What is Kp? What goes on in a Kp equation?

#K_P# is the equilibrium constant conventionally used to describe gas-phase reactions in which both products and reactants are generated simultaneously.

Conventionally, it is reported in implied units of #atm#.

It is used in a mass action expression to determine equilibrium partial pressures. Here is a nice example...

#1"NH"_4"Cl"(s) rightleftharpoons color(red)(1)"NH"_3(g) + color(red)(1)"HCl"(g)#

#"I"" "-" "" "" "" "" "0" "" "" "0#
#"C"" "-" "" "" "" "+P_i" "+P_i#
#"E"" "-" "" "" "" "color(white)(//.)P_i" "" "P_i#

In this case, we see a solid goes into the expression as #bb1#, because its standard concentration is equal to its molar density. (It has nothing to do with coefficients.)

I have purposefully highlighted the coefficients, as they become the exponent for gaseous reactants.

The mass action expression is therefore:

#K_P = ((P_(NH_3))^color(red)(1)(P_(HCl))^color(red)(1))/1#

But since the coefficients for both products are the same, #P_(NH_3) = P_(HCl)#. Thus, both exponents are the same as well.

As a result, if the #K_P# at a certain temperature is #6.25#, then

#6.25 = P_(NH_3)P_(HCl) = P_i^2#

So, the partial pressures at equilibrium are each:

#=> P_i = P_(NH_3) = P_(HCl) = sqrt(6.25) = ul"2.5 atm"#

CHALLENGE: What is the total pressure at equilibrium? HINT: What is Dalton's law?