Van’t-Hoff-Le Chatelier principle

The rate of the forward reaction in (i.e. the number of C molecules formed per unit of time) is favored by a small volume since the probability of reactants A and B meeting is higher. However, the backward reaction is determined by the rate of dissociation of C, and is independent of volume. According to the Van t-Hoff-Le Chatelier principle, it is thus possible to shift the equilibrium concentrations of A, B, and C by changing the container volume. The formation of C is favored by a small volume, and vice versa. If the volume increase or decrease is made at a constant rate, dV/dt — fey, the concentration of A, B, and C change over time as ... 

Both thermodynamic and stoichiometric considerations are involved in maximising the formation of SO3. To decide how to optimise the equilibrium, consider the van T Hoff-Le Chatelier Principle, which states that when an equilibrium system is subjected to stress, the system will tend to adjust itself in such a way to partly relieve the stress. The stresses are in the form of an increase or decrease in the temperature, pressure or concentration of a product or reactant. In all catalytic reactions, the function of the catalyst is to increase the rate of the reaction. 

Increasing the system pressure will force the system to form more product SO3, since it occupies less volume and hence will result in a lower pressure (van t Hoff - Le Chatelier Principle). Super-atmospheric plants are not suitable for sulphonation applications. 

The principle has been enunciated, more especially in connexion with chemical reactions, by van t Hoff, under the name of the Principle of Mobile Equilibrium, and by Le Chatelier, as the Principle of Reaction. 

What does this equation tell us Suppose that the reaction is endothermic, then AH° is positive. If T2 > T, then 1/T2 < 1/T, and the term in braces is also positive. Therefore, In (K2/K,) is positive, which implies that K,/K, > 1 and therefore that K2 > K,. In other words, an increase in temperature favors the formation of product if the reaction is endothermic. We predict the opposite effect for an exothermic reaction because AHr° is then negative. Therefore, the van t Hoff equation accounts for Le Chatelier s principle for the effect of temperature on an equilibrium. 

Words that can be used as topics in essays 5% rale buffer common ion effect equilibrium expression equivalence point Henderson-Hasselbalch equation heterogeneous equilibria homogeneous equilibria indicator ion product, P Ka Kb Kc Keq KP Ksp Kw law of mass action Le Chatelier s principle limiting reactant method of successive approximation net ionic equation percent dissociation pH P Ka P Kb pOH reaction quotient, Q reciprocal rule rule of multiple equilibria solubility spectator ions strong acid strong base van t Hoff equation weak acid weak base... 

From (8.34c) or (8.35), it is easy to see that if the chosen reaction is endothermic (AH° > 0), then a T increase tends to promote product formation (the reaction shifts right ). Conversely, if the reaction is exothermic (AH° < 0), a temperature increase promotes formation of reactants (the equilibrium shifts left ). Such conclusions appear intuitive from the perspective of Le Chatelier s principle, and indeed we shall show in Section 8.6 that such Le Chatelier-like conclusions arise from deep theoretical roots that permeate the Van t Hoff equation and many other thermodynamic relationships. 

When gases dissolve in water without chemical reaction there is generally an evolution of heat. Hence by Le Chatelier s principle an increase in temperature usually leads to a decreased solubility. The effect of temperature on the absorption coefficient may be determined from an equation analogous to the van t Hoff equation ... 

Retention in chromatography is controlled by thermodynamic equilibria. The partition ofthe analyte between the mobile and the stationary phase is in control of the retention factor. This partition can be described by the laws of reversible thermodynamics. Therefore, we also borrow the thermodynamic description of the temperature dependence of equilibria. This is the so-called van t Hoff equation, which is the quantitative expression of the Le Chatelier principle. According to this, the temperature dependence of the retention factor k can be described by 2.9, with R being the general gas constant, AH° the molar enthalpy (heat tone) related to the transition of the analyte from mobile to stationary phase, AS° the molar entropy change for this transition, andj( the so-called phase ratio of the packed stationary phase in the column. 

Equilibrium constant dependence on temperature is different for exothermic and endothermic reactions, Van t Hoff called this conclusion mobile equilibrium, a principle that Le Chatelier generalized in the same period. From the equation (11) and the relation of K with the rate constants he obtained the phenomenological equation for the dependence of the rate constant k with temperature ... 

At about the same time as van t Hoffs book appeared, the Frenchman Henri Louis Le Chatelier (1850-1936) stated the principle named after him, that when a change is imposed on a system in dynamic equilibrium, the system will respond in such a way as to tend to reduce the imposed change. Van t Hoffs principle of mobile equilibrium was thus shown to be a special case of Le Chatelier s principle. 

Now the synthesis of many chemicals, particularly ammonia, is governed by chemical equilibrium considerations, and the effects of temperature, pressure and concentration upon the reaction are of paramount importance. In the past, many have sought to summarise the effect of temperature and pressure by application of Le Chatelier s deceptively simple (or, as some critics would say, simply deceptive) principle. A statement of the principle is given in the Glossary. A sound set of rules, based on Van t Hoffs principal laws of chemical... 

For an endothermic reaction, AH° is positive, whereas for an exothermic reaction, AH° is negative. The van t Hoff equation predicts that the AH° of a reaction defines the effect of temperature on the equilibrium constant. For an endothermic reaction, K increases as T increases for an exothermic reaction, K decreases as T increases. These predictions are in agreement with Le Chatelier s principle, which states that increasing the temperature of an equilibrium reaction mixture causes the reaction to proceed in the direction that absorbs heat. The van t Hoff equation is used for the determination of the AH° of a reaction by plotting InTT against /T. The slope of the resulting line corresponds to —AH°/R (Fig. 1.10). It is also possible to determine the AS° of the reaction from the y-intercept, which corresponds to AS°/R. It is important to reiterate that this treatment applies only for cases where the heat capacities of the reactants and products are equal and temperature independent. 

Use the van t Hoff equation to argue which side of a chemical reaction is favored as the temperature increases for (a) an exothermic process and (b) an endothermic process. Do the directions conform with Le Chatelier s principle ... 

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