Consider the following reaction at constant P. Use the information here to deter
ID: 507278 • Letter: C
Question
Consider the following reaction at constant P. Use the information here to determine the value of Ssurr at 355 K. Predict whether or not this reaction will be spontaneous at this temperature.
2NO(g) + O2(g) 2NO2(g) H = -114 kJ
Consider the following reaction at constant . Use the information here to determine the value of at 355 . Predict whether or not this reaction will be spontaneous at this temperature.
2(g) + (g) 2(g) = -114
Explanation / Answer
We have to know the value of G for the raction to determine whether the reaction is spontaneous or not. For a spontaneous reaction, the value of Gibb's energy change must be negative while it is positive for a non-spontaneous reaction. The relation for calculating Gibbs's energy change is:
G = H-TS
Now, first we have to calculate the value of S.
S(reaction) = S(product) - S(reactant)
We can look up the values of entropies from standard tables and these are:
NO = 210.7 J/mol/K
NO2 = 240.0 J/mol/K
O2 = 205.0 J/mol/K
Now turn to the equation: 2 NO(g) + O2(g) 2 NO2(g)
S = 2*240.0 - (2*210.7 + 205.0) = -146.4 J/mol/K
It can be seen that entropy change for the system is negative, so it must be positive for the surrondings, though it won't have the same value at 355 K as these values are for standard states (T=293 K). Nevertheless they give us a good estimation. Here's how we do it:
Stotal=Suniv= Ssys + Ssurr>0
so, Ssurr = +146.4 J/mol/K (for standard state)
Now look at each options of the question
Options A and B are absolutely incorrect as entropy change cannot be that big, i.e, in units of kJ/K. Entropy changes are always in units of J/K and from our work we know that Ssurr is positive so it cancels out the possibility of D and E being correct. So C is the correct answer