Let us have an example, in the laboratory oxygen can be produced through the decomposition of potassium chlorate (KClO3):
2KClO3(s) → 2KCl(s) + 3O2(g)
This reaction is very slow even if heated strongly, however mixing black manganese dioxide (MnO2) with KClO3 before heating causes the reaction to occur more faster. After the reaction the MnO2 remain unchanged and the overall chemical reaction still remains the same.There are three general types of catalysis: the homogeneous catalysis, heterogeneous catalysis and enzymes catalysis.
Homogeneous Catalysis
A homogeneous catalyst is a catalyst that is present in the same phase as the reacting molecules usually liquid solution and in gas phase. For example in the decomposition of aqueous hydrogen peroxide, H2O2(aq) into water and oxygen:
2H2O2(aq) → 2H2O(l) + O2(g)
Without the catalyst, this reaction is extremely very slow. One of the substances that can catalyze the reaction is the bromide ion, Br-(aq). The bromide ion reacts with hydrogen peroxide in acidic solution, as shown in the reaction below:
Hydrogen peroxide will again react with Br2(aq) forming O2. Therefore, the reaction is an example of a reaction having a multistep mechanism having two elementary steps.
The overall reaction is shown below:
In general, a catalyst lowers the overall activation energy for a chemical reaction.
Heterogeneous Catalysis
A heterogeneous catalyst is a catalyst that exists in a different phase from the reacting molecules. It is usually a solid that is combined either with liquid or gaseous reactants. Many industrially important reactions are catalyzed by the surfaces of solids like the Haber process, the synthesis of ammonia, and the manufacture of nitric acid.
The Haber Process
Ammonia is an important inorganic compound in the fertilizer industry, the manufacture of explosives, and much other application, It is formed by the reaction of N2 and H2, and the reaction is exothermic,
2Br-(aq) + H2O2(aq) + H+ → Br2(aq) + 2H2O(l)
Hydrogen peroxide will again react with Br2(aq) forming O2. Therefore, the reaction is an example of a reaction having a multistep mechanism having two elementary steps.
Br2(aq) + H2O2(aq) → 2Br-(aq) + 2H+(aq) + O2(g)
The overall reaction is shown below:
2H2O2(aq) → 2H2O(l) + O2(g)
The catalyst in the reaction is the Br-(aq), since it was left unreacted after the reaction. Br2 on the other hand is the intermediate since it was formed in the first step and consumed in the second step.In general, a catalyst lowers the overall activation energy for a chemical reaction.
Heterogeneous Catalysis
A heterogeneous catalyst is a catalyst that exists in a different phase from the reacting molecules. It is usually a solid that is combined either with liquid or gaseous reactants. Many industrially important reactions are catalyzed by the surfaces of solids like the Haber process, the synthesis of ammonia, and the manufacture of nitric acid.
The Haber Process
Ammonia is an important inorganic compound in the fertilizer industry, the manufacture of explosives, and much other application, It is formed by the reaction of N2 and H2, and the reaction is exothermic,
N2(g) + 3H2(g) → 2NH3(g) ∆Ho = -92.6 kJ/mol
This reaction is very slow at room temperature and if the temperature is increased the rate also increases but it lowers the rate of the formation of NH3(g), instead it promotes the formation of N2(g) and H2(g).
Fritz Haber, a German chemist, tried hundreds of compounds that will catalyze the reaction of N2 and H2 to form NH3 and discovered iron plus a few percent of oxides and aluminum at about 500oC. This procedure is known as the Haber Process.
In heterogeneous catalysis, the surface of the solid catalyst is the site of the reaction. In the Haber process, the dissociation of N2 and H2 is done on the metal surface of the catalyst. The two reactant molecules behave very differently on the metal surface, they are highly reactive. Studies show that H2 dissociates into atomic hydrogen at -196oC while N2 molecules dissociate at about 500oC. The highly reactive N and H combine rapidly at high temperatures forming NH3.
Fritz Haber, a German chemist, tried hundreds of compounds that will catalyze the reaction of N2 and H2 to form NH3 and discovered iron plus a few percent of oxides and aluminum at about 500oC. This procedure is known as the Haber Process.
In heterogeneous catalysis, the surface of the solid catalyst is the site of the reaction. In the Haber process, the dissociation of N2 and H2 is done on the metal surface of the catalyst. The two reactant molecules behave very differently on the metal surface, they are highly reactive. Studies show that H2 dissociates into atomic hydrogen at -196oC while N2 molecules dissociate at about 500oC. The highly reactive N and H combine rapidly at high temperatures forming NH3.
N + 3H → NH3
Manufacture of Nitric Acid
Nitric acid is also one of the inorganic compounds that has many useful uses like production of fertilizer, dyes, drugs and explosives. This is prepared by heating ammonia and molecular oxygen in the presence of platinum-rhodium catalyst at about 800oC. This process is known as Ostwald process.
Nitric acid is also one of the inorganic compounds that has many useful uses like production of fertilizer, dyes, drugs and explosives. This is prepared by heating ammonia and molecular oxygen in the presence of platinum-rhodium catalyst at about 800oC. This process is known as Ostwald process.
4NH3(g) + 5O2(g) → 4NO(g) + 6H2O(g)
The nitric oxide even without the catalyst oxidizes to nitrogen dioxide:
2NO(g) + O2(g) → 2NO2(g)
And when dissolve in water, NO2 forms both nitrous acid and nitric acid.
2NO2(g) + H2O(l) → HNO2(aq) + HNO3(aq)
On heating, the nitrous acid is converted to nitric acid.
3HNO2(aq) → HNO3(aq) + H2O(l) + NO(g)
NO(g) can be recycled to produce NO2 and the same process occurs in step 2 above.
Enzyme Catalyst
Enzymes are called biological catalysts. Enzymes increase the rate of biological reaction by factors ranging from 106 to 1018. Enzymes are very selective in reactions that they catalyze, and some are absolutely specific, operating for only one substrate (substance) in only one reaction. The enzyme catalyzes are usually homogeneous with the substrate and enzymes present in the same aqueous solution. Most enzymes are large proteins with molecular weights ranging from 10,000 to about 1 million amu.
Enzyme Catalyst
Enzymes are called biological catalysts. Enzymes increase the rate of biological reaction by factors ranging from 106 to 1018. Enzymes are very selective in reactions that they catalyze, and some are absolutely specific, operating for only one substrate (substance) in only one reaction. The enzyme catalyzes are usually homogeneous with the substrate and enzymes present in the same aqueous solution. Most enzymes are large proteins with molecular weights ranging from 10,000 to about 1 million amu.
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