Created By : awnish

Reviewed By : Rajashekhar Valipishetty

Last Updated : Apr 10, 2023

Free Enthalpy Calculator is helpful to find the chnage in enthalpy of a reaction. We have two simple methods to calculate the enthalpy change like reaction scheme and enthalpy formula. Students have to select one method to calculate the enthalpy, enter the required input details and hit the calculate button to avail change in enthalpy value easily.

Choose a Calculation
Internal energy - reactants (Q1)
Volume of reactants (V1)
Internal energy - products (Q2)
Volume of products (V2)
Constant pressure (p)

Steps to Calculate Enthalpy of a Reaction

Below-listed are the useful steps to compute the enthalpy change of a chemical reaction using either enthalpy formula or using the chemical reaction scheme. Follow these steps for obtaining the result easily.

  • Using Enthalpy Formula:
    • Obtain the internal energy, volume of the reactants, products and pressure.
    • Subtract the products volume from the reactants volume and multiply it by the constant pressure.
    • Subtract the internal energy of the products from the reactants.
    • Add the result in step 2 with the step 3 to get the chnage in enthalpy.
  • Using Reaction Scheme
    • Get any balanced chemical reaction.
    • Find the standard enthapies of the each of the substance in the reaction.
    • Find the sum of enthapies of reactants and sum of products enthapies.
    • Subtract total enthapies products from the reactants to check the change in enthalpy.

What is Enthalpy?

Enthalpy measures the total amount of energy of a thermodynamics system which is in the form of heat or volume multiplied by the constant pressure.

The formula to calculate the enthalpy is along the lines:

H = Q + pV


Q is the internal energy

p is the vpressure

V is the volume

H is the enthalpy.

To calculate the change in enthapy, you need initial and final values with constant pressure. The formula for enthalpy change is ΔH = (QΓéé - QΓéü) + p * (VΓéé - VΓéü) or

ΔH = ΔQ + p * ΔV


QΓéü, QΓéé are the internal energies of the products and reactants.

VΓéü, VΓéé are the volume of the products and reactants

p is the pressure

ΔH is change in enthalpy

ΔQ is change in inyternal energy

ΔV is the change in volume

Standard Enthalpy of Formation

It is another method used to compute the enthalpy change. The standard enthalpy of formation of a compound is ΔH°f. It is nothing but the change in enthalpy for forming one mole of substance in the standard state.

The standard enthalpy of formation formula is given as

ΔH°reaction = ∑ΔH°f(products) - ∑ΔH°f(reactants)


∑ΔH°f(reactants) is the sum of the standard enthalpes of the reactants.

∑ΔH°f(products) is the sum of the standard enthalpies of the products

ΔH°reactionis the enthalpy change

The standard enthalpy of each chemical substance is collected from the standard enthalpy of formation table mentioned below:

Substance Change in Enthalpy in kJ/mol
OΓéé(g) 0
SOΓéé(g) −296.83
SOΓéâ(g) −395.72
HΓééO(l) −285.8
CuΓééO(s) −168.6
Mg²Γü║(aq) -466.85


Question: Calculate the change in enthalpy for 2H2 + O2 → 2H2O?


Given chemical reaction is 2H2 + O2 → 2H2O

The standard enthalpy of formation of substances is here.

H2 = 0, O2 = 0, H2O = −285.8

Change in enthalpy formula is ΔH°reaction = ∑ΔH°f(products) - ∑ΔH°f(reactants)

∑ΔH°f(reactants) = 2(0) + 0 = 0

∑ΔH°f(products) = 2(-285.8) = -571.7

ΔH°reaction = -517.7 - 0 = -517.7

Therefore, change in enthalpy of the reaction is -517.7 kJ.

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FAQ's on Enthalpy Calculator

1. What is meant by Enthalpy?

Enthalpy is the measurement of energy in a thermodynamic system. The amount of enthalpy is equal to the sum of heat absorbed by the system and work done.

2. What is the Enthalpy Formula?

The formula to calculate enthalpy is H = Q + pV.

The formula to find change in enthalpy is ΔH = ΔQ + p * ΔV.

3. What is enthalpy of chemical reaction?

The bonds between atoms can form or dissolve during chemical reaction to either absorb or release heat energy. The heat either absorbed or emitted under the constant pressure from a device is called the enthalpy. The reaction enthalpy is the change in enthalpy arising from a chemical reaction.

4. Why enthalpy is useful?

Enthalpy is important because it tells about the amount of heat energy in a system. An enthalpy shift shows how much enthalpy was lost or gained in terms of a chemical reaction. In simple words, enthalpy means system heat energy.