Reaction spontaneity effect

Effect of Temperature, Pressure, and Concentration on Reaction Spontaneity... 

EFFECT OF TEMPERATURE. PRESSURE. AND CONCENTRATION ON REACTION SPONTANEITY... 

For many reactions entropy effects are small and it is the enthalpy that mainly determines whether the reaction can take place spontaneously. However, in certain... 

In a catalysed system, the spontaneous reaction may also occur, but in many cases it is insignificant compared to the catalysed rate of reaction. The effect of a catalyst is to accelerate the rate of both the forward and reverse reaction, allowing equilibrium to be reached much more quickly. The concentration of the reactants is still important, as this affects the probability that the catalyst will interact with the reactants and trigger the reaction. 

The net cell reaction is an equilibrium reaction. The cell voltage directly relates to the spontaneity of a reaction. The effect of concentration on cell voltages can easily be explained by Le ChMelier s principle. Let us consider the following reaction ... 

Let us give the general definition of chemical induction, which reflects its specific features. At chemical induction, chemical reaction spontaneously proceeding in the system performs the effective work for another, non-spontaneous chemical reaction in the same system to proceed, which may not run without the first one. 

Ducry and Roberge reported controlled nitration of phenol in a glass microreactor with a channel width of 500 pm and an internal volume of 2.0 ml [2]. Nitration was most efficient and controlled under nearly solvent-free conditions at 20 °C without the addition of sulfuric acid or acetic acid (Scheme 4.2). Under these concentrated conditions, autocatalysis spontaneously started in the mixing zone, allowing safe control of the reaction. Undesirable polymer formation, which is significant in batch reactions, was effectively suppressed by a factor of 10. 

In an opposite sense to these reactions, pressure effects occur in systems where hot molecules or hot radicals are produced . Here quenching of the hot species may be followed in competition with spontaneous decomposition or isomerisation of the hot species (A ). 

You may be wondering whether entropy has an effect on reaction spontaneity. Recall that the law of disorder states that the entropy of the universe must increase as a result of a spontaneous reaction or process. Therefore, the following is true for any spontaneous process. 

The influence of spontaneity Recall from Chapter 16 that reaction spontaneity is related to change in free energy, AG. If AG is negative, the reaction is spontaneous under the conditions specified. If AG is positive, the reaction is not spontaneous. Let s now consider whether spontaneity has any effect on reaction rates. Are more spontaneous reactions faster than less spontaneous ones ... 

As you saw in Sample Problem 20.4, one way to calculate AG is from enthalpy and entropy changes. Because AH and AS usually change little with temperature if no phase changes occur, we can use their values at 298 K to examine the effect of temperature on AG and, thus, on reaction spontaneity. 

Figure 20.10 The effect of temperature on reaction spontaneity. The two terms that make up AG are plotted against T. The figure shows a relatively constant AH and a steadily increasing TAS (and thus more negative -TAS) for the reaction between CU2O and C. At low T, the reaction Is nonspontaneous (AG > 0) because the positive AH term has a greater magnitude than the negative TAS term. At 352 K,...

At this time, an even more streamlined approached to oxazinone 1 was investigated. A modified Strecker reaction between 4-fluoro-benzaldehyde and A-benzylethanolamine (NBEA) afforded aminonitrile 16, which was extracted into IPAC (Scheme 7). The subsequent Pinner reaction was effected by addition of HCl (g) to this solution. Imidate hydrochloride 17 HC1 was formed and residual water caused spontaneous hydrolysis to lactone hydrochloride l-HCI. This slurry could be filtered to afford the crystalline lactone hydrochloride (containing ca. 15 wt% NH4CI) or alternatively and preferably it could be treated with aqueous KHCO3 to freebase the amine. The resulting IPAC stream fed directly into the crystallization-induced resolution procedure that had been established. 

FIGURE 3.6 Effect of temperature on reaction spontaneity. The two iines cross when the energy contribution becomes iess than the entropy contribution. 

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