Question

1) why is the change in the enthalpy a meaningful quantity for many chemical processes?

2) Enthalpy is said to be a state function. What is it about state functions that makes them particularly useful?

3) During a constant pressure process the system absorbs heat from the surrounding. Does the enthalpy of the system increase or decrease during the process?

Answer 1

1)Enthalpy is defined as 'Total heat content of a system' in thermodynamics. It is denoted by H.

Change in Enthalpy thus indicates change in total heat content of the system. Now, as per the first law of Thermodynamics, energy can neither be created not destroyed.

Thus, if change in Enthalpy is positive, it means that total heat content has been added in the system during the process and heat has to be supplied to carry out the process, either as thermal energy or as mechanical work, depending on the case. Conversely, if change in Enthalpy is negative, it means that heat content has been removed from the system (and to the surroundings) during the process. Therefore, heat will be gained during the process, either has thermal energy or as mechanical work.

Change in enthalpy is also meaningful in chemical reactions. This is because , in a chemical reaction :

H_reaction = H _products - H _reactants

Thus, change in enthalpy for a chemical reaction is given by difference between enthalpy of products and enthalpy of reactants. If this change in enthalpy is negative, it means enthalpy of products is less than enthalpy of reactants (and reactants are actually more stable than the products). Therefore, the excess heat is given out of the process as the reaction proceeds and the reaction becomes 'exothermic' , that is one which gives out heat.

Conversely, it change in enthalpy is positive, reaction is endothermic, or one which requires heat to proceed.

2) State functions are funtions which depend only on 'state' , that is, current state of the system. They don not depend upon the path taken to reach that state. Therefore, the value of state functions only depend upon parameters used to completely define a system , and not on any other factors.

This makes them particularly useful in thermodynamics.

3)At constant pressure, change in enthalpy is equal to the energy interchanged with the surroundings

H = Q

Here, Since heat is absorbed, enthalpy of the system will increase during the process.