Measurement of ??U and ??H — Definition

Definition

Imagine a chemical reaction happening. It either releases energy (exothermic) or absorbs energy (endothermic). This energy change can be quantified in two primary ways in chemistry: as a change in internal energy ($\Delta U$) or as a change in enthalpy ($\Delta H$).

Think of internal energy, $U$, as the total energy contained within a system, including kinetic and potential energies of its molecules. When a reaction occurs in a sealed, rigid container, like a strong steel bomb, its volume cannot change.

In this scenario, all the heat exchanged is directly related to the change in internal energy, $\Delta U$. We measure this using a device called a 'bomb calorimeter'. It's essentially a sealed container where the reaction takes place, surrounded by a known amount of water.

By measuring the temperature change of the water, we can calculate the heat released or absorbed by the reaction, which directly gives us $\Delta U$. \n\nNow, consider most reactions you do in a lab, like mixing two solutions in an open beaker.

These reactions usually happen at constant atmospheric pressure, not constant volume. In such cases, the system can expand or contract, meaning it can do work on the surroundings or have work done on it.

To account for this pressure-volume work, we use a different thermodynamic quantity called enthalpy, $H$. Enthalpy is defined as $H = U + PV$, where $P$ is pressure and $V$ is volume. The change in enthalpy, $\Delta H$, represents the heat exchanged at constant pressure.

This is typically measured using a 'coffee-cup calorimeter', which is a simpler device, often just two Styrofoam cups nested together with a lid and a thermometer. The reaction occurs in the solution inside the cups, and since the cups are open to the atmosphere, the pressure remains constant.

Again, by monitoring the temperature change of the solution, we can calculate the heat flow, which directly gives us $\Delta H$. \n\nSo, in simple terms, $\Delta U$ tells us the heat change when a reaction is forced to occur without any volume change, while $\Delta H$ tells us the heat change when a reaction occurs under normal, constant pressure conditions, allowing for volume changes.

Both are crucial for understanding the energy profile of chemical processes.