Tharmochemistry PDF
Tharmochemistry PDF
Tharmochemistry PDF
THERMOCHEMISTRY
All chemical processes are associated with energy changes in chemical reactions. A thermochemical equation,
represents both the material change and the energy. While writing a thermochemical equation, the heat evolved
in case of exothermic reaction, or the heat absorbed in case of endothermic reaction, is indicated on the product
side of the balanced chemical equation. For example,
1
(i) H2(g) + O 2 (g) H2O() H = –285.91 kJ
2
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THERMOCHEMISTRY
Ex. A gas mixture of 4 litres of ethylene and methane on complete combustion at 25ºC produces 6 litres of CO2 . Find
out the amount of heat evolved on burning one litre of the gas mixture. The heats of combustion of ethylene and
methane are – 1464 and –976 kJ mol-1 at 25ºC.
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THERMOCHEMISTRY
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SOLVED EXAMPLES
Ex. 1 From the following data at 25ºC
Reaction r Hº/KJ mol–1
1 435.88
(i) H 2 (g) H(g) rH° =
2 2
1 1
(ii) OH(g) H2(g) + O2 (g) rH° = –42.09
2 2
1 495.04
(iii) O (g) O(g) rH° =
2 2 2
By adding equation (i), (ii) & (iii),
we get
OH(g) H(g) + O(g)
435.88 495.04
rH° – 42.09 +
2 2
or rH° = 423.37 kJ/mol.
(b) The desired equation is
H2O(g) 2H(g) + O(g)
1 495.04
(i) O (g) O(g) rH° =
2 2 2
(ii) H2(g) 2H(g) rH° = 435.88
1
(iii) H2O(g) H2(g) + O (g) rH° = 241.88
2 2
The net equation is,
H2O(g) 2H(g) + O(g) ∆rH° = 925.28 kJ/mol
(c) The desired equation is
H2O(g) H(g) + OH(g)
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THERMOCHEMISTRY
2. If H is the change in enthalpy and E the change in internal energy accompanying a gaseous
reaction
(A) H is always greater than E
(B) H < E only if the number of moles of the products is greater than the number of the reactants
(C) H is always less than E
(D)H < E only if the number of moles of the products is less than the number of moles of the reactants
R
P R P R P
H H H H
(A) (B) R (C) (D)
P
5. 2C O 2 2CO; H 220 kJ Which of the following statement is correct for this reaction
(A) Heat of combustion of carbon is 110 kJ (B) Reaction is exothermic
(C) Reaction needs no initiation (D) All of these are correct
6. Given, H2(g) + Br2(g) 2HBr(g), H01 and standard enthalpy of condensation of bromine is H02, standard
enthalpy of formation of HBr at 250C is
(A) H01 / 2 (B) H01 / 2 + H02 (C) H01 / 2 H02 (D) (H01H02) / 2
9. The heat of combustion of sucrose (C12H22O11) is 1350 kcal. How much of heat will be liberated when 17.1 g of
sucrose is burnt ?
(A) 67.5 kcal (B) 13.5 kcal (C) 40.5 kcal (D) 25.5 kcal
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CHEMISTRY FOR JEE MAIN & ADVANCED
1. Use the given standard enthalpies of formation to determine the heat of reaction of the following reaction :
2 LiOH(s) + CO2(g) Li2CO3(s) + H2O(l)
Hof LiOH(s) = – 487.23 kJ/mole
Hof Li2CO3(s) = – 1215.6 kJ/mole
Hof H2O(l) = – 285.85 kJ/mole
Hof CO2(g) = – 393.5 kJ/mole
(A) +303.4 (B) – 133.5 (C) – 198.6 (D) +198.6
3. Determine Ho of the following reaction using the listed heats of formation :
4 HNO3(l) + P4O10(s) 2 N2O5(s) + 4 HPO3(s)
o
H f HNO3(l) = –174.1 kJ/mole
Hof N2O5(s) = –43.1 kJ/mole
Hof P4O10(s) = –2984.0 kJ/mole
Hof HPO3(s) = –948.5 kJ/mole
(A) –176.3 (B) – 199.8 (C) +276.2 (D) – 242.4
4. If x1, x2 and x3 are enthalpies of H–H, O=O and O–H bonds respectively, and x4 is the enthalpy of vaporisation
of water, estimate the standard enthalpy of combustion of hydrogen.
