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A spontaneous process is the time-evolution of a system in which it releases free energy (usually as heat) and moves to a lower, more thermodynamically stable energy state. The sign convention of changes in free energy follows the general convention for thermodynamic measurements, in which a release of free energy from the system corresponds to a negative change in free energy, but a positive change for the surroundings.

A spontaneous process is capable of proceeding in a given direction, as written or described, without needing to be driven by an outside source of energy. The term is used to refer to macro processes in which entropy increases; such as a smell diffusing in a room, ice melting in lukewarm water, salt dissolving in water, and iron rusting.

The laws of thermodynamics govern the direction of a spontaneous process, ensuring that if a sufficiently large number of individual interactions (like atoms colliding) are involved then the direction will always be in the direction of increased entropy (since entropy increase is a statistical phenomenon).

Entropy is a chemical concept that is very difficult to explain, because a one-sentence definition will not lead to a comprehensive statement. Thus, few people understand what entropy really is. You are not alone if you have some difficulty with this concept.

The word entropy is used in many other places and for many other aspects. We confine our discussion to thermodynamics (science dealing with heat and changes) and to chemical and physical processes.

We have defined energy as the driving force for changes; entropy is also a driving force for physical and chemical changes (reactions). Entropy, symbol S, is related to energy, but it a different aspect of energy. This concept was developed over a long period of time. Human experienced chemical and physical changes that cannot be explained by energy alone. A different concept is required to explain spontaneous changes such as the expansion of a gas into an available empty space (vacuum) and heat transfer from a hot body into a cold body. These changes cause an increase in entropy for the system under consideration, but energy is not transferred into or out of the system.

Traditionally, the entropy concept is associated with the second and third laws of thermodynamics. Entropy is related to the energy distribution of energy states of a collection of molecules, and this aspect is usually discussed in statistical mechanics.

Second Law of Thermodynamics
When a system receives an amount of energy q at a constant temperature, T, the entropy increase S is defined by the following equation.

S = q / T.

Entropy is the amount of energy transferred divided by the temperature at which the process takes place. Thus, entropy has the units of energy unit per Kelvin, J K-1. If the process takes place over a range of temperature, the quantity can be evaluated by adding bits of entropies at various temperatures. This sum can take the form of integration if the temperature various continuously. You have learned the concept of integration in a calculus course.

Third Law of Thermodynamics
By definition, the change in entropy can be evaluated by measuring the amount of energy transferred. Entropy contained in a system, say in a mole of a pure substance, is a theoretical quantity that takes account of all heat transferred to it since the lowest attainable temperature, 0 K. At absolute zero Kelvin, the substance contains no removable energy. A substance is in a completely ordered crystalline state, at which the molecules contains no removable vibrational, rotational, translational, or even thermal disorder energy.

As energy q is absorbed by a substance, its temperature increases by T. If the heat capacity is C, then

q = C T

Gibb’s Free Energy Change, G

Gibb's Free Energy change, G, allows us to decide if a reaction is spontaneous. G has the advantage that it can be calculated entirely from measurements on the reacting system without regard to the surroundings. The equation is:

G= H - T Ssys

The equation also shows that G is dependent on temperature, so a chemical reaction that is not spontaneous at one temperature might be spontaneous at another.

A negative value for G indicates that the reaction is spontaneous.G values can be used in Hess's Law calculations just like H values.

1. It allows us to decide if a reaction is spontaneous. 2. It is a chemical concept that is defined as a driving force for physical and chemical changes (reactions). 3. What law states that when a system receives an amount of energy q at a constant temperature, T, the entropy increases S? 4. Entropy, _________, is related to energy, but it a different aspect of energy. 5. A __________ is the time-evolution of a system in which it releases free energy (usually as heat) and moves to a lower, more thermodynamically stable energy state. 6. A ___________ for G indicates that the reaction is spontaneous.G values can be used in Hess's Law calculations just like H values. 7. Traditionally, the entropy concept is associated with the ________ and ________ laws of thermodynamics. 8. ____________ is the amount of energy transferred divided by the temperature at which the process takes place. 9. A substance is in a completely ordered _________, at which the molecules contains no removable vibrational, rotational, translational, or even thermal disorder energy. 10. Entropy is related to the _____________ of energy states of a collection of molecules, and this aspect is usually discussed in statistical mechanics.

Activity #1
Why do some reactions take place of their own accord while others do not?
There is a natural tendency for physical and chemical systems to become more disordered…

Think of your room at home. It has just been tidied for you and is very orderly. You may get up in a morning without making your bed. You read books and magazines leaving them open at various pages on your bed and across the floor. You change your clothes taking garments from your wardrobe but leaving those you have worn strewn about the room.
Day by day the disorder gets worse. The repetition that existed amongst your books and magazine on shelves and your clothes hanging in a wardrobe is being lost. This tendency towards disorder is a natural one. There are many more ways books, magazines and clothes can be placed randomly than arranged in an orderly pattern. So, disorder is favored. And, work must be done to restore order!
Activity #2
Why do some reactions take place of their own accord while others do not?

