Enthalpy Definition in Physical Chemistry

In more detail

The math formula for enthalpy is H = U + PV. The letter U stands for internal energy, P is pressure, and V is volume. Most chemistry experiments take place in open beakers or flasks on a lab bench.

Because they are open to the room, the atmospheric pressure stays perfectly constant. Under these open conditions, the change in enthalpy (ΔH) exactly equals the heat absorbed or released. You can easily measure this heat flow using a simple device called a calorimeter.

If a reaction releases heat into the room, it has a negative ΔH value. We call this an exothermic reaction, and it will feel hot to the touch. If a chemical reaction absorbs heat, it has a positive ΔH value.

We call this an endothermic reaction, and it makes the surroundings feel cold. Enthalpy is also known as a thermodynamic state function. This means the total heat change depends only on your starting and ending points.

It does not matter how many chemical steps the reaction takes to get there. This helpful rule is famously known in chemistry as Hess’s law.

Key facts

Symbol

H (ΔH for change in enthalpy)

Defining equation

H = U + PV

SI unit

joule (J), commonly kJ/mol for reactions

Negative ΔH

Exothermic reaction (releases heat)

Positive ΔH

Endothermic reaction (absorbs heat)

Field

Physical Chemistry

Example

Think about the combustion reaction when you burn methane gas on a kitchen stove. The balanced equation is CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l). The enthalpy change (ΔH) for this specific chemical reaction is -890.4 kJ/mol. The negative sign is a very important detail here. It clearly tells us that the reaction releases heat into the surrounding kitchen. Because it is an exothermic reaction, we can use that released heat to boil water. If you somehow ran this reaction in reverse, the enthalpy value would simply flip. The reverse reaction would be endothermic and require an input of +890.4 kJ/mol.

Frequently asked questions

Why do chemists use enthalpy instead of internal energy?

Most reactions happen in open vessels where atmospheric pressure stays constant. Enthalpy automatically accounts for the energy used if the chemicals expand and push the air away. This makes it much easier to measure heat flow directly.

How is a change in enthalpy (ΔH) different from a change in temperature?

Temperature measures how fast molecules are moving at one specific moment. Enthalpy measures the total heat energy absorbed or released as chemical bonds break and form during a reaction.

What does it mean that enthalpy is a state function?

It means the total enthalpy change for a reaction is always the same. It does not matter if the reaction happens in one fast step or five slow steps.