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

Question

Noah Aldrich · Accepted Answer

#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?

Natalie Anton · Answer

undefined Kp is the equilibrium constant in relation to the concentrations of reactants and products in a chemical reaction at a specific temperature. The Kp equation expresses the ratio of the partial pressures of products to reactants, each raised to the power of their respective coefficients in the balanced chemical equation.