x x x x
(A) x1 + 2 –2x3 + x4 (B) x1 + 2 –2x3 – x4 (C) x1 + 2 –x3 + x4 (D) 2x1 – x1 – 2 – x4
2 2 2 2
5. Use the given bond enthalpy data to estimate the Ho (kJ) for the following reaction.
(C – H = 414 kJ, H – Cl = 431 kJ, Cl – Cl = 243 kJ, C – Cl = 331 kJ).
CH4(g) + 4 Cl2(g) CCl4(g) + 4 HCl(g)
(A) 620 (B) 330 (C) 420 (D) 105
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CHEMISTRY FOR JEE MAIN & ADVANCED
1. Columm - I Columm - II
(A) C (s, graphite) + O2(g) CO2(g) (p) Hºcombustion
1
(C) CO(g) + O (g) CO2(g) (r) Hºatomization
2 2
(D) CH4(g) C(g) + 4H(g) (s) Hºsublimation
3. Match the reaction (In Column I) with relation between H and E (in Column II) :
Column-I Column-II
(A) C(s) + O2(g) CO2(g) (p) H = E + RT
(B) N2(g) + 3H2(g) 2NH3(g) (q) H = E
(C) NH4HS(s) NH3(g) + H2S(g) (r) H = E – 2RT
(D) PCl5(g) PCl3(g) + Cl2 (s) H = E + 2RT
(E) 2SO2(g) + O2(g) 2SO3(g) (t) H = E – RT
4. Column-I Column-II
(A) S(g) + O2(g) SO2(g) ; H (p) Heat of solution
(B) CH4(g) + 2O2(g) CO2(g)
+ 2H2O(l) ; H (q) Heat of neutralisation
(C) NaOH(s) + aq NaOH (aq) ; H
(r) Heat of formation
(D) NaOH(aq) + HCl(aq) NaCl (aq)
+ H2O(l) ; H (s) Heat of combustion
Comprehension # 1
The commercial production of "Water gas" utilises the endothermic reaction C(s) + H2O (g) H2(g) + CO(g). The
heat required for this reaction is generated by combustion of coal to CO2 using stoichiometric amount of air. (79%
N2 by volume and 21%O2 by volume). The superheated steam undergoes 75% conversion. Using the following
data, answer the question that follows
Hf [CO(g)] = – 110.53 kJ.mol; Hf [H2O(g)] = – 241.81 kJ/mol; Hf [CO2(g)] = – 314.0 kJ/mol
1. The amount of heat liberated when one litre of product gases are burnt at 373 K and at 1 atm is
(A) 3.6kJ (B) 3.9 kJ (C) 4.43 kJ (D) 5.34 kJ
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CHEMISTRY FOR JEE MAIN & ADVANCED
3. The specific heats of iodine vapour and solid are 0.031 calg–1 ºC–1 and 0.055 calg–1 ºC–1 respectively. If heat of
sublimation of iodine is 24 cal/g at 200ºC, what is its value at 250ºC ?
4. Predict the standard reaction enthalpy of 2 NO2(g) N2O4(g) at 100°C. H° at 25°C is –57.2 kj. mol–1 Cp(NO2) =
37.2 j. mol–1 K–1 Cp(N2O4) = 77.28 J. mol–1 k–1.
