TYPES OF CHEMICAL REACTIONS at a glance

  Chemical reactions are reactions in which elements or/and compounds combine chemically to form new substances.

There are different types of chemical reactions, they include 

1.   Combinations reactions 

2.   Decomposition reactions 

3.   Displacement reaction

4.   Double decomposition reaction

5.   Thermal Dissociation reaction 

6.    Reversible reapction

7.  Catalytic reaction and 

8. Oxidation and reduction reaction 

Combination Reaction 

A combination reaction is a chemical reaction in which two or more substances combine to form one single product.

It is also called a synthesis reaction.

General Form

A + B → AB

Examples

1. Formation of magnesium oxide
   
   2Mg_(s) + O_2(g) →2MgO_(s)
   
   

2. Formation of water
   
   2H_2(g) + O_2(g) →2H₂O_(l)
   
   

3. Formation of ammonia  
   
   N_2(g) + 3H_2(g) →2NH_3(g)
   
   

4. Formation of calcium oxide
   
   CaO_(s) + CO_2(g)→ CaCO_3(s)

🧠 Important Tip

In a combination reaction, many reactants give one product.

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Decomposition Reaction

A decomposition reaction is a chemical reaction in which one compound breaks down into two or more simpler substances when heat, electricity, or light is applied.

General Form

AB → A + B

Types and Examples

1. Thermal Decomposition (by heat)

CaCO_3(s) {heat} CaO_(s) + CO_2(g)

(Calcium carbonate breaks into calcium oxide and carbon dioxide.)

2. Electrolytic Decomposition (by electricity)

2H₂O_(l) {electricity} 2H_2(g) + O_2(g)

3. Photochemical Decomposition (by light)

2AgCl_(s) {sunlight} 2Ag_(s) + Cl_2(g)

🧠 Important Tip

 In a decomposition reaction.

 compound splits into two or more products,

Displacement Reactions 

A displacement reaction is a chemical reaction in which a more reactive element replaces a less reactive element from its compound.

It usually occurs between a metal and a salt solution.

General Equation

A + BC → AC + B
(Where A is more reactive than B)

Examples

1. Zinc and copper (II) sulphate
   Zn_(s) + CuSO_4(aq) → ZnSO_4(aq) + Cu_(s)

2. Zinc displaces copper because zinc is more reactive.

3. Iron and copper (II) sulphate
   
   Fe_(s) + CuSO_4(aq) → FeSO_4(aq) + Cu_(s)
   
   

4. Copper and silver nitrate

5. Cu_(s) + 2AgNO_3(aq) → Cu(NO₃)_2(aq) + 2Ag_(s)
   
   

Important Points

• Only a more reactive metal can displace a less reactive metal.

• The reaction depends on the reactivity series.

🧠 Important Tip

If a metal is higher in the reactivity series, it will displace a metal below it from solution.

Double Decomposition Reaction

A double decomposition reaction (also called double displacement or metathesis reaction) is a chemical reaction in which two compounds exchange their ions to form two new compounds.

General Form

AB + CD → AD + CB

Examples

1. Reaction between sodium chloride and silver nitrate
   
   NaCl_(aq) + AgNO_3(aq) → AgCl_(s) + NaNO_3(aq)
   
   

2. Reaction between barium chloride and sodium sulphate
   
   BaCl2(aq) + Na₂SO_4(aq) →BaSO_4(s) + 2NaCl_(aq)

3. Reaction between hydrochloric acid and sodium hydroxide
   
   HCl_(aq) + NaOH_(aq) → NaCl_(aq) + H₂O_(l)

(This is also a neutralization reaction.)

Important Points

• The reaction usually occurs in aqueous solution.

• One of the products is often a precipitate, gas, or water.

🧠 Important Tip

If two compounds exchange ions to form new compounds, it is a double decomposition reaction.

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Thermal Dissociation

Thermal dissociation is a process in which a compound splits into simpler substances when heated, and the reaction is reversible.

When the temperature is lowered, the products can recombine to form the original compound.

