Chem A level help - rate equations

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#1
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State which of the proposed mechanisms could be a possible mechanism and which could not be a correct mechanism. Do not assume that there is one answer in each case!

Overall reaction: P + 2 Q  R
Rate equation: rate = k [P]

Possible mechanism 1:
step 1 (rds) P + Q  PQ
step 2 PQ + Q  R

Possible mechanism 2:
step 1 2 Q  Q2
step 2 (rds) Q2 + P  R

Answer:
mechanism 1 = not possible; mechanism 2 = not possible

Why?????
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danielhaslam19
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#2
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Pay attention to the rate equation. Rate = k[P]. This means that the rate depends on P only.

Now take a look at each mechanism. For the first mechanism, the slow (rate-determining) step shows a reaction between P and Q. This means that the rate depends on P and Q, so the reaction would theoretically be rate = k[P][Q]. This is obviously wrong.

For the second one, the rate determining step shows a reaction between Q2 and P. Again, look at how the RDS involves the reaction of Q with P. This reaction depends on concentration of both, so the rate equation couldn't just depend on P.


^^^ Hope I made sense. I'm doing A2 and it's been a couple of months since I went over rate equations so apologies if this is a little mistaken lol
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