acid

What are the chemical properties of an acid?

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What are the chemical properties of an acid?

Chemical properties of acids:

  1. Acids react with reactive metals.
    What are the chemical properties of an acid 1

    • Acid + metal → salt + hydrogen
    • Copper and silver do not react with dilute acid.
      What are the chemical properties of an acid 3
  2. Acids react with bases.
    What are the chemical properties of an acid 4

    • Acid + base → salt + water
      What are the chemical properties of an acid 5
  3. Acids react with carbonates.
    What are the chemical properties of an acid 6

    • Acid + carbonate → salt + water + carbon dioxide
      What are the chemical properties of an acid 7

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To study the chemical reactions of acids experiment

Aim: To study the chemical reactions of acids.
Materials: 2 mol dm-3 hydrochloric acid, 2 mol dm-3 ethanoic acid, magnesium, copper(II) carbonate, iron(III) oxide, limewater, wooden splint and filter paper.
Apparatus: Test tubes, delivery tube, spatula, test tube holder, Bunsen burner, stopper, evaporating dish and filter funnel.
Safety measure: Acids are corrosive. Wear safety glasses.

A. Reactions of acids with bases

Procedure:

  1. About 5 cm3 of dilute hydrochloric acid is poured into a test tube.
  2. The acid is warmed using a gentle flame.
  3. Iron(III) oxide powder is added bit by bit with stirring to the hot acid.
  4. Any change that occurs is observed and recorded.
  5. The unreacted iron(III) oxide is removed by filtration.
  6. The filtrate is evaporated in an evaporating dish until one-third of the original volume remains.
  7. The hot solution is allowed to cool for crystals to form.
  8. The crystals formed are removed by filtration and dried by pressing the crystals between sheets of filter paper.
  9. The features of the crystals are noted and recorded.
  10. Steps 1 to 9 are repeated using dilute ethanoic acid to replace dilute hydrochloric acid.

Observations:

AcidObservation
Hydrochloric acidThe reddish-brown solid dissolves to form a brown solution. Reddish-brown crystals are obtained.
Ethanoic acidThe reddish-brown solid dissolves to form a brown solution. Reddish-brown crystals are obtained.

Discussion:

  1. Iron(III) oxide is an example of a base.
  2. It reacts with acids to form salts and water.
    What are the chemical properties of an acid 9
  3. The crystals are iron(III) chloride and iron(III) ethanoate.
  4. Iron(III) salts dissolve in water to produce brown solutions.

B. Reactions of acids with metals

Procedure:

  1. About 5 cm3 of dilute hydrochloric acid is poured into a test tube.
  2. One spatulaful of magnesium powder is added to the acid.
  3. A lighted wooden splint is brought to the mouth of the test tube to ignite the gas liberated.
  4. All observations are recorded.
  5. The unreacted magnesium is filtered out.
  6. The filtrate is evaporated until one-third of the original volume remains.
  7. The hot solution is allowed to cool for crystals to form.
  8. The crystals formed are removed by filtration and dried by pressing the crystals between sheets of filter paper.
  9. The features of the crystals are noted and recorded.
  10. Steps 1 to 9 are repeated using dilute ethanoic acid to replace dilute hydrochloric acid.

Observations:

AcidObservation
Hydrochloric acidThe grey solid dissolves to form a colourless solution. A colourless gas that burns with a ‘pop’ sound is produced. White crystals are obtained.
Ethanoic acidThe grey solid-dissolves to form a colourless solution. A colourless gas that burns with a ‘pop’ sound is produced. White crystals are obtained.

Discussion:

  1. Magnesium is a reactive metal.
  2. Reactive metals react with acids to form salts and hydrogen. Unreactive metals such as copper and silver do not react with dilute acids.
    What are the chemical properties of an acid 10
  3. The hydrogen gas liberated can be identified using a lighted wooden splint. Hydrogen gas burns with a ‘pop’ sound.
  4. The white crystals are magnesium chloride and magnesium ethanoate. They dissolve in water to form colourless solutions.

C. Reactions of acids with metal carbonates

Procedure:

  1. About 5 cm3 of dilute hydrochloric acid is poured into a test tube.
  2. One spatulaful of copper(II) carbonate powder is added to the acid.
  3. The test tube is quickly closed with a stopper fitted with a delivery tube dipped into limewater.
  4. Any change that occurs is recorded.
  5. The unreacted copper(II) carbonate is filtered out.
  6. The filtrate is evaporated until one-third of the original volume remains.
  7. The hot solution is allowed to cool for crystals to form.
  8. The crystals formed are removed by filtration and dried by pressing the crystals between sheets of filter paper.
  9. The features of the crystals are noted and recorded.
  10. Steps 1 to 9 are repeated using dilute ethanoic acid to replace dilute hydrochloric acid.

Observation:

AcidObservation
Hydrochloric acidThe green solid dissolves with effervescence to form a blue solution. A colourless gas which turns limewater milky is produced. Blue crystals are obtained.
Ethanoic acidThe green solid dissolves with effervescence to form a blue solution. A colourless gas which turns limewater milky is produced. Blue crystals are obtained.

Discussion:

  1. Copper(II) carbonate is a metal carbonate.
  2. It reacts with acids to form salts, water and carbon dioxide.
    What are the chemical properties of an acid 11
  3. Copper(II) salts crystallise as blue crystals and dissolve in water to produce blue solutions.
  4. When carbon dioxide gas is bubbled into limewater, a white precipitate of calcium carbonate is formed. The white precipitate causes the limewater to turn milky.

Conclusion:

  1. An acid reacts with a base to produce a salt and water.
  2. An acid reacts with a reactive metal to produce a salt and hydrogen gas.
  3. An acid reacts with a metal carbonate to produce a salt, water and carbon dioxide gas.

Chemical properties of alkalis

  1. Alkalis react with acids.
    What are the chemical properties of an acid 12

    • Alkali + acid → salt + water
    • Alkali neutralises acids.
      What are the chemical properties of an acid 13
  2. Alkalis react with metal ions.
    What are the chemical properties of an acid 14

    • Alkali + metal ion → metal hydroxide
    • Most metal hydroxides are insoluble in water and are precipitated.
    • Transition metal ions form coloured precipitates.
      What are the chemical properties of an acid 15

 

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

People also ask

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