Question 1

When zinc metal reacts with dilute sulfuric acid, hydrogen gas is produced. Which type of chemical reaction is this?

Accepted Answer

This reaction is a displacement reaction (also called single displacement). Zinc is more reactive than hydrogen and displaces hydrogen from the acid. The balanced equation is: Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g). In a displacement reaction, a more reactive element replaces a less reactive element in a compound. This is a standard Class 10 NCERT example used to demonstrate reactivity series and displacement reactions.

Question 2

When zinc metal is added to dilute hydrochloric acid, hydrogen gas is evolved and zinc chloride is formed. Which type of chemical reaction is this, and what is the oxidation state change of zinc?

Accepted Answer

This is a displacement reaction (also called single displacement) because zinc, being more reactive than hydrogen, displaces hydrogen from the acid. In this reaction, zinc undergoes oxidation: its oxidation state changes from 0 (in elemental form) to +2 (in ZnCl₂). Hydrogen is reduced from +1 to 0. The balanced equation is: Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g). Since zinc loses electrons (oxidized) and hydrogen gains electrons (reduced), this is also a redox reaction. Therefore, answer is B.

Question 3

When 2.3 g of sodium metal reacts with water, hydrogen gas is produced. If the reaction is: 2Na + 2H₂O → 2NaOH + H₂↑, how many moles of H₂ gas are produced? (Atomic mass: Na = 23)

Accepted Answer

This question tests the student's ability to balance a chemical equation, calculate moles from mass, and use stoichiometry to find the product quantity. The setup requires understanding that chemical reactions follow fixed molar ratios.

Step 1: Calculate moles of sodium.
Molar mass of Na = 23 g/mol
Moles of Na = 2.3 g ÷ 23 g/mol = 0.1 mol

Step 2: Use stoichiometry from the balanced equation.
From the equation: 2Na + 2H₂O → 2NaOH + H₂↑
Molar ratio: 2 mol Na : 1 mol H₂

Step 3: Calculate moles of H₂.
If 2 mol Na produces 1 mol H₂, then 0.1 mol Na produces:
0.1 mol Na × (1 mol H₂ / 2 mol Na) = 0.05 mol H₂

Therefore, the answer is C (0.05 mol).

The most common trap is selecting option A (0.1 mol), which occurs when students forget to apply the stoichiometric ratio and assume a 1:1 molar relationship between reactant and product. This reflects a fundamental misunderstanding of how balanced equations dictate the proportions of substances involved in a reaction.

AFCAT Chemical Reactions

Chemical Reactions— Rules & Concept

Key Points to Remember

Exam-Specific Tips

Chemical Reactions — Practice Questions

60-Second Revision — Chemical Reactions