Types of Reactions

As you already know, the formation of water, electrolysis of water and processes such as respiration and digestion, are all chemical reactions. But, did you know that they are all different types of reactions? Some produce heat whereas some combine and give rise to a single product. Let’s go ahead and learn about the different types of reactions.

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Types of Reactions

During a chemical reaction, atoms do not disappear from the mixture and appear elsewhere. Nor do they change from one element to atoms of another element. In fact, during chemical reactions, bonds between atoms are broken and re-made to give rise to new substances. Let’s understand how this happens in different types of reactions.

Browse more Topics under Air

• Balanced Chemical Equations
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Combination Reaction

These are reactions where two or more reactants combine to give rise to a single product. For example, calcium oxide vigorously reacts with water to produce calcium hydroxide, also known as slaked lime. Slaked lime is used to whitewash walls.

CaO(s) + H₂O(l) → Ca(OH)₂(aq)

(Quicklime) → (Slaked lime)

Let’s look at some other examples of combination reactions –

• Formation of water from hydrogen and oxygen.

2H₂(g) + O₂(g) → 2H₂O(l)

• Burning of coal.

C(s) + O₂(g) → CO₂(g)

Exothermic Reaction

These are types of reactions where along with products, heat is also generated. For example, in the above reaction between calcium oxide and water, a large amount of heat gets produced making the reaction mixture warm. Other examples of exothermic reactions are –

• Burning of natural gas

CH₄(g) + 2O₂ (g) → CO₂ (g) + 2H₂O (g)

• Do you know how we get the energy to do our daily activities? The food we consume is broken down into simpler substances during digestion. For example, bread, potatoes rice etc contain carbohydrates which get broken down to its simpler form glucose. During respiration, glucose combines with oxygen in our cells and provides energy. Therefore, respiration is also an exothermic process.

C₆H₁₂O₆(aq) + 6O₂(aq) → 6CO₂(aq) + 6H₂O(l) + energy
(Glucose)

• Another example of an exothermic reaction is the decomposition of vegetable matter to compost.

Decomposition Reaction

In these types of reactions, a single reactant breaks down to give simpler products. For example, when ferrous sulfate crystals are heated, they lose water and the colour of the crystals changes. This is because ferrous sulfate decomposes to ferric oxide, sulfur dioxide, and sulfur trioxide.

Note: When a decomposition reaction is carried out by heating, it is known as thermal decomposition. Other examples of decomposition reactions include –

• Thermal decomposition of calcium carbonate to produce calcium oxide and carbon dioxide is very important in many industries. The product calcium oxide, which is also called lime or quicklime, is used to manufacture cement.

• Thermal decomposition of lead nitrate to produce lead oxide, nitrogen dioxide, and oxygen. The brown fumes observed when you heat lead nitrate over a flame are that of nitrogen dioxide.

• Electrolysis of water i.e. the decomposition of water into hydrogen and oxygen gas when an electric current is passed through water.

2H₂O → 2H₂ + O₂

• Decomposition of silver chloride or silver bromide when exposed to sunlight. Silver chloride decomposes to give silver and chlorine whereas silver bromide gives silver and bromine. These reactions are used in black and white photography.

Note: Decomposition reactions use up energy in the form of light, heat or electricity. Therefore, they are called endothermic reactions since they use up or absorb energy.

Displacement Reaction

In these type of reactions, one element displaces or removes another element from a compound. Let’s understand this reaction using the following experiment as an example.

Experiment

• Clean three iron nails by rubbing them with sandpaper.
• Mark two test-tubes as (A) and (B) and fill each tube with 10ml of copper sulfate solution.
• Dip two iron nails into the tube (B) for 20 minutes carefully using a thread and keep one nail aside for comparison.
• After 20 minutes, remove the iron nails from the copper sulfate solution.
• Now, note the intensity of the colour of the copper sulfate solution in the two tubes.
• Also, note the colour of the iron nails dipped in the solution versus the one kept aside.

Observations

• The copper sulfate solution in tube B is much lighter compared to the solution in tube A.
• The iron nails dipped in the copper sulfate solution have turned brown compared to the nail kept aside.

Conclusion

The above observations are a result of the following reaction –

Fe(s) + CuSO₄(aq)          →       FeSO₄(aq) + Cu(s)
(Copper sulfate)       (Iron sulfate)

In the above reaction, iron has displaced or removed copper from the copper sulfate solution. This is a displacement reaction. Some other examples of displacement reactions are –

Zn(s) + CuSO₄(aq)        →      ZnSO₄(aq) + Cu(s)
(Copper sulfate)        (Zinc sulfate)

Pb(s) + CuCl₂(aq)    →          PbCl₂(aq) + Cu(s)
(Copper chloride)      (Lead chloride)

Note: Both zinc and lead are more reactive than copper. Therefore, they displace copper in the above reactions.

Double Displacement Reaction

These are types of reactions, where there is an exchange of ions between the reactants. Let’s understand this using the following example.

• When equal amounts of sodium sulfate and barium chloride solutions are mixed, a white, insoluble substance is formed. This substance is called a ‘precipitate’. Therefore, this is also known as a precipitation reaction. Here, sodium sulfate and barium chloride exchange SO₄^2- and Cl^– ions to give barium sulfate which forms the precipitate and sodium chloride that stays in solution. Here, two ions have been displaced – SO₄^2- and Cl^–, therefore, this is a double displacement reaction.

Na₂SO₄(aq)       +       BaCl₂(aq)       →          BaSO₄(s)        +       2NaCl(aq)

(Sodium sulfate)      (Barium chloride)         (Barium sulfate)    (Sodium chloride)

Oxidation And Reduction

A substance is said to be oxidized if it gains oxygen or loses hydrogen during a reaction. Contrarily, if a substance loses oxygen and gains hydrogen during a reaction, it is said to be reduced. Let’s look at the following examples.

• When the copper powder is heated, its surface gets coated with black copper oxide. This happens because oxygen is added to copper to give copper oxide. Therefore, here copper is oxidized.

Now, if you pass hydrogen gas over heated copper oxide, the reverse reaction takes place and the black coating turns brown giving copper.

In the above reaction, CuO loses oxygen and is, therefore, is reduced. On the other hand, hydrogen gains oxygen and is being oxidized. Such reactions, where one reactant is oxidized and the other is reduced are called ‘oxidation-reduction reactions’ or ‘redox reactions’.

Other examples of redox reactions are –

•  ZnO + C → Zn + CO

Here, ZnO is reduced to Zn and carbon is oxidized to CO.

• MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂

Here, MnO₂ is reduced to MnCl₂ whereas HCl is oxidized to Cl₂.

Solved Examples for You

Question: In the following reaction, identify the substances that have been oxidized and reduced

4Na(s) + O₂(g) → 2Na₂O(s)

Solution: In the above reaction, sodium gains oxygen and is oxidized to Na₂O.

Question: Fill in the blanks from the following options. Fe₂O₃ + 2Al → Al₂O₃ + 2Fe. This reaction is an example of ________________.

1. Double displacement reaction
2. Decomposition reaction
3. Combination reaction
4. Displacement reaction

Solution: This reaction is an example of displacement reaction. This is because aluminium (Al) displaces iron (Fe) in the compound Fe₂O₃ to form Al₂O₃.