[Catalist] Equilibrium and temperature

Fri Nov 24 15:48:05 AEDT 2017

The syllabus tells us the predictions of the effect of a change in temperature can be made by applying LCP:

-          the effects of changes in temperature, concentration of species in solution, partial pressures of gases, total volume and the addition of a catalyst on equilibrium systems can be predicted using Le Châtelier’s Principle

However, what is the expectation in terms of explaining the effect of a change in temperature?

-          the effect of changes of temperature on chemical systems initially at equilibrium can be predicted by considering the enthalpy changes for the forward and reverse reactions; this can be represented on energy profile diagrams and explained by the changes in the rates of the forward and reverse reactions

The final part of this syllabus point “explained by the changes in the rates of the forward and reverse reactions” is open to interpretation.  Do students merely need to state that the rate of one reaction initially increase/decreases to a greater extent than the other or must they explain the relative changes in rate?

It would be great to get some clarity regarding this from SCSA.

Regards,
Simon

Mr Simon Bosustow
Teacher - Science Department
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St Hilda's Anglican School for Girls
PO Box 34 Mosman Park
Western Australia  6912
(08) 9285 4100
www.sthildas.wa.edu.au

From: Catalist [mailto:catalist-bounces at lists.stawa.net] On Behalf Of Tania Stafford
Sent: Friday, 24 November 2017 12:21 PM
To: catalist at lists.stawa.net
Subject: Re: [Catalist] Equilibrium and temperature

The mechanics behind this are quite complex and students don’t really get into it until studying chemistry at university. That is why Le Chetalier’s principle is used often to explain temperature change rather than collision theory and enthalpy. Your standard answer is all that expected in the ATAR papers.
Tania

From: Catalist <catalist-bounces at lists.stawa.net<mailto:catalist-bounces at lists.stawa.net>> on behalf of Andrew Bland <abl at gmas.wa.edu.au<mailto:abl at gmas.wa.edu.au>>
Reply-To: "catalist at lists.stawa.net<mailto:catalist at lists.stawa.net>" <Catalist at lists.stawa.net<mailto:Catalist at lists.stawa.net>>
Date: Friday, 24 November 2017 at 12:11 pm
To: "Catalist at lists.stawa.net<mailto:Catalist at lists.stawa.net>" <Catalist at lists.stawa.net<mailto:Catalist at lists.stawa.net>>
Subject: [Catalist] Equilibrium and temperature

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

T:  +61 8 9752 5252

F:  +61 8 9752 5299

A:  PO Box 920, Busselton WA 6280

E:  abl at gmas.wa.edu.au<mailto:abl at gmas.wa.edu.au>

W: www.gmas.wa.edu.au<http://www.gmas.wa.edu.au>

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