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Thermodynamics Part 1: Thermochemistry - Calorimetry experiments - Enthalpies of reaction,formation, combustion and bond dissociation are explained with exempler calculations

 ANSWERS to the ∆H enthalpy Q's below * EMAIL * GCSE notes Energy Changes, exothermic/endothermic reactions

Question 1

(a) Define the standard enthalpy of (i) combustion; (ii) formation

(b) Given the following standard enthalpies of combustion (298K, 1 atmos.):

C_(s) -393 kJ mol^-1; H_2(g) -285.6 kJ mol^-1; C₂H_6(g) -1560 kJ mol^-1

(note: the first two enthalpies correspond to the enthalpy of formation of carbon dioxide and water respectively) 

Calculate the standard enthalpy of formation of ethane, C₂H₆(g)

(c) Given the following bond enthalpies (bond energies) in kJ mol^-1;

C-H 412; C-C 347; O-H 464; O=O 498; C=O 805 (for CO₂); C-O 358

calculate the enthalpy of combustion of ethane. (you will not get -1560 kJ mol^-1, see later)

Question 2

Given the following standard enthalpies of combustion (298K, 1 atmos.):

C_(s) -393 kJ mol^-1; H_2(g) -285.6 kJ mol^-1; C₈H_18(l) -5512 kJ mol^-1;

Calculate the enthalpy of formation of octane, C₈H_18(l)

Question 3

(a) Given the following standard enthalpies of combustion (298K, 1 atmos.):

C_(s) -393 kJ mol^-1; H_2(g) -285.6 kJ mol^-1;

and the enthalpy of formation of cyclohexane, C₆H_12(l) -156 kJ mol^-1;

Calculate the enthalpy of combustion of cyclohexane, C₆H_12(l)

(b) Using the appropriate bond energies from Q1c calculate the theoretical enthalpy of combustion of cyclohexane.

Question 4

(a) Given the following standard enthalpies of combustion (298K, 1 atmos.):

C_(s) -393 kJ mol^-1; H_2(g) -285.6 kJ mol^-1;

and the enthalpy of formation of ethanoic acid, CH₃COOH_(l) -487 kJ mol^-1

Calculate the enthalpy of combustion of ethanoic acid, CH₃COOH_(l)  

(b) Using the bond enthalpies listed in Q1c calculate the enthalpy of combustion of ethanoic acid but noting that the bond enthalpy for C=O is +743 kJmol^-1 when it is NOT in the carbon dioxide molecule.

Question 5

Given the following standard Enthalpies of Formation (298K, 1 atmos.):

NH_3(g) -46.2 kJ mol^-1; HCl_(g) -92.3 kJ mol^-1; NH₄Cl_(s) -315.0 kJ mol^-1;

Calculate the enthalpy change for the reaction:

NH₄Cl_(s) ====> NH_3(g) + HCl_(g)

Question 6

Given the following standard Enthalpies of Formation in kJ mol^-1 (298K, 1 atmos.):

CH_4(g) -74.9; CH₃Br_(l) -36.0; HBr_(g) -36.2

Calculate the enthalpy change for the reaction:

Br_2(l) + CH_4(g) ====> CH₃Br_(l) + HBr(g)