Reaction of Metals with Acids - Class 10 Science

Class 10 Science – Chapter 2

Acids, Bases and Salts | अम्ल, क्षारक एवं लवण

Reaction of Metals with Acids

Reaction of Metals with Dilute Acids

1. What happens when a metal reacts with a dilute acid?

When a reactive metal reacts with a dilute acid, generally a salt and hydrogen gas are formed.

General Reaction:

Metal + Dilute Acid → Salt + Hydrogen Gas ↑

हिंदी: जब कोई क्रियाशील धातु तनु अम्ल के साथ अभिक्रिया करती है, तो सामान्यतः लवण और हाइड्रोजन गैस बनती है।

Important: सभी metals dilute acids के साथ समान प्रकार से react नहीं करते। बहुत कम reactive metals जैसे copper सामान्य dilute HCl के साथ hydrogen नहीं निकालते।

2. Why is Hydrogen Gas Produced?

Acids provide H⁺ ions in aqueous solution. A sufficiently reactive metal loses electrons and hydrogen ions gain electrons to form hydrogen gas.

Reduction:

2H⁺ + 2e⁻ → H₂

Thus hydrogen gas is evolved.

3. Examples of Metals Reacting with Acids

Zn + 2HCl → ZnCl₂ + H₂ ↑

Mg + 2HCl → MgCl₂ + H₂ ↑

Fe + 2HCl → FeCl₂ + H₂ ↑

इन सभी reactions में: Metal + Hydrochloric Acid → Metal Chloride + Hydrogen

4. Reactivity Matters

Metal	Reaction with dilute HCl	Hydrogen
Magnesium (Mg)	Very vigorous	Produced
Zinc (Zn)	Moderate	Produced
Iron (Fe)	Slow	Produced
Copper (Cu)	No reaction under ordinary conditions	Not produced

5. Important Concept

Metals above hydrogen in the reactivity series can generally displace hydrogen from dilute non-oxidising acids such as HCl and dilute H₂SO₄.

Reactivity Series:

K > Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Hg > Ag > Au

Exam Point: Zinc is above hydrogen in the reactivity series, therefore zinc can displace hydrogen from dilute HCl.

6. Observation in Laboratory

When zinc granules are added to dilute HCl:

• Bubbles are observed.
• Hydrogen gas is evolved.
• Zinc gradually dissolves.
• Zinc chloride solution is formed.
• The reaction mixture may become warm.

7. Test for Hydrogen Gas

Hydrogen gas can be tested using a burning splint. When a burning splint is brought near hydrogen gas, it burns with a characteristic ‘pop’ sound.

2H₂ + O₂ → 2H₂O

8. 30 MCQs – Concept

Q1. What is generally formed when a reactive metal reacts with a dilute acid?
A) Salt and hydrogen
B) Salt and oxygen
C) Base and hydrogen
D) Water only

Answer: A

Reactive metal + dilute acid generally gives salt and H₂ gas.

Q2. Which gas is generally evolved when zinc reacts with dilute HCl?
A) Oxygen
B) Hydrogen
C) Carbon dioxide
D) Chlorine

Answer: B

Zinc displaces hydrogen from hydrochloric acid.

Q3. Which salt is formed when zinc reacts with HCl?
A) ZnSO₄
B) ZnCl₂
C) ZnCO₃
D) ZnO

Answer: B

Zn combines with chloride ions to form zinc chloride.

Q4. Which metal reacts vigorously with dilute HCl?
A) Magnesium
B) Copper
C) Silver
D) Gold

Answer: A

Magnesium is highly reactive and reacts rapidly with dilute HCl.

Q5. Hydrogen is produced because H⁺ ions are:
A) Oxidised
B) Reduced
C) Neutralised only
D) Evaporated

Answer: B

H⁺ ions gain electrons to form H₂.

Q6. Which metal does not normally liberate hydrogen from dilute HCl?
A) Zn
B) Mg
C) Fe
D) Cu

Answer: D

Copper is below hydrogen in the reactivity series.

