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# First order reactions Watch

1. For a first order reaction the gradient changes so concentration affects rate. However the half life is constant.

I don't understand how the rate can be changing but the half life remains constant? Why doesn't rate impact half life? Also p I need to know exactly what happens to rate over time or just that the concentration does effect rate?

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2. Imagine you had 64 particles.

In a unit of time half of them convert to another form. 32 have popped, 32 remain. The rate is 32 per unit time.

In another unit of time, half of the remaining 32 pop. The rate is 16 per unit time.

Half life is simply defined as the time it takes for half to pop.

For zero order reactions half life decreases and for second order it increases. It is only constant for first order reactions.
3. (Original post by maths_4_life)
For a first order reaction the gradient changes so concentration affects rate. However the half life is constant.

I don't understand how the rate can be changing but the half life remains constant? Why doesn't rate impact half life? Also p I need to know exactly what happens to rate over time or just that the concentration does effect rate?

Posted from TSR Mobile
The half life depends on the rate constant k. For a single first order reaction, the gradient is constant so the half-life is the same throughout the reaction.
4. (Original post by Pigster)

In a unit of time half of them convert to another form. 32 have popped, 32 remain. The rate is 32 per unit time.

In another unit of time, half of the remaining 32 pop. The rate is 16 per unit time.

Half life is simply defined as the time it takes for half to pop.

For zero order reactions half life decreases and for second order it increases. It is only constant for first order reactions.
Decay might be a better word to use...?? Science isn't my thing so I may be wrong, i just remember hearing it like 5 years ago while doing GCSEs.....

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Updated: April 5, 2016
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