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Question:

The decomposition of phosphine (PH3) on tungsten at low pressure is a first-order reaction. It is because the: rate is independent of the surface coverage; rate of decomposition is very slow; rate is inversely proportional to the surface coverage; rate is proportional to the surface coverage

rate is inversely proportional to the surface coverage

rate is independent of the surface coverage

rate of decomposition is very slow

rate is proportional to the surface coverage

Solution:

The decomposition of PH3 on tungsten at low pressure is a first order reaction because surface area coverage is proportional to partial pressure of PH3. Hence, rate is proportional to the surface coverage.