Ammonia is produced by the Haber process, in which nitrogen and hydrogen are rea

Question

Ammonia is produced by the Haber process, in which nitrogen and hydrogen are reacted directly using an iron mesh impregnated with metal oxides as a catalyst. For the reaction N2(g) + 3H2(g) 2NH3(g) equilibrium constants as a function of temperature are 300C, 4.34 x 10^-3 atm-2 500C, 1.45 x 10^-5 atm-2 600C, 2.25 x 10^-6 atm-2

a. Is the reaction exothermic or endothermic? b. Typical conditions used for producing ammonia are temperatures of 400 to 500C and total pressures of 200 to 600 atm. Why are these conditions chosen?

Explanation / Answer

we know that

ln (k2/k1) = (dH/R) (1/T1 - 1/T2)

given

k1 = 4.34 x 10-3

T1 = 300 + 273 = 573 K

k2 = 1.45 x 10-5

T2 = 500 + 273 = 773 K

so

using those values

we get

ln ( 1.45 x 10-5 / 4.34 x 10-3) = (dH/8.314) ( 1/ 573 - 1/ 773)

dH = -104979 J

dH = -104.979 x 1000 J

dH = -104.979 kJ

we know that

if dH > 0 , it is endothermic

id dH < 0 , it is exothermic

so

the given haber process is exothermic

b)

let the reaction be written as

N2 + 3 H2 ---> 2 NH3 + heat

now

we can see that

the reactants have very high number of gaseous moles

so

according to Le Chatlier principle

increasing pressure will shift the equilibrium towards the products , in this ammonia

So

high pressure is used in habers process

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