Chemical reactions are the heart of many processes in nature and industry. Understanding their thermodynamics helps us predict if they'll happen on their own or need a push. We'll look at key concepts like , , and .
We'll also explore how reactions progress, from start to finish. This includes energy profiles, reaction coordinates, and what happens when reactions reach equilibrium. These ideas are crucial for grasping chemical reaction equilibria.
Thermodynamic Quantities
Gibbs Free Energy and Spontaneity
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Gibbs free energy (G) measures the maximum reversible work that can be performed by a system at constant temperature and pressure
Changes in Gibbs free energy (ΔG) determine the spontaneity of a process
If ΔG<0, the process is spontaneous and will occur naturally
If ΔG>0, the process is non-spontaneous and requires an input of energy to proceed
If ΔG=0, the system is at equilibrium, and no net change occurs
Gibbs free energy is related to enthalpy (H) and entropy (S) by the equation: G=H−TS, where T is the absolute temperature
Enthalpy and Heat of Reaction
Enthalpy (H) is a state function that represents the total heat content of a system
Changes in enthalpy (ΔH) during a chemical reaction are known as the heat of reaction
For an , ΔH<0, indicating heat is released by the system to the surroundings
For an , ΔH>0, indicating heat is absorbed by the system from the surroundings
The heat of reaction can be measured using calorimetry experiments (constant-pressure calorimetry)
states that the total enthalpy change for a reaction is independent of the pathway and depends only on the initial and final states
Entropy and Spontaneity
Entropy (S) is a measure of the disorder or randomness of a system
According to the , the entropy of the universe always increases for a spontaneous process
Changes in entropy (ΔS) contribute to the spontaneity of a process
If ΔS>0, the process is spontaneous and will occur naturally
If ΔS<0, the process is non-spontaneous and requires an input of energy to proceed
Entropy increases with increasing temperature, volume, and the number of particles in a system (ideal gas equation)
Reaction Dynamics
Reaction Coordinate and Energy Profile
A reaction coordinate is a geometric parameter that describes the progress of a chemical reaction from reactants to products
An energy profile is a plot of the potential energy of the system as a function of the reaction coordinate
The energy profile shows the barrier that must be overcome for the reaction to proceed
The difference in potential energy between the reactants and products represents the heat of reaction (ΔH)
Catalysts lower the activation energy barrier without being consumed in the reaction, increasing the reaction rate (enzymes in biological systems)
Spontaneity and Thermodynamic Equilibrium
The spontaneity of a reaction is determined by the sign of ΔG, which depends on both ΔH and ΔS
A reaction will proceed spontaneously in the direction that decreases Gibbs free energy until equilibrium is reached
At thermodynamic equilibrium, ΔG=0, and the forward and reverse reaction rates are equal
The (K) is related to the standard Gibbs free energy change (ΔG∘) by the equation: ΔG∘=−RTlnK, where R is the gas constant and T is the absolute temperature
The equilibrium position can be shifted by changing the temperature, pressure, or concentrations of reactants or products (Le Chatelier's principle)
Reference States
Standard State and Standard Thermodynamic Quantities
The standard state of a substance is defined as the pure substance at a pressure of 1 atm (or 1 bar) and a specified temperature (usually 25°C or 298 K)
Standard thermodynamic quantities, such as standard enthalpy of formation (ΔHf∘), standard entropy (S∘), and standard Gibbs free energy of formation (ΔGf∘), are defined for substances in their standard states
These values are used to calculate the standard enthalpy change (ΔH∘), standard entropy change (ΔS∘), and standard Gibbs free energy change (ΔG∘) for a reaction
Standard thermodynamic quantities can be found in reference tables (thermodynamic tables) and are essential for predicting the spontaneity and equilibrium of chemical reactions under standard conditions
The standard heat of formation (ΔHf∘) is the enthalpy change when one mole of a compound is formed from its constituent elements in their standard states (Hess's law)