Introduction:
The first law of thermodynamics is the law of conservation of energy, which states that energy cannot be created nor destroyed, but it can be converted from one form to another. This law applies to all systems, including chemical reactions and physical processes.

Internal Energy:
The internal energy of a system is the sum of all the kinetic and potential energies of its particles. This energy is a state function, which means that it depends only on the current state of the system and not on how it got there. The internal energy of a system can be changed by adding or removing heat or work from it.

Heat and Work:
Heat is the transfer of thermal energy between two objects at different temperatures. Work is the transfer of energy from one system to another through mechanical means, such as by pushing a piston or lifting a weight. Both heat and work can change the internal energy of a system.

First Law of Thermodynamics:
The first law of thermodynamics states that the total energy of a closed system remains constant. This means that the energy entering the system must equal the energy leaving the system. In other words, the change in internal energy of a system is equal to the heat added to the system minus the work done by the system.

Conclusion:
The first law of thermodynamics leads to the concept of internal energy because it shows that the total energy of a system is conserved, and that changes in internal energy can be caused by the transfer of energy through heat and work. Understanding the relationship between heat, work, and internal energy is essential for understanding how energy is transferred in chemical reactions and physical processes.