Enthalpy And Thermochemistry

Firstly the heat capacity of any substance is largely dependent on size. More descriptively the heat capacity of a substance is defined as the amount of heat energy required to raise it's temperature by 1K, Which can easily be expressed in the more useful form of the molar heat capacity which is it's heat capacity divided by the amount of the substance present (in moles). This however must not be confused with the substances SPECIFIC heat capacity: The amount of heat energy required in order to raise the temperature of 1 gram of a substance by 1K.

Other factors such as whether or not the process is conducted under constant volume(within a sealed test tube) or pressure(within an open test tube). Heat capacity calculated at a constant at a constant volume is refered to as isochoric and a heat capacity calculated at a constant pressure is called isobaric. It is also useful to know that at constant volumes and pressure, the heat capacities of Cv and Cp are roughly equal as the volume change is very small with a change in temperature.

There can now be a relationship between the change in internal energy and the heat capacities under conditions of constant volume. It is obvious to see that under constant volume the change in internal energy is equal to the heat energy absorbed by the system, dU=q. But we also know that q=C_VdT. So q=C_VdT=dU

dU=C_VdT.

Heat capacity varies according to whether it expands or not (as some of the energy is used in the expansion). For this reason the heat capacity taken at a constant pressure must be larger than the heat capacity taken at a constant volume due to some of the energy being used to counteract the external pressure of the atmosphere. A relationship is therefore evident:

C_p=C_v+nR

ENTHALPY,H

Lets consider a reaction that takes place at constant temperature and pressure(in a test tube in the lab). It could be visualised as inputting heat energy into the reaction will increase the internal energy...? Although this initially makes sense it is untrue as some of the heat supplied is used to push back or produce work on the atmosphere (in order to release a gas). Therefore we find it convienient to ignore any work of expansion, thus introducing enthalpy (H). Enthalpy can be calculated using the following equation:

H=U+pV (this makes sense as pV>0 therefore the H of that system is >U).

Due to enthalpy being considered a state function, the molar enthalpy is a better way of expressing this relationship:

dH=dU+pdV

At a constant pressure, The enthalpy change becomes simply equal to the amount of heat energy transferred. We know that the enthalpy will increase if we increase the temperature so enthalpy can be measured by assessing the amount of heat energy required in order to raise the temperature by a certain amount.

dH=(-p_exdV=q)=pdV but at constant pressure: dH=q

THERMOCHEMISTRY IN ENTHALPY

So the enthalpy change of a general reaction is considered as the amount of heat energy taken in or precipitated out during a reaction, under the conditions of constant pressure. But many other enthalpy changes must be considered for more specific processes:

THE STANRDARD ENTHALPY OF FORMATION is the enthalpy change associated when one mole of a compund is produced from its nessecary elements, all being in their reference, or more stable states and conducted under standard conditions (101.325 KPa and 298K).

THE ENTHALPY OF IONISATION is described as being the minimum, average amount of energy required to remove 1 electron from each atom of 1 mole of perfectly gaseous atoms to produce 1 mole of perfectly gaseous 1⁺ ions.

THE BOND DISSOCIATION ENTHALPY is the enthalpy change for breaking a covalent bond between 2 certain gaseous atoms, to form 2 gaseous free radicals. Although the commonly used MEAN BOND ENTHALPY is the bond dissociation enthalpy of a particular bond, averaged over a series of related compounds.

THE ENTHALPY OF ATOMISATION is the standard enthalpy change when one mole of gaseous atoms is formed.

THE ENTHALPY OF LATTICE DISSOCIATION is the enthalpy change produced when one mole of a solid ionic lattice dissociates into gaseous ions.

THE ENTHALPY OF SOLUTIONis the heat energy change for a process in which one mole of an ionic solid dissolves in an amount of water large enough to ensure that the ions will ave no interactions once they are dissolved in the water.

THE STANDARD ENTHALPY OF COMBUSTION is defined as the heat energy change when 1 mole of a substance is completely burned in oxygen under standard conditions, all reacts in their reference states and water being assumed to be completely liquid.