IB Chemistry HL 1.2.1 Bond Energies Questions

Apply bond-enthalpy data to calculate ΔH, compare estimates with measured values and quantify uncertainty in the result.

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.

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

[Maximum number: 4]

Organic compounds have many industrial applications.

(a)

Ethene reacts with steam to produce ethanol.

\mathrm{C}_{2} \mathrm{H}_{4}(\mathrm{~g})+\mathrm{H}_{2} \mathrm{O}(\mathrm{~g}) \rightarrow \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}(\mathrm{~g})

[ 4 ]

(i)

Calculate the enthalpy, in kJ , of the reaction using section 12 of the data booklet.

[ 3 ]

ALTERNATIVE 1:

Bonds broken:

$1 \mathrm{C}=\mathrm{C}+4 \mathrm{C}-\mathrm{H}+2 \mathrm{O}-\mathrm{H} / 614+4(414)+2(463) / 3196$ «kJ»

Bonds formed:

1 C-C+1 C-O+5 C-H+1 O-H /

346+358+5(414)+463 /

3237 «kJ»
$\Delta H=\text { «3196-3237 }=\text { » }-41 \text { «kJ» » }$

ALTERNATIVE 2:

Bonds broken:
$1 \mathrm{C}=\mathrm{C}+1 \mathrm{O}-\mathrm{H} / 614+463 /$

1077 «kJ»

Bonds formed:
$\begin{aligned} & 1 \mathrm{C}-\mathrm{C}+1 \mathrm{C}-\mathrm{O}+1 \mathrm{C}-\mathrm{H} / 346+358+414 / & 1118\langle\mathrm{~kJ}\rangle{ }^{2} & \Delta H=\langle 1077-1118=\rangle-41\left\langle\mathrm{~kJ} \|{ }^{2} \right. \end{aligned}$

Award [3] for correct final answer.

Award [2 max] for +41 «kJ».

(ii)

Outline why the enthalpies calculated in (i) and (ii) are different.

[ 1 ]

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IB Chemistry HL 1.2.1 Bond Energies Questions question 1

Question (a)

Question (i)

Question (ii)