Types of Reactions Combination - UNSOLVED PRACTICE SET

Q7. Define a combination reaction. Write one example of a combination reaction that is: (i) exothermic, and (ii) endothermic.

Q8. Write balanced chemical equations for the following combination reactions:

(a) Calcium oxide reacts with water.

(b) Hydrogen burns in chlorine to form hydrogen chloride.

Q9. Your grandfather applies a paste of limestone (calcium oxide) and water on the walls of your village home. After a few days, the walls become hard and white. Explain the chemical reaction involved and why it is a combination reaction.

Q10. Why are combination reactions called "synthesis reactions"? Give one example each from: (i) metal + non-metal, and (ii) non-metal + non-metal.

Q11. Distinguish between a combination reaction and a decomposition reaction with one example of each.

Q12. When iron filings are heated with sulphur powder, a black solid is formed. Identify the product and write the balanced chemical equation. What type of reaction is this?

Q13. Define combination reaction with examples. Explain with balanced chemical equations how combination reactions occur between: (i) two elements, (ii) an element and a compound, and (iii) two compounds. Why are most combination reactions exothermic? Give a reason.

Q14. A student performs the following experiments in the laboratory:

Experiment A: Burning magnesium ribbon in air

Experiment B: Adding water to calcium oxide

Experiment C: Passing hydrogen gas over heated copper oxide

Experiment D: Mixing ammonium chloride and sodium hydroxide solutions

(a) Identify which experiments show combination reactions.

(b) Write balanced chemical equations for the combination reactions.

(c) For each combination reaction, state whether it is exothermic or endothermic.

(d) Explain why Experiment D is NOT a combination reaction, even though two substances are mixed.

Q15. "Combination reactions are the building blocks of complex substances in nature and industry." Discuss this statement by explaining: (i) how combination reactions form essential compounds like water and ammonia, (ii) their role in the formation of minerals and rocks, (iii) industrial applications such as the production of sulphuric acid and ammonia, and (iv) how understanding combination reactions helps in predicting the products of chemical reactions.

Q16. A cement factory produces calcium oxide (quicklime) by heating limestone. The quicklime is then used in various construction activities where it combines with water.

(a) Write the balanced chemical equation for the combination reaction of calcium oxide with water.

(b) If the factory produces 280 kg of calcium oxide per day, calculate the mass of calcium hydroxide formed when all the quicklime reacts with water. (Molar masses: CaO = 56 g/mol, Ca(OH)₂ = 74 g/mol)

(c) This reaction is highly exothermic. If 65 kJ of heat is released per mole of CaO, calculate the total heat released daily.

(d) Why is this exothermic property useful in construction work during cold weather?

(e) If a construction worker uses 50 kg of quicklime, how much water is needed for complete reaction? (Molar mass of H₂O = 18 g/mol)

Q17. In a chemistry laboratory, a teacher demonstrates combination reactions using the following setup:

Setup 1: Hydrogen gas + Chlorine gas (with sunlight) → Hydrogen chloride

Setup 2: Carbon + Oxygen (limited) → Carbon monoxide

Setup 3: Carbon + Oxygen (excess) → Carbon dioxide

Setup 4: Nitrogen + Hydrogen (high pressure, catalyst) → Ammonia

(a) Write balanced chemical equations for all four reactions.

(b) In Setup 2 and 3, the same reactants give different products. What does this tell you about combination reactions?

(c) For Setup 4, if 42 g of nitrogen reacts with excess hydrogen, calculate the mass of ammonia produced. (Molar masses: N₂ = 28 g/mol, NH₃ = 17 g/mol)

(d) Why is a catalyst needed in Setup 4? What is the role of a catalyst in a chemical reaction?

(e) Calculate how many molecules of ammonia are produced from 42 g of nitrogen.

Q18. During Diwali, firecrackers contain chemicals that undergo combination reactions to produce light, sound, and colour.

(a) Magnesium in firecrackers burns with a dazzling white light. Write the balanced equation for this combination reaction.

(b) If a firecracker contains 4.8 g of magnesium, calculate the mass of magnesium oxide produced. (Molar masses: Mg = 24 g/mol, MgO = 40 g/mol)

(c) Potassium nitrate in firecrackers decomposes to provide oxygen for combustion. However, the initial reaction of sulphur with oxygen is a combination reaction. Write this equation.

(d) Why do firecracker manufacturers need to understand combination reactions for safety purposes?

(e) If 3.2 g of sulphur burns completely, calculate the mass of sulphur dioxide produced. (Molar masses: S = 32 g/mol, SO₂ = 64 g/mol)