Let's be clear about something:
A spontaneous reaction is one that can 'possibly' take place, as opposed to a nonspontaneous process that cannot occur. However, a spontaneous process may not actually occur, or may be so slow that we think it does not occur. A spontaneous process is one that is thermodynamically feasible (energetically feasible) but may or may not be kinetically feasible. Thermodynamics tells us what cannot happen, but never guarantees that anything will happen.
Can we use change in Enthalpy, H, to predict whether reactions will go of their own accord (occur spontaneously)? The answer is 'No', as we are familiar with both exothermic and endothermic reactions that take place readily in the laboratory.
Our reasoning with regard to Entropy gives us the idea that chemical reactions occur spontaneously if S is calculated to be a positive value. But...
Try allowing some hydrogen chloride and ammonia gases to mix. Immediately, a finely powdered white solid (ammonium chloride, looking like white fumes), is formed.
The reaction has occurred spontaneously, though clearly there is a decrease in entropy when a solid product is formed from two gaseous reactants.
NH3(g) + HCl(g) NH4Cl(s) Ssys = -284 J K-1 mol-1
Explanation #1
Order is characterized by repetition; disorder by a lack of patterns, an absence of organization, by randomness and chaos.
The degree of disorder or randomness of a substance is measured by its entropy. The greater the disorder, the higher the entropy. The symbol for entropy, for no obvious reason, is S. Entropy is a property of a system. As with enthalpy, we consider changes in entropy:
S = Sfinal - Sinitial
S = Sproducts - Sreactants
S depends only on initial and final states - it is not dependent on the path taken by the change to the system. For an increase in entropy S has a positive value; for a decrease in entropy S has a negative value.
An entropy value (unlike an enthalpy value) can be calculated for an element or compound...
Like enthalpy, H, the entropy of a substance depends upon conditions, such as temperature, pressure and the amount. Standard conditions are 298K, 100000 Pa (Nm-2), and 1 mole. Some Standard Molar Entropy values, Sare given below: Substance | S (J K-1 mol-1) | Carbon (graphite) (s) | 5.7 | Carbon (diamond) (s) | 2.4 | Carbon dioxide (g) | 213.6 | Ammonia (g) | 192.0 | Hydrogen chloride (g) | 187.0 | Ammonium chloride (s) | 95.0 | Sodium chloride (s) | 72.1 | Helium (g) | 126.0 | Argon (g) | 154.7 | Ethanol (l) | 160.7 | Water (l) | 69.9 | Water (g) | 188.7 |

Explanation #2
Here we have considered the entropy change involving only the reactants and product, that is, only of the system itself. In this case, S should be written as Ssys. The reaction is exothermic (H° = -176 kJ mol-1), and the energy given out increases the entropy of the surroundings, Ssurr. (Standard conditions will not be shown throughout.)
We need to consider the total entropy change, Stotal,
Stotal = Ssys + Ssurr to decide if a reaction is spontaneous.
It can be shown (look up Trouton's Rule) that
Ssurr = -H / T
(This calculates a positive value for Ssurr as the reaction is exothermic.)
Ssurr = - - 176000/298 = + 590 J K-1 mol-1
Stotal = -284 + +590 = +306J K-1 mol-1
Thus, the reaction between HCl and NH3 is spontaneous because the total entropy change has a positive value.
The second law of thermodynamics looks mathematically simple but it has so many subtle and complex implications that it makes most chemistry majors sweat a lot before (and after) they graduate. Fortunately it’s practical, down- to- earth applications are easy and crystal clear. We can build on those to get to very sophisticated conclusions about the behavior of material substances and objects in our lives.

Definition of Spontaneity

• Definition: A process is spontaneous if it occurs when it is left to itself in a universe. A process is non- spontaneous if it requires action from outside a universe to occur.
• Consequences:
1. If a process is spontaneous, the reverse one is non- spontaneous.

2. A non- spontaneous process can occur if it is coupled with another process that is spontaneous. Example: If we add to the universe a pump powered by an electrical motor and a battery then gas flowing to a full reservoir becomes a part of overall spontaneous process in the universe that includes: (i) gas flowing to a full reservoir, (ii) battery discharge, and (iii) heating of surroundings.

What Do We Need to Know to Find Out If a Process Is Spontaneous?
•According to the 2nd Law of Thermodynamics, in order to determine whether the process is spontaneous or not we need to find out the sign of Suniv = Ssys + Ssur. For this we need to know the values Ssys and Ssur.

Ssur can be found easily due to its definition: Ssur = qsur/Tsur, in an experiment where a system is placed in a calorimeter (large one to keep Tsur= const)

•Finding Ssys is trickier but possible using the second part of the 2nd Law: In reversible processes, Suniv does not change:
Suniv = Ssys + Ssur = 0

Trouton’s Rule:
For many liquids at their normal boiling points, S°sys vap 80 to 90 J mol-1 K-1

The rational for this will be better to obtain from the microscopic definition of entropy: the entropy change is dominated by moving molecules from a semi- ordered liquid state to a gas state. Different molecules behave similarly in the same phase.

Substance | Η° vap, kJmol-1 | Tbp, K | S°vap, Jmol-1K-1 | Methane | 9.27 | 111.75 | 83.0 | Carbon tetrachloride | 30 | 349.85 | 85.8 | Cyclohexane | 30.1 | 353.85 | 85.1 | Benzene | 30.8 | 353.25 | 87.2 | Hydrogene Sulphide | 18.8 | 213.55 | 88.0 | Water | 40.7 | 373.15 | 109 |

Hydrogen bonds of water in liquid make it much better “structured” than molecules of other liquids.

Free Energy (Gibbs Function, G)
We can multiply both parts of the last expression by T:
T Suniv = TS - H = - (H -TS)
Definition:Free energy function is:
G = H - TS
Free energy change is:
G = -TSuniv = H - TS

G connects the change of entropy in the universe,
Suniv, with thermodynamic parameters of the system only,
H, S, and T.

1. Free Energy Change 2. Entropy 3. Second Law of Thermodynamics 4. Symbol S 5. Spontaneous Process 6. Negative value 7. Second and third 8. Entropy 9. Crystalline state 10. Energy distribution http://entropy.pdf

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