Hack tech fact

Are the molecularity and reaction order different for elementary reactions?    It may be stated once again that the total number of molecules or atoms which participate in a reaction as represented by the chemical equations is known as the molecularity of the reaction. The sum of the power to which the concentrations are raised in the rate law is termed as the order of a reaction. Let us consider an elementary reaction of the type.        2A + B --> Products.          Since the rate of a reactio n is proportional to number of collisions between the atoms or molecules of the reactants, and the number of collisions is proportional to the concentration of each reactant species, the rate of the above reaction                alfa [A²] [B²].          rate = k [A²] [B]  Molecularity of reaction =       2 + 1 = 3    It shows that the molecularity and order of an elementary reaction are the same.

Molecularity of a reaction:   It may be defined as the number of reacting species (atoms, ions or molecules) that collide simultaneously to form the product in the rate determining step of a chemical reaction.       Now, we have two types of chemical reaction, namely: (1) Elementary reactions, android (2) Complex reactions.

Difference between rate of reaction and rate constant:   1- Rate of reaction (r) = Rate constant (k)×(conc), when concentration is one molar, r = k. 2- Rate of reaction depends on the concentrations of reactants and the temperature whereas the rate constant does not depend on the temperature. 3- The rate of reaction at a particular instant is the instantaneous rate of change of concentration of any one of reactant or product at that instant. On the other hand the rate constant is a constant of proportionality, and it equals the rate of reaction when the molar concentration of the reactants is unity.

Units of rate:  We have the rate of reaction= Change in concentration of the reaction or product _____________________________               Time taken   Thus,  reaction rate has the units of concentration divided by time. Concentration in moles per litre (molar concentration, M =mol dm-³), and time may be in seconds, (s), minutes (min), hours (h), days (d) or possibly years (y).  Therefore, the units of reaction rates may be        mol dm-³ s-¹ or mols/ liter, sec or Ms-¹  mol dm-³ min-³ or moles/liter min, or M min-¹  mol dm-³h-¹ or moles/ liter- hour or Mh-¹ and so on  In gaseous reaction, pressure is normally used and thus rate is expressed in atm sec-¹ or atm mim-¹.

 Variation of reactant concentration with time:     As stated above, the rate of a reaction is measured by the rate of decrease in concentration of a reactant or the rate of increase in concentration of product with time. Thus, the rate at which the concentration of reacting substance changes with time is known as rate of reaction of velocity of reaction. This means that the concentration of a reactant decreases with time. Let us study now at what rate the concentration varies. Decomposition of N²O⁵ in gaseous phase or its solution in CCl⁴ takes place at a convenient rate as a in        2N²O⁵ --> 4NO² + O² 

 Modified collision theory:  Since the collision theory fails in a number of cases giving the rate of reaction values of various gaseous reaction, it is concluded that collisions between molecules cannot be the only factor, required in determining the rate of a reaction. So, Arrhenius modified the simple collision theory. According to this modified theory.  (I) The collisions between all molecules cause chemical reaction. (II) The collisions between the molecules which possess energy of activation cause the chemical reaction. It means that if the energy of the colliding molecules is less than the energy of activation, no reaction take place. If the energy is equal to or greater than energy of activation, reaction occurs Thus, eqn (iv), k=ze -Ea/RT  is modified, to account for the deviations from the collision theory, in the form k=pZ e -Ea/RT             Where p is the probability factor which is a measure of an increase or decrease of the speed of reactions. The value of p varies from

 Collision theory of unimolecular reactions:  This theory was proposed independently by max trautz in 1916 and William Lewis in 1918. This theory qualitatively explain how chemical reactions occur and why reaction rates differ for different reaction. According to this theory, the reacting molecules must collide to react as a result of which the chemical reaction occurs. But, in fact, not all collisions are effective. Only a certain percentage of total number of collisions produce any significant chemical reaction. In other words, successful (i.e., effective) collisions give rise to chemical reaction. It is just like that a few number of crackers burst with sound and hence effective while others are ineffective producing no sound. The effective collisions between the reacting molecules causing any chemical change depends on two conditions (e.g., postulates) :   (I) the colliding molecules must have enough energy, known as activation energy to cause a reaction, and. (II) the reacting mol

Effect of      temperature     reaction rate:     As discussed in sec 4.1.6, the rates of chemical reactions increase with increase in temperature. The rate constant becomes almost doubled for a rise in temperature of 10°C, so the ratio of rate constants of reaction at two different temperature differing by 10°C is termed as temperature coefficient, i.e,                                               Temperature coefficient =               k ( t+10 )               ------------- is 2 to 3.                   Kt         This shows that as an approximate rule, the rate constant becomes double for a rise in temperature of 10°C.

 Reaction of high molecularity are rare:   As discussed in the preceding section, the rate of a reaction is proportional to the number of collisions between the reactants molecules. With the increase in number of reactant molecules the chances of collisions between the molecules become less. It means that the number of collisions between 3 reacting molecules becomes less than that between two molecules. Thus, when a reaction involves more than 3 molecules the number of collisions decreases and hence the probability of the occurrence of the reaction become less. It is, therefore, the reaction of higher molecularity, i.e., greater than 3 are rare. In such cases, complex reaction occuring in various steps involving one, or two or at the best 3 molecules are considered.