# internal energy

(noun)

## Definition of internal energy

A property characteristic of the state of a thermodynamic system, the change in which is equal to the heat absorbed minus the work done by the system.

Source: Wiktionary - CC BY-SA 3.0

## Examples of internal energy in the following topics:

• ### Comparison of Enthalpy to Internal Energy

• Internal energy encompasses both potential and kinetic energy.
• Internal energy is a state function, meaning its value is dependent only on the current state of the system.
• Internal energy is generally represented as the sum of work and heat done by or to the system.
• Therefore a positive $\Delta U$ value means there is a net gain of energy by the system, while a negative $\Delta U$ value means there is a net loss of energy in the system.Because the internal energy encompasses only the energy contained within a thermodynamic system, the internal energy of isolated systems cannot change.
• Simply put, enthalpy accounts for heat flow within a system.Sometimes, measuring the internal energy of a system may be an inaccurate gauge of the change in energy.
• Internal energy and enthalpy are both measurements that quantify the amount of energy present in a thermodynamic system.
• ### The Three Laws of Thermodynamics

• Thermodynamics defines macroscopic variables (such as temperature, internal energy, entropy, and pressure) that describe average properties of material bodies and radiation, explains how they are related and by what laws they change with time.
• In the context of chemistry, the internal energy is the sum of the kinetic energy of the molecules, and the potential energy represented by the chemical bonds between the atoms and any other intermolecular forces that may be operative.The first law of thermodynamics, also known as Law of Conservation of Energy, states that energy can be neither be created nor destroyed; it can only be transferred or changed from one form to another.
• A way of expressing the first law of thermodynamics is that any change in the internal energy (∆U) of a system is given by the sum of the heat (q) that flows across its boundaries and the work (w) done on the system by the surroundings: $ΔU = q + w$This law says that there are two kinds of processes, heat and work, that can lead to a change in the internal energy of a system.
• If heat flows into a system or the surroundings to do work on it, the internal energy increases and the sign of q or w is positive.
• Conversely, heat flow out of the system or work done by the system will be at the expense of the internal energy, and will therefore be negative.
• The laws of thermodynamics define fundamental physical quantities (temperature, energy, and entropy) that characterize thermodynamic systems.
• ### Enthalpy of Reactions

• In thermodynamics, work (w) is defined as the process of an energy transfer from one system to another.
• The amount of energy for a closed system is written as follows:$\Delta U = \Delta q + \Delta w$where U is the total energy of the system, q is heat, and w is work.
• Since no work is performed, the energy of the system, U, is solely dependent on the heat, or enthalpy, of the system.
• Because some heat was either given off or absorbed to perform work, we cannot assume that the amount of measurable heat is equal or proportional to the internal energy of the system.
• Therefore, we introduce enthalpy (H), which is the sum of the internal energy of the system and the work performed on or by the system.
• The enthalpy of a reaction measures energy changes in a reaction and is dependent on reaction conditions.
• ### Free Energy and Work

• Similarly to mechanics, where potential energy is defined as capacity to do work, different potentials have different meanings.
• The Gibbs free energy is the maximum amount of non-expansion work that can be extracted from a closed system.
• The work is done at the expense of the system's internal energy.
• Energy that is not extracted as work is exchanged with the surroundings as heat, with the value –TΔS.
• The appellation “free energy” for G has led to so much confusion that many scientists now refer to it simply as the "Gibbs energy."
• The Gibbs free energy is the maximum amount of non-expansion work that can be extracted from a closed system.
• ### Heat and Work

• Definition of Heat Heat is defined as the energy transferred from one system to another by thermal interaction.
• The high temperature body loses thermal energy/triangle E and the low temperature body acquires the same amount of energy.
• Definition of Work Work is the transfer of energy by any process other than heat.
• The product of volume (V) and pressure (P) is an energy.