Have you ever tried to set fire to the sugar? It would seem highly exothermic reaction C12N22O11+12°2 →12SO2+ 11H2O should go easy. But, not - with strong heating the sugar melts, gets a brown color and caramel smell, but does not light up. But, you can burn the sugar. For this purpose it is necessary to sprinkle it by ashes of tobacco and make the fire - then the sugar burns. A similar effect on the reaction provide some other substances such as salts of lithium, oxide or chromium (III).
Chemical reactions that are "unwilling" to proceed by themselves or go with a very low rate and require additional "stimulus" — the presence of substances what remain unchanged, happen everywhere. This is, firstly, all the chemical processes that are in the cells activity. They occur only in the presence of enzymes, and the absence in the body of at least one of them violates the metabolism and cause to serious illness or not associated with life.
In addition, such reactions include the majority of large-scale processes used in the chemical industry. Getting a sulfuric acid, oil processing, the synthesis of ammonia is unthinkable without the participation of "foreign matter", called catalysts.
How our world would look without catalysts? It would be much estetichnee, because many chemical reactions would not happen. However, nobody would not to study chemistry: a life can not appear in such a world.
Catalysts allow chemical reactions process under much milder conditions. Moreover, in the presence of catalyst are the reactions that are impossible without it in any conditions.
The amount of catalyst required for the conversion of huge masses of the reactants in the reaction products, is disproportionately small. One molecule of enzyme catalyzes the decomposition of 5 million molecules of sugar for 1 sec.!
Catalysis and its secrets
But what's the hidden secret of chemicals — catalysts? Let's look at why sugar and other organic matter does not spontaneously turn into carbon dioxide and water is a much more energetically favorable (say "thermodynamically stable") substances. Isn't it amazing? Because if you put, say, a ball to the top of the hill, it is more energetically favorable position is rolled down. If its to protect the barrier, it will not roll down. To be the bottom and reduce its potential energy, the ball needs to overcome the barrier, and for this it needs to bring extra energy.
All existing chemical substances, even a highly thermodynamically unstable, surrounded on its energy "peaks" such barriers. Sometimes the energy required to overcome them comparable with the kinetic energy of thermal motion of molecules. Then rather mixing of reagents and the reaction occurs at room temperature. Heating the reaction mixture, it is possible to overcome the barrier a little higher. But sometimes it's too high, and in this case you will either have to find ways of delivering the required energy to the molecules of the reactants, or to try to bypass the energy barrier.
How to do it? It turns out that catalyst, like an experienced guide, knew the place well, the conduct reaction in a different way. Its mechanism has a strong changes. There are many ways to overcome energy "uphill". Every catalyst working in a particular reaction, chooses to process way. The new route can be much longer than the original: the number of intermediate stages and reaction products sometimes increases several times. But the amount of energy required at each stage, is significantly less than in the absence of a "conductor". In the end, after long way with the help of a catalyst, the reaction gives the desired result much faster.
However, the "foreign substance" can back affect for the reaction, which lead it to difficult overcome the energy barrier. Then the process slows down. Such "negative" catalysis is called inhibition (from Latin. inhibeo — "stop", "keep"), and "catalysts operating in reverse" — inhibitors.
Why you need to slow down the rate of reaction? There are processes that a we need, but there are also processes such which may be detrimental for as, our belongings and the environment. For example the rusting - corrosion of metals, rotting food. Such reactions can be explosions of various chemicals that are sensitive to motion or concussion. You need to consider that the chemical reactions that format only one substance is quite rare. Basically reactions produce more than one substance. Especially this phenomenon is observed in organic chemistry.
In live organisms and many other processes in our environment is often required to get only one desired substance or the reaction product in the reaction process. In this case is used catalysis. Competent selection of catalyst do enables chemical processes only at one direction and with the receipt of the required substances, thus eliminating the release of other side effects of reactions.
There are enzymes - catalysts in our body, which work in exactly the same way. Science has possibility to get a catalytic reaction by analogy with enzymes .
Despite the analogy and the knowledge the secrets catalysis remain completely unsolved.
Types of catalystsWhat to use as the catalyst!?
What are types of catalysts?! It can be a wide variety of substances: gases, ions and various complexes; metals and oxides; simple organic compounds and complex natural and synthetic polymers; even... water, sand, clay!
But the most important factor is the physical state of catalyst and interacting substances.
When the catalyst, the reactants and the reaction products are in the same one phase (e.g., liquid, gas) where there are no boundaries, the reagents are much easier to get in contact with the catalyst. In this case, catalyst is in the form of molecules or ions. This catalysis called a homogeneous (from Greek "Homogenic" — "homogeneous"). In the case of heterogeneous catalysis (from Greek "heterogenic" — "heterogeneous"), especially if the catalyst is a solid, in the course of the reaction affects not the individual molecule, but a part of the surface and subsurface layers. This interaction of the reactants with many atoms or molecules of the catalyst, built in a certain way, enhances the catalytic effect. Reactions between solids are always heterogeneous.
Homogeneous catalysis and heterogeneous catalysis differ substantially in their mechanism. Probably, it has relation with the "one-sidedness" — pre-emptive explanation or homogeneous or heterogeneous catalysis.
With the catalysts every year billions of tons of chemical products are produced worldwide. About 90% of industrial chemical processes — are catalytic processes.
First of all, in the early nineteenth century, discovered the catalytic properties of noble metals: platinum, palladium, etc. So far, these metals are used, for example, in the exhaust afterburning gas of cars (they convert the poisonous carbon monoxide, CO, is formed by the incomplete oxidation of the fuel to carbon dioxide CO2.,), the oxidation ammonia NH3 and in other processes. Of course, scientists are working to replace more expensive metals to cheap.
From homogeneous catalytic reactions the most famous catalysis by acids and bases as well as salts and complexes of metals. The easiest acid catalyst is a hydrogen ion. It is the smallest. Among the first studied catalytic reactions was the hydrolysis of starch ("shredded" it to molecules sugars) in an acidic solution. Now acid catalysis is very common in organic chemistry: hydrogen ions well accelerate various reaction of alcohols, aldehydes, fatty acids and other substances.
However, the technology at chemical production use heterogeneous catalysis. For example, the reaction proceeds in the gas phase with solid catalyst (porous iron in the synthesis of ammonia). Since the catalyst is in a different phase, the reaction products can easy separated from it. It is noteworthy that the same acid-base catalysis can be made heterogeneous: different aluminosilicates and metal oxides depending on the type of processing and its composition demonstrate the properties of the acid or base.