A spontaneous process is a physical or chemical change that occurs with no outside intervention. However, for many spontaneous processes, some energy must be supplied to start the reaction. Ex) When iron is left in moist air, it rusts slowly according to the following equation.
4Fe(s) + 3O2(g) à 2Fe2O3(s) ∆H = ­1625 kJ
This is an __________, ___________ reaction. If exothermic spontaneous
you want to reverse the reaction to decompose the rust, the sign of the ∆H changes and the reaction becomes _____________. This reaction is _____
not endothermic
spontaneous and does not occur under ordinary conditions.
2Fe2O3(s) à 4Fe(s) + 3O2(g)
∆H = + 1625 kJ
Entropy
Entropy is a measure of the disorder or randomness of the particles that make up a system.
Because many exothermic processes occur spontaneously, scientists originally concluded that all exothermic processes were spontaneous and all endothermic processes were non­spontaneous. It was later discovered that ice melting at room temperature is a spontaneous endothermic process so something other than ∆H must influence whether a chemical process will be spontaneous. entropy
That something is called _____________________.
This tendency toward disorder or randomness is called the law of disorder which states that spontaneous processes always occur so that the entropy of the universe increases
It is represented by S. Think about how an air freshener scent spreads throughout an entire room. The particles tend to move from a low state of disorder (remaining in the same place) to a high state of disorder (spreading out)
This law is the second law of thermodynamics. Recall, the first law was the law of conservation of energy (energy cannot be created or destroyed, just converted from one form to another).
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Predicting the Changes in Entropy
Finding the change in entropy follows the same pattern as finding the change in enthalpy:
∆Ssystem = Sproducts ­ Sreactants
If Sproducts > Sreactants then ∆Ssystem is ____________. positive
This means the entropy ______________. increased
1. For changes of state, entropy increases from solid to liquid to gas. This is because solid particles are tightly packed, while liquids have more movement, and gases have unrestricted movement.
Solid < Liquid < Gas
∆Ssystem = +'ve
If Sproducts < Sreactants then ∆Ssystem is ____________. negative
This means the entropy ______________.
decreased
2. Dissolving a gas in a solvent always decreases the entropy. This is because dissolving a gas limits the particles’ movements and randomness.
CO2 (g) à CO2 (aq)
∆Ssystem = ­'ve
3. There is usually an increase in entropy when the number of gaseous particles in the products is greater than the number of gaseous particles in the reactants. More particles = more movement and randomness. (This is only if there are no changes of state.)
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2SO3(g) à 2SO2(g) + O2(g)
∆Ssystem = +'ve
4. Entropy usually increases when a solid or liquid is dissolved in a solution. This is because the solute particles become spread out throughout the solution thus, creating more randomness and disorder. (There are some exceptions to this rule.)
NaCl (s) à Na1+ (aq) + Cl1­ (aq)
5. Entropy increases with an increase in temperature. This is because an increase in temperature increases the kinetic energy of the substance which causes the molecules to move faster (more random motion).
Temperature
∆Ssystem = +'ve
∆Ssystem = +'ve
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Enthalpy, Entropy and Spontaneity
Recall that the law of disorder states that the increase
entropy of the universe must __________ as a result of a spontaneous reaction. Therefore, for any spontaneous process ∆Suniverse > 0.
In nature, ∆Suniverse tends to be positive for reactions under the following conditions:
1.The reaction is exothermic, which means ∆Hsystem is negative. 2. The entropy of the system increases, so ∆Ssystem is positive.
exothermic
Thus, ____________ chemical reactions increase in entropy
accompanied by an __________________
are ALL spontaneous.
Determining Spontaneity Using Gibbs Free Energy
Willard Gibbs, a physicist at Yale University, created a formula to calculate whether a reaction will be spontaneous or not. He called it Gibbs free energy (or Free energy is the energy available to free energy). do work. The free energy change, denoted ∆G
system is the difference between the change in enthalpy and the product of the Kelvin temperature and the change in entropy.
Example: Calculating Free Energy Change
How do changes in enthalpy and entropy affect free energy change and spontaneity for the following reaction?
N2 (g) + 3H2 (g) à 2NH3 (g)
Given, T = 298 K
H0system = ­91.8 kJ (convert to J first!) ∆S0system = ­197 J/K (NOTE: we can see from the reaction that S will decrease because there are 4 gaseous particles on the reactant side and only 2 on the product side)
∆G0system = ∆H0system ­ T∆S0system
The sign of the free energy change tells you whether or not a reaction is spontaneous at a constant specified temperature and pressure.
If ∆Gsystem is negative, the reaction is ____________ spontaneous
under the specific temperature and pressure. If ∆Gsystem is positive, the reaction is non­spontaneous
__________________ under the specific conditions.
∆G0system = ∆H0system ­ T∆S0system
∆G0system = (­91800 J) ­ (298K)(­197 J/K)
∆G0system = ­33094 J
Even though entropy decreased, ∆G0system is still negative. Therefore, the above reaction is ________________ under standard conditions!
spontaneous
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How ∆H0system and ∆S0system Affect Reaction Spontaneity
­ ∆H0system
+ ∆S0system
+ ∆H0system
ALWAYS Spontaneity depends spontaneous upon temperature
­ ∆S
0
Spontaneity depends upon temperature
system
NEVER spontaneous Did You Know That?
Many of the chemical reactions that enable plants and animals to live and grow are non­spontaneous (∆G0system is positive). In living systems, non­
spontaneous reactions often occur in conjunction with other reactions that are spontaneous. Reactions of this type that occur together are called coupled reactions
______________________. In these reactions, free energy released by one or more spontaneous reactions is used to drive a non­spontaneous reaction.
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