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In the hydrogenation of C2H4 on a Ni surface both the alkene and H2 are adsorbed on similar active sites of the catalyst surface. Which of the following figures depict the reaction ?

  • A.Graph of d[C2H6]/dt against H2 pressure at constant C2H6 pressure: a horizontal straight line, rate independent of H2 pressure
  • B.Graph of d[C2H6]/dt against H2 pressure at constant C2H6 pressure: rate rises from the origin and levels off to a plateau
  • C.Graph of d[C2H6]/dt against H2 pressure at constant C2H6 pressure: rate rises from zero to a maximum then falls back to zero
  • D.Graph of d[C2H6]/dt against H2 pressure at constant C2H6 pressure: a straight line rising from the origin

Answer: C

The correct plot is C, a rise to a maximum followed by a fall.

When both reactants compete for the same sites (the Langmuir-Hinshelwood mechanism) the rate is proportional to the product of their fractional coverages:

\[ \begin{aligned} r &= \frac{k K_{A}K_{B} p_{A} p_{B}}{D^{2}} \\ D &= 1 + K_{A}p_{A} + K_{B}p_{B} \end{aligned} \]

At constant ethylene pressure the rate first rises with pH2, because more hydrogen is adsorbed. At high pH2 the denominator grows as p2, hydrogen crowds the alkene off the surface and the rate falls. A constant rate (A), a saturation curve (B) and a straight line (D) describe zero-order, single-site or first-order behaviour, not competitive adsorption. The axis labels print C2H6 for ethylene.

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