Chemistry (Part-I) (UPSC)

Total Questions: 50

41. Consider the following statements with reference to the periodic table of chemical elements: [2001]

1. Ionisation potential gradually decreases along a period.
2. In a group of elements, electron affinity decreases as the atomic weight increases.
3. In a given period, electronegativity decreases as the atomic number increases.

Which of these statement (s) is/are correct?

Correct Answer: (b) 2 only
Solution:The electron affinity of a molecule or atom is the energy change when an electron is added to the neutral atom to form a negative ion. This property can only be measured in an atom in gaseous state.

X + e⁻ → X

Down a group, the electron affinity decreases because of a large increase in the atomic radius, electron-electron repulsion and the shielding effect of inner electrons against the valence electrons of the atom. As one moves from left to right across a period in the periodic table, the electronegativity increases due to the stronger attraction that the atoms obtain as the nuclear charge increases. There will be an increase of ionization energy from left to right in a given period.

42. Match List-I (Oxidation number) with List II (The element) and select the correct answer using the codes given below the lists: [2002]

List-I (Oxidation number)List-II (The elements)
A. 21. Oxidation number of Mn in MnO₂
B. 32. Oxidation number of S in H₂S₂O₇
C. 43. Oxidation number of Ca in CaO
D. 64. Oxidation number of Al in NaAlH₄

 

Code:ABCD
(a)3412
(b)4312
(c)3421
(d)4321

 

Correct Answer: (a) 3
Solution:The oxidation number characterises the oxidation state of an element in a compound. It is a full number, positive or negative, which indicates the amount of electron loss or gain by this element in the given compound, with respect to the neutral atom. Oxidation number of calcium is 2 in calcium oxide (CaO). Oxidation number of Aluminium is +3 in Sodium aluminium hydride (NaAlH₄) is a chemical compound used as a reducing agent.
Oxidation number of manganese is 4 in Manganese dioxide (MnO₂).
Oxidation number of sulphur is 6 in Pyrosulfuric Acid (H₂S₂O₇).

43. The following item consists of two statements. One labeled as the 'Assertion (A)' and the other as "Reason (R). You have to answer this item using the code given below: [2002]

Assertion: Synthetic detergents can lather well in hard water.
Reason (R): Synthetic detergents form soluble calcium and magnesium salts with hard water.

Correct Answer: (a) Both A and R are true and R is the correct explanation of A
Solution:Synthetic detergents are sodium salt of long chain sulphonic acid or alkyl hydrogen sulphate. Hardness in water is due to presence of chlorides, sulphates and nitrates salts of calcium and magnesium. Now detergents are capable of forming soluble salts even with the calcium and magnesium ions present in hard water and forms lather easily in hard water.

44. Which one of the following statements is NOT correct ? [2003]

Correct Answer: (d) Barium and strontium are found free in nature.
Solution:Barium and Strontium are alkaline earth metals. They are quite reactive in nature, they do not occur in free state. Strontium is a relatively abundant element in the Earth's crust. The most common minerals containing strontium are celestine and strontianite. Celestine contains primarily strontium sulfate (SrSO₄), while strontianite contains mostly strontium carbonate (SrCO₃). The most common naturally occurring minerals containing barium are the very insoluble barium sulfate, BaSO₄ (barite), and barium carbonate, BaCO₃ (witherite).

45. (Questions 45-46): The following Items consist of two statements. One labeled as the 'Assertion (A)' and the other as 'Reason (R)'. You have to answer these items using the code given below: [2003]

Assertion (A): In the periodic table of chemical elements, electron affinity is always found to increase from top to bottom in a group
Reason (R): In a group, the atomic radii generally increase from top to bottom.

Correct Answer: (d) A is false but R is true
Solution:Electron Affinity increases across a period from left to right because of increasing effective nuclear charge and decreasing size of atoms. Electron Affinity decreases down the group due to increased size of atoms. Thus Assertion is false.

The atomic radii of elements increases from top to bottom in a group. In moving down a group, the nuclear charge is increasing as the atomic number increases and we expect that the size of the atom should decrease. Thus, Reason is true. Hence the correct answer is (d).

