For each of the following reactions, calculate Delta G degree_rxn Delta H_rxn de

Question

For each of the following reactions, calculate Delta G degree_rxn Delta H_rxn degree, and Delta_rxn degree a. Which of the above reactions is/are exothermic? How do you know? b. Which of the above reactions is / are spontaneous under standard conditions? How do you know? c. Which of the above reactions could be made spontaneous by increasing the temperature of the reaction? Temperature necessary to make the reaction spontaneous. d. Which of the above reactions will always be spontaneous, no matter what the temperature? Why?

Explanation / Answer

Q1

Reaciton 1:

N2 + O2 = 2NO

dH = 2*NO2 - (N2 + O2) = 2*90.4 = 180.8 kJ/mol

dS =  2*NO2 - (N2 + O2) = 2*210.5 - (191.6 + 205) = 24.4 J/K

dG = dH - T*dS = 180.8 -298*24.4 /1000 = 173.5288 kJ/mol

a)

this reaction must be EXOTHERMIC, since it ABSORBS energy, and DH is positive

b)

This is NOT spontaneous as writte, since dG > 0, meaning that entropy of universe decreases

c)

If

dG = dH - T*dS

since dH is positive, and dS is positive, if we increase T then eventually

dH < T*dS

meaning that dG < 0, so this will be spontaneous at high T

d

Reactions that

dG = dH - T*dS <0

have a negativ enethalpy of reaction and positive entorpy of reaction will always be negative, i.e. always spontaneosu

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