Chemical Concept

Concept of a catalyst:

A catalyst is a substance that speeds up a chemical reaction, but remains unchanged at the end of the reaction and is not consumed by the reaction.

Species involved in any reaction must undergo a rearrangement of chemical bonds in order for the reaction to occur.

There are two steps in the chemical reaction, the slow step, which is also termed as the transition state, where it contains chemical species that is an intermediate between the reactants and the products, and the fast step, which is the determining step for the products formed.

Energy is required to form the transition state, and the energy is most commonly referred to the Activation Energy, or otherwise known as Ea. Reactants that has energy lower than Ea are not able to pass through the transition state to react and become products, hence the need for catalysts.

Catalysts are able to provide a different energy path with lower Ea for the reaction. Hence, a catalyst is able to speed up the chemical reaction by lowering the Ea. In any given time interval, the presence of a catalyst allows a greater portion of the reactant species to acquire sufficient energy to pass through the transition state and become products.

Figure 1

Figure 1 above shows the diagram of an uncatalysed chemical reaction. Only those particles represented by the area on the right of the Activation Energy will react when they collide. The great majority, represented by the area on the left do not have enough energy, and hence will not react and will simply bounce apart.

Figure 2

From Figure 2, it can be seen that with a lower activation energy of the catalysed reaction represented by the new activation energy, the probability of particles with that kinetic energy (KE) increases, which is equivalent to a greater portion of the reactant species acquiring sufficient energy to pass through the transition states to form products. This is shown by the total area on the right of the new activation energy, and this increased fraction of molecules leads to an increase in area, and hence allowing greater chances of a “fruitful” collision leading to product formation, thus speeding up the rate of reaction.

Another property of catalyst is that it is unable to shift the position of a chemical equilibrium, hence leading to both the forward and backward reaction being accelerated at the same time so that the equilibrium constant Keq is unchanged. However, by removing products from the reaction mixture as they form could lead to the overall rate of product formation to be increased in practice.

Catalyst in general can be classified as organic or inorganic or petrochemical. Organic catalysts are typically biological in nature (i.e Enzymes), whereas inorganic catalyst exist as small molecules.

An example of organic catalyst are enzymes. Enzymes are proteinaceous molecules that helps to break or form chemical bonds. Such examples of enzymes are kinases, isomerase and phosphatases, just to name a few. Kinases are enzymes that are responsible for the phosphorylation (or adding a phosphate group) of other biomolecules. This process is crucial for the survivability of a cell, and thus, life. These enzymes are produced inside a living cell, through the complex process of transcription of genes and translation of mRNA. Due to its biological nature, enzymes are sensitive to both pH and temperature.

On the other hand, inorganic catalyst are merely small molecules that promotes diverse reactions such as extraction, purification of ores or decomposition of molecules such as hydrogen peroxide. Hydrogen peroxide can be decomposed to oxygen and water with the aid of Manganese (IV) oixide or Iron (Fe3+).

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What is important to note in the rather complex equation above is that Iron is not used, but rather, simply aids in proceeding the chemical reaction.

Lastly, in the petrochemical industry, catalyst is used through catalytic cracking, which is the process of breaking larger hydrocarbon molecules into smaller and more useful bits. It is an example of heterogeneous catalysis, which refers to the catalyst being in a different phase from the reactants. The catalyst is recovered after the process and the cracked mixture is separated by cooling and further fractional distillation. The hydrocarbon molecules are broken up in a fairly random way to produce mixtures of smaller hydrocarbons of alkanes and alkenes and one example is the cracking of hydrogen carbon C15H32.

There may be many other ways in which this particular molecule might break up and this is only one of the many ways. The ethene and propene are important materials for making plastics or producing other organic chemicals and the octane is one of the molecules found in petrol (gasoline).

royalty-free-border-clipart-illustration-1056108

Catalyst in the OZONE problem:

The issue with the ozone layer is that the emission of CFCs speeds up the reaction, driving the conversion of ozone to oxygen. Since the nature of ozone is irreversible, that is, ozone decomposes to oxygen and oxygen could form ozone, a net equilibrium is established. CFCs, as catalyst drives the process forward, breaking ozone into oxygen and thus, accelerating the ‘depletion’ observed.

The catalyst in CFCs in question here is Chlorine.

Chemical equation
CFCl3 + UV Light ==> CFCl2 + Cl
Cl + O3 ==> ClO + O2
ClO + O ==> Cl + O2
The free chlorine atom is then free to attack another ozone molecule
Cl + O3 ==> ClO + O2
ClO + O ==> Cl + O2
and again …
Cl + O3 ==> ClO + O2
ClO + O ==> Cl + O2
and again… for thousands of time

In the equation above, chlorine is guilty for the breaking down of ozone into oxygen. The chlorine excited from CFCl3 reacts with ozone in the atmosphere producing hypochlorite (ClO) and oxygen. This hypochlorite would then react with free oxygen atoms producing oxygen molecules, releasing the chlorine atom. This chlorine atom is thus free to repeat the cycle. Since the chlorine atom is not used up in the reaction, it is said to be a catalyst.

 

 

(References from:
http://www.merriam-webster.com/dictionary/chain%20reaction
http://www.chemguide.co.uk/physical/basicrates/catalyst.html
http://www.job-stiftung.de/pdf/versuche/H2O2_Decomposition.pdf
http://www.majordifferences.com/2013/09/difference-between-enzymes-and.html#.VQhILSlGyFI
http://www.chemguide.co.uk/physical/catalysis/petrochem.html#top)