Rate inversely proportional to time Watch

youssef9461
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Why is the assumption that the rate of reaction is inversely proportional to the time for a certain volume of gas to be produced only valid if less than 10% of the acid is used up?
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charco
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(Original post by youssef9461)
Why is the assumption that the rate of reaction is inversely proportional to the time for a certain volume of gas to be produced only valid if less than 10% of the acid is used up?
You need to be more specific. What exactly are you discussing here?
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youssef9461
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(Original post by charco)
You need to be more specific. What exactly are you discussing here?
Well, it's actually a question in Kinetics...In a reaction A + B ------> C, the value of [A] fell from 0.12 mol dm^-3 to 0.012 mol dm^-3 in 60 s. Explain why it is inaccurate to state that the rate of reaction is 0.0018 mol dm^-3/s... and the answer is that the assumption that the rate of reaction is inversely proportional to the time only valid if less than 10% of the reactant is used up while in the question 90 % is used up.
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charco
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(Original post by youssef9461)
Well, it's actually a question in Kinetics...In a reaction A + B ------> C, the value of [A] fell from 0.12 mol dm^-3 to 0.012 mol dm^-3 in 60 s. Explain why it is inaccurate to state that the rate of reaction is 0.0018 mol dm^-3/s... and the answer is that the assumption that the rate of reaction is inversely proportional to the time only valid if less than 10% of the reactant is used up while in the question 90 % is used up.
The rate of a chemical reaction is not constant and proportional (in some way) to the concentration of the reactants.

During the course of the reaction the concentration falls from 0.12 to 0.012, i.e. by a factor of 10. Clearly the initial rate will be much faster than the fnal rate - by a factor of 10 if the rate is directly proportional to the concentration.

Hence the rate cannot be stated to be 0.0018 mol dm-3 s-1, rather the average rate. If only a small quantity of the reactants are used up, then the average will be fairly close to the initial, but if a large amount of reactants are used up then the rate of change of concentration will deviate vastly from the initial.

As the rate is inversely proportional to the time taken, then the approximation only holds true near the original concentrations.
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youssef9461
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(Original post by charco)
The rate of a chemical reaction is not constant and proportional (in some way) to the concentration of the reactants.

During the course of the reaction the concentration falls from 0.12 to 0.012, i.e. by a factor of 10. Clearly the initial rate will be much faster than the fnal rate - by a factor of 10 if the rate is directly proportional to the concentration.

Hence the rate cannot be stated to be 0.0018 mol dm-3 s-1, rather the average rate. If only a small quantity of the reactants are used up, then the average will be fairly close to the initial, but if a large amount of reactants are used up then the rate of change of concentration will deviate vastly from the initial.

As the rate is inversely proportional to the time taken, then the approximation only holds true near the original concentrations.
Thanks a lot! I get it now...
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Vinny C
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(Original post by charco)
You need to be more specific. What exactly are you discussing here?
Cold fusion.
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