1. Do we include a catalyst when writing donw the rate
equation?
2.Explain the following observations as fully as you can.
When ammonia is passed over a platinum gause, the rate of
decompisition into nitrogen and hydrogen is independent of the
partial pressure of ammonia. However, at very low pressure, the
rate is directly proportional to the partial pressure of
ammonia.
Just these 2 questions, thanks.
1. Yes. Since the rate of reaction is affected by the presence and
molarity of a catalyst. For any given reaction, the rate
constant k is said to be affected only by temperature. This is
true. But when a catalyst is employed, the reaction pathway
involves an alternate mechanism with a lower activation energy
(at the rate determining elementary step), which means to say that
this may be considered a different reaction altogether, and hence
has a different (a much larger) rate constant k value.
2. At sufficiently high pressure, the catalyst (ie. platinum) is
saturated with its substrate (ie. ammonia, the excess reactant),
and hence altering (eg. further increasing) the partial
pressure (or molarity) of ammonia (ie. the excess
reactant) does not alter the rate of reaction, ie. the
reaction is zero order with respect to ammonia. At lower partial
pressures (or molarities) of ammonia, the limiting reactant is the
ammonia itself, and increasing partial pressure (or molarity)
of ammonia (ie. the limiting reactant) will result in an increase
in rate of reaction, ie. the reaction is first order with
respect to ammonia.