Question

What is the difference between ∆H and ∆U? What is the sign of ∆H for exothermic and endothermic reactions? Under what circumstances ∆H =∆U?

Answer 1

∆H and ∆U denotes the change in enthalpy and change in internal energy of a system respectively. The enthalpy is used to refer to the total heat content of a system and internal energy of a system is the energy contained within the system.

Complete step by step solution:
- Let’s start with the concept of enthalpy. Enthalpy is the heat energy which is evolved or absorbed during a chemical reaction and is represented by H. ∆H is the enthalpy change in a system and the unit is joules or kilojoules.
- Internal energy of a system refers to the addition of kinetic and potential energy of that particular system and is represented by U and the change in internal energy is given by ∆U.
- The change in enthalpy ∆H of a reaction is connected to the change in internal energy ∆U at constant pressure with the volume change ∆V by the given relation
$\Delta H=\Delta U+P\Delta V$
The difference between ∆H and ∆U can be given as
$\Delta H-\Delta U=P\Delta V$
- A system which releases heat to the surroundings, an exothermic reaction, will have a negative ∆H, since the enthalpy of the products is lower than the enthalpy of the reactants of the system and a system of reactants which absorbs heat from the surroundings in an endothermic reaction has a positive ∆H, since the enthalpy of the products is higher than the enthalpy of the reactants of the system.
For endothermic reaction ∆H> 0
For exothermic reaction ∆H< 0
- When the ∆V or change in volume is zero(∆V=0) (for solids and liquids the volume change is negligible) or when none of the products or reactants are in gaseous states or when number of moles of reactants is equal to the number of products , change in internal energy becomes equal to change in enthalpy i.e., ∆H =∆U

Note: Keep in mind that the measurement of internal energy and enthalpy change is normally carried out by an experimental approach called calorimetry and these techniques are established on thermometric procedures.