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Unit 9: Equilibrium and Advanced Thermodynamics—Balance in Chemical Reactions

Section 10: Temperature and Equilibrium

As discussed in Unit 7, Section 6, exothermic reactions release energy and endothermic reactions absorb energy. Because exothermic reactions give off heat, we can consider heat a product of the reaction, and we can even include it in the chemical equation for convenience:

Aaq equilibrium arrows Baq + heat

If the reaction is endothermic, we consider heat a reactant:

heat + Aaq equilibrium arrows Baq

It is now possible to predict the direction of shift when the reaction is heated or cooled. If an exothermic reaction is heated, the reaction shifts left; if cooled, it shifts right. If an endothermic reaction is heated, the reaction shifts right; if the reaction is cooled, it shifts to the left. It is important to remember that heat isn't really a product or a reactant of the reaction; this is just a way to understand how temperature affects equilibrium. So, we can pretend it is like Le Châtelier's Principle from Section 9 of this unit to get an idea of which way the reaction will shift. However, what really happens when the temperature changes is that the value of the equilibrium constant (K) changes, and the reaction has to move to shift to that new ratio. Whether K goes up or down is a function of whether the reaction is endothermic or exothermic.

Uric Acid Crystals

Figure 9-13. Uric Acid Crystals

© Wikimedia Commons, Creative Commons License 3.0. Author: Bobjgalindo, 18 March 2005.

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Uric Acid Crystals

Figure 9-13. Uric Acid Crystals

Gout is a painful condition that occurs when crystals of uric acid precipitate out inside the joints. This image shows these crystals photographed under polarized light.

These equilibrium shifts explain why some substances are more or less soluble at higher temperatures. Dissolving CO2 in water is exothermic; cooling the water shifts the reaction to the right, allowing more gas to dissolve. Warming the solution causes a shift to the left, and CO2 comes out of the solution; this explains why soda goes flat as it comes to room temperature.

Conversely, dissolving uric acid (a waste product in the bloodstream) is endothermic. It dissolves more at higher temperatures, and less at lower temperatures. In parts of the body that become cold such as the feet, the solubility of uric acid can decrease so much that it precipitates out as tiny needle-shaped crystals. (Figure 9-13) This leads to an extremely painful form of arthritis called "gout." Attacks of gout also occur more often at night because the ambient temperature is lower.


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