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Why 1/T is used as a measure of rate of the reaction?

I am stuck with this question:

Reciprocal time (1/t) is used as a measure of rate in this experiment. ( Reaction is first order with respect to the reactant )

Suggest the assumption on which this depends. Refer in your answer to the shape of a typical graph of reactant concentration against time.

The answer in the mark scheme comes with a Graph of concentration vs time, which looks like an asymptote on the positive y and x axis:

Ans: the methyl red is decolourised early in the reaction
when the concentration of reactant vs time graph is
(almost) linear so the gradient of the tangent is the
same as the (change in) concentration / time

Allow : assumes that the reaction rate is
constant for a particular run using conc v time
graph as shown on :
https://www.thestudentroom.co.uk/attachment.php?d=1610091450&attachmentid=990042

do not award : just ‘concentration is
proportional to time’

Here, I understand that the graph of the concentration against time
has a constant gradient at the beginning which means the rate of the progressing reaction would also be constant.

However, I dont really know why 1/T is used as a measure of rate of the reaction. This
is because the answer in the mark scheme does not seem related to the question to me. Probably because I dont get a point :frown:

By the way, I have an idea but not so sure:
If the rate of the reaction for a particular run ( the gradient ) is constant, then the change concentration per unit time must also be constant. So the rate of the that reaction should be directly proportional to 1/T, since the change in concentration is constant in the formula : rate = change in conc / time taken. So I could write 1/T instead of rate? Please let me know if and where I am wrong.

I would highly appreciate your help! :smile:
(edited 3 years ago)
Reply 1
Hi. I have put together a few graphs to try and explain this..please see the attachments
Reply 2
Original post by tahmidbro
I am stuck with this question:

Reciprocal time (1/t) is used as a measure of rate in this experiment. ( Reaction is first order with respect to the reactant )

Suggest the assumption on which this depends. Refer in your answer to the shape of a typical graph of reactant concentration against time.

The answer in the mark scheme comes with a Graph of concentration vs time, which looks like an asymptote on the positive y and x axis:

Ans: the methyl red is decolourised early in the reaction
when the concentration of reactant vs time graph is
(almost) linear so the gradient of the tangent is the
same as the (change in) concentration / time

Allow : assumes that the reaction rate is
constant for a particular run using conc v time
graph as shown on :
https://www.thestudentroom.co.uk/attachment.php?d=1610091450&attachmentid=990042

do not award : just ‘concentration is
proportional to time’

Here, I understand that the graph of the concentration against time
has a constant gradient at the beginning which means the rate of the progressing reaction would also be constant.

However, I dont really know why 1/T is used as a measure of rate of the reaction. This
is because the answer in the mark scheme does not seem related to the question to me. Probably because I dont get a point :frown:

By the way, I have an idea but not so sure:
If the rate of the reaction for a particular run ( the gradient ) is constant, then the change concentration per unit time must also be constant. So the rate of the that reaction should be directly proportional to 1/T, since the change in concentration is constant in the formula : rate = change in conc / time taken. So I could write 1/T instead of rate? Please let me know if and where I am wrong.

I would highly appreciate your help! :smile:

Minor nit-picking, but 1/T would be 1/temperature, rather than 1/t which would be 1/time.
Reply 3
Original post by BDavies1
Hi. I have put together a few graphs to try and explain this..please see the attachments

Thanks a lot!
I watched the second attachment.
It seems my idea is correct :

Original post by tahmidbro
By the way, I have an idea but not so sure:
If the rate of the reaction for a particular run ( the gradient ) is constant, then the change concentration per unit time must also be constant. So the rate of the that reaction should be directly proportional to 1/T, since the change in concentration is constant in the formula : rate = change in conc / time taken. So I could write 1/T instead of rate?


Would love to know further information relating the topic :smile:

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