Analyzing Heat and Enthalpy

Heat and enthalpy relationship are the two important topics of thermodynamics. But, the basic idea of heat and enthalpy is also an essential part of general physics. Heat is a form of energy, and therefore heat always exists and only changes by transforming. Measuring heat flow is important in understanding enthalpy or the changes in the inner energy of a system, whether it is chemical or physical.

Thus, the concepts of heat and enthalpy are closely connected in chemistry, physics, and thermodynamics. Here we will tell you all about heat and enthalpy and how they are related.

What is Heat?

The capacity to do any work is called energy. This inner energy helps any system to work. And this work can be done by the object itself or with the help of outside catalysts. When an object works, its energy level reduces or elevates according to work. These changing energy levels within the system also increase or decrease the object’s temperature, which differs from its surroundings. And by this process, the inner energy of an object transforms as temperature, what we call heat. 

Heat is a quantifiable form of energy, and q is the symbol of heat. Also, according to the temperature gradient, heat always flows from high temperature to low temperature. 

What is Enthalpy?

The total amount of energy of any system is the inner energy of that system. This inner energy is the compilation of kinetic energy and potential energy of the molecules present in the system. The energy of a system can be changed by working or heating the system. Though, the amount of heat transferred to the system may not be equal to the changed inner energy of the system.

Enthalpy is a process of this energy transformation, and the amount of heat flow counts it. The symbol H is used to denote enthalpy. 

Change in Enthalpy

H = U + PV 

This equation can help us to understand the enthalpy change. Here, H is the symbol of Enthalpy, U is the symbol of the inner energy of a system, and P is the pressure. V is the volume of the system. 

So, this equation explains that heat transferred as energy with a stable pressure is equal to the change in enthalpy. And the phrase PV explains the needed energy to change the volume of the system in the presence of stable pressure. 

Therefore, by subtracting the reactants’ enthalpy from the product’s enthalpy, you will be able to understand the amount of changing enthalpy obtained at a given pressure and temperature in a system.

Relation between heat and enthalpy

Many people think of heat and enthalpy as interchangeable terms. While enthalpy refers to a property of a system that is not quantifiable, heat is not a property of a system and can be easily quantified. Heat is not directly related to enthalpy. Rather heat flow is equated with enthalpy.

Heat flow refers to the transition of energy to or from any system. As we cannot quantify enthalpy, we can equate it with the heat flow against constant pressure. Thus heat and enthalpy relationships are not straightforward. Still, the amount of heat transferred in a system during a chemical process against a constant pressure is equivalent to the enthalpy change of that system.

Heat of formation

The heat of formation is also known as the standard enthalpy of formation. This refers to the formation of 1 molecule of a system from its constituent elements during enthalpy change when all other substances are in their standard states. 

The heat of formation for gas could be following the ideal gas equation against the stable pressure of 1 bar.

The standard state is the pure liquid or solid for a liquid or solvent at a constant pressure of 1 bar.

There are also other different standards of enthalpy for gaseous systems and elements like water etc.

Example: standard heat of formation or enthalpy of formation of carbon dioxide-

C(graphite)+ O2 (g)= CO2 (g)

Examples of Enthalpy change

The enthalpy connected with any phrase change can be measured. Each phase change has a particular associated heat and a specific name. 

For Instance, 

The transformation from solid to liquid is termed as fusion. And the enthalpy for this phrase change is  Hfusion.  Therefore this change in enthalpy is,

Hfusion =Hf–Hi=Hliquid–Hsolid

Or for example, the enthalpy change for melting can also be stated as the transformation of solid to liquid at a melting temperature. For H2 O, this will be,

H2O(s, 0C)H2O(l, 0C)

Conclusion

The concept of heat and enthalpy and their relationship are integral to thermodynamics. But these two topics also have a significant contribution to the study of physics and chemistry. In simple words, heat is a form of energy, and it can be measured through the escalation and reduction of the temperature of a system. 

On the other hand, enthalpy is a part of a system, and generally, enthalpy refers to the inner energy of a system. Therefore, measuring enthalpy can not be done directly. To measure enthalpy change, you need to understand the heat and enthalpy relationship and change heat flow in a system.