(g)(sodium aluminate)

Reaction with carbon dioxide

When carbon dioxide gas is bubbled through aqueous solutions of the caustic alkalis, the carbonates are formed.

2NaOH(aq) +CO2(g) → Na2CO3(aq) + H2O(l). With excess of the gas, the hydrogencarbonates are formed.Na2CO3(aq) + H2O(l) + CO2(g) → 2NaHCO3(aq)

Reaction with chlorine

Chlorine reacts with excess of cold dilute caustic alkalis to form the hypochlorite, (NaClO or KClO). 2NaOH(aq) + Cl2(g) → NaCl(aq) + NaClO(aq) + H2O(l).2KOH(aq) + Cl2(g) → KCl(aq) + KClO(aq) + H2O(l)

If excess chlorine is bubbled through hot concentrated solutions of caustic alkalis, the chlorates are formed, (NaClO3 or KClO3).

6NaOH(aq) + 3Cl2(g) → 5NaCl(aq) + NaClO3(aq) + 3H2O(l).6KOH(aq) + 3Cl2(g) → 5KCl(aq) + KClO3(aq) + 3H2O(l)

Applications of acid-base neutralization in everyday life

Acid-base neutralization has many applications in everyday life. The following are some of these applications:

Indigestion and pain relief

The dilute hydrochloric acid produced in your stomach is used for digestion and killing bacteria that might have been swallowed together with food or taken with water. However, excess acid causes indigestion, which can be painful. To ease the pain, we take an anti-acid treatment. Anti-acids are a broad group of compounds with no toxic effects on the body. They are used to neutralize the effects of acid indigestion.

Some of these anti-acids such as milk of magnesia [insoluble magnesium hydroxide, Mg(OH)2] help to neutralize and hence counteract the excess acid in the stomach. This treatment, therefore, prevents indigestion and pains. The neutralization reaction equation is:

Mg(OH)2(aq) + 2HCl(aq)→ MgCl2(aq) + 2H2O(l)

Other anti-acids such as “Alka-Seltzer” contain soluble materials, including sodium hydrogencarbonate. These tablets also contain some citric acid (a solid acid). On adding water, the acid and some of the sodium hydrogencarbonate react, producing carbon dioxide gas. This helps to spread and dissolve the other less soluble material. When taken, more sodium hydrogencarbonate neutralizes the excess hydrochloric acid in the stomach, thus easing digestion.

Some anti-acid tablets also contain painkiller to relieve pain. “Soluble aspirin” tablets dissolve and work in a similar way to “Alker-Seltzer” tablets. Vitamin C (ascorbic acid) can be added to the tablets. Note that it is important to add water to start the action of the acid.

Descaling kettles

The limescale (CaCO3) is formed inside boilers, kettles and water heaters when hard water is boiled. The limescale can be removed by treatment with an acid that is strong enough to react with CaCO3, but not strong enough to damage the metal. Vinegar can be used to discale kettles. Commercial “discalers” use other acid solutions such as methanoic acid

Prevention of tooth decay

Food remnants sticking onto teeth (plaque), after eating especially sugary food is acted upon by bacteria in your mouth. The pH of a sugar solution is 7. However, bacteria in your mouth break down the sugar in plaque to form acids, for example lactic acid. These acids lower the pH. Tooth decay begins when the pH falls below 5.8. The acid attacks the tooth enamel.

To help prevent tooth decay many types of toothpaste contain basic substances to neutralize the acids produced by these bacteria in your mouth. The pH of these basic substances is alkaline (higher than 7). The pH of saliva is slightly alkaline (pH 7.4), so it can also help to counteract the acid, particularly after a meal. After eating a sweet, for example, it takes about 15 minutes for saliva to raise the pH above 5.8, and stop further decay.

Soil treatment

Most plants grow best when pH of the soil is close to 7. They prefer the pH of between 6.5 and 7.0. If the soil pH is below 6.0, the soil is too acidic. Above the pH of 8.0, the soil is too alkaline. If the soil is too acidic or too alkaline, the plants grow poorly or not at all.

Chemicals can be added to the soil to adjust its pH. Most often, if the soil is too acidic, it is usually treated by liming. In this context, liming means addition of quicklime (calcium oxide), slaked lime (calcium hydroxide) or powdered chalk or limestone (calcium carbonate) to an acidic soil. These compounds (bases) have the effect of neutralizing the acidity of the soil.

If the soil is too alkaline, acids such as sulphuric acid, nitric acid or hydrochloric acid may be added to the soil to neutralize excessive alkalinity. However, these compounds are very expensive and hence uneconomical to apply on large-scale basis.

Insect stings treatment

When a bee stings someone, it injects an acid liquid into the skin. The bee sting, which is acidic in nature, can be neutralized by rubbing on calamine solution, which contains zinc carbonate or baking soda, which is sodium hydrogencarbonate. These compounds are basic in nature and so have the effect of neutralizing the acid in the sting.

Wasp stings are alkaline in nature, and can be neutralized with vinegar, which contains ethanoic acid. Ant and nettle stings contain methanoic acid. These may be neutralized by rubbing an extract squeezed from crushed onion leaves (which contain basic compounds) on the affected skin. The acid in the sting can also be neutralized by applying weak alkalis such as ammonia solution, ash extract, baking powder, etc.

Factory wastes treatment

Liquid wastes from factories often contain acid. If it reaches a river, lake or ocean, the acid will kill fish and other aquatic life. This can be prevented by adding slaked lime (calcium hydroxide) to the waste, to neutralize the acid before being dumped into water bodies.