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

##### 1 Answer

**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**

*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,

As a result, if the

#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?*