General Form

AB →  A + B

Examples

1. Ammonium chloride
   
   NH₄Cl_(s) {heat} NH_3(g) + HCl_(g)
   
   

2. Dinitrogen tetroxide
   N₂O_4(g){heat} 2NO_2(g)
   

3. Calcium carbonate
   CaCO_3(s){heat} CaO_(s) + CO_2(g)
   

🧠 Important Tip

If a substance breaks on heating and reforms on cooling, it shows thermal dissociation.

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Reversible Reaction

A reversible reaction is a chemical reaction in which the products can react together to reform the original reactants.

It occurs in both forward and backward directions at the same time.

Symbol

A + B C + D

Examples

1. Formation of ammonium chloride
   
   NH_3(g) + HCl_(g) → NH₄Cl_(s)

2. Dinitrogen tetroxide and nitrogen dioxide

3. N₂O_4(g) ⇌ 2NO_2(g) 

4. Haber process
   
   N_2(g) + 3H_2(g)⇌ 2NH_3(g)
    

🧠 Important Tip

If a reaction can go both forward and backward, it is a reversible reaction.

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Catalytic Reaction

A catalytic reaction is a chemical reaction in which a substance called a catalyst increases the rate of the reaction without being used up or changed permanently.

Catalyst

A catalyst is a substance that speeds up a chemical reaction but remains unchanged at the end of the reaction.

Examples

1. Decomposition of hydrogen peroxide
   
   

2.   2H₂O_2(aq) ---{MnO₂}--->2H₂O_(l) + O_2(g)
   
   

3. Haber process (manufacture of ammonia)
   
   N_2(g) + 3H_2(g) ---{Fe}---> 2NH_3(g)
   
   

4. Contact process (manufacture of tetraoxosulphate VI acid)
   
   2SO_2(g) + O_2(g) ---{V₂O₅}---> 2SO_3(g)
   
   

🧠 Important Tip

A catalyst alters the rate of a reaction but is not used up in the process.

          Characteristics of a catalyst 

I. A catalyst alters the rate of a chemical reaction 

ii. It is not affected by the reaction it catalysts 

Iii. A catalyst will affect a reaction even in small amounts 

iv. Solid catalysts are more effective when grinded to powder

v. A catalyst does not affect the equilibrium position of a reversible reaction.

 

Objective Questions 

1. A reaction in which two or more substances combine to form a single product is called:

A. Decomposition reaction
B. Combination reaction
C. Displacement reaction
D. Reversible reaction

2. Which of the following is an example of a combination reaction?

A. CaCO₃ → CaO + CO₂
B. Zn + CuSO₄ → ZnSO₄ + Cu
C. 2Mg + O₂ → 2MgO
D. AgNO₃ + NaCl → AgCl + NaNO₃

3. A reaction in which a compound breaks into simpler substances is called:

A. Catalytic reaction
B. Combination reaction
C. Decomposition reaction
D. Thermal dissociation reaction

4. Which reaction is a decomposition reaction?

A. 2H₂ + O₂ → 2H₂O
B. CaCO₃ → CaO + CO₂
C. Fe + CuSO₄ → FeSO₄ + Cu
D. HCl + NaOH → NaCl + H₂O

5. In a displacement reaction:

A. Two compounds exchange ions
B. One element replaces another in a compound
C. Heat causes breakdown
D. Products react to form reactants

6. Which of the following is a displacement reaction?

A. Zn + CuSO₄ → ZnSO₄ + Cu
B. H₂ + Cl₂ → 2HCl
C. NH₄Cl ⇌ NH₃ + HCl
D. AgNO₃ + NaCl → AgCl + NaNO₃

7. Double decomposition reactions involve:

A. Combination of elements
B. Exchange of ions between compounds
C. Breakdown by heat
D. Use of catalysts only

8. Which equation represents a double decomposition reaction?

A. NaOH + HCl → NaCl + H₂O
B. 2KClO₃ → 2KCl + 3O₂
C. 2Na + Cl₂ → 2NaCl
D. Mg + H₂SO₄ → MgSO₄ + H₂

9. A reaction that proceeds in both forward and backward directions is called:

A. Catalytic reaction
B. Irreversible reaction
C. Reversible reaction
D. Displacement reaction

10. Which symbol indicates a reversible reaction?

A. →
B. ⇌
C. +
D. =

11. Which of the following is a reversible reaction?

A. H₂ + I₂ ⇌ 2HI
B. CaCO₃ → CaO + CO₂
C. Zn + CuSO₄ → ZnSO₄ + Cu
D. 2Mg + O₂ → 2MgO

12. A catalyst:

A. Stops a reaction completely
B. Changes into another substance
C. Speeds up a reaction without being used up
D. Lowers the yield of products

