# Is Kp and Kc only effected by temperature?

Are Kp and Kc only effected by temperature?

So in exothermic and endothermic equilibrium reactions you are increasing or decreasing temperature, which will effect which side is favourable.

But then so does pressure if one side has fewer molecules?

• – Philipp Jun 3 '17 at 12:58
• @Philipp is this statement in my textbook wrong then "increasing the pressure forces the equilibrium in such a way as to reduce the total pressure, that is, to the side with fewer molecules in other words KP increases. So increasing the total pressure increases the yield of ammonia" – Aaron Jun 3 '17 at 13:23
• It sounds fishy, at least the part "in other words KP increases". They are talking about Le Chatelier's principle but that does not entail a pressure dependence of the equilibrium constant. However, the partial pressures/activities/relative amounts of the reactants and products in the reaction mixture which feature in the expression for $K$ do not need to be constant with pressure only their reaction quotient has to be in order to preserve the contant $K$. Thus, pressure (depending on how it is administered) does influence the equilibrium composition of the reaction mixture. – Philipp Jun 3 '17 at 15:14
• You could have a look at page 221 of the 9th edition of the book "Physical Chemistry" by Atkins. It describes this in more detail. – Philipp Jun 3 '17 at 15:18
• As $K_p$ is defined by standard states, i.e. 1 atm (or 1 bar) it must be independent of pressure. The T dependence is from the van't Hoff isochore $d\ln(K_p)/dT = \Delta H^o/RT^2$ (assuming $\Delta H$ constant). If the equilibrium constant is defined in terms of mole fraction x then $K_x=K_pp^{-\Delta n}$ where $\Delta n =a+b-c-d$ is the change in number of moles in reaction $\ce{aA + bB <=>cC + dD}$. Sometimes the equilibrium constant is called $K_p$ when it should be $K_x$ or $K_c=K_p(RT)^{-\Delta n}$. – porphyrin Jun 3 '17 at 16:08

## 1 Answer

According to LeChatelier's principle on increasing temperature in an exothermic reaction "which is at it's equlibrium initially" the reaction proceeds in backward direction and in endothermic reaction it proceeds in forward direction, this can be thought as in exothermic reactions the heat should be provided to the products to yield the reactants and in endothermic the heat should be supplied to reactants to yield the products! And the value of equilibrium constants depend only upon temperature and the pressure only effect the equilibrium conditions and not the value of Kp because partial are so adjusted to provide the same value of Kp at the same temperature.