Aim for FiveChemistry

Short free-response question

Adding hydrogen to ethene

  • Unit 6
  • 4 points
  • About 9 minutes

You can use a calculator on this question, just like on exam day.

A shorter multipart question that sticks to one or two topics. Each part usually asks for one calculation, claim or explanation. On the exam: 4 questions (Questions 4–7), part of the 1-hour-45-minute free-response section; calculator allowed.

The question and its sources

Ethene reacts with hydrogen gas in the presence of a metal catalyst: C₂H₄(g) + H₂(g) → C₂H₆(g). In C₂H₄ the two carbon atoms are joined by a double bond; in C₂H₆ they are joined by a single bond.

Table 1. Average bond enthalpies

BondAverage bond enthalpy (kJ/mol)
C=C614
C–C348
C–H413
H–H436

Source: Standard average values, rounded

Suggested time: 9 minutes

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Part (a)

1 point

Use the bond enthalpies in Table 1 to calculate ΔH° for the reaction.

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Part (b)

1 point

Is the reaction exothermic or endothermic? Explain in terms of the energy associated with the bonds broken and the bonds formed.

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Part (c)

1 point

Calculate the amount of heat released when 5.00 g of C₂H₄ reacts completely with excess H₂.

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Part (d)

1 point

The value of ΔH° calculated from standard enthalpies of formation is −136 kJ/mol_rxn. Explain why the value calculated from bond enthalpies differs from this value.

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