How is the first law of thermodynamics related to enthalpy changes?

The first law of thermodynamics is related to enthalpy changes as it governs the conservation of energy in chemical reactions.

The first law of thermodynamics, also known as the law of energy conservation, states that energy cannot be created or destroyed, only transferred or transformed. This principle is directly related to enthalpy changes in chemical reactions. Enthalpy is a measure of the total energy of a thermodynamic system. It includes the internal energy, which is the energy required to create a system, and the amount of energy required to make room for it by displacing its environment and establishing its volume and pressure.

In a chemical reaction, the change in enthalpy (ΔH) is the total energy absorbed or released by the reaction. It is the difference between the enthalpy of the products and the enthalpy of the reactants. If the reaction absorbs energy, it is endothermic and ΔH is positive. If the reaction releases energy, it is exothermic and ΔH is negative.

The first law of thermodynamics ensures that the total energy change in the universe remains zero. This means that the energy absorbed or released in a chemical reaction must be accounted for elsewhere in the universe. For example, in an exothermic reaction, the energy released by the reaction is absorbed by the surroundings, causing an increase in temperature. Conversely, in an endothermic reaction, the energy absorbed by the reaction is taken from the surroundings, causing a decrease in temperature.

In summary, the first law of thermodynamics and enthalpy changes are intrinsically linked. The law of energy conservation ensures that the energy change in a chemical reaction, as measured by the change in enthalpy, is balanced by an equal and opposite energy change in the surroundings. This fundamental principle underpins our understanding of energy transfer in chemical reactions and is a key concept in thermodynamics.