Consider the hypothetical reaction A+B-->C at 25 degrees C, for which changeS=+1
ID: 522367 • Letter: C
Question
Consider the hypothetical reaction A+B-->C at 25 degrees C, for which changeS=+108 J/K and changeH=+1500 kJ.
A) Entropy change is favorable for this reaction at 25 degrees C-true,
B)Enthalpy change is favorable for this reaction at 25 degrees C- false
C)This reaction will be spontaneous below a certain temperature- false
D)This reaction can be expected to have high energy of activation-false
E)as temperature is decreased, activation energy for this reaction will increase and as a result it will become nonspontaneous- false
I know the answers, but if someone could please explain theoretically why these are the answers, that would be great. I'm pretty certain I'm correct, but I would like to make sure I have the reasons right.
Explanation / Answer
The given reaction is
A + B -----> C
The reaction takes place at 25C, i.e, T = 298 K.
We know that
G = H – T*S
Given H = +1500 kJ and S = +108 J/K, find G at T = 298 K.
G = (+1500 kJ) – (298 K)*(+108 J/K) = (+1500 kJ) – (32184 J) = (+1500 kJ) – (32184 J)*(1 kJ/1000 J) = (+1500 kJ) – (32.184 kJ) = 1467.816 kJ
Since G > 0, the reaction is unfavorable at 25C.
a)The entropy change is positive, hence the entropy change is favorable for the reaction (ans).
b) A positive enthalpy change is unfavorable for a reaction, hence second statement is false (ans).
c) The reaction will be spontaneous (favorable) only when G < 0. The condition for G = 0 is
T = H/S = (+1500 kJ)/(+108 J/K) = (+1500 kJ)*(1000 J/1 kJ)/(+108 J/K) = 13888.9 K.
The reaction will be spontaneous only above the temperature deduced above, around 14000 K. Thus, the third statement is false (ans).
d) The activation energy is given by
Ea = H + R*T
Since H is highly positive and T is positive, hence Ea will have a high positive value. Therefore, the 4th. statement is true (ans).
e) As temperature is decreased, Ea decreases. Moreover, G becomes more positive and hence the reaction becomes more non-spontaneous. Therefore, the statement is partly correct or false at best.