Bond Energies, Endothermic and Exothermic Reactions

Thermodynamics

the branch of science concerned with the energy changes accompanying physical and chemical changes.

System

whatever we are studying

Surroundings

All the rest of the universe outside the boundaries of the system

Energy

It’s the capacity to do work or the capacity to transfer heat.

The energy of a system can change by having heat or work enter or leave the system.

BUT, if heat or work enters the system, an equivalent amount of energy leaves the surroundings.

First Law of Thermodynamics or Law of Conservation of Energy

The total amount of energy of the system plus the surroundings remains constant.

Two basic forms of energy

Kinetic Energy

Potential Energy

Kinetic Energy

E_K

The energy of motion (vibrational, rotational, translational).

Potential Energy

E_P

The stored energy that matter possesses because of its position, condition, composition or electric charge.

Potential Energy Diagrams

Used to illustrate the change in potential energy of a system during a chemical reaction.

Potential Energy Diagram for an Exothermic Reaction

In this reaction, the temperature of the SURROUNDINGS increased (+ΔT, ↑Ek)

Law of Conservation of Energy 🡪 there must be a decrease in potential energy of the SYSTEM.

Therefore, there is a net release of energy from the system to the surroundings, so it is an exothermic reaction.

Potential Energy Diagram for an Endothermic Reaction

In this reaction, the temperature of the SURROUNDINGS decreased (-ΔT, ↓Ek)

Law of Conservation of Energy 🡪 there must be an increase in potential energy of the SYSTEM

Therefore, there is a net absorption of energy by the system from the surroundings, so it is an endothermic process