The entropy of the system increases (∆S > 0) as the gas molecules spread out in a larger volume. On the other hand, phases changes from solid to liquid or from liquid to gas also lead to an increased entropy of the system.
There are processes that lead to the decrease in the entropy of the system. Example of this is condensing a gas or freezing of liquid, this result in the increase in the order of the system and lead to the decrease of the entropy (∆S < 0). A reaction that leads to a decrease in the number of molecules generally leads to the decrease in entropy. Consider the example:
2NO(g) + O2(g) → 2NO2(g)
The entropy change decreases (∆S < 0) since three molecules of gas react to form two molecules of gas. The formation of N-O bonds imposes more order on the system, the fact that the atoms of the system are more tied up in the products than in the reactants that leads to the decrease in the entropy of the system. The formation of new bonds decreases the number of degrees of freedom, or forms of motions, available to the atoms ; that is; the atoms are less free to move in random motion because of the formation of new bonds. In other words, the greater the number of degrees of freedom of a system, the greater its entropy.
The degrees of freedom of molecules are associated with three different types of motion for the molecules:
1. Translational motion is a motion in which the entire molecule can move in one direction only, as in the movements of gas molecules. The molecules in a gas have more translational motion than those in the liquid, which in turn have more translational motion than the molecules in solid.
2. Vibrational motion is another motion that exist within molecule, in which molecules move periodically toward and away from one another.
3. Rotational motion is a motion of molecules as though they were spinning like tops.
The greater the energy that is stored in translational, vibrational, or rotational motion, the greater the entropy.
If the thermal energy of a system is decreased by lowering the temperature, the energy stored in translational, vibrational, and rotational forms of motion decreases. As less energy is stored, the entropy of the system decreases. If the temperature is keep lowered, there will come a time in which the motions are shutdown, a point of perfect order. This occurs when the temperature reaches the absolute zero ( 0 K). The entropy of a pure crystalline substance at absolute zero is zero, this is called the third law of thermodynamics.
But if the temperature is increased the reverse occurs. Entropy increases when the temperature is increased. Just like when we increase the temperature in a solid substance, the substance will soon melt and the atoms or molecules are free to move about the entire volume of the substance. The added degrees of freedom of the individual molecules greatly increase the entropy of the substance.
Generally entropy increases when temperature is increased. In other words, the entropies of the phases of a given substance follow the order Ssolid < Sliquid < Sgas.
In general, the entropy is expected to increase for processes in which:
1. Liquids or solutions are formed from solids.
2. Gases are formed either from solid or liquids.
3. The number of molecules of gas increases during a chemical reaction.
Below are the examples:
Choose the sample of matter that has greater entropy in each pair:
1. 1 mol of NaCl(s) or 1 mol of HCl(g) at 25oC
2. 2 mol of HCl(g) or 1 mol of HCl(g) at 25oC
3. 1 mol of HCl(g) or 1 mol of Ar(g) at 25oC
4. 1 mol of N2(s) at 24K or 1 mol of N2(g) at 298 K
Answer:
1. 1 mol of HCl since gaseous substance are more disordered than solid substance
2. 2 mol of HCl since it contains more molecules
3. 1 mol of HCl since it has capacity to stored more energy than Ar. The structure of HCl molecules can vibrate and rotate.
4. Gaseous nitrogen sample because it is more disordered than the solid nitrogen
The degrees of freedom of molecules are associated with three different types of motion for the molecules:
1. Translational motion is a motion in which the entire molecule can move in one direction only, as in the movements of gas molecules. The molecules in a gas have more translational motion than those in the liquid, which in turn have more translational motion than the molecules in solid.
2. Vibrational motion is another motion that exist within molecule, in which molecules move periodically toward and away from one another.
3. Rotational motion is a motion of molecules as though they were spinning like tops.
The greater the energy that is stored in translational, vibrational, or rotational motion, the greater the entropy.
If the thermal energy of a system is decreased by lowering the temperature, the energy stored in translational, vibrational, and rotational forms of motion decreases. As less energy is stored, the entropy of the system decreases. If the temperature is keep lowered, there will come a time in which the motions are shutdown, a point of perfect order. This occurs when the temperature reaches the absolute zero ( 0 K). The entropy of a pure crystalline substance at absolute zero is zero, this is called the third law of thermodynamics.
But if the temperature is increased the reverse occurs. Entropy increases when the temperature is increased. Just like when we increase the temperature in a solid substance, the substance will soon melt and the atoms or molecules are free to move about the entire volume of the substance. The added degrees of freedom of the individual molecules greatly increase the entropy of the substance.
Generally entropy increases when temperature is increased. In other words, the entropies of the phases of a given substance follow the order Ssolid < Sliquid < Sgas.
In general, the entropy is expected to increase for processes in which:
1. Liquids or solutions are formed from solids.
2. Gases are formed either from solid or liquids.
3. The number of molecules of gas increases during a chemical reaction.
Below are the examples:
Choose the sample of matter that has greater entropy in each pair:
1. 1 mol of NaCl(s) or 1 mol of HCl(g) at 25oC
2. 2 mol of HCl(g) or 1 mol of HCl(g) at 25oC
3. 1 mol of HCl(g) or 1 mol of Ar(g) at 25oC
4. 1 mol of N2(s) at 24K or 1 mol of N2(g) at 298 K
Answer:
1. 1 mol of HCl since gaseous substance are more disordered than solid substance
2. 2 mol of HCl since it contains more molecules
3. 1 mol of HCl since it has capacity to stored more energy than Ar. The structure of HCl molecules can vibrate and rotate.
4. Gaseous nitrogen sample because it is more disordered than the solid nitrogen
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