# chemistry

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Many reaction involving heterogeneous catalysts are zeroth order, that is rate=k. An example is the decomposition of phosphine (PH3) over tungsten (w):

4Ph3---> P4 + 6H2

It is found That the reaction is independent of PH3. As lon 1 g as phosphine's pressure is sufficiently high >_

• chemistry -

• chemistry -

He says we have To explain it. But I still don't knowwhat he wants us to explain?

• chemistry -

You know what zero order is; that means the rate doesn't depend upon the concn. However, SOME of it must be present, that is, the concn of PH3 obviously can't be zero. Also, it doesn't much matter how much is present. In this case, however, you can look at the equation and see that there are 4 mols on the left and 6 on the right (if P4 is a solid) or 7 on the right if T is high enough for everything to be gaseous. So you must have a pressure of PH3 high enough to force the equilibrium to the right. After the reaction starts at a measurable speed, adding more PH3 makes no difference.

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