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Entropy ­ Answers
1. In an endothermic reaction, the chemicals gain energy and the surroundings lose an equal quantity of energy. Clearly
the feasibility of process or change cannot solely depend on the enthalpy change and in order to explain further you
need to consider the concept of ENTROPY and some of its ramifications. The entropy of a system is a measure of the
number of ways a system can be arranged. This provides the energy for the endothermic reaction and can be
expressed by the equation:
Stotal = Ssystem + Ssurr
2. The second law of thermodynamics determines:
Whether a physical or chemical change is likely to happen at a particular temperature
Whether redox reactions will take place
The position of equilibrium
3. Entropy is the random dispersal of molecules and of energy quanta between molecules. The greater the disorder
the greater the entropy. If a system becomes more disorder S becomes positive.
4. The second law of thermodynamics states that spontaneous changes result in an increase in entropy. Entropy
increases when:
Solids melt
Liquids boil
Solids dissolve in water
The number of gas molecules increases
The temperature increase
5. The third law of thermodynamics states that the entropy of a perfect crystalline substance at absolute zero (0 K).
6. The entropy change of the system can be calculated from the formula:
Ssystem = nS(products) - nS(reactants)
7. As the crystalline substance is heated, it gains in entropy until its melting temperature is reached. On melting, there
is a large jump in entropy, followed by a steady increase as the liquid is heated to its boiling temperature. There is
another large jump in entropy as its physical state changes, followed by a gradual increase as the gas is heated.
8. The standard entropy of a substance depends on:
State ­ As the temperature increases, and the substance changes state, it becomes more disordered and
entropy increases
Complexity ­ As the complexity of a substance increases, entropy increases. E.g. the entropy of ethane is
greater than that of methane.
9. When an exothermic reaction takes place, heat energy is transferred to the surrounding air, causing an increase in
the disorder of the air molecules. As the temperature increases, the molecules have a greater range of energy, so
are more disordered. This tells us that:
Ssurr is positive for all exothermic reactions
Ssurr is negative for all endothermic reactions
The entropy change in the surroundings, caused by transfer of heat, depends on the value of the heat change, i.e. if
the surroundings are hot, the entropy increase is smaller than if its cold because the molecules have high entropy
and are already in chaotic motion. Therefore Ssurr is also inversely proportional to the temperature of the
Ssurr = -T
10. Changes are thermodynamically feasible if the total entropy change is positive:
Ssystem Ssurr Feasible
Positive Positive (exo) Always
Negative Negative (endo) Never
Negative Positive (exo) If the value of H/T> Ssystem (more likely at low temps)
Positive Negative (endo) If the value Ssystem >H/T (more likely at high temps)
If Stotal is positive, the change is said to be thermodynamically spontaneous, so the change will occur, providing
the kinetics are favourable. If Stotal is negative, the reverse reaction is thermodynamically spontaneous. The value
of Stotal can be altered by changing the temperature.

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Spontaneous endothermic reactions:
When solid ammonium carbonate is added to pure ethanoic acid, bubbles of gas are rapidly produced. This
appears to be a violent reaction, but the temperature actually falls during it. Even though the reaction is
endothermic, entropy of the system increases because a gas is produced. This makes Stotal positive and the
reaction thermodynamically stable
Hydrated barium hydroxide reacts with solid ammonium chloride in a rapid endothermic reaction at room
temperature.…read more


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