Question

Examine the reaction: Ag2CO3(s)  Ag2O(s) + CO2(s) at standard conditions (for ease in the calculation, use 300 K as the standard temperature. a) Perform a calculation to determine if Ag2CO3(s) is thermodynamically stable (unreactive) under standard conditions. b) If it is stable, is there a temperature at which the reaction becomes unstable (a reaction takes place). Condition 1: a) ▲H° = 60.0 kJ and ▲S° = − 0.200 kJ/K Answer______________________ b) At what temperature, if any, does Ag2CO3(s) become unstable? Answer___________ Condition 2: a) ▲H° = − 60.0 kJ and ▲S° = − 0.200 kJ/K Answer______________________ b) At what temperatures, if any, does Ag2CO3(s) become unstable? Answer___________  

Answer 1

Given delta H ⁰ = 60 .0 kj

delta S ^{​​​​​​0 =} -0.200 kj /k

delta G ⁰= delta H ⁰- T delta S ⁰

delta G⁰ = 60kj - [ 300k ( - 0.200 kj/k) ]

delta G⁰ = 60 kj - (-60 kj) = 60+60 =120kj

delta G⁰ is positive , it means the forward reaction is not spontaneous. So, the reactant doesn't decompose.

Answer : under standard conditions AgCO₃ is thermodynacially stable

Given delta S⁰ = - 0.200 kj/k

delta H⁰= - 60 kj

delta S⁰ is negative means change entropy is less i. e. entropy is decreasing. delta H⁰ is negative, means the reaction is exothermic large amount of heat is released. Exothermic reactions take place at low temperature . Thus at low temperature even if entropy change is small i. e. The reaction is accompanied by decrease of entropy, the factor T delta S remains very small( negligible because both T and S are small so product Tdelta S is small), therefore , the value of delta G will have significantly large negative value. Thus, the reaction will be spontaneous in the forward reaction.

Answer : At low temperature (lower than 300K) delta G is negative, silver carbonate(Ag₂CO₃ ​​​​​​) decomposes to form silver oxide (Ag₂O) and carbon dioxide (CO₂).Thus, it is thermodynacially unstable at low temperature .

Low temperature delta H T delta S delta G

- ( negative) negligible - ( negative)

delta G = delta H - T delta S

delta G = negative - ( negligible)

delta G = negative