Q: Carbonisation of coal: identify the correct description of the process Select the statement that correctly defines carbonisation of coal (coke-making by heating coal in the absence of air). Choose the correct option.

Concept Carbonisation of coal (coking) is the strong heating of coal in the absence of air to drive off volatiles and obtain coke. The options given describe other operations: (a) Pulverisation (size reduction), not carbonisation. (b) Briquetting (agglomeration), not carbonisation. (c) Refers to wood (charcoal making) at relatively low temperature; the question is about coal. Final Answer None of the above

Q: Avogadro’s law and gas density: proportionality with molecular mass For two gases at the same temperature and pressure, state how their densities compare with their molecular masses. Choose the correct relationship.

Concept At the same T and p, Avogadro’s law implies equal molar volumes. Density ρ is ρ = (mass)/(volume) = (M molar mass)/(molar volume). With constant molar volume, ρ ∝ M. Step ρ1/ρ2 = M1/M2 Final Answer Directly proportional to molecular mass

Question 1

Reversibility of basic thermodynamic processes: evaluate the statement  Statement: “The constant-pressure, constant-volume, and general p·vⁿ processes are regarded as irreversible processes.” Decide whether this statement is correct.

Accepted Answer

Concept/Approach A process type (isobaric, isochoric, polytropic p·vⁿ) can be carried out reversibly or irreversibly depending on how it is executed (quasi-static with negligible gradients vs. finite gradients, friction, etc.). Thus, none of these process labels inherently implies irreversibility. Final Answer Disagree — the statement is incorrect.

Question 2

Fuels: name the process—coal strongly heated for 42–48 hours in a closed vessel without air  Identify the standard term for this process used to produce coke and by-products.

Accepted Answer

Concept/Approach Prolonged heating of coal in absence of air in a retort/oven produces coke, tar, gas, etc.; this is called carbonization (also termed destructive distillation). Final Answer Carbonization (destructive distillation) of coal

Question 3

First law at constant volume: decide correctness of the statement  Statement: “When a gas is heated at constant volume, the heat supplied increases the internal energy of the gas.”

Accepted Answer

Concept/Approach At constant volume, boundary work is zero: W = ∫ p dV = 0. First law: Q = ΔU + W ⇒ Q = ΔU. Heat supplied raises internal energy. Final Answer Agree — the heat at constant volume increases internal energy.

Question 4

Otto cycle on a T–s (temperature–entropy) diagram: choose the correct description  Identify the correct schematic description of how the Otto cycle appears on a T–s plot.

Accepted Answer

Concept/Approach Otto cycle: isentropic compression → constant-volume heat addition → isentropic expansion → constant-volume heat rejection. On a T–s diagram: isentropes are vertical lines; constant-volume processes appear as sloped curves (entropy changes with temperature at v = const). Final Answer Two vertical (isentropic) lines joined by two constant-volume heat-addition/rejection curves.

Question 5

Internal energy change in an isothermal process: evaluate the statement  Statement: “There is no change in internal energy in an isothermal process.” Decide whether this is correct for an ideal gas.

Accepted Answer

Concept/Approach For an ideal gas, internal energy depends only on temperature: ΔU = m cv ΔT. Isothermal ⇒ ΔT = 0 ⇒ ΔU = 0. Final Answer Agree — for an ideal gas, isothermal implies no change in internal energy.

Question 6

Liquid fuels: identify the lightest and most volatile common liquid fuel  Choose the fuel that is lightest and most volatile among the listed options.

Accepted Answer

Concept/Approach Volatility decreases from gasoline → kerosene → diesel → heavy fuel oils. Gasoline has the lowest boiling range and highest volatility. Final Answer Petrol (gasoline)

Question 7

Stoichiometry: oxygen required for CO to CO₂ conversion  Reaction: CO + (1/2) O₂ → CO₂. For 1 kg of CO producing 11/7 kg of CO₂, compute the required mass of O₂ (in kg). Choose the correct value.

