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 >_
You haven't asked a question.
He says we have To explain it. But I still don't knowwhat he wants us to explain?
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.
To determine the rate constant for a zeroth-order reaction, you need to measure the initial rate of the reaction under different conditions. In this case, the reaction is the decomposition of phosphine (PH3) over tungsten.
To experimentally measure the rate of a zeroth-order reaction, you can vary the concentration of the reactants and measure the initial rate of the reaction. However, in the case of a zeroth-order reaction, the rate is independent of the concentration of the reactants.
Given that the reaction is independent of PH3 (phosphine), it implies that the rate of the reaction remains constant regardless of the concentration of phosphine. This suggests that the reaction rate is solely dependent on the concentration of the W (tungsten) catalyst.
To determine the rate constant (k), you need to perform experiments where you maintain a constant concentration of phosphine but vary the concentration of the W catalyst. By measuring the initial rate in each experiment and plotting the rate versus the concentration of the catalyst, you can determine the rate constant (k), which represents the rate of the reaction per unit concentration of the catalyst.
To summarize, in order to determine the rate constant for a zeroth-order reaction like the decomposition of phosphine over tungsten, you need to perform experiments where you vary the concentration of the catalyst (W) while keeping the concentration of the reactant (PH3) constant. Then, you can measure the initial rate of the reaction and plot it against the concentration of the catalyst to determine the rate constant.