ScienceCBSE Class 10 · Board exam 2024, set 31/3/1 · 5 marks · Long answer

CBSE Class 10 Science 2024: question 34(b)

Set in the CBSE Class 10 board exam of 2024, paper 31/3/1, section D, question 34(b), for 5 marks. Try it first; the hints and our step-by-step solution are below.

Q34(b) · 5 marks · Long answerAcids, Bases and Salts

Name and state in brief the process which is used to prepare sodium hydroxide from sodium chloride. In this process along with the main product two gases 'X' and 'Y' are also given off at the two electrodes. Name 'X' and 'Y' specifying the name of their respective electrode at which each gas is obtained. One of these gases when reacts with dry calcium hydroxide produces a compound 'Z' which is widely used in water treatment plants and textile industries. Name Z and write chemical equation for the reaction involved in its formation.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2024 board paper, set 31/3/1.

Hints

Open one hint at a time and try again before the next.

Hint 1

Think about what happens when electricity is passed through brine. The name of the process comes from its two main products.

Hint 2

Chlorine is formed at one electrode and hydrogen at the other. Remember that negative chloride ions move towards the positive electrode.

Hint 3

Which product of this process reacts with slaked lime to make a compound used for disinfecting water?

Step-by-step solution

Each step shows the marks it earns, as an examiner would award them. Checked by an independent re-solve from the question paper.

Name of the process: Sodium hydroxide is made from sodium chloride by the chlor-alkali process. It is called this because its products are chlorine (chlor) and sodium hydroxide (alkali).
1 mark
The process in brief: Electricity is passed through brine, a concentrated aqueous solution of sodium chloride. The brine decomposes to give sodium hydroxide, chlorine and hydrogen:
$2 NaCl (aq) + 2 H_{2} O (l) \to 2 NaOH (aq) + C l_{2} (g) + H_{2} (g)$
Sodium hydroxide solution forms near the cathode.
½ mark
Gas X: X is **chlorine gas ( $C l_{2}$ ). It is given off at the anode** (positive electrode).
1 mark
Gas Y: Y is **hydrogen gas ( $H_{2}$ ). It is given off at the cathode** (negative electrode).
1 mark
Compound Z: Chlorine (gas X) reacts with dry slaked lime, $Ca (OH)_{2}$ , to form **bleaching powder (calcium oxychloride), $CaOC l_{2}$ ** = Z.
Z is used to disinfect drinking water in treatment plants and to bleach cotton and linen in the textile industry.
½ mark
Equation for the formation of Z:
$Ca (OH)_{2} + C l_{2} \to CaOC l_{2} + H_{2} O$
1 mark

Answer: Chlor-alkali process ( $2 N a C l + 2 H_{2} O \to 2 N a O H + C l_{2} + H_{2}$ ); X = $C l_{2}$ at anode, Y = $H_{2}$ at cathode; Z = bleaching powder $C a O C l_{2}$ ; $C a (O H)_{2} + C l_{2} \to C a O C l_{2} + H_{2} O$

Where the marks are

Names the chlor-alkali process (1 mark)
States that electrolysis of brine gives NaOH, or writes the balanced equation (½ mark)
X = chlorine, given off at the anode (½ + ½ marks)
Y = hydrogen, given off at the cathode (½ + ½ marks)
Z = bleaching powder / $C a O C l_{2}$ (½ mark)
Balanced equation $C a (O H)_{2} + C l_{2} \to C a O C l_{2} + H_{2} O$ (1 mark)

Revise the chapter

Chapter: Acids, Bases and Salts (notes, formulas, where marks are lost and every past question)

Practise this chapterBoard-style questions from Acids, Bases and Salts, one at a time, marked against the answer key and marking points.Back to the full paper2024 board paper, set 31/3/1: every question, by section.

The question is transcribed from the official paper; figures are described in words. The solution, hints and chapter tag are ours. Spotted an error? Write tosupport@questionbuddy.in.

CBSE Class 10 Science 2024: question 34(b)

Hints

Step-by-step solution

Revise the chapter

More questions from Acids, Bases and Salts