In general, the atomic radii decrease with increase (from left to right) in a period. For example, in the second period, the atomic radii decrease from Li to Ne through Be, C, N, O, and F. The effective nuclear charge increases across the period from left to right. Due to this reason elements on the right side of the periodic table are small in size because they experienced the higher effective nuclear charge.

46. You have to answer these items using the code given below: [2003]

Assertion (A): Coal-based thermal power stations contribute to acid-rain.
Reason (R): Oxides of carbon are emitted when coal burns.

Correct Answer: (b) Both A and R are individually true but R is not the correct explanation of A
Solution:Coal based thermal power plants contribute to acid rain because SO₂ and NO₂ are emitted from these plants which form H₂SO₄ and HNO₃ in atmosphere, that cause acid rain. Oxides carbon are emitted when coal burns, but it does not contribute to acid rain.
• Coal based thermal power plants are known to pump out a lot of greenhouse gases and ash, which are by-products of burning fossil fuels.
• Coal-based thermal power stations pollute the atmosphere by adding NOx, SO2, SPM, and CO.
• Carbon monoxide is one of the main gases that is released from the burning of fossil fuels. Out of all the gases released from a thermal power plant, carbon monoxide is the main one, and thermal power plants are one of the main contributors to the increased carbon monoxide levels throughout the world.
• Sulfur dioxide is another gas that is released from power plants.
• Nitrogen oxides are another set of gases that are released into the atmosphere by thermal power plants. Thermal power plants are also one of the biggest contributors to global nitrogen oxide levels.
• The other big pollutant to the atmosphere is ash. Ash often contains harmful particulate matter, as well as heavy metals.

47. Consider the following statements: [2004]

1. Baking soda is used in fire extinguishers.
2. Quick lime is used in the manufacture of glass.
3. Gypsum is used in the manufacture of plaster of Paris.

Which of the statements given above is/are correct?

Correct Answer: (d) 1, 2 and 3
Solution:Baking soda is a great as a fire extinguisher for electrical fires and grease fires. When baking soda is heated it releases carbon dioxide and produces water.
2NaHCO₃ → H₂O + CO₂ + Na₂CO₃

Since carbon dioxide is heavier than air and does not support combustion like oxygen does, it smothers the fire while water that is formed cools the fire to below ignition temperature. Calcium oxide, chemical compound, CaO is also called, quicklime. It is widely used in making porcelain and glass. Plaster of Paris is a type of building material based on calcium sulphate hemihydrate nominally CaSO₄. 1/2H₂O. It is created by heating gypsum to about 300°F (150°C).
2CaSO₄. 2H₂O → 2CaSO₄. 1/2H₂O + 3H₂O
(released as steam)

48. Salts of which of the following elements provide colours to fireworks? [2004]

Correct Answer: (c) Strontium and barium
Solution:Creating firework colors is a complex endeavor, requiring considerable art and application of physical science. Strontium and barium both are alkaline earth metal and are extremely reactive. They both impart characteristic color to flame. Strontium salts impart a red color to fireworks. Strontium compounds are also important for stabilizing fireworks mixtures. Barium is used to create green colors in fireworks, and it can also help stabilize other volatile elements.

49. Which are the materials generally employed as solder in soldering operations in electronics? [2006]

Correct Answer: (b) Lead and tin
Solution:Solder is a fusible metal alloy with a melting point or melting range of 90 to 450 degree Celsius (190 to 840°F), used in a process called soldering where it is melted to join metallic surfaces. It is especially useful in electronics and plumbing. Solder is an alloy of lead and tin.

50. Which one of the following non-metals is not a poor conductor of electricity? [2007]

Correct Answer: (b) Selenium
Solution:Among the given non-metals, selenium is not a poor conductor of electricity. Unlike the other options, selenium has the ability to conduct electricity to some extent, especially when exposed to light, which makes it a photoconductor. This property sets it apart from sulfur, bromine, and phosphorus, which are generally poor conductors of electricity.