Alkalis

What is the definition of a base in chemistry

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What is the definition of a base in chemistry?

Bases
Bases are substances that are soapy to touch and bitter in taste.
Substances containing a base are called basic substances. Sodium hydroxide (NaOH) and calcium hydroxide [Ca(OH)2] are examples of bases used in the laboratory. Corn starch, fresh egg white, etc., are other examples of bases.
Bases may have a strong irritating odour and should be used with caution as they can harm the skin and eyes.

the-definition-of-an-acid-in-chemistry
Some bases

A base is a substance, usually the oxide or the hydroxide of a metal, which can react with an acid to produce salt and water.

For example, sodium oxide (Na2O), calcium oxide (CaO), cupric oxide (CuO), iron oxides (FeO, Fe2O3 etc.), sodium hydroxide (NaOH) and calcium hydroxide (Ca(OH)2 are all bases.

Certain substances are also called bases, though they do not fit into the above definition. For example, ammonia (NH3). It forms salt with an acid without giving water. So, it should not be treated as a base. But ammonium hydroxide (NH4OH), the aqueous solution of NH3, is a base as it reacts with an acid to give salt and water

NH4OH + HCl  →  NH4Cl + H2O

Alkalis :
Bases that are soluble in water are called alkalis. For example, sodium hydroxide, potassium hydroxide, calcium hydroxide are soluble in water. Therefore, they are alkalis. But bases like copper hydroxide (Cu(OH)2 ferric hydroxide (Fe(OH)3), aluminium hydroxide (Al(OH)3 do not dissolve in water. They are, therefore, not alkalis.

Hence, all alkalis are bases, but all bases are not alkalis. Some of the bases are listed here in Table.

Oxides

Soluble hydroxidesInsoluble hydroxides

Sodium monoxide (Na2O)

Sodium hydroxide (NaOH)

Ferric hydroxide (Fe(OH)3).

Calcium oxide (CaO)

Potassium hydroxide (KOH)

Aluminium hydroxide

(Al(OH)3)

Cupric oxide (CuO)

ZnO

Calcium hydroxide (Ca(OH)2)

Ammonium hydroxide

NH4OH

Lime water, baking soda and washing soda are all bases.

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What is the definition of an acid and a base?

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What is the definition of an acid and a base?

 

What are acids?

  • The Swedish chemist, Arrhenius proposed the following definition of an acid.
    An acid is a substance which ionises or dissociates in water to produce hydrogen ions, H+.
  • For example, hydrochloric acid, HCl(aq) is a solution of hydrogen chloride in water, obtained by dissolving pure hydrogen chloride gas in water.
    HCl(g) → HCl(aq)
    As the gas dissolves in water, the hydrogen chloride molecule reacts with water and ionises to produce hydrogen ion, H+.
    HCl(aq) → H+(aq) + Cl–(aq)
    The hydrogen ion then attaches itself to a water molecule to form the hydroxonium ion, H3O+.
    H+(aq) + H2O(l) → H3O+(aq)
    Hence, the overall equation for the ionisation of hydrogen chloride is given below.
    HCl(aq) + H2O(1) → H3O+(aq) + Cl–(aq)
    What is the definition of an acid and a base 1
  • For the sake of convenience, the term ‘hydrogen ion is used to replace ‘hydroxonium ion and H+(aq) is used in place of H3O+(aq).
  • Hence, the ionisation of hydrochloric acid in water can be represented as:
    What is the definition of an acid and a base 2
  • Other acids ionise similarly in water. Examples:
    What is the definition of an acid and a base 3
    What is the definition of an acid and a base 4
  • Hydrochloric acid is known as a monoprotic acid. This acid contains only one ionisable hydrogen atom, producing only one hydrogen ion (proton) per molecule of acid.
  • Polyprotic acids can produce more than one hydrogen ion per molecule of acid. Sulphuric acid is a diprotic acid, whereas phosphoric acid is a triprotic acid.
  • Basicity of an acid is the number of ionisable hydrogen atoms per acid molecule.
  • A number of non-metal oxides react with water to produce acidic solutions which contain hydrogen ions and turn blue litmus paper red. They are called acidic oxides.
    (a) Carbon dioxide reacts with water to form carbonic acid.
    CO2(g) + H2O(l) → H2CO3(aq)
    (b) Sulphur trioxide reacts with water to form sulphuric acid.
    SO3(g) + H2O(l) → H2SO4(aq)
    (c) Dinitrogen pentoxide reacts with water to form nitric acid.
    N2O5(g) + H2O(l) → 2HNO3(aq)
  • Not all non-metal oxides are acidic oxides. Only those that are able to react with water can produce acidic solutions. For example, carbon monoxide does not react with water. Therefore, carbon monoxide is classified as a neutral oxide.
  • Acids are classified into two groups, mineral acids and organic acids.
  • Mineral acids are obtained frdm minerals, whereas organic acids are extracted from animal and plant materials.

