Enthalpy vs Heat
For the study purposes in chemistry, we divide the universe into two; as a system and surrounding. At any time, the part we are interested is the system, and the rest is surrounding. Heat and enthalpy are two terms describing the energy flow and properties of a system.
The capacity of a system to do work is the energy of that system. Work can be done on the system or work can be done by the system. Then the energy of the system is increased or decreased accordingly. Energy of a system can be changed, not only by the work itself, by other means too. When the energy of a system changes as a result of temperature difference between the system and its surroundings, we refer to that energy transferred as heat (q); that is, energy has been transferred as heat. Heat transfer takes place from high temperature to low temperature, which is according to a temperature gradient. And this process continues until the temperature between the system, and the surrounding reaches same level. Heat transferring processes can be of two types. They are endothermic processes and exothermic processes. Endothermic process is a process in which energy is getting into the system from the surroundings as heat. In an exothermic process, heat is transferred from the system to the surroundings as heat.
In thermodynamics, the total energy of a system is called internal energy. Internal energy specifies the total kinetic and potential energy of molecules in the system. Internal energy of a system can be changed either by doing work on the system, or heating it. The change in internal energy is not equal to the energy transferred as heat, when the system is capable of changing its volume.
Enthalpy, which is denoted as H is a thermodynamic property of a system. It is defined as,
H= U + pV
Where, U is the internal energy, p is the pressure of the system and v is its volume.
This equation shows that the energy supplied as heat at a constant pressure is equal to the change in enthalpy. The term pV accounts for the energy required by the system to change volume against the constant pressure. So simply, enthalpy is the heat of a reaction at constant pressure.
The enthalpy change (∆H) for a reaction in a given temperature and pressure is obtained by subtracting the enthalpy of reactants from the enthalpy of products. If this value is negative, then the reaction is exothermic. If the value is positive, then the reaction is said to be endothermic. The change in enthalpy between any pair of reactants and products is independent of the path between them. Moreover, enthalpy change depends on the phase of the reactants. For example, when the oxygen and hydrogen gases react to produce water vapor, the enthalpy change is -483.7 kJ. But, when the same reactants react to produce liquid water, the enthalpy change is -571.5 kJ.
2H2 (g) +O2 (g) → 2H2O (g); ∆H= -483.7 kJ
2H2 (g) +O2 (g) → 2H2O (l); ∆H= -571.7 kJ
What is the difference between Enthalpy and Heat?
- Heat is the form of energy transfer from a high temperature to a lower temperature. Enthalpy is the heat transfer at a constant pressure.
- Enthalpy cannot be measured directly. Rather, the heat added or lost from the system gives the enthalpy change.
- Enthalpy is a function of the state, where heat isn’t.