Thermodynamics

Thermodynamics: Spontaneity to Free Energy

Exploring the principles of thermodynamics and their application in determining spontaneity and free energy in chemical reactions.

Thermodynamics is the branch of physics that deals with the relationships between heat, work, temperature, and energy.
It is a macroscopic science, meaning that it does not consider the microscopic structure of matter. Instead, it focuses on the behavior of systems as a whole.

The four laws of thermodynamics are:

• First law of thermodynamics: The total energy of a system and its surroundings is always constant.

• Second law of thermodynamics: The total entropy of a system and its surroundings always increases.

• Third law of thermodynamics: The entropy of a system at absolute zero is zero.

• Zeroth law of thermodynamics: If two systems are each in thermal equilibrium with a third system, then they are in thermal equilibrium with each other.

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key concepts in thermodynamics:

• System: A system is any part of the universe that we are interested in studying. The rest of the universe is then considered to be the surroundings.

• State: The state of a system is described by a set of macroscopic variables, such as pressure, volume, temperature, and internal energy.

• Process: A process is a change in the state of a system.

• Heat: Heat is the transfer of energy between a system and its surroundings due to a temperature difference.

• Work: Work is the transfer of energy between a system and its surroundings due to a force acting through a distance.

• Internal energy: The internal energy of a system is the sum of all the kinetic and potential energies of the particles in the system.

• Entropy: Entropy is a measure of the disorder of a system

​Examples of how thermodynamics is used in the real world:

• Power plants: Power plants use thermodynamics to convert heat from a fuel source into electrical energy.

• Engines: Engines use thermodynamics to convert heat from a fuel source into mechanical energy.

• Refrigerators: Refrigerators use thermodynamics to remove heat from a food compartment and transfer it to the outside environment.

• Chemical reactions: Thermodynamics can be used to predict the spontaneity of chemical reactions and to calculate the amount of heat released or absorbed by a reaction.

• Biological processes: Thermodynamics can be used to study the energy flow through biological systems and to understand how they maintain homeostasis.

Spontaneous (Favorable) Processes

• A spontaneous process occurs naturally without outside forces
• Spontaneity is not related to the rate of a reaction
• If a reaction is spontaneous in one direction, it is nonspontaneous in the reverse direction
• Spontaneous reactions often have a decrease in energy
• Greater dispersal of matter and energy can drive a spontaneous process

Spontaneous (Favorable)Reactions

Spontaneous reactions occur naturally without outside forces. They can be slow or fast, and they can be reversible or irreversible. They do not require an increase in energy. Examples include rusting of iron and the combustion of gasoline.

​In chemistry, a spontaneous process is one that occurs without the need for external intervention. It proceeds naturally in the direction that favors an increase in the entropy of the system, which is a measure of disorder or randomness. The spontaneity of a chemical process is determined by the change in Gibbs free energy (ΔG) of the system.

Gibbs Free Energy (ΔG)

The Gibbs free energy is a thermodynamic potential that combines enthalpy (H) and entropy (S) to determine the spontaneity of a process at constant temperature and pressure. It is represented by the equation:

ΔG = ΔH - TΔS

where:

• ΔG is the change in Gibbs free energy

• ΔH is the change in enthalpy (heat content)

• T is the temperature in Kelvin

• ΔS is the change in entropy

If ΔG is negative, the process is spontaneous and will proceed in the forward direction. If ΔG is positive, the process is non-spontaneous and will not occur without external intervention. If ΔG is zero, the system is at equilibrium.

​Factors Affecting Spontaneity

The spontaneity of a chemical process is influenced by several factors:

1. Enthalpy Change (ΔH): Exothermic reactions (ΔH < 0) tend to be spontaneous as they release heat, while endothermic reactions (ΔH > 0) require energy input and are less likely to be spontaneous.

2. Entropy Change (ΔS): Processes that increase entropy (ΔS > 0) are generally spontaneous, as they favor a more disordered state. Processes that decrease entropy (ΔS < 0) are less likely to be spontaneous.

3. Temperature (T): Temperature plays a crucial role in determining spontaneity. At high temperatures, the TΔS term becomes more significant, making processes with positive entropy change more favorable.

​Examples of Spontaneous Processes

1. Ice melting at room temperature: This process is spontaneous as it increases entropy (solid to liquid) and is exothermic (ΔH < 0).

2. Salt dissolving in water: This process is spontaneous as it increases entropy (solid to dissolved ions) and is slightly exothermic (ΔH < 0).

3. Combustion reactions: These reactions are spontaneous as they release a large amount of heat (ΔH < 0) and increase entropy (more gaseous products).

​Non-spontaneous Processes

Non-spontaneous processes can occur if external energy is provided.

For example, electrolysis of water requires electrical energy to split water molecules into hydrogen and oxygen.

Entropy

Entropy is a measure of the disorder or randomness of a system. It is a fundamental concept in thermodynamics, and it has a wide range of applications in other fields, such as physics, chemistry, biology, and information theory.

One way to think about entropy is to imagine a room full of gas molecules. If all the molecules are moving in the same direction, the room is in a very ordered state and the entropy is low.

However, if the molecules are moving in all different directions, the room is in a more disordered state and the entropy is high.

Factors Influencing Entropy

Phase, temperature, and particle composition are key factors that influence entropy. Entropy is a measure of disorder or randomness in a system. It increases with an increase in temperature and the number of possible arrangements of particles in a given phase. Understanding these factors helps us comprehend the behavior of systems and their spontaneity.

Multiple Choice

The thermodynamic quantity that expresses the degree of disorder in a system is _

• entropy

• internal energy

• heat flow

• enthalpy

• bond energy

Multiple Choice

Which one of the following is always positive when a spontaneous process occurs?

• ΔH univ

• ΔH surr

• ΔS surr

• ΔS univ

• ΔS sys

Multiple Choice

ΔS will be positive for the reaction

• 2H2 (g) + O2 (g) -> 2H2O (g)

• 2NO2 (g) -> N2O4 (g)

• BaF2 (s) ->Ba2+ (aq) + 2F- (aq)

• 2Hg (l) + O2 (g) ->2HgO (s)

Multiple Choice

Which one of the following processes produces a decrease in the entropy of the system?

• dissolution of solid KCl in water

• mixing of two gases into one container

• freezing water to form ice

• melting ice to form water

Multiple Choice

Of the following, the entropy of gaseous __________ is the largest at 25eC and 1 atm.

• C2H2

• H2

• C2H6

• CH4

• C2H

Multiple Choice

What can be said about a chemical system that has reached a minimum in free energy?

• The system has achieved equilibrium.

• The reaction is complete.

• The system entropy is zero.

• The reaction is very fast.

Multiple Choice

A reaction that is not spontaneous at low temperature can become spontaneous at high temperature if ΔH is __________ and ΔS is __________.

• +, +

• -, -

• +, -

• -, +

• +, 0

Multiple Choice

For a reaction to be spontaneous under standard conditions at all temperatures, the signs of ΔH^Ο and ΔS^Ο must be __________ and __________, respectively.

• +, +

• +, -

• -, +

• -, -

• +, 0

Multiple Choice

In the Haber process, ammonia is synthesized from nitrogen and hydrogen: N2 (g) + 3H2 (g) ->2NH3 (g)

∆G at 298 K for this reaction is -33.3 kJ/mol.

The value of ∆G at 298 K for a reaction mixture that consists of 1.9 atm N2, 1.6 atm H2, and 0.65 atm NH3 is

• 3.86 x10³

• -1.8

• -40.5

• -7.25 x10³

• -104.5