Q7. The general reaction is:
A) Metal + acid → salt + H₂
B) Metal + acid → base + O₂
C) Metal + acid → water only
D) Metal + acid → CO₂ only

Answer: A

Q8. Which metal is above hydrogen in the reactivity series?
A) Copper
B) Silver
C) Zinc
D) Gold

Answer: C

Q9. The bubbles observed in Zn + HCl reaction are due to:
A) O₂
B) H₂
C) CO₂
D) Cl₂

Answer: B

Q10. Hydrogen gas gives which test?
A) Milky solution
B) Pop sound
C) Blue colour
D) Green flame

Answer: B

Q11. Zinc + HCl is an example of:
A) Displacement reaction
B) Combination reaction
C) Decomposition only
D) Neutralisation

Answer: A

Zinc displaces hydrogen from HCl.

Q12. Which ion is reduced to form hydrogen gas?
A) Cl⁻
B) Zn²⁺
C) H⁺
D) OH⁻

Answer: C

Q13. Which product remains dissolved in the solution in Zn + HCl?
A) ZnCl₂
B) H₂
C) O₂
D) Zn metal

Answer: A

Q14. Which reaction is correctly balanced?
A) Zn + HCl → ZnCl₂ + H₂
B) Zn + 2HCl → ZnCl₂ + H₂
C) 2Zn + HCl → ZnCl₂ + H₂
D) Zn + HCl → 2ZnCl₂ + H₂

Answer: B

Q15. Which gas burns with a pop sound?
A) Hydrogen
B) Nitrogen
C) Carbon dioxide
D) Oxygen

Answer: A

Q16. Which metal is below hydrogen in the reactivity series?
A) Zn
B) Fe
C) Mg
D) Cu

Answer: D

Q17. What is the formula of hydrochloric acid?
A) H₂SO₄
B) HCl
C) HNO₃
D) CH₃COOH

Answer: B

Q18. What is the formula of zinc chloride?
A) ZnCl
B) ZnCl₂
C) Zn₂Cl
D) Zn₂Cl₂

Answer: B

Q19. What happens to zinc during the reaction?
A) It gains electrons
B) It loses electrons
C) It remains unchanged
D) It becomes hydrogen

Answer: B

Zn is oxidised to Zn²⁺ by losing electrons.

Q20. Zinc changes into:
A) Zn⁺
B) Zn²⁺
C) Zn³⁺
D) Zn⁻

Answer: B

Q21. Which process occurs at zinc atoms?
A) Reduction
B) Oxidation
C) Neutralisation
D) Crystallisation

Answer: B

Q22. Which process occurs at H⁺ ions?
A) Oxidation
B) Reduction
C) Combustion
D) Sublimation

Answer: B

Q23. Zn + 2HCl produces how many moles of H₂ per mole of Zn?
A) ½
B) 1
C) 2
D) 3

Answer: B

Q24. Which observation confirms gas evolution?
A) Bubbles
B) Crystal formation only
C) No change
D) Solidification

Answer: A

Q25. Which substance is oxidised in Zn + HCl?
A) H⁺
B) Zn
C) Cl⁻
D) H₂

Answer: B

Q26. Which substance is reduced?
A) Zn
B) H⁺
C) Cl⁻
D) ZnCl₂

Answer: B

Q27. The reaction Zn + 2HCl → ZnCl₂ + H₂ is:
A) Redox and displacement
B) Only neutralisation
C) Only decomposition
D) Only crystallisation

Answer: A

Q28. Which metal can displace hydrogen from dilute HCl?
A) Au
B) Ag
C) Zn
D) Cu

Answer: C

Q29. The gas collected in this reaction is:
A) Acidic
B) Hydrogen
C) Chlorine
D) Carbon dioxide

Answer: B

Q30. Which equation represents zinc with dilute HCl?
A) Zn + HCl → ZnCl + H
B) Zn + 2HCl → ZnCl₂ + H₂
C) 2Zn + HCl → ZnCl₂ + H₂
D) Zn + HCl → ZnCl₂ + H₂O

Answer: B

9. 30 Subjective Questions – Complete Solutions

1 Mark Q1. What happens when a reactive metal reacts with dilute acid?

A salt and hydrogen gas are generally formed.

1 Mark Q2. Which gas is evolved when zinc reacts with dilute HCl?

Hydrogen (H₂) gas.

