Bond enthalpy questions

Assignment: Bond enthalpy short answer questions

This page of questions can be marked as direct student access either for assigning as a test or for students to work on in their own time. If you do not wish to use student access, links to downloadable versions of the questions and, separately the worked answers, can be found at Printable versions of written tasks.

Use the values of bond enthalpies given in Section 11 of the IB Chemistry data booklet.

 

All hydrocarbons burn according to the following equation (the state of the hydrocarbon, CxHy, at standard conditions varies according to its mass and structure):

CxHy + (x + ¼y)O2(g) → xCO2(g) + ½yH2O(l)

Explain, in terms of bond enthalpies, why the combustion of all hydrocarbons is exothermic.

5 lines
Show model answer

 

 

Nitrogen and hydrogen react to produce ammonia in the Haber process:

N2(g) + 3H2(g) ⇄ 2NH3(g)

Calculate the enthalpy of formation of ammonia.

5 lines
Show model answer

 

 

Methane reacts with chlorine in the presence of ultraviolet light to produce chloromethane:

CH4(g) + Cl2(g) → CH3Cl(g) + HCl(g)

Calculate the enthalpy change for this reaction.

5 lines
Show model answer

 

 

Consider the three compounds cyclohexane, C6H12, cyclohexene, C6H10 and benzene, C6H6.

(a) Calculate the enthalpy change for the hydrogenation reaction of gaseous cyclohexene to form gaseous cyclohexane.
C6H10(g) + H2(g) → C6H12(g)

3 lines
Show model answer

(b) Both cyclohexene and cyclohexane are liquids at standard conditions. The enthalpies of vaporization of cyclohexene and cyclohexane are + 33.5 kJ mol−1 and + 32.0 kJ mol−1 respectively. Use these values to calculate the enthalpy of hydrogenation of cyclohexene, C6H10(l) + H2(g) → C6H12(l), and compare your value with the literature value of − 121.9 kJ mol−1.

10 lines
Show model answer

(c) Assuming that benzene consists of alternate C=C and C-C bonds, use average bond enthalpies to calculate the enthalpy of hydrogenation of benzene, C6H6(l) + 3H2(g) → C6H12(l) . (Benzene is a liquid at standard conditions and the enthalpy of vaporization of benzene is + 33.9 kJ mol−1).

5 lines
Show model answer

(d) It might be expected that the enthalpy of hydrogenation of benzene would be three times the value of the enthalpy of hydrogenation of cyclohexene as three double bonds are hydrogenated. In fact the value is – 210 kJ mol−1. The reason for the difference is that the alternate double and single bonds give increased stability to the benzene ring through a process known as delocalization. Estimate the value of the delocalization energy of benzene in kJ mol−1.

5 lines
Show model answer

 

 

All materials on this website are for the exclusive use of teachers and students at subscribing schools for the period of their subscription. Any unauthorised copying or posting of materials on other websites is an infringement of our copyright and could result in your account being blocked and legal action being taken against you.