This equation can be used to calculate K° at different temperatures, if we assume that ΔrH° and ΔrS° for a reaction have the same values at all temperatures. This is a good, but not perfect, assumption, and we will use it in this course unless specified otherwise. It is not a good assumption if there is a phase change for a reactant or a product within the temperature range of interest.
Dividing both sides of the equation by “-RT” gives:
A plot of lnK° vs. is called a van’t Hoff plot. The graph has a slope of . Therefore, if the concentrations of reactants and products are measured at various temperatures so that K° can be calculated at each temperature, the reaction enthalpy change can be obtained from a van’t Hoff plot.
Based on the equation for the van’t Hoff plot, an exothermic reaction (ΔrH° < 0) has K° decreasing with increasing temperature, and an endothermic reaction (ΔrH° > 0) has K° increasing with increasing temperature. This shows that the magnitude of ΔrH° will dictate how rapidly K° changes as a function of temperature.
For example, suppose that K°1 and K°2 are the equilibrium constants for a reaction at temperatures T1 and T2, respectively:
Subtracting the two equations yields:
Thus calculating ΔrH° from tabulated ΔfH° values and measuring the equilibrium constant at one temperature would allow us to calculate the equilibrium constant at any other temperature (assuming that ΔrH° and ΔrS° are independent of temperature).
Using this equation, you can also estimate ΔrH° without constructing a van’t Hoff plot if K° was determined at only two temperatures (but the result will not be as accurate).
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