[Catalist] Equilibrium and temperature

Fri Nov 24 18:42:19 AEDT 2017

[http://www.ibchem.com/IB/ibfiles/energy/ene_img/boltz1.gif]
A colleague showed me this a few years back when we were chatting at Future Science.

As temp increases the proportion of particles with energy greater than Ea increases and reaction rate increases. So both forward and reverse rates increase.
However, if you look at the above diagram, if the Ea on the above diagram represented the higher Ea or the endothermic reaction in the equilibrium situation this thread is talking about, you could estimate that the rate of the endothermic reaction would be much greater at T2 because the area under the curve at T2 is probably 5 times larger (yellow plus orange) than it is at T1 (orange only).
If you took the Exothermic reaction to have a very low Ea (which is not labelled on this diagram), but let’s say it was the first mark on the Ek scale  then the area under the curve at T1 would account for 90% of particles and at T2 probably 95% - so the exothermic rate would be faster, but only by 1 and a bit, certainly not 5 times the increase as occurred for the endothermic reaction. You can try this exercise putting the exo Ea closer to the higher endo Ea and the same thing happens – a bigger increase in rate for the endothermic (higher Ea) reaction compared to the exothermic (lower Ea) reaction.

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Dr Brian Plewright HOLA Science

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From: Catalist [mailto:catalist-bounces at lists.stawa.net] On Behalf Of Andy Gray
Sent: Friday, 24 November 2017 2:20 PM
To: catalist at lists.stawa.net
Subject: Re: [Catalist] Equilibrium and temperature

Hi Andrew - although outside of the ATAR course, http://www.chem1.com/acad/webtext/thermeq/TE5.html<https://apac01.safelinks.protection.outlook.com/?url=http%3A%2F%2Fwww.chem1.com%2Facad%2Fwebtext%2Fthermeq%2FTE5.html&data=02%7C01%7CBrian.Plewright%40education.wa.edu.au%7C9c3406a1be384387947008d53303fa34%7Ce08016f9d1fd4cbb83b0b76eb4361627%7C0%7C0%7C636471014561608595&sdata=tGzM7qNhFLqRP6MhV5%2FohDoX4bXeUFbX36%2BkWAuU2Pg%3D&reserved=0> is pretty good at explaining the relationship between changes in Gibbs free energy and K. Also, https://aatishb.com/entropy/<https://apac01.safelinks.protection.outlook.com/?url=https%3A%2F%2Faatishb.com%2Fentropy%2F&data=02%7C01%7CBrian.Plewright%40education.wa.edu.au%7C9c3406a1be384387947008d53303fa34%7Ce08016f9d1fd4cbb83b0b76eb4361627%7C0%7C0%7C636471014561608595&sdata=DoOETuiFwwziN6IrYWbCo8%2FiBiPI9bYviKPuvdnAGoU%3D&reserved=0> uses sheep to explain entropy and it's AWESOME!

Enjoy the weekend :)

On 24 November 2017 at 12:11, Andrew Bland <abl at gmas.wa.edu.au<mailto:abl at gmas.wa.edu.au>> wrote:
Dear chemistry teachers,

When a system is heated, both the forward and reverse reaction rates increase, “but the endothermic reaction increases more than the exothermic reaction. The endothermic reaction is always affected more by a temperature change than the exothermic reaction. This is true even when a system is cooled…” Chem for WA 2.

My question is why is the endothermic reaction rate always effected more than the exothermic rate? I cannot find an explanation in any of my texts, only a re-stating of this in various forms.

Thanks

Andrew

Andrew Bland

Head of Year 10 and Head of Geographe House

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