Acids Bases and Salts
NCERT Exemplar Problems
Long Answer Questions
Question 43: In the following schematic diagram for the preparation of hydrogen gas, what would happen if following changes are made?
- In place of zinc granules, same amount of zinc dust is taken in the test tube
- Instead of dilute sulphuric acid, dilute hydrochloric acid is taken
- In place of zinc, copper turnings are taken
- Sodium hydroxide is taken in place of dilute sulphuric acid and the tube is heated.
Answer: (a) When zinc dust is taken instead of zinc granules to react with sulphuric acid, the hydrogen gas is formed. But the rate of reaction increases in the case of zinc dust compared to the zinc granules, because of increase in surface area as increase in surface are increases the rate of reaction. Thus, when zinc dust is used in the place of zinc granules, hydrogen gas is produced at a faster rate.
(b) Zinc granules give hydrogen gas; along with zinc chloride; when they react with hydrochloric acid.
Zn + 2HCl ⇨ ZnCl2 + H2O
Thus, when hydrochloric acid is used at the place of sulphuric acid, zinc chloride is formed in place of zinc sulphate; along with hydrogen gas and reaction takes place at the same rate.
(c) Copper does not react with dilute acids in normal condition because copper lies at lower position in reactivity series and is considered as a noble metal.So, if copper turning is taken in place of zinc granules, no reaction will take place.
(d) If sodium hydroxide is taken in place of dilute sulphuric acid and the tube is heated, sodium zincate is formed along with hydrogen gas. But heating of test tube will increase the rate of formation of hydrogen gas as heating of reaction mixture increases the rate of reaction.
Zn + 2NaOH ⇨ Na2ZnO2 + H2
Question 43: For making cake, baking powder is taken. If at home your mother uses baking soda instead of baking powder in cake,
(a) How will it affect the taste of the cake and why?
Answer: The chemical name of baking soda is sodium bicarbonate which gives sodium carbonate, carbon dioxide and water on heating. If baking soda is used instead of baking powder, in making of cake the formation of sodium carbonate would make the taste of cake bitter.
2NaHCO3 + Heat ⇨ Na2CO3 + CO2 + H2O
(b) How can baking soda be converted into baking powder?
Answer: By mixing of edible oil, usually tartaric acid, baking soda is converted into baking powder. Other mild edible oil is also used instead of tartaric acid.
(c) What is the role of tartaric acid added to baking soda?
Answer: Tartaric acid gives hydrogen ion when water is added to it. Hydrogen ion produced by tartaric acid reacts with sodium bicarbonate and gives carbon dioxide which makes the dough soft and spongy. If sodium carbonate is formed because of heating, tartaric acid neutralizes it and forms sodium tartarate which has pleasant smell and good taste.
2NaHCO3 + C4H6O6 ⇨ 2CO2 + 2H2O + Na2C4H4O6