6. Diborane is a potential rocket fuel which undergoes combustion according to the reaction
B2H6(g) + 3O2(g) B2O3(s) + 3H2O(g)
From the following data, calculate the enthalpy change for the combustion of diborane
2B(s) + (3/2) O2(g) B2O3(s) H = – 1273 kJ mol–1
H2(g) + (1/2)O2(g) H2O(l) H = – 286 kJ mol–1
H2O() H2O(g) H = 44 kJ mol–1
2B(s) + 3H (g) B H (g)
2 2 6
H = 36 kJ mol–1
7. Find out the heat evolved in combustion if 112 litres (at STP) of water gas (mixture of equal volume of H2(g) and CO (g)).
H2(g) + 1/2 O2 (g) H2O(g) H = – 241.8 kJ
CO (g) + 1/2 O2(g) CO2(g) H = – 283 kJ
9. One litre sample of a mixture of CH4 and O2 measured at 25ºC and 740 torr, was allowed to react at constant pressure
in a calorimeter. The complete combustion of CH4 to CO2 and water caused a temperature rise in calorimeter of 0.667
K. Calculate mole % of CH4 in original mixture.
[Given : Heat of combustion of CH4 is – 210.8 Kcal/mol. Total heat capacity of the calorimeter = 1260 cal/K]
10. The heat of combustion of ethyl alcohol is –300 kcal. If the heats of formation of CO2 (g) and H2O() are –94.3 and
–68.5 kcal respectively, calculate the heat of formation of ethyl alcohol.
11. The standard enthalpy of decomposition of the yellow complex H3NSO2 Into NH3 and SO2 is + 40 kJ mol–1. Calculate
the standard enthalpy of formation of H3NSO2. H0f(NH3) = – 46.11 kJ mol–1, H0f(SO2) = – 296.83
12. When 12.0 g of carbon (graphite) reacted with oxygen to form CO and CO2 at 250C and constant pressure, 313.8 kJ
of heat was released and no carbon remained. If H0f (CO, g) = – 110.5 kJ mol–1 and H0f (CO2,g) = – 393.5 kJ mol–1,
alculate the mass of oxygen consumed.
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THERMOCHEMISTRY
1. The enthalpies of combustion of carbon and carbon monoxide are – 393.5 and – 283 kJ mol–1 respectively. The
enthalpy of formation of carbon monoxide per mole is [AIEEE 2004]
(A) 110.5 kJ (B) 676.5 kJ (C) – 676.5 kJ (D) – 110.5 kJ
2. If the bond dissociation energies of XY, X2 and Y2 (all diatomic molecules) are in the ratio of 1 : 1 : 0.5 and Hf for the
formation of XY is - 200 KJ mol–1. The bond dissociation energy of X2 will be [AIEEE 2005]
–1 –1 –1 –1
(A) 100 KJ mol (B) 200 KJ mol (C) 300 KJ mol (D) 800 KJ mol
3. The enthalpy changes for the following processes are listed below: [AIEEE 2006]
Cl2(g) 2Cl(g); 242.3 kJ mol–1 ; I2(g) 2I(g); 151.0 kJ mol–1
ICl(g) I(g) + Cl(g); 211.3 kJ mol–1 ; I2(s) I2(g); 62.76 kJ mol–1
Given that the standard state for iodine and chlorine are I2(s) and Cl2(g), the standard enthalpy of formation for ICl(g) is
(A) -16.8 kJ mol–1 (B) +16.8 kJ mol–1 (C) +244.8 kJ mol–1 (D) -14.6 kJ mol–1
4. (HU) for the formation of carbon monoxide (CO) from its elements at 298 K is [AIEEE 2006]
–1 –1
(R = 8.314 J K mol )
(A) 1238.78 J mol–1 (B) -2477.57 J mol–1 (C) 2477.57 J mol–1 (D) -1238.78 J mol–1
5. The standard enthalpy of formation (Hf°) at 398 K for methane, CH4(g) is 74.8 kJ mol–1. The additional information
