Question

1. For each of the following processes, give the algebraic sign of ÆHo, ÆSo, and ÆGo. No calculations are necessary; use your common sense.

a.The splitting of water to give oxygen and hydrogen gas. 


b. The combustion of octane. 


c.The explosion of dynamite 


Answer 1

We have to predict Ho, So and Go for each of the situations:

The Gibbs Free Energy is Go is related to entropy and enthalpy by following expression:

  Go = Ho - TSo

The sign of Go should be positive for a product favoured reaction and thus spontaneous reactions are having negative free energy of formation.

The sign of So shoul be positive for a entropy favoured reaction

The sign of Ho shoul be positive for a enthalpy favoured reaction

(a) The decomposition of liquid water to give gaseous oxygen and hydrogen.

   2H2O 2H2 + O2

In the decomposition process, energy is absorbed to break the Hydrogen bond and thus the reaction is endothermic in nature(Ho = negative). Through decomposition, the number of moles of gases increases.In gaseous state, particles are more free to move(solid have fixed positions for paticles) and so entropy increases (So = positive). And therefore, Go is negative.

(b) The combustion of octane:

Combustion of gasoline in the engine is basically combustion of octane.

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

For combustion process, heat is always liberated i.e an exothermic reaction (Ho =positive).

The combustion reaction is always spontaneous in nature (Go =negative).

Here, the number of moles in product is greater then that of reactant thus entropy also increases (So =positive).

(c) The explosion of dynamite.

Dynamite is a mixture of nitroglycerin, C3H5N3O9 and diatomaceous earth. The decomposition gives gaseous products such as water, CO2 and heat is evolved.

The explosion of dynamite is exothermic reaction as heat is liberated into the atmosphere (Ho =positive).

The explosion is non spontaneous in nature i.e they does not occur on there own (Go =positive)

As count of atoms increases, the entropy is said to be increased (So =positive)..