Conditions That Affect the Rate of a Chemical Reaction

4 Conditions That Affect the Rate of a Chemical Reaction 

4 State and explain the relationship between reaction rate and temperature. 

5 Using an energy-reaction graph, explain how a catalyst affects reaction rate. 

Driving from one city to another over rural roads and through towns takes a certain amount of time. If an interstate highway were built between the cities, you would 

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When the conditions that affect the rate of a chemical reaction are changed in a system at equilibrium, the rates of the forward and the reverse reaction may be affected differently. If these rates become different, more reactants or products are produced. The direction of the shift of an equilibrium can be predicted qualitatively using LeChatelier s principle. LeChatelier s principle states that if a stress is applied to a system at equilibrium, the equilibrium will tend to shift in a direction to relieve that stress. 

The Collision Theory of Chemical Reactions Energy Changes During a Molecular Collision Conditions That Affect the Rate of a Chemical Reaction The Development of a Chemical Equilibrium Le Chatelier s Principle The Equilibrium Constant The Significance of the Value of K... 

However, some sets of reactants can undergo both a forward and a reverse reaction under the same set of conditions. This circumstance leads to a state called chemical equilibrium. Before we take up equilibrium, however, we have to learn about the factors that affect the rate of a chemical reaction. 

Heat is a form of energy that affects the rate at which chemical reactions take place. The shelf life of a product can therefore be considerably reduced by exposure to elevated temperatures. Odor deterioration and discoloration will develop much faster under these conditions, and this is exploited in the accelerated testing of perfumes for stability in final products. Some products are manufactured at quite high temperatures, and perfumers have to keep this in mind in the formulation of their fragrances. 

For reactions other than simple decompositions to occur, particles must come into contact in a favorable orientation and with enough energy for activation. Thus, the rate of a reaction depends on the collision frequency of the reactants and on the collision efficiency. Any change in reaction conditions that affects the collision frequency, the coUision efficiency, or the collision energy affects the reaction rate. At least five important factors influence the rate of a chemical reaction. 

For rate processes in which the Arrhenius parameters are independent of reaction conditions, it may be possible to interpret the magnitudes of A and ii, to provide insights into the chemical step that controls the reaction rates. However, for a number of reversible dissociations (such as CaCOj, Ca(OH)2, LijSO Hp, etc.) compensation behaviour has been foimd in the pattern of kinetic data measured for the same reaction proceeding under different experimental conditions. These observations have been ascribed to the influence of procedural variables such as sample masses, pressure, particle sizes, etc., that affect the ease of heat transfer in the sample and the release of volatile products. The various measured values of A and cannot then be associated with a particular rate controlling step. Galwey and Brown [52] point out that few studies have been specifically directed towards studying compensation phenomena. However, many instances of compensation behaviour have been recognized as empirical correlations applicable to kinetic data... 

In the last seaion, we found that for a reaction between two particles to form products, the particles must collide with enough energy to reach the activated complex and with an orientation that allows the new bond or bonds to form as the old bond or bonds break. Any factor that affects these conditions will also affect the rate of the reaction. Let s look again at the chemical reaction between oxygen atoms and ozone molecules. 

At the end of this chapter on gas-liquid reaction accompanied by a rise in temperature vMch may be great enough to affect the rate of gas absorption substantially, it may be observed that fundamental background formulations have been developped these few last years ai this must be completed now by experim tal studies. As in the case of the important work in isothermal conditions (more directed on kinetic scheme) that has also been developped recently and that we have not presented here, the suggested ccmi-planentary work necessitates of course the simultaneous knowledge or determination of the physico-chemical parameters such as the solubility, mass and thermal diffusivity... and of the interfacial mass transfer parameters. Let us see now diat has been done recently on that topics. 

The nonequivalence of the rates of photosensitized reactions in heterogeneous nanophases of glassy polymers is proved in experiments with naphthalene phosphorescence decay. For example, it is shown [13] that in aerated PMMA films fluorescence of singlet-excited naphthalene molecules N can be decayed by tinuvin P, but this does not affect the rate of naphthalene dissociation. The latter is consumed in the process, the rate of which is not defined by the concentration of particles responsible for fluorescence. Under such conditions, according to definition by the authors [13], primary chemical acts are inevitable. However, in the absence of oxygen tinuvin P slows the photochemical process down in accordance with a decrease of singlet-excited naphthalene molecule concentration. 

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