Turn on thread page Beta
 You are Here: Home

# finding the rate constant help! watch

1. The thermal decomposition of hexafluoropropene (C3F6), in the temperature range 825-950K and in the initial pressure range 6.7 – 54.7 kN m–2, was analysed as a function of reaction time using gas-phase chromatography and IR spectroscopy. The half-life (t1/2) of C3F6 was measured as a function of the initial C3F6 concentration at 872 K: the results for the initial pressures of 6.8 kN m–2, 13.6 kN m–2 and 27.2 kN m–2 were 4200 s, 3840 s and 3480 s, respectively.

I've worked out that this reaction is first order

And the half life equation would be

ln2/t = k to find out the rate constant

But I'm not sure what t (half-life) would be?

Thank you
2. (Original post by Bloom77)
The thermal decomposition of hexafluoropropene (C3F6), in the temperature range 825-950K and in the initial pressure range 6.7 – 54.7 kN m–2, was analysed as a function of reaction time using gas-phase chromatography and IR spectroscopy. The half-life (t1/2) of C3F6 was measured as a function of the initial C3F6 concentration at 872 K: the results for the initial pressures of 6.8 kN m–2, 13.6 kN m–2 and 27.2 kN m–2 were 4200 s, 3840 s and 3480 s, respectively.

I've worked out that this reaction is first order

And the half life equation would be

ln2/t = k to find out the rate constant

But I'm not sure what t (half-life) would be?

Thank you
With first order kinetics the half-life is constant (i.e. independent of initial concentration).
3. (Original post by charco)
With first order kinetics the half-life is constant (i.e. independent of initial concentration).
Oh, so it would simply be 4200(s)/2 = 2100s <- half life
4. (Original post by Bloom77)
Oh, so it would simply be 4200(s)/2 = 2100s <- half life
The fact that the half-life changes with initial pressure suggests that it's not first order kinetics.

I would explore second order.
5. (Original post by charco)
The fact that the half-life changes with initial pressure suggests that it's not first order kinetics.

I would explore second order.
Thanks
I thought pressure is synonymous with concentration in the gas phase which is why I assumed first order

but how do you work out it's second order from the data
6. (Original post by Bloom77)
Thanks
I thought pressure is synonymous with concentration in the gas phase which is why I assumed first order

but how do you work out it's second order from the data
Yes, in the gas phase you can deal with pressures as if they are concentrations, but why does this lead you to believe that the process is first order?

The evidence from half-lives is that it is not first order.

Reply
Submit reply
Turn on thread page Beta

### Related university courses

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:
Updated: May 19, 2017
Today on TSR

### Edexcel C4 Maths Unofficial Markscheme

Find out how you've done here

### 1,715

students online now

Exam discussions

### Find your exam discussion here

Poll

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

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