Accepted Answer

Given/Reaction CO + (1/2) O₂ → CO₂ Molar masses: CO = 28, O₂ = 32, CO₂ = 44 (kg per kmol). Step-by-step calculation 28 kg CO needs (1/2)×32 = 16 kg O₂ and forms 44 kg CO₂.Per 1 kg CO: O₂ required = 16/28 = 4/7 kg; CO₂ formed = 44/28 = 11/7 kg (matches the statement). Final Answer 4/7 kg of O₂

Question 8

Atomic masses: identify the element with the smallest atomic mass among the given choices  Compare standard atomic masses at natural isotopic abundance for Oxygen, Sulphur, Nitrogen, and Carbon. Choose the correct option.

Accepted Answer

Given data Approximate standard atomic masses (u): O ≈ 16, S ≈ 32, N ≈ 14, C ≈ 12. Concept/Approach Atomic mass reflects the weighted average mass of naturally occurring isotopes. To find the minimum, compare the known approximate values. Step-by-step check Compare: 12 (C) < 14 (N) < 16 (O) < 32 (S) Common pitfalls Do not confuse atomic number with atomic mass; also, sulphur's mass is roughly double oxygen's. Final Answer Carbon

Question 9

Carbonisation of coal: identify the correct description of the process  Select the statement that correctly defines carbonisation of coal (coke-making by heating coal in the absence of air). Choose the correct option.

Accepted Answer

Concept Carbonisation of coal (coking) is the strong heating of coal in the absence of air to drive off volatiles and obtain coke. The options given describe other operations: (a) Pulverisation (size reduction), not carbonisation. (b) Briquetting (agglomeration), not carbonisation. (c) Refers to wood (charcoal making) at relatively low temperature; the question is about coal. Final Answer None of the above

Question 10

Avogadro’s law and gas density: proportionality with molecular mass  For two gases at the same temperature and pressure, state how their densities compare with their molecular masses. Choose the correct relationship.

Accepted Answer

Concept At the same T and p, Avogadro’s law implies equal molar volumes. Density ρ is ρ = (mass)/(volume) = (M molar mass)/(molar volume). With constant molar volume, ρ ∝ M. Step ρ1/ρ2 = M1/M2 Final Answer Directly proportional to molecular mass

Question 11

Entropy in an irreversible cyclic process: behavior over one complete cycle  Evaluate how the entropy of the system changes over a full irreversible cycle. Choose the correct statement.

Accepted Answer

Concept Entropy (S) is a state function. For any complete cycle (start and end at the same state), the change in entropy of the system is zero: ΔSsystem = 0, even if the processes are irreversible. Important nuance Clausius inequality gives ∮ δQ/T < 0 for an irreversible cycle, but this integral is not equal to ΔSsystem over the cycle (which is zero by definition of a state function). The inequality reflects entropy generation in surroundings/universe, not a net change of the system after completing the loop. Common pitfalls Mixing up entropy of the system with entropy of the universe. The latter increases for overall spontaneous irreversible processes. Final Answer Remains constant over the cycle

Question 12

Adiabatic process: identify the correct defining properties  An adiabatic process for a gas is characterized by no heat transfer; infer its implications for temperature and energy balance. Choose the correct option.

Accepted Answer

Concept Adiabatic process: Q = 0. From the first law, ΔU = Q − W (sign convention: W is work done by the system). With Q = 0, ΔU = −W (or equivalently, the change in internal energy equals the negative of work by the system; equals work on the system). Temperature generally changes when a gas compresses/expands adiabatically. Why all are true (a) is the definition; (b) typically occurs in adiabatic expansion/compression; (c) follows directly from the first law with Q = 0. Final Answer All of the above

Question 13

Pressure units conversion: value of 1 mm of mercury in SI units (N/m²)  Convert a pressure head of 1 mm Hg to pascals (N/m²) using the standard definition. Choose the correct value.

