internal energy

Examples of internal energy in the following topics:

• Comparison of Enthalpy to Internal Energy

  • In thermodynamics, the total energy contained by a given thermodynamic system is referred to the internal energy (U).
  • Because the internal energy encompasses only the energy contained within a thermodynamic system, the internal energy of isolated systems cannot change.
  • Enthalpy (H) encompasses both the internal energy of a system and the energy associated with displacing the system's surroundings.
  • Sometimes, measuring the internal energy of a system may be an inaccurate gauge of the change in energy.
  • Therefore, to account for both the possible volume change at constant pressure and the internal energy, enthalpy is used.

• Free Energy and Work

  • 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.
  • ΔG is the maximum amount of energy which can be "freed" from the system to perform useful work.
  • The impossibility of extracting all of the internal energy as work is essentially a statement of the Second Law.

• Internal Energy and Enthalpy

  • Substituting this in for work in the above equation, we can define the change in internal energy for a chemical system:
  • Let's examine the internal energy change, $\Delta U$, at constant volume.
  • At constant volume, $\Delta V=0$, the equation for the change in internal energy reduces to the following:
  • This internal energy is often very difficult to calculate in real life settings, though, because chemists tend to run their reactions in open flasks and beakers that allow gases to escape to the atmosphere.
  • The enthalpy of reaction is defined as the internal energy of the reaction system, plus the product of pressure and volume.

• The Three Laws of Thermodynamics

  • For example, turning on a light would seem to produce energy; however, it is electrical energy that is converted.
  • A way of expressing the first law of thermodynamics is that any change in the internal energy (∆E) 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:
  • 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 do work on it, the internal energy increases and the sign of q and w are positive.
  • Conversely, heat flow out of the system or work done by the system (on the surroundings) will be at the expense of the internal energy, and q and w will therefore be negative.

• Standard Free Energy Changes

  • The standard Gibbs Free Energy is calculated using the free energy of formation of each component of a reaction at standard pressure.
  • These same definitions apply to standard enthalpies and internal energies.
  • In order to make use of Gibbs energies to predict chemical changes, it is necessary to know the free energies of the individual components of the reaction.
  • The energy units will need to be the same in order to solve the equation properly.
  • Calculate the change in standard free energy for a particular reaction.

• Heat and Work

  • When energy is exchanged between thermodynamic systems by thermal interaction, the transfer of energy is called heat.
  • Heat is transfer by conduction occurs when an object with high thermal energy comes into contact with an object with low thermal energy.
  • The high temperature body loses thermal energy, and the low temperature body acquires this same amount of thermal energy.
  • For a closed system, the change in internal energy (∆U) is related to heat (Q) and work (W) as follows:
  • This means that the total energy within a system is affected by the sum of two possible energy transfers: heat and work.

• Reaction Energetics

  • To summarize, bond energy is energy that must be introduced to break a bond, and is not a component of a molecule's potential energy.
  • The energy needed to raise the reactants to the transition state energy level is called the activation energy, ΔE‡.
  • The activation energy is drawn in red in each case, and the overall energy change (ΔE) is in green.
  • The overall activation energy is the difference in energy between the reactant state and the highest energy transition state.
  • At room temperature, indeed at any temperature above absolute zero, the molecules of a compound have a total energy that is a combination of translational (kinetic) energy, internal vibrational and rotational energies, as well as electronic and nuclear energies.

• The Hydrogen Economy

  • As such, hydrogen is not a primary energy source, but an energy carrier.
  • The feasibility of a hydrogen economy depends on issues including the use of fossil fuel, the generation of sustainable energy, and energy sourcing.
  • Fuel cells are electrochemical devices capable of transforming chemical energy into electrical energy.
  • Although H2 has high energy density based on mass, it has very low energy density based on volume.
  • The main source of hydrogen is fossil fuel reforming, but this method ultimately leads to higher emissions of carbon dioxide than using the fossil fuel in an internal combustion engine.

• Changes in Energy

  • The concept of entropy can be described qualitatively as a measure of energy dispersal at a specific temperature.
  • This is because some energy is expended as heat, limiting the amount of work a system can do.
  • This is because the thermal energy from the warm surroundings spreads to the cooler system of ice and water.
  • The entropy of the room decreases as some of its energy is dispersed to the ice and water.
  • The second law of thermodynamics shows that in an isolated system internal portions at different temperatures will tend to adjust to a single uniform temperature and thus produce equilibrium.

• Introduction

  • The magnetic moment of the lower energy +1/2 state is aligned with the external field, but that of the higher energy -1/2 spin state is opposed to the external field.
  • The difference in energy between the two spin states is dependent on the external magnetic field strength, and is always very small.
  • The international unit for magnetic flux is the tesla (T).
  • Even with these high fields, the energy difference between the two spin states is less than 0.1 cal/mole.
  • The following diagram displays energy differences for the proton spin states (as frequencies).