Hi, I have a question regarding NJC 2013 Prelim P1 Q14.
How to derive half life? Since it's a first order reaction, I
know k = (ln2)/half-life.
I know that 10 mins into reaction, Pa + Pb = 600Pa,
And at the end of reaction, Pa + Pb = 400Pa.
What should I do with the data given?
At end of reaction, partial pressure of B is 400 Pa. Hence initial
partial pressure of A is 800 Pa. Hence, 1 half-life into the
reaction, partial pressure of A is 400 Pa, and B is 200 Pa. That's
a total pressure of 600 Pa. Hence, each half-life must be 10 min.
Since half-life = ln 2 / k, hence k = ln 2 / half-life, which is
calculated out to be 0.0693 min-1.
For A level purposes, such questions will be kept mathematically
simple (such as in this case), to avoid necessitating the use of
calculus and integrated rate laws.
