The standard enthalpy of a reaction which has a symbol of ΔHr⦵ refers to the enthalpy change that happens in a given system when matter gets transformed by a chemical reaction. What is meant by the standard enthalpy of a reaction? If you get a negative value for q, this means that you have an exothermic reaction which means that it releases heat into the surroundings. If you get a positive value for q, this means that you have an endothermic reaction which means that it absorbs heat from the surroundings. Therefore, a better version of this equation is:īut for constant pressure, the standard enthalpy change simply refers to transferred heat: In this equation, the delta symbol or ∆ means “a change in.” Normally, you would hold the pressure as a constant value. Therefore, a standard enthalpy change is: If you want to define enthalpy in the most precise way, it refers to the sum of internal energy “U” plus the product of the pressure (P) and the volume (V).

For this situation, use this simple formula: However, if you already know the enthalpy of both the reactants and the products, the calculation becomes easier and simpler. But if you want to perform a manual calculation, there are different methods for this and they depend on the date you have as well as the specific situation.įor most of the calculations, one significant piece of data you must use is Hess’ Law. You can use a delta h calculator, a heat of reaction calculator or a standard enthalpy calculator to calculate standard enthalpy change automatically. How do you calculate standard enthalpy change? ΔQ refers to the change in the internal energy P refers to pressure which remains constant V₂ and Q₂ refer to the volume and internal energy of the reactants V1 and Q1 refer to the volume and internal energy of the reaction of the product ΔH = (Q₂ – Q₁) + p * (V₂ – V₁) or ΔH = ΔQ + p * ΔV when simplified In such a situation, the change in enthalpy is: Let’s assume that the pressure remains constant when the reaction occurs. Therefore, by definition, enthalpy is the total heat absorbed by a system along with the work done when it’s expanding:īut when it comes to calculating the change in enthalpy, you must consider both the initial and final state. This is a simplified explanation of energy transfer which refers to the energy in the form of work or heat done in an expansion. The change of enthalpy is the more intriguing quantity as this refers to the total energy exchanged within a given system. Enthalpy is a state function which means that it only depends on a system’s equilibrium state.

It comes in the form of either volume or heat multiplied by pressure. This refers to the measurement of the total energy of a given thermodynamic system.

To understand this enthalpy change calculator better, let’s learn more about enthalpy.

Enthalpy calculator thermodynamics