Enthalpy

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State Function

Enthalpy change, entropy change and free energy change are state function. This means that they all depend only on the change between the initial and final state of the system not on the process by which the change occurs. For a chemical reaction, this means that the thermodynamics state functions are independent of reaction pathway.


Original definition

This is the heat change which occurs when 1 mol of a substance reacts completely with oxygen to form products at 298 K and 1 atm. The function H was introduced by the Dutch physicist Heike Kamerlingh Onnes in early 20th century in the following form:


where

E represents the energy of the system


In the absence of an external field, the enthalpy may be defined, as it is generally known, by:


where (all units given in SI)

H is the enthalpy (joules)

U is the internal energy, (joules)

p is the pressure of the system, (pascals)

V is the volume, (cubic metres) is absorbed

Note: The Basic Rule of enthalpy: when bonds are formed, energy is released When bonds are broken, energy


Heats of reaction

The total enthalpy of a system cannot be measured directly; the enthalpy change of a system is measured instead. Enthalpy change is defined by the following equation:

ΔH = H_f - H_i


where

ΔH is the enthalpy change

H_f is the final enthalpy of the system, measured in joules. In a chemical reaction, Hfinal is the enthalpy of the products.

H_i is the initial enthalpy of the system, measured in joules. In a chemical reaction, Hinitial is the enthalpy of the reactants.


For an exothermic reaction at constant pressure, the system's change in enthalpy is equal to the energy released in the reaction, including the energy retained in the system and lost through expansion against its surroundings. In a similar manner, for an endothermic reaction, the system's change in enthalpy is equal to the energy absorbed in the reaction, including the energy lost by the system and gained from compression from its surroundings. A relatively easy way to determine whether or not a reaction is exothermic or endothermic is to determine the sign of ΔH. If ΔH is positive, the reaction is endothermic, that is heat is absorbed by the system due to the products of the reaction having a greater enthalpy than the reactants. On the other hand if ΔH is negative, the reaction is exothermic, that is the overall decrease in enthalpy is achieved by the generation of heat. Although enthalpy is commonly used in engineering and science, it is impossible to measure directly, as enthalpy has no datum (reference point)


Hess's Law

Hess's Law says that if a reaction can be described a sereies of steps , the change of enthalpy for the overall reaction is simply the sum of the change of entahlp values for all steps



Heat Capicity and specific Heat

Heat capacity is measure of how much the temperature of an object is raised when it absorbs heat