The Overall Change in Entropy

In an exothermic reaction, such as the synthesis of ammonia or a combustion reaction, the heat released by the reaction increases the disorder of the surroundings. In some cases, the entropy of the system may decrease, as when a gaseous reactant is converted into a solid or liquid. However, provided that AH is large and negative, the release of energy as heat into the surroundings increases their entropy so much that it dominates the overall change in entropy and the reaction is spontaneous (Fig. 7.18). 

Remember that AG is a measure of the overall change in entropy at constant temperature and pressure, AS is the change in entropy of the system and —AH/T is the change in entropy of the surroundings (Section 7.9). 

The overall change in entropy ASov is an algebraic sum of the partial components of elementary processes - propagation (ASp), decomposition of the initiator (ASa) and termination (ASt). Assuming typical kinetic equation, the following equation is valid ... 

Suppose that the temperature of a sample is increased from Tt to T2. The overall change in entropy is the sum (integral) of all these infinitesimal changes ... 

The second law according to Equation 8.11 simplifies to the equation AS = Sgen > 0. The process is possible if the overall change in entropy of the streams is positive and indeed it is, as follows from applying Equation 8.12. 

Hopefully, you will recall that we can express the overall change in entropy in terms of parameters describing changes within the system alone, using (for a fixed pressure and temperature) the Gibbs free energy, Equation 10-21 in the preceding section (AG = -TA AZf-TAS). 

Consider the reversible melting of 1 mol of ice in a large, isothermal water bath at 0°C and 1 atm pressure. The enthalpy of fusion of ice is 6.01 kj/mol. Calculate the entropy change in the system and in the surroundings, and the overall change in entropy of Ihe universe for this process. 

Determine the overall change in entropy for the following process using 1.00 mole of He ... 

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