13. Which reaction uses a catalyst?

A. 2H₂O₂ → 2H₂O + O₂ (MnO₂ used)
B. CaCO₃ → CaO + CO₂
C. Zn + H₂SO₄ → ZnSO₄ + H₂
D. NaCl + AgNO₃ → AgCl + NaNO₃

14. Thermal dissociation occurs when:

A. Light breaks compounds apart
B. Electricity decomposes substances
C. Heat causes a reversible decomposition
D. Two elements combine together

15. Which of the following is an example of thermal dissociation?

A. NH₄Cl ⇌ NH₃ + HCl
B. 2Mg + O₂ → 2MgO
C. Cu + 2AgNO₃ → Cu(NO₃)₂ + 2Ag
D. HCl + NaOH → NaCl + H₂O

16. Which reaction type usually produces only one product?

A. Combination reaction
B. Double decomposition reaction
C. Displacement reaction
D. Thermal dissociation reaction

17. In the reaction: Fe + CuSO₄ → FeSO₄ + Cu
    Iron is:

A. Reduced
B. Displaced
C. The catalyst
D. Thermally decomposed

18. Which reaction type often forms a precipitate?

A. Combination reaction
B. Double decomposition reaction
C. Catalytic reaction
D. Thermal dissociation reaction

19. The reaction: AgNO₃ + NaCl → AgCl + NaNO₃
    produces a:

A. Gas
B. Catalyst
C. Precipitate
D. Metal oxide

20. Which statement about catalysts is correct?

A. They are consumed during reactions
B. They slow down reactions
C. They remain chemically unchanged after reaction
D. They change products formed

21. Which reaction involves the breakdown of a compound by heat?

A. Combination reaction
B. Catalytic reaction
C. Thermal decomposition reaction
D. Displacement reaction

22. The reaction: 2KClO₃ → 2KCl + 3O₂
    is an example of:

A. Combination reaction
B. Displacement reaction
C. Decomposition reaction
D. Double decomposition reaction

23. Which of the following is NOT a displacement reaction?

A. Mg + CuSO₄ → MgSO₄ + Cu
B. Zn + 2HCl → ZnCl₂ + H₂
C. NaOH + HCl → NaCl + H₂O
D. Fe + CuSO₄ → FeSO₄ + Cu

24. A reversible reaction reaches equilibrium when:

A. The reaction stops completely
B. Forward and backward reactions occur at equal rates
C. Only products are formed
D. Reactants disappear completely

25. Which reaction is aided by manganese(IV) oxide?

A. Decomposition of hydrogen peroxide
B. Neutralization reaction
C. Displacement reaction
D. Combination reaction

26. Which reaction type commonly requires heating?

A. Thermal decomposition reaction
B. Double decomposition reaction
C. Combination reaction only
D. Displacement reaction only

27. In a double decomposition reaction:

A. Elements exchange positions
B. Compounds exchange ions
C. Only gases are formed
D. Heat is always produced

28. Which of the following reactions is catalytic?

A. 2H₂O₂ → 2H₂O + O₂ (MnO₂ present)
B. NaOH + HCl → NaCl + H₂O
C. Mg + O₂ → MgO
D. CaCO₃ → CaO + CO₂

29. The reaction: CaO + CO₂ → CaCO₃
    is a:

A. Combination reaction
B. Decomposition reaction
C. Double decomposition reaction
D. Thermal dissociation reaction

30. Which of the following reactions is reversible?

A. H₂ + Cl₂ → 2HCl
B. NH₄Cl ⇌ NH₃ + HCl
C. Mg + 2HCl → MgCl₂ + H₂
D. AgNO₃ + NaCl → AgCl + NaNO₃