required to determine the average energy for C - H bond formation would be : [AIEEE 2007]
(A) the dissociation energy of H2 and enthalpy of sublimation of carbon
(B) latent heat of vapourisation of methane
(C) the first four ionization energies of carbon and electron gain enthalpy of hydrogen
(D) the dissociation energy of hydrogen molecule, H2
7. The standard enthalpy of formation of NH3 is – 46.0 kJ mol–1. If the enthalpy of formation of H2 from its atoms is –
436 kJ mol–1 and that of N2 is – 712 kJ mol–1, the average bond enthalpy of N – H bond in NH3 is [AIEEE 2010]
(A) – 964 kJ mol–1 (B) + 352 kJ mol–1 (C) + 1056 kJ mol–1 (D) – 1102 kJ mol–1
8. The value of enthalpy change (H) for the reaction
C2H5OH(l) + 3O2(g) 2CO2(g) + 3H2O(I)
at 27°C is –1366.5 kJ mol–1. The value of internal energy change for the above reaction at this temperature will be :
[AIEEE 2011]
(A) –1369.0 kJ (B) –1364.0 kJ (C) –1361.5 kJ (D) –1371.5 kJ
9. Consider the reaction :
4NO2(g) + O2(g) 2N2O5(g), rH = –111 kJ.
If N2O5(s) is formed instead of N2O5(g) in the above reaction, the rH value will be:
(given, H of sublimation for N2O5 is 54 kJ mol–1) [AIEEE 2011]
(A) + 54kJ (B) + 219 kJ (C) – 219 kJ (D) – 165 kJ
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THERMOCHEMISTRY
MOCK TEST
R
P R P R P
H H H H
(A) (B) R (C) (D)
P
2. If H is the change in enthalpy and E the change in internal energy accompanying a gaseous
reaction
(A) H is always greater than E
(B) H < E only if the number of moles of the products is greater than the number of the reactants
(C) H is always less than E
(D) H < E only if the number of moles of the products is less than the number of moles of the reactants
5. The heat of combustion of sucrose (C12H22O11) is 1350 kcal. How much of heat will be liberated when 17.1 g of
sucrose is burnt ?
(A) 67.5 kcal (B) 13.5 kcal (C) 40.5 kcal (D) 25.5 kcal
6. One mole of anhydrous MgCl2 dissolves in water and librates 25 cal/mol of heat. Hhydration of MgCl2 = –30 cal/mol.
Heat of dissolution of MgCl2.H2O is
(A) +5 cal/mol (B) –5 cal/mol (C) 55 cal/mol (D) –55 cal/mol
7. Enthalpy of neutralization of H3PO3 acid is – 106.68 kJ/mol using NaOH. If enthalpy of neutralization of HCl by NaOH
is –55.84 kJ/mol. Calculate Hionization of H3PO3 into its ions :
(A) 50.84 kJ/mol (B) 5 kJ/mol (C) 2.5 kJ/mol (D) None of these
8. The bond dissociation energy of gaseous H 2 , Cl 2 and HCl are 104, 58 and 103 kcal mol –1
respectively. The enthalpy of formation for HCl gas will be
(A) – 44.0 kcal (B) – 22.0 kcal (C) 22.0 kcal (D) 44.0 kcal
9. Heat of hydrogenation of ethene is x1 and that of benzene is x2. Hence, resonance energy is :
(A) x1 – x2 (B) x1 + x2 (C) 3x1 – x2 (D) x1 – 3x2
10. AB, A 2 and B2 are diatomic molecules. If the bond enthalpies of A 2, AB & B2 are in the ratio
1 : 1 : 0.5 and enthalpy of formation of AB from A2 and B2 is – 100 kJ/mol–1. What is the bond enthalpy of A2.
(A) 400 kJ/mol (B) 200 kJ/mol (C) 100 kJ/mol (D) 300 kJ/mol
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11th Class Modules Chapter Details