Internal energy and enthalpy

Introduction to thermodynamics

In the study of chemistry and especially thermodynamics, there are two important concepts you will encounter, internal energy and enthalpy. These terms are useful for understanding how energy changes during a chemical reaction and how heat interacts with a system under different circumstances.

Understanding internal energy

Internal energy is a concept that attempts to describe the total energy contained within a substance. It includes all the kinetic energy (due to the motion of the particles) and potential energy (due to interactions between particles) present in the particles of the substance. In a chemical reaction, internal energy is essentially the energy stored in chemical bonds and the random motion of atoms and molecules.

Imagine a closed system where no particle can go in or out, and think of it as a box filled with gas molecules. The internal energy of this box can be thought of as the sum of all the kinetic energies of the molecules and the energy resulting from their interactions.

Formula of internal energy

The total internal energy U of a system is represented as the sum of the kinetic energy and potential energy of all its components.

U = KE + PE

Where:

• KE is the total kinetic energy.
• PE is the total potential energy.

Understanding enthalpy

Enthalpy is another form of energy, but it is more closely related to energy changes in chemical reactions that occur at constant pressure. Enthalpy is defined as the internal energy including the product of the pressure and the volume of the system.

In simple terms, if you have a system like an inflated balloon, as it expands, its volume changes, doing work against the outside atmospheric pressure. This work is part of the enthalpy change.

Formula of enthalpy

Enthalpy H is expressed mathematically as:

H = U + PV

Where:

• H is the enthalpy.
• U is the internal energy.
• P is the pressure.
• V is the volume.

Relationships and differences

Both internal energy and enthalpy are state functions, meaning they depend only on the current state of the system, not on how the system got to that state. They provide us with a means of understanding heat transfer and work done in a system.

The differentiating factor is that while internal energy considers all types of energy in a closed system, enthalpy considers the energy required to create space or volume against a certain pressure in a system.

Visual example: heating gas

Consider a container of gas. Heating the gas increases its internal energy because the gas molecules move more quickly. If the gas expands when heated, work is done by the system (by pushing against the container walls or the atmosphere), which involves a change in enthalpy.

In this simplified illustration, the movement of the partition inside the container appears to cause an increase in volume, as the gas expands to the right due to heating.

Further information on enthalpy: Heat of reaction

In practice, chemists are often interested in the change in enthalpy during a reaction, known as the heat of reaction. This is the difference in enthalpy between the products and the reactants and is expressed as:

ΔH = H_products - H_reactants

Exothermic reactions: If ΔH is negative, the reaction is exothermic, meaning it releases heat into its surroundings. For example, combustion reactions are usually exothermic.

Endothermic reactions: Conversely, if ΔH is positive, the reaction is endothermic, requiring heat from the surroundings. An example of this would be the melting of ice or the dissolving of ammonium nitrate in water.

Example calculation for enthalpy change

Let's consider a practical example of how enthalpy changes during a chemical reaction. Suppose we have the combustion of propane:

C3H8 + 5O2 → 3CO2 + 4H2O

The enthalpy change for the reaction can be determined using the standard enthalpy of formation:

ΔH = [3(ΔHf_CO2) + 4(ΔHf_H2O)] - [ΔHf_C3H8 + 5(ΔHf_O2)]

The resulting ΔH will tell us whether heat is absorbed or released, providing important information about the energy profile of the reaction.

Conclusion

Understanding internal energy and enthalpy is essential when delving deeper into thermodynamic processes in chemical reactions. These concepts serve as the basis for studying energy transformations and understanding the complexities of reactions from a thermodynamic perspective. With internal energy, we assess the molecular energy content within a system, and with enthalpy, we evaluate changes during reactions at constant pressure, including phase changes and chemical transformations. These concepts significantly impact the way chemists predict reaction behavior, write balanced thermochemical equations, and estimate energy transferred in laboratory or industrial processes.

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