You are Here: Home

# First order reactions

Announcements Posted on
TSR's new app is coming! Sign up here to try it first >> 17-10-2016
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?

Posted from TSR Mobile
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.....

## Register

Thanks for posting! You just need to create an account in order to submit the post
1. this can't be left blank
2. this can't be left blank
3. this can't be left blank

6 characters or longer with both numbers and letters is safer

4. this can't be left empty
1. Oops, you need to agree to our Ts&Cs to register

Updated: April 5, 2016
TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Today on TSR

### How does exam reform affect you?

From GCSE to A level, it's all changing

### Who would you like to thank?

Poll
Study resources

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.