Chemical Kinetics : Study of fast Reactions

 

Study of fast reactions

Rates of chemical reactions differ form very slow to very fast. The rates of moderate speed reactions lying in between these two extreme reactions. However, rates of some instantaneous reactions are so fast that they occur within 10–12 second or in even less time. For example,

Neutralization reactions have half life of 10–10 sec; 

H+(aq) + OH–(aq) → H2O(l)

Photosynthesis has half life of ; 

6CO2 + 6H2O \frac { chlorophyll }{ hv }   C6H12O6 + 6O2

Some precipitation reactions have very short half life; 

AgNO3 + KCl → AgCl↓ + KNO3

Isomerisation of retinal in vision has half life of 10–12 sec

Rates of such reactions cannot be studied by ordinary methods because change in concentration cannot be measured during this short interval of time. However, modern techniques such as flow methods, relaxation methods, flash photolysis, laser technique and spectrophotometric methods are used to study such fast reactions.

(1)     Photosynthesis in plants : Plants obtain their food for growth by the combination of CO2 and H2O in presence of chlorophyll and light (a fast reaction) which leads to the preparation of carbohydrate and this phenomenon is known as photosynthesis. The studies on photosynthesis involve flash photolysis technique. The following mechanism has been proposed for photosynthesis. First step of the reaction mechanism involves the excitation of chlorophyll molecule by absorbing photon of red light. The excited chlorophyll molecule transfers its energy in the form of an electron to nearby reactant molecule A within  second.  The reactant molecule which accepts this energy is known as electron acceptor.

Chlorophyll\quad \underrightarrow { \quad hv\quad } \quad \underset { Excited\quad molecule }{ Chlorophyll }

Chlorophyll + A → \underset { A\quad is\quad CO_{ 2 }or\quad H_{ 2 }O }{ Chlorophyll } +A^{ - }+Energy

The electron acceptor (A) transfer this electron to another electron acceptor molecule (B). A–

The process leads to release of energy which is used for the series of reaction to yield the synthesis of energy rich molecule of carbohydrates from CO2 and water.

6CO2 + 6H2O \underrightarrow { \quad Energy\quad }   C6H12O6 + 6O2

(2)     Isomerisation of retinal in vision : The mechanism involves two steps

(i)      The retinal molecule (a light sensitive molecule present in the retina of eye) gets excited on exposure to light and undergoes geometrical isomerization and the energy absorbed is stored as chemical energy. The process takes place within 10–12 sec.

(ii)     As soon as the first step gets completed, the retinal is converted back into its original form within 10–12 sec and the energy released is used to send signals to the brain which ultimately causes the sensation of vision.

 

Differences between Photochemical and Thermochemical reactions

Photochemical reactions

Thermochemical reactions

These reactions are initiated by light radiation.

These reactions are initiated by heat energy.

They cannot occur in dark.

They do occur in dark.

Temperature does not have any significant effect on the rates of such reactions and temperature coefficient is low.

The temperature does have a marked effect on the rates of these reactions and temperature coefficient is generally high.
The value of ΔG may be +ve or – ve. ΔG is – ve for such reactions. 

 

Important tips

  • Chemiluminiscence : It is the emission of light in chemical reaction at ordinary temperature e.g., the light emitted by glow worms (fire flies) is due to the oxidation of a protein Luciferin present in them.
  • Fluorescence and Phosphorescence : There are some substances which when exposed to light or radiation, absorbs light and immediately start re-emitting the energy. The glow continue as long as the source of light is there. Such substances are called fluorescent substances and phenomenon as fluorescence e.g., fluorite (CaF2) fluorescein dye etc.

On the other hand, there are some substances which continue to glow for some time even after the source of light is cut off. Such substances are called phosphorescent substances and phenomenon as phosphorescence e.g., Zinc sulphide and sulphides of other alkaline earth metals.

  • Bioluminiscence : Certain living organisms emit light and show the phenomenon of chemiluminiscence. It is known as Bioluminiscence e.g., light emission in fire flies.
  • Example of fourth order reaction, 4KClO3 ⇌ 3KClO4 + KCl
  • Grothus-Draper law : When light falls on a substance, a part of light is absorbed, a part is reflected and a part is transmitted. Only that part of light which is absorbed causes a particular reaction to occur.
  • Stark’s Einstein law of photochemical equivalence : According to this law, every atom or molecule taking part in a photochemical reaction absorbs only one quantum of radiation.(E = hv). The energy absorbed by one mole of reacting molecules is known as one Einstein.
  • Free energy (ΔG) for thermochemical reactions is always negative but remember, ΔG for photochemical reactions may not always be negative. It is because a part of the light energy absorbed by the reactants is converted into free energy of the products. In the following photochemical reactions for which is positive and still they are spontaneous
  • Ozanisation of oxygen (b) Synthesis of carbohydrates (c) Decomposition of HCl to H2 and Cl2
  • There are some substances which when added to a reaction mixture helps to start the photochemical reaction but do not undergo any chemical change are called photosensitizer and this process is called photosensitization. A photosensitizer simply acts as a carrier of energy. For example (i) Dissociation of H2 in the presence of mercury vapours. (ii) Photosynthesis in presence of chlorophyll.