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This Demonstration shows the effect of temperature on chemical equilibrium by changing temperature in an exothermic and in an endothermic reaction, specifically:

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and .

The initial concentrations of all species are fixed.

The final concentrations in a reaction are related to initial concentrations by the equation for the equilibrium constant [1]:

and .

This allows us to obtain the final concentrations for the exothermal reaction using: , and ; for the endothermal reaction using: and .

Changing the temperature in an equilibrated system will stimulate the establishment of a new equilibrium. In exothermic reactions, heat can be included among the products of reaction. Le Chatelier's principle states that lowering the temperature can be seen as subtracting products, so it will cause a shift toward the product. Endothermic reactions work in the exact opposite way: as heat can be considered one of the reactants, lowering the temperature causes a shift toward the reactants.

Opacity of the arrows is proportional to the forward and backward reaction rate. Iodine in the gas phase is colored purple, while the products are colorless. For the endothermic reaction, nitrogen dioxide assumes a brown-yellow coloration. Thus the reaction rate can be monitored by the intensity of the color.

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Contributed by: D. Meliga, L. Lavagnino and S. Z. Lavagnino (July 2020)
Additional contribution by: S. Porta
Open content licensed under CC BY-NC-SA


Snapshots


Details

Snapshot 1: system at the equilibrium: the initial concentration fulfils the equilibrium constant equation, so there is no change

Snapshot 2: Le Chatelier's principle: raising the temperature in the equilibrium state causes a shift of an endothermic reaction toward the product side

Snapshot 3: lowering the temperature of an exothermic reaction increases the value of and consequently shifts the equilibrium toward the product side

Snapshot 4: raising the temperature of an endothermic reaction increases the value of and consequently shifts the equilibrium toward the product side

References

[1] C. H. P. Lupis, Chemical Thermodynamics of Materials, New York: North-Holland, 1983.

[2] S. V. Lavagnino. Chemical Equilibrium [Video]. (Jul 10, 2020) www.youtube.com/watch?v=TDBQOF7M-W8&list=PLswwssc6Q2yac7AM3x5UjmesLQaye-xMP&index=3.



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