Number of effective collisions

The temperature dependence of a rate is often described by the temperature dependence of the rate constant, k. This dependence is often represented by the Arrhenius equation, /c = Aexp(- a/i T). For some reactions, the temperature relationship is instead written fc = AT" exp(- a/RT). The A term is the frequency factor for the reaction, which reflects the number of effective collisions producing a reaction. a is known as the activation energy for the reaction, and is a measure of the amount of energy input required to start a reaction (see also Benson, 1960 Moore and Pearson, 1981). 

Decreasing the temperature would decrease the number of effective collisions between CO( ) and 02(g). Decreasing the pressure (accomplished by either increasing the volume or decreasing the number of molecules) would also favor decreased effective collisions. Note that this problem was NOT at equilibrium. 

The rate of a reaction is proportional to the effective collisions in a unit of time. If the number of effective collisions increases, so does the rate of reaction. 

The nonbonding electron cloud of the attached fluorine atoms would tend to repel some of the incident fluorine molecules as they approach the carbon skeleton. This reduces the number of effective collisions, making it possible to increase the total number of collisions and still not accelerate the reaction rate as the reaction proceeds toward completion. This sheath of fluorine atoms is one of the reasons for the inertness of Teflon and other fluorocarbons and also explains the greater success commonly reported in the literature when the hydrocarbon to be fluorinated is partially fluorinated in advance by some other process or is prechlorinated. 

Concentration. In most reactions, the rate increases when the concentration of either or both reactants is increased. This is understandable on the basis of the collision theory. If we double the concentration of one reactant, it will collide in each second twice as many times with the second reactant as before. Since the rate of reaction depends on the number of effective collisions per second, the rate is doubled (Fig. 20.2). 

As the frequency and the number of effective collisions between reacting particles increases, the rate of the reaction... 

If the attacking species X has a partial pressure [X] the number of effective collisions with the particle surface in unit time will be kA[X],... 

A chemical reaction i the result of collisions of sufficient energy and proper orientation. The rate of reaction, therefore, must be the rate at which these effective collisions occur, the number of effective collisions, let us say, that occur during each second within each cc of reaction space. We can then express the rate as the product of three factors. (The number expressing the probability th at a... 

Changes in concentration What happens if the industrial chemist injects additional carbon monoxide into the reaction vessel, raising the concentration of carbon monoxide from 0.30000M to 1. OOOOOM The higher carbon monoxide concentration immediately increases the number of effective collisions between CO and H2 molecules and unbalances the equilibrium. The rate of the forward reaction increases, as indicated by the longer arrow to the right. 

The number of effective collisions increases dramatically because the fraction of molecules (calculated using the Boltzmann distribution) with energy greater than the activation energy is greater. 

It is believed that proximity and orientation are critical to the way enzymes accelerate reactions. The key is to increase the number of effective collisions between reactive partners. Not only are the reactive partners undergoing a huge number of collisions within the active site of the enzyme, but the active site is also orienting them correctly. The slow step for some enzymes is just getting the reactants into the active site, the diffusion-controlled limit. 

A catalyst is a substance that speeds up a reaction and can be recovered unchanged. An enzyme catalyst increases reaction rates by the dramatic amplification of the number of effective collisions between the reactants within the enzyme s active site. Enzymes increase the rate of the chemical step by using binding to stabilize the transition state more than the reactants or products. 

Ring Strain versus the Number of Effective Collisions, The General Order is 5 > 6 > 3 > 7 > 4-Membered Transition States... 

Highly Increased Number of Effective Collisions Active Site Preorganization of Solvation and General Acid/Base Catalysts Avoiding High-Energy Intermediates Electrostatic Catalysis by Metal Ions Covalent Catalysis by Enzyme-Bound Electrophiles and Nucleophiles Coupling ATP Hydrolysis to Drive Equilibria... 

Understand collision theory (why concentrations are multiplied, how temperature affects the fraction of collisions exceeding a, and how rate depends on the number of effective collisions) and transition state theory (how is used to form the transition state and how a reaction energy diagram depicts the progress of a reaction) ( 16.6) (SP 16.7) (EPs 16.41-16.52)... 

When reaction rates calculated using collision theory are compared to the experimental rates, the agreement is usually poor. In some cases, the agreement is within a factor of 2 or 3, but in other cases the calculated and experimental rates differ by 10 to 10. The discrepancy is usually explained in terms of the number of effective collisions, which is only a fraction of the total collisions owing to steric requirements. The idea here is that in order for molecules to react, (1) collision must occur, (2) the... 

The number of effective collisions taking place between the reacting molecules in a given period of time. The greater the number of collisions, the faster the reaction. 

The Effect of Molecular Structure on Rate At ordinary temperatures, the enormous number of collisions per second between reactant particles is reduced by six or more orders of magnitude by counting only those with enough energy to react. And, even this tiny fraction of all colhsions is typically much larger than the number of effective collisions, those that actually lead to product because the atoms that become bonded in the product make contact. Thus, to be effective, a collision must have enough energy and the appropriate molecular orientation. 

Direct measurements of activation energies for these reactions are sparse. Consequently, to find the approximate dependence of the reactivity (i.e. of the number of effective collisions and of the activation energy) on the molecular structure of RCl in the Na + RCl system, the pre-exponential factors A in the Arrhenius equation have been assumed to be A = 5 10 cm mol- s"i for all reactions. The activation energies are then deduced from the measured rate constants and from A (see [511]). 

An increase in temperature increases the average kinetic energy of the particles in a substance this can result in a greater number of effective collisions when the substance is allowed to react with another substance. 

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