IB Chemistry SL 1.2.1 Bond Energies Questions

Use real exam questions to count bonds, calculate enthalpy change from mean bond enthalpies and explain approximation or uncertainty.

Exam points

Identify and count the covalent bonds broken and formed in a displayed molecular equation, including multiple bonds and stoichiometric coefficients.
Calculate ΔH from mean bond enthalpies as total energy for bonds broken minus total energy for bonds formed, with correct sign and units.
Explain why bond-enthalpy estimates differ from measured or formation-enthalpy values because tabulated bond enthalpies are averages and apply to specified gaseous bonds or states.
Propagate uncertainty from the bond-enthalpy inputs to report the percentage uncertainty of a calculated ΔH when the question supplies those uncertainties.

Learn this topic

Study 1.2.1—Bond energies conceptsOpen the corresponding Concept lesson for explanations, examples and exam connections.

[Maximum number: 4]

Alkenes, such as A (shown below), are important intermediates in the petrochemical industry because they undergo addition reactions to produce a wide variety of products, such as the conversion shown below.

Figure for Question IB Chemistry SL 1.2.1 Bond Energies Questions question 1 — IB Chemistry SL

In the gas phase, A reacts with hydrogen to form D.

Determine the enthalpy change, in $\mathrm{kJ} \mathrm{mol}^{-1}$ , for the reaction of A with hydrogen, using Table 10 of the Data Booklet, and state whether the reaction is exothermic or endothermic.

bonds broken: $(\mathrm{E}(\mathrm{C}=\mathrm{C})+\mathrm{E}(\mathrm{H}-\mathrm{H})=612+436=) 1048\left(\mathrm{~kJ} \mathrm{~mol}^{-1}\right)$ ;
Accept (6956+436=) 7392 if all bonds in alkene broken.
bonds formed: $\mathrm{E}(\mathrm{C}-\mathrm{C})+2 \times \mathrm{E}(\mathrm{C}-\mathrm{H})=347+(2 \times 413)=1173\left(\mathrm{~kJ} \mathrm{~mol}^{-1}\right)$ ;

Marking guidance:

Accept 7517 if all the bonds in the product are summed.
$\Delta H=1048-1173 / 7392-7517=-125\left(\mathrm{~kJ} \mathrm{~mol}^{-1}\right) ;$
Award [3] for correct final answer.
Award [2] for +125 .
exothermic;

Apply ECF if sign of $\Delta H$ incorrect.
Do not award a mark for "exothermic" if $\Delta H$ given as positive.

All question bank results loaded

Learn this topic

IB Chemistry SL 1.2.1 Bond Energies Questions question 1