Enthalpy and Entropy: Overview and Applications in Thermodynamics (copy)

Enthalpy and Entropy

Understanding the concepts of enthalpy and entropy is crucial in thermodynamics, particularly for predicting the behavior of reactions. This guide provides a comprehensive overview of how to find and measure enthalpy and entropy, as well as their implications in spontaneous reactions.

Determining Enthalpy Changes

Enthalpy changes can be measured experimentally, with calorimetry being a primary method discussed in practical sessions. However, most enthalpy values can be found in reliable thermodynamic data tables, which compile the work of previous researchers.

Resources for Thermodynamic Data

One of the best resources for thermodynamic data is the National Institute for Standards and Technology (NIST) Chemistry WebBook. This online database provides a wealth of thermodynamic and other related data, making it invaluable for students and professionals in the field. The address is NIST Chemistry WebBook.

Standard Enthalpy

Standard Enthalpies

Standard enthalpy refers to the energy change per mole of reaction measured under Standard Temperature and Pressure (STP). STP is defined as a temperature of 298 K and a pressure of 1 bar. Standard enthalpies can include various types, such as:

  • Enthalpy of Formation

  • Enthalpy of Combustion

  • Enthalpy of Neutralization

Standard Enthalpy of Combustion (ΔH°ₐ)

This specifically refers to the enthalpy change during the complete combustion of one mole of a substance in oxygen. For example:

CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l)
This reaction indicates that methane reacts with oxygen to produce carbon dioxide and water, releasing energy.

Standard Enthalpy of Formation (ΔH°ₓ)

This is the enthalpy change when one mole of a compound forms from its elements at standard conditions. For instance:

2C(s) + 3H₂(g) + ½O₂(g) → C₂H₅OH(l)
In this case, ethyl alcohol is formed from its elemental constituents, which involves a specific amount of energy change.

Measuring Entropy

Entropy is another key thermodynamic property that can also be experimentally measured. The standard entropy of a substance reflects the change in entropy per mole as the substance is heated from 0 K to 298 K. Unlike enthalpy, absolute values for entropy can be quantified.

Standard Entropy Change of Reaction

The standard entropy change for a reaction is defined as the difference in entropy between products and reactants. This differential approach helps in understanding the randomness or disorder associated with chemical reactions.

Gibb's Free Energy

Role of Gibb's Free Energy

Gibb's Free Energy combines enthalpy and entropy to determine the spontaneity of a reaction. A reaction is said to be spontaneous if it leads to a system changing from a less stable state to a more stable one.

  • Gibb's Free Energy Equation: ΔG = ΔH - TΔS
    Where ΔG represents the change in free energy, ΔH is the change in enthalpy, T is the absolute temperature, and ΔS is the change in entropy.

A negative ΔG indicates a spontaneous reaction (entropy of the universe increases), while a positive ΔG indicates non-spontaneity. At equilibrium, ΔG equals zero, signaling that the system is at a balanced state.

Example Calculation

For an example reaction:

3HC ≡ CH(g) → C₆H₆(g)
Given ΔH = -597.3 kJ mol⁻¹ and ΔS = -0.33 kJ mol⁻¹ K⁻¹, we can determine spontaneity at 300 K:

  1. Apply the equation: ΔG = ΔH - TΔS

ΔG = -597.3 - (300 × -0.33)
ΔG = -597.3 + 99 = -498.3 kJ mol⁻¹

The negative result indicates that the reaction is spontaneous.

Predicting Spontaneity at Different Temperatures

At elevated temperatures, the influence of the entropy term (-TΔS) increases, which can eventually offset the negative enthalpy value in some reactions, turning them from spontaneous (ΔG < 0) to non-spontaneous (ΔG > 0).

Activation Energy and Spontaneity

A key concept to remember is activation energy; reactions, even if spontaneous, often require an initial input of energy (activation energy) to proceed. For example, combustion reactions like that of coal are spontaneous in theory but require ignition energy to commence.

Kinetically Controlled Reactions

Some spontaneous reactions, such as the oxidation of sugar, exhibit high activation energy, making them kinetically controlled. Even though thermodynamically favorable (large negative ΔG), they may not occur readily under normal conditions due to the activation barrier.

Conclusion

The principles of thermodynamics give crucial insight into reaction behavior, particularly in fields such as forensic chemistry. Conditions affecting reactions, such as temperature and concentration, must be understood and considered in practical applications.