Haber Process
Dinitrogen (N2) is unreactive because of its triple bond - cant be used by plants.
N₂ molecules must be broken apart in to produce chemical species with N-C, N-O, or N-H bonds via fixation.
This need had been forseen. In the early 20th century Fritz Haber and Carl Bosch had developed the recipe for large-scale synthesis of ammonia.
Uses high temperature and pressure with a catalyst.
The image the Haber’s ammonia synthesis chematically.
Reaction Quotient
aA + bB <-> cC + dD
Q = [C]^c [D]^d / [A]^a [B]^b
Not solids or water.
Law of Equilibrium
For a given reaction at a specified temperature, all equilibrium mixtures have the same value of the reaction quotient (Q). This particular value of Q is called the equilibrium constant (K).
Activity-Based Equilibrium Constants
- Reaction quotients are most correctly expressed as functions of activities of species rather than as concentrations.
- Activities are measures of effective concentration, and as a ratio compared with 1 mol L-1, are unitless.
- In sufficiently dilute solutions, there is negligible difference between the concentrations and activities of species.
Thermodynamic activity
sometimes called effective concentration and is the ratio of the concentration to a standard value.
The Relationship between Q and K in Reaction Mixtures
aA + bB <-> cC + dD
- Q < K. Equilibrium is attained if the net reaction changes reactants into products.
- Q > K. Equilibrium is attained if the net reaction changes products into reactants.
- Q = K. The reaction mixture is at equilibrium.
Spontaneous Reaction Direction
the direction in which the net reaction occurs to move the concentrations in the reaction mixture to equilibrium.
As a reaction comes to equilibrium, the mixture has a lower Gibbs free energy than at any other point in time before equilibrium
Product favoured: K > 1
Reactant favoured: K < 1
Estimating Equilibrium Constants
determined with the help of an ICE table, which lists initial, changes, and equilibrium concs
Disturbing Reactions at Equilibrium
Accomplished by changing:
* The concentrations of reactants or products (or
both) so the value of Q ≠ K.
- The temperature, which changes the equilibrium constant K, so Q ≠ K.
Le Chatelier’s principle
Counteracts an imposed change on a system
Effect of concentration change
For a gas phase raction where the number of reactant molecules and product molecules is different:
- Decreasing the volume of a mixture leads to the reaction that decreases the number of molecules in the system.
- Increasing the volume of a mixture leads to the reaction that increases the number of molecules in the system. For a reaction with the same number of reactant and product molecules:
- Changing the volume a volume does not affect the relative amounts of reactants and products, because Q = K after the volume change.
Effect of Changing the Temperature
If the reaction mixture is at equilibrium (Q=K), and the temperature is increased:
* The value of K decreases, which means Q > K.
* The net reaction will increases Q, i.e. the endo direction REVERSE of written above.
When T is raised, the next reaction occurs that decreases the temperature (Endorthermic).
Similarly, if T is decreased, the net reaction will increase the temperature (exothermic)
Applying the Principles: The Haber-Bosch Process
At ambient temperatures, the reaction is
exothermic and product-favoured (K > 1), but is slow.
- The reaction goes faster as the temperature
increases… the equilibrium constant decreases - It is necessary to balance reaction rate (faster at higher temperature) with product yield (greater at lower temperature).
- A catalyst increases the reaction rate. The optimal temperature for the catalyst is ~450°C.
- The reaction done under pressure to increase the equilibrium ammonia concentration.
- Ammonia is continually liquefied by cooling in a chamber, and removed.
- The mixture of gases is then recycled into the reaction chamber
