Papers are made from the cellulose, a complex compound made of a highly reduced biomolecule called glucose. In the presence of fire paper burns within minutes whereas when books are kept for years they start turning brown. Do you know that both the processes are related to activation energy of the reaction and chemically involve the decomposition of the paper by the oxidation?
Why pages of books require years to oxidise and turn brown whereas in the presence of fire it oxidises completely within minutes? As we mentioned in the article, ‘Why learn chemical reactions?’ any chemical reaction involves the breaking of the existing chemical bonds and making of the new bonds between the atoms and molecules.
At the end of reaction, all the products formed settle at the lowest energy state possible by releasing all the energy they can give off. This energy is lost in multiple forms like light, heat, electrical potential etc. and also energy can be released in more than one forms in a single reaction.
Activation energy and instability of chemical bonds
In the process of chemical reaction, most of the bonds in stable compound have already given off maximum energy. If the product of one reaction has to be a reactant of the other reaction to produce new compound, it is essential to break the bonds formed earlier. This is achieved by the energy supplied by external sources. It can be an absorption of energy from the surrounding or from the manual supply of the energy by heating or radiation.
Heat is the most common form of the energy that is absorbed by the bonds in the reacting molecules. Heating increases the kinetic energy of the molecules ultimately escalating molecular vibrations, bending and stretching of the bonds. At this state bonds in the reactants are highly unstable and compounds are highly reactive. Increased kinetic energy of the molecules, increases the probability of random collisions between the participating reactants, ultimately increasing the conversion rate of the reactants to products.
Thus it is clear that in the beginning, EVERY REACTION needs an input of certain amount of energy for the initiation of the reaction. Hence that certain amount of energy required for a particular reaction is called as an ACTIVATION ENERGY. And the state at which the bonds have highest amount of energy, are highly unstable and prone to breakage is called as TRANSITION STATE of the reaction.
Activation energy and reaction rate
As we discussed, paper can be burnt in the presence of the fire within minutes whereas paper from books needs years for natural oxidation. Fire provides immediate and ample amount of energy to overcome the barrier of activation energy and the paper undergoes quick oxidation. The papers from the book need more time to acquire energy from the surrounding to undergo spontaneous oxidation, requiring more time for the same process.
The activation energy of any reactions tells an important information about the rate of the reaction. The rate of reaction is a speed at which the reactants are converted to the end product. It is pretty obvious that, for any spontaneous reaction with higher Ea, more time will be required to overcome the energy barrier spontaneously. Which is why reactions with higher Ea tend to be slower and reactions with lower Ea tend to be faster. Consider the oxidation of the wood (high Ea) and the oxidation of the petrol and gas (low Ea).
Activation energy and catalysis
In the living systems there are millions of chemical reactions going on in the cells per minute. Each biological reaction has different Ea that produce end products at different rates. In order to maintain the phenomenon of homeostasis, cells are required to synthesise all the molecules at the required time and rate. Thus, it is must to speed up the all the biological reactions at remarkably higher rates to meet the required rates. This process of speeding up of the chemical reactions is called as catalysis. Enzymes are the most important bio-catalysts in the biological systems and they can speed up the reactions by a factor of 10^5 to 10^14.
Many times protein complexes use the energy released by one reaction to catalyse other reactions. This process is called as energy coupling. By using the energy coupling cells produce one of the most important molecule for life, ATP. In the next article we will be discussing how the catalysis aids in overcoming the barrier of activation energy and increase the rate of reaction. The process is facilitated by special agents called as catalysts.