1 Mark Q3. Write the formula of hydrochloric acid.

HCl

1 Mark Q4. Write the formula of zinc chloride.

ZnCl₂

1 Mark Q5. Name the type of reaction between zinc and dilute HCl.

Displacement reaction; it is also a redox reaction.

2 Marks Q6. Write the balanced equation for zinc with dilute hydrochloric acid.

Zn + 2HCl → ZnCl₂ + H₂↑

2 Marks Q7. Why is hydrogen gas evolved when zinc reacts with HCl?

Zinc is more reactive than hydrogen and displaces hydrogen from HCl. H⁺ ions gain electrons and form H₂ gas.

2 Marks Q8. Name the products formed when zinc reacts with dilute HCl.

Zinc chloride (ZnCl₂) and hydrogen gas (H₂).

2 Marks Q9. Why does copper not produce hydrogen with dilute HCl?

Copper is below hydrogen in the reactivity series and therefore cannot displace hydrogen from dilute HCl under ordinary conditions.

2 Marks Q10. How can hydrogen gas be tested?

Bring a burning splint near the gas. Hydrogen burns with a characteristic pop sound.

3 Marks Q11. Explain the reaction of zinc with dilute HCl.

Zinc reacts with dilute hydrochloric acid to form zinc chloride and hydrogen gas.

Zn + 2HCl → ZnCl₂ + H₂↑

This is a displacement as well as redox reaction.

3 Marks Q12. What observations are seen when zinc granules are added to dilute HCl?

1. Bubbles of gas are produced.
2. Zinc gradually dissolves.
3. Hydrogen gas is evolved and ZnCl₂ remains in solution.

3 Marks Q13. Why is the reaction between zinc and HCl called a displacement reaction?

Zinc displaces hydrogen from hydrochloric acid.

Zn + 2HCl → ZnCl₂ + H₂

Therefore it is a displacement reaction.

3 Marks Q14. Explain oxidation and reduction in Zn + 2HCl reaction.

Oxidation:

Zn → Zn²⁺ + 2e⁻

Reduction:

2H⁺ + 2e⁻ → H₂

Thus Zn is oxidised and H⁺ is reduced.

3 Marks Q15. Why do metals above hydrogen in the reactivity series react with dilute acids?

Such metals are sufficiently reactive to lose electrons and displace H⁺ ions from dilute non-oxidising acids, resulting in hydrogen gas formation.

4 Marks Q16. Explain the reaction of Mg, Zn and Fe with dilute HCl.

Magnesium:

Mg + 2HCl → MgCl₂ + H₂

Zinc:

Zn + 2HCl → ZnCl₂ + H₂

Iron:

Fe + 2HCl → FeCl₂ + H₂

All three can liberate hydrogen, but their reaction rates differ.

4 Marks Q17. Why does zinc displace hydrogen from hydrochloric acid?

Zinc is above hydrogen in the reactivity series. It readily loses electrons:

Zn → Zn²⁺ + 2e⁻

H⁺ ions accept these electrons:

2H⁺ + 2e⁻ → H₂

Therefore hydrogen gas is evolved.

4 Marks Q18. Differentiate between the reactions of zinc and copper with dilute HCl.

Zinc: Reacts with dilute HCl and produces H₂.

Zn + 2HCl → ZnCl₂ + H₂

Copper: Does not react with dilute HCl under ordinary conditions because Cu is below H in the reactivity series.

4 Marks Q19. Explain the laboratory test for hydrogen gas.

Hydrogen gas is collected and a burning splint is brought near it. Hydrogen burns rapidly with a characteristic pop sound.

2H₂ + O₂ → 2H₂O

The pop sound is used as the characteristic test for hydrogen.

4 Marks Q20. Explain why the equation Zn + HCl → ZnCl₂ + H₂ is not balanced.

Left side: Zn = 1, H = 1, Cl = 1 Right side: Zn = 1, H = 2, Cl = 2 Therefore H and Cl atoms are unequal. Adding coefficient 2 before HCl balances it:

Zn + 2HCl → ZnCl₂ + H₂

5 Marks Q21. Write the balanced equation and explain the reaction of zinc with dilute HCl.

When zinc reacts with dilute hydrochloric acid, zinc chloride and hydrogen gas are formed.

