The Effect of Temperature on Reaction Spontaneity

In most cases, the enthalpy contribution (AH) to the free energy change (AG) is much larger than the entropy contribution TAS). For this reason, most exothermic reactions are spontaneous the negative AH helps make AG negative. However, the temperature of a reaction influences the magnitude of the TAS term, so, for many reactions, the overall spontaneity depends on the temperature. 

Let s examine the four combinations of positive and negative AH and AS two are independent of temperature and two are dependent on temperature  

Reaction is spontaneous at all temperatures AH 0, AS 0. Both contributions favor the spontaneity of the reaction. AH is negative and AS is positive, so —TAS is negative thus, AG is always negative. Most combustion reactions are in this category. The decomposition of hydrogen peroxide, a common disinfectant, is also spontaneous at all temperatures  

Reaction is spontaneous at higher temperatures AH 0 and AS 0. Here, AS favors spontaneity —TAS 0), but AH does not. For example, 

With a positive AH, the reaction will occur spontaneously only when —TAS is large enough to make AG negative, which will happen at higher temperatures. The oxidation of NjO occurs spontaneously at T 994 K. 

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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,...

The Eree Energy Change and the Work a System Can Do 671 The Effect of Temperature on Reaction Spontaneity 671 Coupling of Reactions to Drive a Nonspontaneous Change 674... 

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

The opposite situation holds for reactions that have negative values for A iiT ° and A S °. These reactions are spontaneous at low temperature because their release of heat disperses energy into the surroundings. The favorable AH ° dominates A G ° as long as T does not become too large, and the reaction is enthalpy-driven. At high temperature, however, the unfavorable A S ° dominates A G °, and the reaction is no longer spontaneous. The effects of temperature on spontaneity are summarized in Table 14-3. 

When considering the effect of temperature on equilibrium constant, the sign and magnitude of AH° are the determining factors. However, the spontaneity of a reaction is determined by the sign and magnitude of AS°, which determines the effect of temperature. 

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. 

Discuss the effect of temperature on the spontaneity of reactions with the following values for AH" and AS". 

Energy is an important component of most equilibrium systems. The input or output of energy in a system causes the temperature to change. Thus, the requirement that an equilibrium system be closed means that the temperature of the system must remain constant. In the next section, you will examine more closely the effects of thermodynamics on equilibrium systems. In particular, you will examine the factors that affect the amount of reactant and product in a reaction and the factors that determine whether or not a reaction is spontaneous. 

After reaction with the monomer to form a new propagating chain the position is formally the same as transfer with monomer. However, the two mechanisms can be distinguished kinetically if realkylation of the catalyst is slow compared with propagation. There is no direct evidence for this reaction although it is well established that the relatively stable alkyls of ms nesium and aluminium form metal hydride bonds on decomposition at elevated temperatures [83]. The existence of spontaneous termination has been deduced from a consideration of the kinetics, and by analogy with the effects of hydrogen on the polymerization. 

Derive the free energy change (AG) from the second law, and explain how AG is related to work explain why temperature (7) affects the spontaneity of some reactions but not others describe how a spontaneous change drives a nonspontaneous one calculate AG°xn from A// and S° values or from AGf values and quantify the effect of T on AG° obtain the T at which a reaction becomes spontaneous ( 20.3) (SPs 20.4-20.6) (EPs 20.35-20.50)... 

FIGURE 7.28 The effect of an increase in temperature on the spontaneity of a reaction under standard conditions. In each case, "spontaneous" is taken to mean AC0 < 0 and "nonspontaneous" is taken to mean AC° > 0. (a) An exothermic reaction with negative reaction entropy becomes spontaneous below the temperature marked by the dotted line, (b) An endothermic reaction with a positive reaction entropy becomes spontaneous above the temperature marked by the dotted line, (c) An endothermic reaction with negative reaction entropy is not spontaneous at any temperature, (d) An exothermic reaction with positive reaction entropy is spontaneous at all temperatures. 

Methyl nitrate is prepared by the effect of nitric acid on methanol in the presence of sulphuric acid. Operating conditions are critical, given the exothermicity of the reaction and the compound thermal instability (see previous table). Moreover, methyl nitrate is very sensitive to impact. It combusts spontaneously at 250-316°C when it is in the vapour state and even when the vapour is diluted by an inert gas. The flame temperature reaches 2600°C. 

Decomposition, Thermal of Explosives ond Propellants. Influence of Pressure and Temperature. It is important to keep in mind the distinction betw the effects of pressure and temperature on the beginning of thermal de-compn of expls and proplnts and on its progress once it is started. Often the effects of pressure and temp are similar as, for example, when hydrocarbon mixtures are subjected to high pressure and temp simultaneously. In some of these cases nonflammable gas mixts might become explosive, "mild reactions become violent and "stable gas mixtures become spontaneously reactive (Ref 10, p 143)... 

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