Chemical Thermodynamics

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C(s) + 2H_2(g) → CH_4(g); Δ H = -74.8 kJ mol^-1Which of the following diagrams gives an accurate representation of the above reaction? [R → reactants; P → products] Answer: 1. Energy diagram (1) Key idea: Δ H=-74.8 kJ/mol means an exothermic reaction: products lie 74.8 kJ below the reactants. Diagram (1)... Hess's law and enthalpies of reactions (bond dissociation, combustion, formation etc.) 1 answer
The standard heat of formation, in kcal/mol of Ba^2+ is :[Given : standard heat of formation of SO_4^2- ion (aq) = -216 kcal/mol, standard heat of crystallisation of BaSO_4(s) = -4.5 kcal/mol, standard heat of formation of BaSO_4(s) = -349 kcal/mol] Answer: 1. -128.5 Key idea: mathrmBa²⁺(aq)+SO₄²⁻(aq)→ BaSO₄(s) has Δ H=-4.5 kcal (crystallisation). Δ H_f(BaSO₄)=Δ H_f(mathrmBa²⁺)+Δ H_f(mat... Hess's law and enthalpies of reactions (bond dissociation, combustion, formation etc.) 1 answer
The work done during reversible isothermal expansion of one mole of hydrogen gas at 25^°C from pressure of 20 atmosphere to 10 atmosphere is:(Given R = 2.0 cal K^-1 mol^-1) Answer: 3. - 413.14 calories Key idea: for reversible isothermal expansion, w=-2.303 nRTlogP₁/P₂ (work done by the gas is negative in this c... First law: work, heat, internal energy, enthalpy 1 answer
In which of the following processes entropy increases?A. A liquid evaporates to vapour.B. Temperature of a crystalline solid lowered from 130 K to 0 K.C. 2NaHCO_3(s) → Na_2CO_3(s) + CO_2(g) + H_2O_(g)D. Cl_2(g) → 2Cl_(g)Choose the correct answer from… Answer: 4. A, C and D Key idea: entropy rises with disorder, i.e. when more gas or more particles are formed. A: liquid to vapour, disorder... Second law, entropy and spontaneity 1 answer
Match List I with List II.List I (Process): A. Isothermal process B. Isochoric process C. Isobaric process D. Adiabatic processList II (Conditions): I. No heat exchange II. Carried out at constant temperature III. Carried out at constant volume IV. C… Answer: 1. A-II, B-III, C-IV, D-I Key idea: the prefix tells the fixed quantity. Isothermal: constant temperature (II). Isochoric: constant... System, surroundings, extensive/intensive properties, state functions, processes 1 answer
The equilibrium concentrations of the species in the reaction A + B ⇌ C + D are 2, 3, 10 and 6 mol L^-1, respectively at 300 K. Δ G^° for the reaction is (R = 2 cal / mol K) Answer: 4. -1381.80 cal Key idea: Δ G^° = -2.303 RTlog K . K = [C][D]/[A][B] = 10 × 6/2 × 3 = 10 , so log K = 1 . Δ G^° = -2.303 × 2 × 300 ×... Gibbs energy change and equilibrium constant 1 answer
Which amongst the following options is the correct relation between change in enthalpy and change in internal energy? Answer: 3. Δ H = Δ U + Δ n_g RT Key idea: H = U + PV , so at constant T and P, Δ H = Δ U + PΔ V = Δ U + Δ n_g RT . Here Δ n_g is the change... First law: work, heat, internal energy, enthalpy 1 answer
Which of the following p-V curve represents maximum work done ? Answer: 3. [graph in figure] Key idea: work done by the gas equals the area under the p-V curve, so find the graph with the largest area. Gr... First law: work, heat, internal energy, enthalpy 1 answer
For irreversible expansion of an ideal gas under isothermal condition, the correct option is : Answer: 3. Δ U=0, Δ S_total≠0 Key idea: for an ideal gas, internal energy depends only on temperature. Isothermal means Δ U=0 . An irreversi... Second law, entropy and spontaneity 1 answer
Which one among the following is the correct option for right relationship between C_P and C_V for one mole of ideal gas ? Answer: 2. C_P - C_V = R Key idea: for one mole of an ideal gas, C_P exceeds C_V by exactly R (Mayer's relation). At constant pressure, heat... Heat capacity and molar heat capacity 1 answer
The correct option for the number of body centred unit cells in all 14 types of Bravais lattice unit cells is : Answer: 4. 3 Key idea: count the body-centred (I) cells among the 14 Bravais lattices. Cubic: primitive, body-centred, face-centred (1 body-... System, surroundings, extensive/intensive properties, state functions, processes 1 answer
