Enthalpy Calculator

What is Enthalpy?

Enthalpy is a measure of total energy in a system. This is typically in the form of heat, but also a form of volume and pressure. Since enthalpy is a measure of the state of a system, it does not change at equilibrium. That is why we look at the change in enthalpy of a system from one state to another. The state of the system has to change in order for the enthalpy to change. This normally happens when work or energy is transferred to a system, typically through heat. Enthalpy changes in both endothermic or exothermic reactions. An endothermic reaction is an act of absorbing energy to change states, and an exothermic reaction is an act of releasing energy or heat. The change in enthalpy will be positive for endothermic and negative for exothermic.

Formula

The formula to calculate the change in enthalpy (ΔH) is:

\[ \Delta H = (Q_2 - Q_1) + p \cdot (V_2 - V_1) \]

Where:

\( \Delta H \) is the change in enthalpy.
\( Q_1 \) and \( Q_2 \) are the internal energies at the initial and final times, respectively.
\( V_1 \) and \( V_2 \) are the volumes at the initial and final times, respectively.
\( p \) is the constant pressure.

Example

Let's say the initial internal energy (Q₁) is 500 J, the final internal energy (Q₂) is 800 J, the initial volume (V₁) is 0.1 m³, the final volume (V₂) is 0.2 m³, and the pressure (p) is 1000 Pa. Using the formula:

\[ \Delta H = (800 - 500) + 1000 \cdot (0.2 - 0.1) = 300 + 100 = 400 \text{ J} \]

So, the change in enthalpy (ΔH) is 400 J.

Enthalpy Calculator for Steam

Formula: \( H = m \times h \)

H: Enthalpy (in kJ)
m: Mass (in kg)
h: Specific enthalpy (in kJ/kg)

Example: \( H = 2 \times 2700 \)

m: 2 kg
h: 2700 kJ/kg

Enthalpy Calculator for HVAC

Formula: \( H = m \times c_p \times \Delta T \)

H: Enthalpy (in kJ)
m: Mass flow rate (in kg/s)
c_p: Specific heat capacity (in kJ/kg·K)
(\Delta T): Temperature change (in K)

Example: \( H = 1.5 \times 1.005 \times 10 \)

m: 1.5 kg/s
c_p: 1.005 kJ/kg·K
(\Delta T): 10 K

Enthalpy Calculator for Air

Formula: \( H = m \times c_p \times \Delta T \)

H: Enthalpy (in kJ)
m: Mass flow rate (in kg/s)
c_p: Specific heat capacity (in kJ/kg·K)
(\Delta T): Temperature change (in K)

Example: \( H = 2 \times 1.005 \times 15 \)

m: 2 kg/s
c_p: 1.005 kJ/kg·K
(\Delta T): 15 K

Enthalpy Calculator for Water

Formula: \( H = m \times c_p \times \Delta T \)

H: Enthalpy (in kJ)
m: Mass (in kg)
c_p: Specific heat capacity (in kJ/kg·K)
(\Delta T): Temperature change (in K)

Example: \( H = 3 \times 4.18 \times 20 \)

m: 3 kg
c_p: 4.18 kJ/kg·K
(\Delta T): 20 K

Enthalpy Calculator for BTU/lb

Formula: \( H = m \times h \)

H: Enthalpy (in BTU)
m: Mass (in lb)
h: Specific enthalpy (in BTU/lb)

Example: \( H = 5 \times 180 \)

m: 5 lb
h: 180 BTU/lb

Enthalpy Calculator for R134a

Formula: \( H = m \times h \)

H: Enthalpy (in kJ)
m: Mass (in kg)
h: Specific enthalpy (in kJ/kg)

Example: \( H = 1.2 \times 250 \)

m: 1.2 kg
h: 250 kJ/kg

Enthalpy Calculator for Chemistry

Formula: \( \Delta H = \Delta U + P \Delta V \)

(\Delta H): Change in enthalpy
(\Delta U): Change in internal energy
P: Pressure
(\Delta V): Change in volume

Example: \( \Delta H = 500 + 101.3 \times 0.02 \)

(\Delta U): 500 kJ
P: 101.3 kPa
(\Delta V): 0.02 m³

Enthalpy Calculator for Saturated Steam

Formula: \( H = m \times h \)

H: Enthalpy (in kJ)
m: Mass (in kg)
h: Specific enthalpy (in kJ/kg)

Example: \( H = 2.5 \times 2700 \)

m: 2.5 kg
h: 2700 kJ/kg