Zero-Order Rxns

[Sammy1024]

I saw these explanations for the answer and just wanted to confirm with one simple statement.

"The rate for a zero-order reaction is constant, so it takes more time to go from 100% reactant to 50% reactant than it does to go from 50% reactant to 25% reactant. This means that for a zero-order reaction, the half-life is decreasing with time."

"Because a zero-order reaction has a constant rate, the first fifty percent of reactants takes longer to decay than the next twenty-five percent (the second half-life). This means that as the concentration increases, the half-life increases for a zero-order reaction."

So going from 100% -> 50% the rate is decreasing and going 50% -> 100% rate is increasing for Zero-Order?

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[DrknoSDN]

So going from 100% -> 50% the rate is decreasing and going 50% -> 100% rate is increasing for Zero-Order?

No. Zero order reactions have a constant rate so the reactants are decreasing linearly.

Source: http://chemwiki.ucdavis.edu/Physical_Chemistry/Kinetics/Reaction_Rates/Zero-Order_Reactions

Zero order reactions can easily be related to catalysts or saturated enzymes.
With a saturated enzyme no matter how much or how little (assuming saturated) substrate there is, the reaction will occur at a specific rate (turnover).
Time from 100 to 50 = Time from 50 to 0.


("The rate for a zero-order reaction is constant, so it takes more time to go from 100% reactant to 50% reactant than it does to go from 50% reactant to 25% reactant. This means that for a zero-order reaction, the half-life is decreasing with time.")

This quote is describing the halflife property of the linearly slope shown above. It takes longer to decrease 50 percent than it does 25 percent.. (because it takes the same time to drop 25% always, and 50% is always going to take twice as long).

Zero order just means the rate is pre-determined and does not depend on how much reactants there are. So Time required for first 10% is the same as all the other 10%'s.

 

[Sammy1024]

So I still don't understand why it says:

This means that for a zero-order reaction, the half-life is decreasing with time.

And then it says:

This means that as the concentration increases, the half-life increases for a zero-order reaction.

Zero-order reaction's half-life decreases with time, but increases with concentration?

 

[DrknoSDN]

So I still don't understand why it says:

This means that for a zero-order reaction, the half-life is decreasing with time.

And then it says:

This means that as the concentration increases, the half-life increases for a zero-order reaction.

Zero-order reaction's half-life decreases with time, but increases with concentration?

For the first part, what is a halflife?
It is the time required to get from X to 1/2X of your reactants... In a zero order reaction every 10% is equal to every other 10%, therefore, to decrease to 25% from 50% will take a shorter time than to get to 50% from your initial concentration. The decrease in time from 50>25 vs 100>50 is basically a decreased halflife... "It takes less time to half your concentration."


For the second part, is just restating the first part. If you have a concentration of something (say 50g/L), if you start the reaction the half-life is going to be less than if you have a higher concentration (say 100g/L). Re-started, increasing the concentration increases the halflife.

Maybe think of it as an engine? If you have more fuel it will take longer to use up half of it.

[Edit]
Maybe think of zero order as any constant rate.
I.E. if a reaction takes 1 minute total. The first halflife is going to take 30 seconds while the 2nd only takes 15 seconds. The reaction is going the same rate the whole time, but you're looking at a smaller portion of the whole time interval.

 

[Sammy1024]

Oh! Hahah! I get it now. How dumb of me! Thank you!! :]