Endothermic reactions, equilibrium

The reaction is endothermic and the equilibrium favors ethylene at low temperatures but shifts to favor acetylene above 1150°C Indeed at very high temperatures most hydro carbons even methane are converted to acetylene Acetylene has value not only by itself but IS also the starting material from which higher alkynes are prepared... 

STRATEGY Raising the temperature of an equilibrium mixture will tend to shift its composition in the endothermic direction of the reaction. A positive reaction enthalpy indicates that the reaction is endothermic in the forward direction. A negative reaction enthalpy indicates that the reaction is endothermic in the reverse direction. To find the standard reaction enthalpy, use the standard enthalpies of formation given in Appendix 2A. 

Because the formation of SO, is exothermic, the reverse reaction is endothermic. Hence, raising the temperature of the equilibrium mixture favors the decomposition of SO, to S02 and 02 as a consequence, the pressures of S02 and 02 will increase and that of SO, will decrease. 

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. 

Suppose this reaction is occurring in a CSTR of fixed volume and throughput. It is desired to find the reaction temperature that maximizes the yield of product B. Suppose Ef > Ef, as is normally the case when the forward reaction is endothermic. Then the forward reaction is favored by increasing temperature. The equilibrium shifts in the desirable direction, and the reaction rate increases. The best temperature is the highest possible temperature and there is no interior optimum. 

In a system at chemical equilibrium, there are always two opposing reactions, one endothermic and the other exothermic. 

These reactors operate near equilibrium, and therefore the first reactor must be heated to high temperatures because this reaction is endothermic, and the second must be cooled to fairly low temperatures. The kinetics of these reactions are very important if one is designing a reactor in detail, but the major features of the process are governed by equilibrium limitations and heat effects. 

E) The sign of AH° is positive, therefore the reaction is endothermic (heat is a reactant). Lowering the temperature will shift the equilibrium in the direction that produces heat (to the left). Adding N2 will not shift equilibrium because N2 is neither a prodnct nor reactant. All other choices will cause the equilibrium to shift to the right. 

The final stress to be considered is a change in temperature. To apply Le Chate-lier s Principle with temperature changes, the sign of AH for the reaction needs to be known. The AH in our example is = +131 kilojoules. This indicates that the forward reaction is endothermic and the reverse reaction is exothermic. When the temperature of a system at equilibrium is increased, the equilibrium will favor the endothermic reaction. One way to think of the effect of temperature is to think of energy as a reactant or product. This is seen when the forward and reverse reactions are written as two separate reactions ... 

The forward reaction is endothermic, and the reverse reaction is exothermic, according to the Le Chatelier s principle. If we change the temperature of the system, it will shift in a way that will decrease the effect of the change. If the temperature of the system is raised, the equilibrium will proceed to the right (products) to decrease the temperature, according to the Le Chatelier s principle. If the reaction mixture is cooled down, the equilibrium will shift to the left (reactants) to increase the temperature. 

In the above reaction, the forward reaction is exothermic, and the reverse reaction is endothermic. If the system is heated (the temperature raised), the equilibrium will shift to the left to counteract the effect of the change. 

For each reaction in a surface chemistry mechanism, one must provide a temperature dependent reaction probability or a rate constant for the reaction in both the forward and reverse directions. (The user may specify that a reaction is irreversible or has no temperature dependence, which are special cases of the general statement above.) To simulate the heat consumption or release at a surface due to heterogeneous reactions, the (temperature-dependent) endothermicity or exothermicity of each reaction must also be provided. In developing a surface reaction mechanism, one may choose to specify independently the forward and reverse rate constants for each reaction. An alternative would be to specify the change in free energy (as a function of temperature) for each reaction, and compute the reverse rate constant via the reaction equilibrium constant. 

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. 

We see that the reaction is endothermic, in keeping with the fact that the equilibrium constant increases with increasing temperature... 

Reactions (2) and (3) are reversible tile concentrations of the five components CH, H20, C02, CO and H2 which result are governed by thermodynamic equilibrium. Raising the reaction temperature shifts the equilibrium for both reactions to the right. Thus at low temperatures the exothermic reaction (]) predominates, while at high temperatures the overall reaction is endothermic, At approximately 500-550°C the reacdon is thermally neutral,... 

At room temperature this reaction is endothermic with an equilibrium constant of about 10 22. At 300° conversions of 20%-50% per pass can be realized and, by recycling the unreacted alcohol, the yield can be greater than 90%. 

Our expression for K in terms of rate constants helps to explain one application of Le Chatelier s principle (Section 9.10). According to Le Chatelier, an increase in temperature shifts the equilibrium composition in the endothermic direction. We can now see why. If the forward reaction is endothermic, the activation energy will be higher for the forward direction than for the reverse direction (Fig. 13.28). The higher activation energy means that the rate constant of the forward reaction depends more strongly on temperature than does the rate constant of the reverse reaction. Therefore, when the temperature is raised, the rate constant for the forward reaction increases more than that of the reverse reaction. As a result, K will increase and the products will become more favored, just as Le Chatelier s principle predicts. 

Ealy, Jr., "Effect of Temperature Change on Equilibrium Cobalt Complex" Chemical Demonstrations, A Sourcebook for Teachers, Vol. 1 (American Chemical Society, Washington, DC, 1988), p. 60-61. Concentrated hydrochloric acid is added to pink [Co(H20)5]2+ until blue [C0CI4]2- is formed. When heated the solution turns darker blue when cooled the solution turns pink, indicating that the reaction is endothermic. Students are asked to examine the equilibrium reaction and predict how the system will shift upon the addition of water. 

An increase in temperature favours the absorption of heat (endothermic) and thus shifts the reaction equilibrium to the right. 

The design of chemical reactors encompasses at least three fields of chemical engineering thermodynamics, kinetics, and heat transfer. For example, if a reaction is run in a typical batch reactor, a simple mixing vessel, what is the maximum conversion expected This is a thermodynamic question answered with knowledge of chemical equilibrium. Also, we might like to know how long the reaction should proceed to achieve a desired conversion. This is a kinetic question. We must know not only the stoichiometry of the reaction but also the rates of the forward and the reverse reactions. We might also wish to know how much heat must be transferred to or from the reactor to maintain isothermal conditions. This is a heat transfer problem in combination with a thermodynamic problem. We must know whether the reaction is endothermic or exothermic. 

You will observe a change in the color depending on whether the equilibrium was established at room temperature or at 100°C (in boiling water). From the color change, you should be able to tell whether the reaction was endothermic or exothermic. 

The local reactor temperature affects the rates of reaction, equilibrium conversion, and catalyst deactivation. As such, the local temperature has to be controlled to maximize reaction rate and to minimize deactivation. In the case of an exothermic (endothermic) reaction, higher (lower) local temperatures can cause suboptimal local concentrations. Heat will have to be removed (added) to maintain more uniform temperature conditions. The mode of heat removal (addition) will depend on the application and on the required heat-transfer rate. 

Reaction (1) is the primary reforming reaction and is endothermic. Reaction (2) is the water-gas shift reaction and is exothermic. Both these reactions are limited by thermodynamic equilibrium. The overall reaction is endothermic and hence requires that additional fuel be combusted to supply heat. The conventional steam reformer is a fired furnace containing catalyst-filled tubes. The hydrocarbon and steam mixture is processed in the catalyst-filled tubes while external burners heat the tubes. Nickel supported on a ceramic matrix is the most common steam reforming catalyst. 

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