Kp Calculator

In some chemical reactions, the change is not permanent - the products can sometimes form the reactants. If both the forward and backward reactions occur simultaneously, then it is known as a reversible reaction. In such cases, you can calculate the equilibrium constant by using the molar concentration (Kc) of the chemicals, or by using their partial pressure (Kp).

To understand the progression of a reversible reaction, in the case the reaction is not at the equilibrium yet, you should use the reaction quotient. In each case, the equations are fairly similar. For the reaction:

a⋅A+b⋅B⇌c⋅C+d⋅Da⋅A+b⋅B⇌c⋅C+d⋅D,

the equilibrium constant in terms of concentration is:

Kc​=[B]b ⋅ [A]a[C]c ⋅ [D]d​Kc​=[B]b ⋅ [A]a[C]c ⋅ [D]d​,

where

• [A][A] and [B][B] are the molar concentrations of the reactants
• [C][C] and [D][D] are the molar concentrations of the products

The equilibrium constant formula in terms of partial pressure is:

Kp​=Pb​b ⋅ Pa​aPc​c ⋅ Pd​d​Kp​=Pb​b ⋅ Pa​aPc​c ⋅ Pd​d​,

where, analogously

• Pa​Pa​ and Pb​Pb​ are partial pressures of the reactants
• Pc​Pc​ and Pd​Pd​ are partial pressures of the products

The relationship between Kp and Kc is:

Kp​=Kc​⋅(R⋅T)ΔnKp​=Kc​⋅(R⋅T)Δn, where

• Kp​Kp​ is the equilibrium constant in terms of pressure.
• Kc​Kc​ is the equilibrium constant in terms of molarity.
• RR is the gas constant.
• TT is the temperature.
• ΔnΔn is the change in the number of moles:

Δn=mol of gaseous products−mol of gaseous reactantsΔn=mol of gaseous products−mol of gaseous reactants

To save time, start calculations with the change in the number of moles. If it's zero (Δn=0Δn=0) then Kc equals Kp. If not, read the last paragraph of this article, use the correct units, and find the answer in less than a few minutes (or seconds if you use our Kp calculator ;) ).

The most important thing to remember when calculating the equilibrium constant in terms of pressure is to only take into account components in the gas phase. For example, the Kp of the following heterogenous reaction:

2H2(g)​+O2(g)​⇌2H2​O(s)​2H2(g)​+O2(g)​⇌2H2​O(s)​

is equal to:

Kp​=PH2​​2 ⋅ PO2​​1​Kp​=PH2​​2 ⋅ PO2​​1​

So, instead of using the pressure of water you just need to input 11. You might be wondering, what if there aren't exactly two reactants, or two products? Then, similarly to the above Kp equation, divide the pressures of the products by the pressures of the reactants, just like we did in the following example:

H2​O(g)​+C(s)​⇌H2(g)​+CO(g)​H2​O(g)​+C(s)​⇌H2(g)​+CO(g)​

Kp​=PH2​O​PH2​​ ⋅ PCO​​Kp​=PH2​O​PH2​​ ⋅ PCO​​

Keep in mind to always double-check the units! They should always be the same!

The conversion between Kc and Kp might be tricky. The equilibrium constant is a unitless number, but give some thought to the gas constant unit. It indicates if the equilibrium constant for partial pressures is calculated in terms of bars, atmospheres, or Pascals. We've prepared a table with the most common pressure units and their corresponding gas constants:

Now, let's have a look at this reversible reaction:

N2(g)​+3H2(g)​⇌2NH3(g)​N2(g)​+3H2(g)​⇌2NH3(g)​

How do we find the equilibrium constant Kp in terms of pressure, in atmospheres, if at 298K298K the equilibrium constant Kc is 2.27⋅10−22.27⋅10−2?

1. Start with establishing the value of the gas constant:

   R=0.082 057 46(14)K⋅molL⋅atm​R=0.082 057 46(14)K⋅molL⋅atm​

2. Then, determine the change in moles:

   Δn=2−(3+1)=−2Δn=2−(3+1)=−2

3. Finally, calculate the value of Kp for the equation:

   Kp​=2.27⋅10−2⋅(0.0820574614⋅298)−2=3.796⋅10−5Kp​=2.27⋅10−2⋅(0.0820574614⋅298)−2=3.796⋅10−5

Now you know the equilibrium constant for an example of the Haber process, and how to calculate Kp! But if you don't feel like doing all that math on your own, you can always put our Kp calculator to good use!

Can't get enough of chemistry? Check out the Avogadro's number calculator next!