INTERNAL ENERGY

As heat and work put energy in and take energy out of the system, certain amounts of the energy is stored within the system which we refer to as its internal energy (U). It is described as the total energy within a system due to physical state under specified conditions. 'U is the sum of all kinetic and potential energy from molecules, atoms and ions in the system'.

This brings us to the first law of thermodynamics, the conservation of energy, which states that 'the internal energy of a system is constant unless it is changed by work or heat transfer'. In general we can simply say that therefore, any changes in U is caused by energy lost or gained through work or heat. It follows that:-

Change in U=q+w (so these three variables are positive if the system gains energy, although when work is done by the system on the surroundings, w is negative ans so U decreases).

Hess's law closely ties with the first law of thermomdynamics. Its states that the enthalpy chenge of a reaction depends solely on the initial and final states of the reaction and is therefore independant of the route taken, which is the background understanding behind such principles as the Born-habar cycle.

Kirchhoff's law is a one that allows us to estimate the enthalpy of a reaction at a desired temperature using data recorded at another temperature. This relationship works on the principle that the heat capacities are constant over the required range

dH^o(of required reaction enthalpy)=dH^o(of known reaction enthalpy)+d_rC_pdT

Internal energy is regarded as a state function, or a property of a system depending on the state that the system is in but which doesn't take into regard how the system reached that state (as explained above in Hess's law). For example when we talk about altitude, we talk about a fixed point but how that point is reached is not of relavence.