Equilibrium law
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If a reaction is repeated at a higher temperature and Kc increases greatly, how is the reaction endothermic?
211208
Ulex writes ....
The simplest way to justify this is to apply Le Chatelier’s Principle in reverse, as it were. If you raise the temperature of a system at equilibrium, providing thermal energy in order to do so, the equilibrium will shift in such a direction as to oppose the imposed change, that is, it will shift in its endothermic direction. It follows, therefore that if Kc increases when you raise the temperature, the reaction must be endothermic in the forward direction.
A more mathematical way of looking at it is to use the relationship between Kp and temperature which is:
ln Kp = C - (delta)H/RT, where C is constant (and delta is the triangular sign we can't do on this email!)
You can see from this that if Kp increases when T increases, the value of (delta)H/RT must decrease so C - (delta)H/RT becomes greater, which can only happen if (delta)H is positive.
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updated: 21 December 2008
