Question 273 marksShort AnswerSection C

(i) While electrolysing water before passing the current some drops of an acid are added. Why ? Name the gases liberated at cathode and anode. Write the relationship between the volume of gas collected at anode and the volume of gas collected at cathode.

(ii) What is observed when silver chloride is exposed to sunlight ? Give the type of reaction involved.

Chemical Reactions and Equations
Decomposition Reaction

Official answer

(i)

  • Reason for adding acid: Pure water is a poor conductor of electricity. A few drops of dilute sulphuric acid are added to make the water conducting (acidified water conducts electricity easily).
  • Gases liberated: Hydrogen gas (H2\text{H}_2) is liberated at the cathode (negative electrode), and oxygen gas (O2\text{O}_2) is liberated at the anode (positive electrode).
  • Volume relationship: The volume of gas collected at the cathode (hydrogen) is double the volume of gas collected at the anode (oxygen).

Volume of gas at cathode:Volume of gas at anode=2:1\text{Volume of gas at cathode} : \text{Volume of gas at anode} = 2 : 1


(ii)

  • Observation: White silver chloride turns grey in sunlight.
  • Type of reaction: Photolytic decomposition reaction (or decomposition reaction).
  • Chemical Equation:

2AgCl(s)→Sunlight2Ag(s)+Cl2(g)2\text{AgCl}(s) \xrightarrow{\text{Sunlight}} 2\text{Ag}(s) + \text{Cl}_2(g)

Answer keywords:acidified waterconductorhydrogen at cathodeoxygen at anodedouble volumewhite turns greyphotolytic decomposition2AgCl2AgCl2

Marking scheme

  1. 1.Explanation of why acid is added to water during electrolysis (0.5 marks)
  2. 2.Naming the gases liberated at the cathode (Hydrogen) and anode (Oxygen) (0.5 marks)
  3. 3.Stating the volume relationship between the gases (Hydrogen is double the volume of Oxygen) (0.5 marks)
  4. 4.Observation when silver chloride is exposed to sunlight (White turns grey) (0.5 marks)
  5. 5.Identifying the type of reaction (Photolytic decomposition) and writing the balanced chemical equation (1 mark)

Hint

Think about the conductivity of pure water versus acidified water. For the volume ratio, look at the stoichiometric coefficients of hydrogen and oxygen in the balanced chemical equation for the decomposition of water. For silver chloride, recall what happens to its white color when exposed to light.

Quick oral answer

Acid is added to water during electrolysis to increase its electrical conductivity because pure water is a poor conductor. Hydrogen gas is collected at the cathode, and oxygen gas is collected at the anode in a 2:1 volume ratio. When white silver chloride is exposed to sunlight, it decomposes into grey silver metal and chlorine gas, which is an example of a photolytic decomposition reaction.

Common mistakes

  1. 1.Confusing the electrodes: writing that oxygen is liberated at the cathode and hydrogen at the anode.
  2. 2.Stating the volume ratio incorrectly as 1:2 instead of 2:1 (cathode to anode).
  3. 3.Forgetting to write the condition 'Sunlight' over the reaction arrow for the decomposition of silver chloride.

Analysis and explanation

During the electrolysis of water, electrical energy is used to decompose water into its constituent gases, hydrogen and oxygen. Since pure water has a very low concentration of ions, it is a poor conductor of electricity. Adding a few drops of dilute acid provides free ions, which significantly increases its electrical conductivity.


The chemical equation for the electrolysis of water is:

2H2O(l)→Electricity2H2(g)+O2(g)2\text{H}_2\text{O}(l) \xrightarrow{\text{Electricity}} 2\text{H}_2(g) + \text{O}_2(g)

From the balanced equation, 2 moles of hydrogen gas are produced for every 1 mole of oxygen gas. Therefore, the volume of hydrogen gas collected at the cathode is twice the volume of oxygen gas collected at the anode.


When silver chloride (AgCl\text{AgCl}) is exposed to sunlight, it undergoes photolytic decomposition. The light energy breaks the chemical bonds in the white silver chloride crystals, decomposing them into grey silver metal and chlorine gas. This reaction is represented as:

2AgCl(s)→Sunlight2Ag(s)+Cl2(g)2\text{AgCl}(s) \xrightarrow{\text{Sunlight}} 2\text{Ag}(s) + \text{Cl}_2(g)

Interesting facts

The photolytic decomposition of silver halides (like silver chloride and silver bromide) was the chemical basis for traditional black-and-white photography.

Water decomposes in a 2:1 ratio by volume because each water molecule (H2OH_2O) contains exactly two hydrogen atoms for every one oxygen atom.

Question details

Q27 of 39 questions

Board
CBSE
Class
Class 10
Subject
Science
Year
2023
Section
Section C
Marks
3
Type
Short Answer
Chapter
Chemical Reactions and Equations
Topic
Decomposition Reaction
Bloom's level
Applying
Syllabus topic
Decomposition reaction

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Frequently asked questions

How many marks does this question carry in CBSE Class 10 Science 2023?

This question carries 3 marks in the CBSE Class 10 Science 2023 examination.

Which chapter does this question come from in Science?

This question is from the chapter "Chemical Reactions and Equations" in the CBSE Class 10 Science syllabus.

What topic does this question cover in Science?

This question covers the topic "Decomposition Reaction" from CBSE Class 10 Science.

What type of question is this in the CBSE Class 10 Science 2023 paper?

This is a Short Answer question from Section C in the CBSE Class 10 Science 2023 paper.