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The Second Law Of Thermodynamics

We learned how to predict the mode of a reaction in terms of whether the reaction will be endothermic or exothermic. In this section, we will explore the dynamics and the factors that influence the spontaneity of reactions. For that, we have to be familiar with the term "entropy."
Entropy (S) is the measure of how disordered a system is. The entropy is dependent on the prevailing conditions such as pressure and temperature. From the MCAT point view, the values of entropy and related aspects that you will be dealing with are at standard state. The standard state refers to 1 atm and 25oC.

Entropy of a system is the measure of disorder or randomness of the system.


Let's talk more about entropy. The higher the disorder is, the greater the entropy of that system. In terms of the phases of matter, think about which phases will have a higher entropy. We can generalize that the gas phase of a substance has higher entropy than its liquid phase, and the liquid phase has higher entropy than its corresponding solid phase. Thus, as the temperature increases the entropy increases. The change in entropy (ΔS) is equal to the final entropy minus the initial entropy.
ΔΣ = Sfinal Sinitial

According to the second law of thermodynamics, there is an overall natural tendency toward disorder. You have to understand that we are talking about the overall entropy. Do not confuse this with the fact that we sometimes look at individual systems or reactions and categorize them as having either increasing entropy or decreasing entropy. But the overall entropy of all the events together is increasing.


Example 10-2

Predict whether the entropy is increasing or decreasing for the following change.

image\14817 ch 10.png



The question asks whether the entropy of the system is increasing or decreasing for the above written equation. Your response should be that there is an increase in entropy. Why? As we discussed, the change in phase is the key aspect to watch here. Carbon dioxide is changing from its solid form to gaseous form. Hence, the randomness of the system is increasing. So the entropy is increasing.


Example 10-3
Predict whether the entropy of the system is increasing as the reaction given below goes to completion.

image\29544 ch 10 ex 10-3.png



The reaction shown above is the combustion reaction of acetylene. This reaction is made use of in welding, commonly called the "acetylene torch." Tremendous amount of heat is liberated as a result of this reaction and is used for welding purposes. As you can see from the equation, the number of moles of the reactants is greater than the number of moles of the products. Thus, as the reaction proceeds, there is a decrease in the number of moles of gas. If you predicted a decrease in entropy, your answer is correct.

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