Table 1 and Table 2 show some examples of mineral and organic acids.

Mineral acid
NameFormula
Carbonic acidH2CO3
Hydrochloric acidHCl
Hydrochlorous acidHClO
Nitrous acidHNO2
Nitric acidHNO3
Sulphurous acidH2SO3
Sulphuric acidH2SO4
Phosphoric acidH3PO4

 

Organic acid
NameFormula
Methanoic acidHCOOH
Ethanoic acidCH3COOH
Propanoic acidC2H5COOH
Ascorbic acidC6H8O5
Citric acidC6H6O7
Lactic acidC3H6O3
Malic acidC4H6O5
Ethanedioic acidH2C2O4

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What are bases?

  1. Arrhenius’ definition of a base:
    A base is a substance which ionises in water to produce ydroxide ions, OH–.
  2. Bases are divided into two categories, ionic bases and covalent bases.
  3. Ionic bases consist of metal oxides and metal hydroxides. Metal oxides contain oxide ions, O2- and metal hydroxides contain hydroxide ions, OH–.
  4. When a soluble metal hydroxide such as sodium hydroxide dissolves in water, it ionises to release the hydroxide ion.
    What is the definition of an acid and a base 5
  5. When a soluble metal oxide dissolves in water, it reacts with water to form the hydroxide ion as one of its products. For example, calcium oxide reacts with water to form calcium hydroxide, which then ionises to produce hydroxide ion.
    What is the definition of an acid and a base 6
  6. Insoluble metal oxides and metal hydroxides are classified as bases because they satisfy the alternative definition of a base.
    A base is a substance that reacts with an acid to form a salt and water only.
  7. (a) For example, magnesium hydroxide reacts with hydrochloric acid to form the salt magnesium chloride and water.
    What is the definition of an acid and a base 7
    (b) On the other hand, copper(II) ‘oxide reacts with nitric acid to produce the salt copper(II) nitrate and water.
    What is the definition of an acid and a base 8
  8. The most common covalent base is ammonia, NH3. Ammonia solution is obtained by dissolving pure ammonia gas in water. When ammonia gas dissolves in water, it reacts with water to produce hydroxide ion.
    What is the definition of an acid and a base 9
    Notice that the ammonia, NH3 molecule has accepted a proton, H+ from water to form the ammonium ion, NH4+.
  9. Bases that are soluble in water are called akalis (Figure).
    What is the definition of an acid and a base 10
  10. An alkali is defined as the following.
    An alkali is a base that is soluble in water and ionises to produce hydroxide ions.
  11. According to this definition, ammonia can be classified as an alkali.

Table shows some examples of bases and alkalis.

Soluble base (alkali)Insoluble base
NameFormulaNameFormula
AmmoniaNH3Magnesium oxideMgO
Sodium oxideNa2OMagnesium hydroxideMg(OH)2
Sodium hydroxideNaOHAluminium oxideAl2O3
Potassium oxideK2OAluminium hydroxideAl(OH)3
Potassium hydroxideKOHZinc oxideZnO
Calcium oxideCaOZinc hydroxideZn(OH)2
Calcium hydroxideCa(OH)2Copper(II) oxideCuO
Barium oxideBaOCopper(II) hydroxideCu(OH)2
Barium hydroxideBa(OH)2Lead(II) oxidePbO

How can we measure the strength of acids and alkalis?

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How can we measure the strength of acids and alkalis?

    1. An acid produces hydrogen ions in aqueous solution. The acidity of a solution is a measure of the concentration of the hydrogen ions in the solution.
  1. A base produces hydroxide ions in aqueous solution. The alkalinity of a solution is a measure of the concentration of the hydroxide ions in the solution.
  2. In 1909, Soren Sorensen proposed the pH scale for measuring acidity or alkalinity of an aqueous solution.

The pH scale

How can we measure the strength of acids and alkalis 1

  1. The pH scale normally has a range of values from 0 to 14 to indicate how acidic or alkaline an aqueous solution is.
  2. The pH value measures the concentration of hydrogen ions or hydroxide ions.
    pH < 7 → acidi solution
    pH = 7 → neutral solution
    pH > 7 → alkaline solution
  3. The lower the pH value, the higher the concentration of hydrogen ions.
    The higher the pH value, the higher the concentration of hydroxide ions.
  4. The pH of values some common solutions used in daily life are shown below.
    How can we measure the strength of acids and alkalis 2
  5. The pH value of an aqueous solution can be measured by using
    (a) Universal Indicator
    (b) pH meter
    (c) acid-base indicators
    How can we measure the strength of acids and alkalis 3

Table shows examples of acid-base indicators.

