Heat Enthalpy

Heat is a part of thermodynamics. Heat enthalpy(H) is the addition of internal energy (U) and the product of its volume (V) and pressure (p), hence H= U + pV. Enthalpy is a measure of the total heat content of the system. This article will study the basic concept of Heat Enthalpy, its example with the solution, and the importance of heat enthalpy. 

Let’s start the article by defining the Heat enthalpy.

Heat Enthalpy

The property of the thermodynamic system is enthalpy. It is the total of the internal energy in the system and the product of its volume and pressure. 

The pressure-volume term measures the work needed to make the system’s physical dimensions. An example is to make room for it by changing the environment. It is very small for liquids and solids at the same conditions and small for gases. Enthalpy depends on the final configuration of pressure, internal energy, and volume, not on the path taken to achieve it.

We can also define enthalpy as the sum of internal energy and its volume and pressure product.

H = U + pV

d H = T d S + P d V 

Where p is pressure, U is internal energy, and V is the system’s volume.

It is an extensive property and is proportional to the size of the homogeneous system. 

As per the International System of Units (SI), the unit of measurement of enthalpy is the joule (J). Calorie and British thermal units (BTU) are other historical conventional units.

We can not measure the total amount of enthalpy of a system directly because certain unknown components that are not easily accessible are involved in the internal energy.

Relation between heat and enthalpy

To check out the relationship between the heat supply and enthalpy increase, we have to recall the 1st law of closed systems

the physics sign convention- dU = δQ − δW

Where 

δQ is the heat provided by conduction, Joule heating, radiation, or friction from stirring by a shaft with paddles or by an externally driven magnetic field acting on an internal rotor. 

The Internal root works based on surroundings but contributes to system-based heat. Therefore, we use it only to some specific conditions with a similar pressure at the surface.

Here, the work stated by p dV. 

where dV = increase of the volume of the system 

p = pressure at the surface. 

Long-range electromagnetic interaction cases need further state variables in their formulation, which is not considered here. 

Here, the first law is-

 dU = δQ – p dV,

Now, dH = dU + d(pV)

So, dH = δQ + V dp + p dV – p dV,

= δQ + V dp 

dp = 0, when the system is under constant pressure, and the rise in enthalpy of the system is equal to the heat added or given off:

dH = δQ.

Example showing Calculation of change in enthalpy

To calculate the enthalpy change when the ice melts into a liquid and the liquid changes into a vapour, we can use the heat of vaporisation of water and the heat of fusion of ice.

The heat of fusion of ice is 333 J/g, and the heat of vaporisation of liquid water is 2257 J/g.

Part A: Part B: Using the calculated values, find the number of grams of ice you can melt using 0.800 kJ of heat. 

Part B: Calculate the change in enthalpy, ΔH, for these two processes.

H2O(s) → H2O(l); ΔH = ?

H2O(l) → H2O(g); ΔH = ?

Solution

1.Now we know that-

1 mol H2O(s) = 18.02 g H2O(s) ~ 6.00 kJ

therefore:

0.800 kJ x 18.02 g ice6.00 kJ = 2.40 g ice melted

2. The heats of fusion and vaporisation are in joules, let’s convert joules into kilojoules. 

1 mol of H2O = 18.02 g (according to the periodic table)

Therefore:

fusion ΔH = 18.02 g x 333 J / 1 g

= 6.00 x 103 J

= 6.00 kJ

vaporisation ΔH = 18.02 g x 2257 J / 1 g

= 4.07 x 104 J

= 40.7 kJ

Hence,

H2O(l) → H2O(g); ΔH = +40.7 kJ

H2O(s) → H2O(l); ΔH = +6.00 kJ

Importance of heat Enthalpy-

• It helps measure the change that allows us to ascertain whether a reaction was exothermic, i.e. released heat, a -ve change in enthalpy or endothermic, i.e. absorbed heat, +ve change in enthalpy. 
• Enthalpy helps us measure the heat of the reaction of a chemical process.
• You can measure an alter in enthalpy of the heat flow by calorimetry.
• It is checked to evaluate a Joule-Thomson expansion or throttling process.
• It is utilised to measure the least power for a compressor.
• During the change in the state of matter, a change in enthalpy happens.

Conclusion

We have read that heat enthalpy is the amount of heat present in the system, which can be high or low as per the requirement of the system. We have also read the many formulas for calculating enthalpy and discussed the importance of enthalpy. Therefore, this Study material notes on Heat Enthalpy must have helped you understand the topic of heat enthalpy and its significance.