Temperature reaction spontaneity, effect

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

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

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. 

A reaction has AH = 98 kJ and A5 = 292 J/K. Investigate the spontaneity of the reaction at room temperature. Would increasing the temperature have any effect on the spontaneity of the reaction ... 

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,...

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

SECTIONS 19.6 AND 19.7 The values of AH and AS generally do not vary much with temperature. Therefore, the dependence of AG with temperature is governed mainly by the value of T in the expression AG = AH — TAS. The entropy term —TAS has the greater effect on the temperature dependence of AG and, hence, on the spontaneity of the process. For example, a process for which AH > 0 and As > 0, such as the melting of ice, can be nonspontaneous (AG > 0) at low temperatures and spontaneous (AG < 0) at higher temperatures. Under nonstandard conditions AG is related to AG° and the value of the reaction quotient, Q AG = AG" + RT In Q. At equilibrium (AG = 0, Q = K), AG = —RT InkT. Thus, the standard free-energy change is directly related to the equilibrium constant for the reaction. This relationship expresses the temperature dependence of equilibrium constants. 

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... 

Discuss foe effect of temperature change on foe spontaneity of foe following reactions at 1 atm. 

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