What is Enthalpy
Enthalpy: A quantity used to describe heat changes at a constant pressure. It is represented by H.
For enthalpy to be measured, the pressure of the system must remain constant (isobaric). This is because most chemical reactions take place at a constant pressure.
q = ∆H
(heat equals the change in enthalpy), meaning ↴
– Positive ∆H → system gains energy → endothermic
– Negative ∆H → system releases energy → exothermic
Enthalpy is a state function, meaning we only care about the initial and final states of enthalpy, not the path it took to get there. Therefore, we can just look at H𝑓 (final enthalpy) and Hᵢ (initial enthalpy) to find the change in enthalpy.
∆H = H𝑓 – Hᵢ
In a chemical reaction, the Hᵢ is the enthalpy of the reactants, and H𝑓 is the enthalpy of the products. Therefore ↴
∆Hᵣₓₙ = ΣH(products) – ΣH(reactants)
– ΔHᵣₓₙ is the change in enthalpy (heat) of the chemical reaction
– ΣH(products) is the sum of the enthalpies of the products
– ΣH(reactants) is the sum of the enthalpies of the reactants
But how do you find the enthalpies of the products and reactants? You will learn below.
Standard Enthalpies
Standard enthalpy of formation is the change in enthalpy from forming one mole of a substance at 25˚C and 1 atm. It is measured in kJ/mol.
Standard enthalpies are represented by H˚. The ˚ indicates standard conditions (25˚C and 1 atm).
There’s no way to look at a molecule and know its standard enthalpy of formation. When doing enthalpy change problems, you will be given data tables with standard enthalpy values.
The only exception is with pure elements. Pure elements in their normal elemental states have an enthalpy of zero (H˚ = 0). For example O₂ has a standard enthalpy of formation of zero (H˚ = 0). So would Au, Cl₂, Na, and any other elemental substance.
∆Hᵣₓₙ = ΣH˚(products) – ΣH˚(reactants)
You saw this equation above, but now it is using standard enthalpy values. ΣH˚(products) is the sum of the standard enthalpies of formation of the products. ΣH˚(reactants) is the sum of the standard enthalpies of formation of the reactants.
So to find the change in enthalpy (heat) of a reaction, add up the standard enthalpies of the products, then subtract the standard enthalpies of the reactants.
∆Hᵣₓₙ is the change in enthalpy of a reaction, however sometimes it may be represented differently for specific types of reactions. For example, ∆H(comb) is the change in enthalpy of a combustion reaction. It is the ∆Hᵣₓₙ for a combustion reaction. In a combustion reaction, ∆H(comb) and ∆Hᵣₓₙ mean the same thing.
Some other common ones are ↴
– ∆H(vap) = heat of vaporization (boiling)
– ∆H(fus) = heat of fusion (melting)
Example Problem
Answer
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