Zn + 2HCl → ZnCl₂ + H₂↑

Zinc displaces hydrogen from HCl. Therefore it is a displacement reaction. It is also a redox reaction because zinc is oxidised and H⁺ is reduced.

5 Marks Q22. Compare Mg, Zn, Fe and Cu with dilute HCl.

Mg reacts vigorously and produces H₂.

Mg + 2HCl → MgCl₂ + H₂

Zn reacts moderately and produces H₂.

Zn + 2HCl → ZnCl₂ + H₂

Fe reacts slowly and produces H₂.

Fe + 2HCl → FeCl₂ + H₂

Cu does not normally react with dilute HCl because it is below hydrogen.

5 Marks Q23. Explain the redox nature of Zn + 2HCl → ZnCl₂ + H₂.

Oxidation of zinc:

Zn → Zn²⁺ + 2e⁻

Reduction of hydrogen ions:

2H⁺ + 2e⁻ → H₂

Therefore oxidation and reduction occur simultaneously. Hence the reaction is a redox reaction.

5 Marks Q24. Explain the importance of the reactivity series in metal-acid reactions.

The reactivity series helps us predict whether a metal can displace hydrogen from a dilute acid. Metals above hydrogen generally can displace H₂, while metals below hydrogen generally cannot. For example, Zn is above H, so:

Zn + 2HCl → ZnCl₂ + H₂

Copper is below H and does not normally liberate hydrogen from dilute HCl.

5 Marks Q25. A student adds zinc to dilute HCl and observes bubbles. Explain the observation scientifically.

The bubbles are hydrogen gas. Zinc loses electrons and H⁺ ions from HCl gain these electrons to form H₂.

Zn → Zn²⁺ + 2e⁻

2H⁺ + 2e⁻ → H₂

Overall:

Zn + 2HCl → ZnCl₂ + H₂

6 Marks Q26. Describe completely what happens when zinc reacts with dilute hydrochloric acid.

Zinc is a reactive metal and lies above hydrogen in the reactivity series. When zinc granules are added to dilute HCl, zinc atoms lose electrons and H⁺ ions gain electrons. Hydrogen gas is released as bubbles and zinc chloride remains in solution.

Zn + 2HCl → ZnCl₂ + H₂↑

Oxidation:

Zn → Zn²⁺ + 2e⁻

Reduction:

2H⁺ + 2e⁻ → H₂

Hence the reaction is displacement as well as redox.

6 Marks Q27. Why do some metals react with dilute acids while others do not?

The ability of a metal to react with dilute acids depends mainly on its position in the reactivity series. Metals above hydrogen can generally displace hydrogen from dilute HCl or dilute H₂SO₄. Metals below hydrogen cannot normally do so. Examples: Zn + 2HCl → ZnCl₂ + H₂ But: Cu + dilute HCl → No reaction under ordinary conditions.

6 Marks Q28. Balance and explain the following equation: Zn + HCl → ZnCl₂ + H₂.

Step 1: Zinc is already balanced. Step 2: ZnCl₂ contains two chlorine atoms, so put 2 before HCl.

Zn + 2HCl → ZnCl₂ + H₂

Step 3: Two hydrogen atoms are now present on the left and two on the right. Therefore the equation is balanced. It represents a displacement reaction and also a redox reaction.

6 Marks Q29. Explain the complete electron-transfer mechanism of Zn and HCl reaction.

Zinc loses two electrons:

Zn → Zn²⁺ + 2e⁻

Hydrogen ions gain these electrons:

2H⁺ + 2e⁻ → H₂

Chloride ions combine with Zn²⁺:

Zn²⁺ + 2Cl⁻ → ZnCl₂

Overall reaction:

Zn + 2HCl → ZnCl₂ + H₂

Thus zinc is oxidised and H⁺ is reduced.

6 Marks Q30. Write a complete answer for the question: “What happens when a metal reacts with a dilute acid? Give the example of zinc and dilute HCl.”