Accepted Answer

Concept Standard atmospheric pressure: 760 mm Hg ≈ 101325 Pa. Therefore, Calculation 1 mm Hg ≈ 101325 / 760 Pa≈ 133.322 Pa ≈ 133.3 N/m² Final Answer 133.3 N/m²

Question 14

Diesel vs. Otto cycle efficiency: limiting behavior with cut-off ratio  State when the Diesel cycle efficiency approaches the Otto cycle efficiency. Choose the correct condition.

Accepted Answer

Concept Diesel cycle has heat addition at constant pressure with a finite cut-off ratio rc. As rc → 1 (i.e., cut-off → 0), the Diesel cycle heat addition tends to constant volume, and the cycle approaches the Otto cycle. Final Answer Cut-off is zero

Question 15

Kinetic theory statement: kinetic energy at absolute zero  Evaluate the statement: “The kinetic energy of gas molecules becomes zero at absolute zero temperature (0 K).” Choose the correct option.

Accepted Answer

Concept For an ideal gas, the average translational kinetic energy per molecule is (3/2) k T. At T = 0 K, this expression gives zero kinetic energy. Note Quantum zero-point energy exists in real systems, but the ideal-gas kinetic theory model used in basic thermodynamics treats the average kinetic energy as proportional to T. Final Answer True

Question 16

Kinetic theory relation: pressure vs. kinetic energy density for an ideal gas  Complete the statement: The pressure exerted by an ideal gas equals what fraction of the kinetic energy of all molecules per unit volume? Choose the correct fraction.

Accepted Answer

Concept Kinetic theory yields p = (2/3) × (total translational kinetic energy per unit volume). Equivalently, the kinetic energy density is (3/2) p. Derivation sketch From molecular impacts on walls and isotropic velocity distribution, p = (1/3) ρ c2, and kinetic energy density is (1/2) ρ c2. Ratio: p / (KE density) = (1/3) / (1/2) = 2/3. Final Answer two-third

Question 17

Thermodynamic property: identify U + p·v  Select the thermodynamic property defined as the sum of internal energy (U) and the product of pressure and volume (p·v). Choose the correct option.

Accepted Answer

Concept Enthalpy is defined as H = U + p v. It is especially convenient for open systems (flow processes) at approximately constant pressure, where enthalpy changes track heat transfer. Why others are incorrect Work is a path function; entropy (S) is a different state function relating heat transfer and temperature. Final Answer enthalpy

Question 18

First law of thermodynamics: judge the correctness of the statement  Statement: “In the first law of thermodynamics, the total energy of the system remains constant.”  Choose whether the statement is correct or incorrect.

Accepted Answer

Concept/Approach The first law is an energy balance: the change in a system's total energy equals heat added minus work done by the system. It does not say the system's energy is always constant; only an isolated system (Q = W = 0) keeps constant energy. Key relation ΔE = Q − WE includes U (internal), KE, and PE. Example At constant volume heating: W = 0 ⇒ ΔE = Q > 0, so energy increases. Final Answer Disagree — the system's energy need not remain constant; it changes per ΔE = Q − W.

Question 19

Specific heats for an ideal gas: compare cp and cv  State how the specific heat at constant pressure (cp) compares to the specific heat at constant volume (cv).  Choose the correct comparison.

Accepted Answer

Concept/Approach At constant pressure, part of the heat goes into doing boundary work during expansion; hence more heat per kelvin is required. For an ideal gas: cp − cv = R and the ratio γ = cp/cv > 1. Final Answer cp is greater than cv.

Question 20

Identify the air-standard cycle from its process set  Processes: one constant-pressure process, one constant-volume process, and two isentropic (reversible adiabatic) processes.  Choose the correct cycle name.

Accepted Answer

Concept/Approach Diesel cycle: isentropic compression → constant-pressure heat addition → isentropic expansion → constant-volume heat rejection. Otto: two isentropes + two constant-volume processes. Carnot: two isotherms + two isentropes. Stirling: two isotherms + two constant-volume processes with regeneration. Final Answer Diesel cycle

Thermodynamics Questions