The correct option for free expansion of an ideal gas under adiabatic condition is : Answer: 1. q = 0, Δ T = 0 and w = 0 Free expansion means expansion into vacuum, so external pressure is zero and work w=-P_extΔ V=0 . Adiaba... First law: work, heat, internal energy, enthalpy 1 answer
For the reaction, 2Cl(g) → Cl_2(g), the correct option is : Answer: 4. Δ_rH < 0 and Δ_rS < 0 2Cl(g)→ Cl₂(g) forms a Cl-Cl bond. Bond formation releases energy, so Δ_rH<0 . Two particles become one, so... Second law, entropy and spontaneity 1 answer
Hydrolysis of sucrose is given by the following reaction.Sucrose + H_2O ⇌ Glucose + FructoseIf the equilibrium constant (K_c) is 2×10^13 at 300 K, the value of Δ_rG^ at the same temperature will be : Answer: 1. -8.314 J mol ⁻¹ K ⁻¹×300 K ×ln(2×10¹³) Standard Gibbs energy and equilibrium constant are related by Δ_rG^ominus=-RTln K . Substi... Gibbs energy change and equilibrium constant 1 answer
In which case change in entropy is negative ? Answer: 4. 2H(g) → H ₂ (g) Entropy measures disorder. Evaporation, sublimation and isothermal expansion all increase randomness ( Δ S>0 ). I... Second law, entropy and spontaneity 1 answer
Under isothermal condition, a gas at 300 K expands from 0.1 L to 0.25 L against a constant external pressure of 2 bar. The work done by the gas is :[Given that 1 L bar = 100 J] Answer: 4. 30 J Key idea: for expansion against a constant external pressure, the magnitude of work is p_extΔ V , and the question asks for... First law: work, heat, internal energy, enthalpy 1 answer
The bond dissociation energies of X_2, Y_2 and XY are in the ratio of 1 : 0.5 : 1. ΔH for the formation of XY is -200 kJ mol^-1. The bond dissociation energy of X_2 will be Answer: 2. 800 kJ mol ⁻¹ Let the bond energies be X-X = a , Y-Y = 0.5a , X-Y = a . For 1/2X₂+1/2Y₂→XY : Δ H=1/2 a+1/2(0.5a)-a=-0.25a . -0.25... Hess's law and enthalpies of reactions (bond dissociation, combustion, formation etc.) 1 answer
Considering Ellingham diagram, which of the following metals can be used to reduce alumina ? Answer: 2. Mg In an Ellingham diagram, a metal can reduce an oxide only if its own oxide-formation line lies below that of the oxide to be r... Gibbs energy change and equilibrium constant 1 answer
A gas is allowed to expand in a well insulated container against a constant external pressure of 2.5 atm from an initial volume of 2.50 L to a final volume of 4.50 L. The change in internal energy Δ U of the gas in joules will be : Answer: 2. -505 J Key idea: well insulated means adiabatic, so q = 0 and Δ U = w . w=-p_extΔ V=-2.5\ atm×(4.50-2.50)\ L=-5\ L atm . Taking 1... First law: work, heat, internal energy, enthalpy 1 answer
For a given reaction, Δ H = 35.5\ kJ\ mol^-1 and Δ S = 83.6\ JK^-1mol^-1. The reaction is spontaneous at : (Assume that Δ H and Δ S do not vary with temperature) Answer: 1. T > 425 K Key idea: a reaction is spontaneous when Δ G = Δ H - TΔ S < 0 . For Δ G<0 : T > Δ H/Δ S . T > dfrac35.5×10³\ mathrmJ mo... Gibbs energy change and equilibrium constant 1 answer
For a sample of perfect gas when its pressure is changed isothermally from p_i to p_f, the entropy change is given by Answer: 1. Δ S = nRln(p_i/p_f) Key idea: for an isothermal ideal gas, Δ S=nRln(V_f/V_i) , and V∝1/p . Δ S=nRlnV_f/V_i=nRlnp_i/p_f The T does... Second law, entropy and spontaneity 1 answer
Consider the following liquid - vapour equilibrium.Liquid ⇌ VapourWhich of the following relations is correct ? Answer: 4. dln P/dT = Δ H_v/RT² Key idea: this is the Clausius-Clapeyron equation for liquid-vapour equilibrium: dln P/dT=Δ H_v/RT² Vapour p... Gibbs energy change and equilibrium constant 1 answer
The correct thermodynamic conditions for the spontaneous reaction at all temperatures is : Answer: 3. Δ H < 0 and Δ S > 0 Key idea: Δ G=Δ H-TΔ S . A reaction is spontaneous at all temperatures when Δ G<0 for every T. This needs Δ H... Second law, entropy and spontaneity 1 answer