Created By : Vaibhavi Kumari

Reviewed By : Rajashekhar Valipishetty

Last Updated : May 17, 2023

Calculate the enthalpy change of a reaction with the Free Enthalpy Calculator. The reaction scheme and the enthalpy formula are two easy ways of calculating the enthalpy change. To obtain a change in enthalpy value, students must choose one technique to calculate the enthalpy, provide the appropriate input details, and hit the calculate button.

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

What is Enthalpy?

The total energy of a thermodynamic system is measured by enthalpy, which can be expressed as heat or as volume multiplied by pressure. It's a state function that only depends on a system's equilibrium state.

The change in enthalpy - the overall energy exchanged inside a system - is a more interesting number. It's a condensed version of the energy transfer (energy is in the form of heat or work done during expansion).

What is an Endothermic or Exothermic Reaction?

Endothermic and exothermic reactions are the two primary types of thermodynamic reactions. Heat is absorbed from the environment in an endothermic reaction. An exothermic one emits heat into the environment.

Both of these reaction types result in changes in energy levels and, as a result, in enthalpy. For this calculator, all you need to remember is

  • The change in enthalpy is positive if the process is endothermic because heat is acquired (absorbed from the surroundings).
  • When a reaction is exothermic, the enthalpy change is negative because heat is lost (released to the surroundings).

Enthalpy Formula

The enthalpy is calculated using the following formula H = Q + pV

  • Where, Q = internal energy
  • p = pressure
  • V = volume
  • H = enthalpy.

You'll need starting and end values with constant pressure to calculate the change in enthalpy. The enthalpy change formula is; ΔH = (Q₂ - Q₁) + p * (V₂ - V₁) or ΔH = ΔQ + p * ΔV

  • Where, Q₁, Q₂ = internal energies of the products and reactants.
  • V₁, V₂ = volume of the products and reactants
  • p = pressure
  • ΔH = change in enthalpy
  • ΔQ = change in internal energy
  • ΔV = change in volume

Standard Enthalpy of Formation

It's a different way of calculating the enthalpy change. A compound's standard enthalpy of production is ΔH°f. It's simply the difference in enthalpy required to create one mole of material in the normal state.

The formula for the standard enthalpy of formation is as follows: ΔH°reaction = ∑ΔH°f(products) - ∑ΔH°f(reactants)

  • Where, ∑ΔH°f(reactants) = sum of the standard enthalpy of the reactants.
  • ∑ΔH°f(products) = sum of the standard enthalpies of the products
  • ΔH°reaction = enthalpy change

The standard enthalpy of production for each chemical compound is obtained from the table below

Substance Change in Enthalpy in kJ/mol
O₂(g) 0
SO₂(g) −296.83
SO₃(g) −395.72
H₂O(l) −285.8
Cu₂O(s) −168.6
Mg²⁺(aq) -466.85

How to Calculate Enthalpy of a Reaction?

The processes for computing the enthalpy change of a chemical reaction using either the enthalpy formula or the chemical reaction scheme are listed below. Follow these methods to get the desired result quickly.

Using the Enthalpy Formula

  • Step 1: Calculate the internal energy, reactant volume, product volume, and pressure.
  • Step 2: Subtract the volume of the products from the volume of the reactants and multiply by the constant pressure.
  • Step 3: Subtract the products' internal energy from the reactants'.
  • Step 4: To calculate the change in enthalpy, multiply the result in step 2 by the result in step 3.

Making Use of Reaction Scheme

  • Step 1: Obtain any chemical reaction that is well-balanced.
  • Step 2: Determine the standard enthalpies of each substance involved in the reaction.
  • Step 3: Calculate the sum of reactant enthalpies and product enthalpies.
  • Step 4: To check the change in enthalpy, subtract total enthalpies products from the reactants.

FAQs on Enthalpy Calculator

1. What is a chemical reaction's enthalpy?

During a chemical process, atoms' bonds can form or dissolve, absorbing or releasing heat energy. The enthalpy of a device is the amount of heat it absorbs or emits under constant pressure. The reaction enthalpy is the enthalpy change caused by a chemical reaction.

2. What is the value of enthalpy?

Enthalpy is significant because it indicates how much heat energy is present in a system. In terms of a chemical reaction, an enthalpy shift tells how much enthalpy was lost or gained. In layman's terms, enthalpy refers to the amount of heat energy stored in a system.

3. Using Hess's law, how do you compute enthalpy?

According to Hess's law, the overall reaction's net change in enthalpy equals the sum of the changes in enthalpy for each intermediate transformation: ΔH = ΔH1+ΔH2+ΔH3.

4. What is ionisation enthalpy?

The enthalpy of ionisation is the change in enthalpy that occurs when an electron is removed from one mole of a gaseous atom.

5. What is the formula for calculating the enthalpy of reaction?

The total of the standard enthalpies of formation of the products minus the sum of the standard enthalpies of formation of the reactants is the standard enthalpy change of formation, according to this equation.

6. What is Enthalpy?

The measurement of energy in a thermodynamic system is enthalpy. The total of the heat absorbed by the system and the work done is the amount of enthalpy.