IndicatorpH rangeColour change
Acid         NeutralAlkali
Methyl orange3.0 – 5.0RedOrangeYellow
Bromothymol blue6.0 – 8.0YellowGreenBlue
Phenolphthalein8.0-10.0ColourlessColourlessPink

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Strong and weak acids

  • The strength of an acid or alkali depends on the degree of dissociation of the acid or alkali in water.
  • The degree of dissociation measures the percentage of acid molecules that ionise when dissolved in water.
  • A strong acid is an acid which ionises or dissociates completely in water to produce a high concentration of hydrogen ions.
  • Hydrochloric acid is a strong acid. It is 100% ionised in water. All the hydrogen chloride molecules that dissolve in the water ionise completely into hydrogen ions and chloride ions.
    How can we measure the strength of acids and alkalis 4
  • A weak acid is an acid which ionises partially in water to produce a low concentration of hydrogen ions.
  • Ethanoic acid is a weak acid. Dilute ethanoic acid is about 0.4% ionised, only 4 out of every 1000 ethanoic acid molecules ionise. As fast as the acid molecules ionise to produce ions, the ions combine back again to give the original acid molecules in a reversible reaction.
    How can we measure the strength of acids and alkalis 5
    How can we measure the strength of acids and alkalis 6
  • Examples of strong and weak acids are shown in Table.
    AcidNameParticles
    Strong acidHydrochloric acid, HClH+, Cl–
    Nitric acid, HNO3H+, NO3–
    Sulphuric acid, H2S04H+, HSO4–, SO42-
    Weak acidCarbonic acid, H2C03H+, HCO3–, CO32-, H2CO3
    Ethanoic acid, CH3COOHH+, CH3COO–, CH3COOH
    Sulphurous acid, H2SO3H+, HSO3–, SO32-, H2SO3
  • Strong acid and weak acid are defined as follows.
    A strong acid is completely ionised in water to produce a high concentration of hydrogen ions.
    A weak acid is partially ionised in water to produce a low concentration of hydrogen ions.
  • The pH of an acid solution changes with concentration of the acid. The concentration of the acid affects the concentration of hydrogen ions produced. For example:
    10 mol dm-3 hydrochloric acid: pH = 1
    01 mol dm-3 hydrochloric acid: pH = 2
  • pH values can be used to compare the acid strength of different acids. All the acids to be compared must be of the same concentration.
    For example:
    10 mol dm-3hydrochloric acid: pH = 1
    10 mol dm-3 ethanoic acid: pH = 3
  • For two different acids of the same concentration, the acid with the lower pH value is the stronger acid, i.e. higher degree of ionisation in water.

Strong and weak alkalis

  • Sodium hydroxide is a strong alkali. It ionises fully when dissolved in water.
    How can we measure the strength of acids and alkalis 7
  • A strong alkali is an alkali which is fully ionised in water to produce a high concentration of hydroxide ions.
  • A weak alkali is an alkali which ionises partially in water to produce a low concentration of hydroxide ions.
  • Ammonia is an example of a weak alkali. It is only partly ionised in water, which means the ionisation of ammonia in water is incomplete. Only a small amount of ammonia molecules are ionised in water to produce ammonium ions and hydroxide ions.
    How can we measure the strength of acids and alkalis 8
  • Examples of strong and weak alkalis are shown in Table.
    AlkaliNameParticles
    Strong alkaliSodium hydroxide, NaOHNa+, OH–
    Potassium hydroxide, KOHK+, OH–
    Barium hydroxide, Ba(OH)2Ba2+, OH–
    Weak alkaliAmmonia, NH3NH3, NH4+, OH–
    Methylamine, CH3NH2CH3NH2, CH3NH3+, OH–
    Hydrazine, N2H4N2H4, N2H5+, OH–
  • Strong alkali and weak alkali are defined as follows.
    A strong alkali is completely ionised in water to produce a high concentration of hydroxide ions.
    A weak alkali is partially ionisesed in water to produce a low concentration of hydroxide ions.
  • The pH of an alkali solution changes with concentration of the alkali. The concentration of the alkali affects the concentration of hydroxide ions produced.
    For example:
    10 mol dm-3m-3 sodium hydroxide solution pH = 13
    01 mol dm-3 sodium hydroxide solution pH = 12
  • pH values can be used to compare the strength of different alkalis. All the alkalis to be compared must be of the same concentration. For example:
    10 mol dm-3 sodium hydroxide: pH = 13
    10 mol dm-3 ammonia solution: pH = 11
  • For two different alkalis of the same concentration, the alkali with the higher pH value is the stronger alkali, i.e. higher degree of ionisation in water.