A reactive metal generally reacts with a dilute acid to form a salt and hydrogen gas. General reaction:

Metal + Dilute Acid → Salt + H₂↑

For zinc and dilute HCl:

Zn + 2HCl → ZnCl₂ + H₂↑

Here zinc displaces hydrogen from hydrochloric acid. Zinc is oxidised to Zn²⁺ while H⁺ ions are reduced to hydrogen gas.

Zn → Zn²⁺ + 2e⁻

2H⁺ + 2e⁻ → H₂

Therefore, the reaction is a displacement as well as a redox reaction. Hydrogen can be identified by its characteristic pop sound on bringing a burning splint near the gas.

Hydrogen Gas Evolution – हाइड्रोजन गैस का उत्सर्जन

1. Formation of Hydrogen Gas

Hydrogen gas is evolved when a sufficiently reactive metal displaces hydrogen ions from a suitable dilute acid.

2H⁺ + 2e⁻ → H₂↑

2. Zinc Reaction

Zn + 2HCl → ZnCl₂ + H₂↑

One zinc atom loses two electrons and two H⁺ ions gain those electrons to form one molecule of H₂.

3. Hydrogen Test

Pop Test: A burning splint brought near hydrogen produces a characteristic pop sound.

2H₂ + O₂ → 2H₂O

4. Important Exam Points

✔ Zn is above H in reactivity series.
✔ Zn displaces H from dilute HCl.
✔ H₂ gas is evolved.
✔ ZnCl₂ remains in solution.
✔ Burning splint gives pop sound.
✔ Reaction is displacement + redox.

Zinc + Dilute HCl – जिंक + तनु HCl

1. Balanced Equation

Zn + 2HCl → ZnCl₂ + H₂↑

2. Word Equation

Zinc + Hydrochloric Acid → Zinc Chloride + Hydrogen

जिंक + हाइड्रोक्लोरिक अम्ल → जिंक क्लोराइड + हाइड्रोजन

3. Why is it Balanced?

Element	Reactants	Products
Zn	1	1
H	2	2
Cl	2	2

Therefore all atoms are equal on both sides.

4. Ionic Explanation

Zn → Zn²⁺ + 2e⁻

2H⁺ + 2e⁻ → H₂

Zn²⁺ + 2Cl⁻ → ZnCl₂

5. Reaction Classification

Displacement: Zinc displaces hydrogen.

Redox: Zinc is oxidised and H⁺ is reduced.

Gas Evolution: H₂ gas is evolved.

Balanced Equation & Applications

1. Balanced Equation

Zn + 2HCl → ZnCl₂ + H₂↑

Coefficient ratio:

Zn : HCl : ZnCl₂ : H₂
1 : 2 : 1 : 1

2. Application / Importance

The reaction is important for understanding:

• Reactivity of metals
• Displacement reactions
• Redox reactions
• Hydrogen gas evolution
• Reactivity series
• Balancing chemical equations

3. Numerical – Foundation Level

Question: According to the equation

Zn + 2HCl → ZnCl₂ + H₂

1 mole of Zn produces how many moles of H₂?

From the balanced equation: 1 mole Zn → 1 mole H₂ Answer = 1 mole H₂

Question: How many moles of HCl are required to react completely with 2 moles of Zn?

Equation:

Zn + 2HCl → ZnCl₂ + H₂

1 mole Zn requires 2 moles HCl. Therefore: 2 × 2 = 4 moles HCl

4. Quick Revision

Question	Answer
Metal used	Zinc (Zn)
Acid used	Dilute HCl
Salt formed	ZnCl₂
Gas formed	H₂
Reaction type	Displacement + Redox
Hydrogen test	Pop sound
Zn oxidation state	0 → +2
H oxidation state	+1 → 0

5. Final Exam Answer

Question: What happens when zinc reacts with dilute hydrochloric acid?

Answer: When zinc reacts with dilute hydrochloric acid, zinc chloride and hydrogen gas are formed.

Zn + 2HCl → ZnCl₂ + H₂↑

Zinc displaces hydrogen from HCl. Therefore, it is a displacement reaction. It is also a redox reaction because zinc is oxidised and H⁺ ions are reduced. Hydrogen gas is identified by its characteristic pop sound.