IB Chemistry SL Reactivity 1.2.1 Bond Energies

Practise estimating reaction enthalpy by counting every gaseous bond broken and formed. Apply ΔH = ΣE(broken) − ΣE(formed), retain stoichiometric factors and explain why average…

[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 1.2.1—Bond energies 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

1.2.1—Bond energies question 1