Question

Thermodynamics of KNO₃Solubility & Voltaic Cells

For each of the following processes, predict the sign (+ or - ) for ?H, ?S, and ?G. Explain your answers conceptually, not with calculations.

1. The decomposition of liquid water to give gaseous oxygen and hydrogen, a process that requires a considerable amount of energy.

2. Dynamite is a mixture of nitroglycerin, C₃H₅N₃O₉, and diatomaceous earth. The explosive decomposition of nitroglycerine gives gaseous products such as water, carbon dioxide, and others; much heat is evolved.

3. The combustion of gasoline in the engine of your car, as exemplified by the combustion of octane.      2 C₈H_18(g)+ 25 O_2(g)? 16 CO_2(g)+ 18 H₂O_(g)

Answer 1

Ans 1

decomposition of liquid water to give gaseous oxygen and hydrogen

2H2O(l) = 2H2(g) + O2(g)

a process that requires a considerable amount of energy .

2H2O(l) + energy = 2H2(g) + O2(g)

The energy is absorbed during the reaction

H > 0 or the sign of H is +

Liquid molecules is being converted into gaseous phase. Disorder of the system increases.

S > 0 or the sign of S is +

From the equation

G = H - TS

H > TS because of small S compared to H

G > 0 or the sign of G is +

Ans 2

decomposition of nitroglycerine

4C3H5(NO3)3(s) = 12CO2(g) + 10H2O (g) + 6N2(g) + O2(g)

heat is evolved

4C3H5(NO3)3(s) = 12CO2(g) + 10H2O (g) + 6N2(g) + O2(g) + Energy

The energy is evolved during the reaction

H < 0 or the sign of H is -

Solid molecules is being converted into gaseous phase. Disorder of the system increases.

S > 0 or the sign of S is +

From the equation

G = H - TS

H > TS because of small S compared to H

G < 0 or the sign of G is -

Ans 3

combustion of gasoline

2 C8H18(g)+ 25 O2(g) = 16 CO2(g)+ 18 H2O(g)

In the combustion reaction, energy is evolved

2 C8H18(g)+ 25 O2(g) = 16 CO2(g)+ 18 H2O(g) + Energy

The energy is evolved during the reaction

H < 0 or the sign of H is -

The phase is same for reactants and products.

Moles of products > moles of reactants

Disorder of the system increases.

S > 0 or the sign of S is +

From the equation

G = H - TS

H > TS because of small S compared to H

G < 0 or the sign of G is -