Entropy, S, is defined as the measure of the disorder or randomness in a system.
The more “mixed up” or “disorderly” or “messy” a system becomes, we say the entropy of the system increases.
4 Factors that affects the Entropy of a chemical system:
1) Change in Phase (Physical State)
Sgas >> Sliquid > Ssolid
A solid has the lowest entropy because their particles are closely packed together and held in regular pattern. Thus, it’s structure is highly ordered.
A liquid has higher entropy than solid because its particles are arranged in an irregular pattern, although the particles are still quite close together.
The entropy of a gas is much greater than that of a liquid because gaseous particles are very far apart from each other and can move freely at great speeds. They are not constrained to be close to each other.
2) Change in Temperature
As temperature increases, the entropy of a system also increases.
As temperature increases, the particles undergo greater vibration (in solids) and more rapid movement (in liquids and gases). This causes more disorderliness. As such, entropy increases.
3) Change in Number of Particles (especially for Gases)
As the number of moles of particles in a system increases, it causes more disorderliness and thus entropy increases.
4) Mixing of Particles
Mixing always leads to increase in disorderliness i.e. entropy increases.
When you dissolve a solute into a solvent to form a solution (a mixture), the entropy increases since the solute particles are now randomly arranged in between the solvent particles.
When you mixed 2 gases together, diffusion will occur randomly between the gases and their orderliness is reduced i.e. entropy increases.
I am sure you learned something useful today regarding how changes in temperature, phase, number of moles of particles and mixing can cause changes in the entropy of a system.
Applications of Entropy:
Next, we are going to look at the application side of how entropy is commonly discussed in everyday Chemistry.
To predict whether the entropy is increasing or decreasing in a particular system, let’s look into a few examples below:
A) Ice at 0 oC –> Liquid Water at 0 oC
Entropy (i.e. randomness) of the system increases because as ice melts to form liquid water, the regular crystalline structure of ice is broken and water molecules move freely in the liquid state i.e. system becomes more random, less orderly.
B) Liquid Water at 25 oC –> Liquid Water at 30 oC
Entropy (i.e. randomness) of the system increases because when temperature is increased, water molecules can now move more freely in the liquid and orderliness is reduced.
C) Na+(g) + Cl–(g) –> NaCl(s)
Entropy (i.e. randomness) of the system decreases because the crystalline structure of NaCl(s) formed is highly ordered and very regular i.e. system becomes more ordered, less random.
D) H2(g) at 3 atm –> H2(g) at 1 atm
Entropy (i.e. randomness) of the system increases when the pressure decreases from 3 atm to 1 atm. This is because H2 molecules are free to move in a larger volume [remember that Volume is inversely proportional to Pressure] and the system becomes less orderly.
GCE A-Level H2 Chemistry Exam-Based Questions on Entropy:
Which of the following substance has a greater entropy in each pair and explain your choice.
i) Diamond or Graphite
Reason: Although both diamond and graphite are solids and have a giant molecular structure, graphite has a less ordered structure.
ii) 1 mole of NaCl(s) or 1 mole of HCl(g) at 25 oC
Answer: 1 mole of HCl(g)
Reason: Gases are more disordered than solids.
iii) 2 moles of HCl(g) or 1 mole of HCl(g) at 25 oC
Answer: 2 moles of HCl(g)
Reason: 2 moles of HCl(g) has twice the entropy (randomness) to that of a sample containing 1 mole of HCl(g).
iv) 1 mole of HCl(g) or 1 mole of Ar(g) at 25 oC
Answer: 1 mole of HCl(g)
Reason: HCl molecules have more ways of displaying itself by rotating in different directions. Argon is a monoatomic gas and exists as a spherical particle.
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Do stay tuned to the upcoming posts as i will be discussing on the applications of Gibbs Free Energy, and how Enthalpy Changes and Entropy are all inter-related when it comes to Thermodynamics.
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