Calculating and Explaining Energy Change

Understanding Energy Change

The energy change in chemical reactions pertains to the differences between the energy needed to break bonds in reactants and the energy released when new bonds are formed in products.
This energy change is represented by the symbol ΔH.
Endothermic reactions absorb heat from the surroundings raising the surrounding temperature, while exothermic reactions release heat to the surroundings, lowering the surrounding temperature.

Calculating Energy Change

To calculate energy change, one must remember the equation ΔH = energy of bonds broken - energy of bonds formed.
The energy needed to break a bond, and the energy released when a bond is formed, is quantified in kJ/mol.
If the energy required to form new bonds in the products is less than the energy required to break the original bonds in the reactants, ΔH is negative, indicating an exothermic reaction.
Conversely, if the energy required to break the bonds in the reactants is greater than the energy released on forming new bonds in the products, ΔH is positive, signifying an endothermic reaction.

Explaining Energy Change

An energy profile diagram maps the energy change during a reaction and can illustrate whether ΔH is positive or negative.
For exothermic reactions, the diagram shows the products having a lower energy than the reactants.
For endothermic reactions, the diagram depicts the energy of products higher than the reactants.

Energy Changes in Real Life

A real-world application of understanding energy changes is in the field of making and using fuels. The efficacy and viability of fuels are often determined by the energy changes that occur during their formation and combustion.
In biological systems, energy changes are integral to life-sustaining processes, such as photosynthesis—an endothermic process—and cellular respiration—an exothermic process.

Effects on Energy Change

Factors such as temperature, pressure, concentration, and the presence of a catalyst can affect the energy change in a reaction.
These factors influence not the energy change itself, but the rate at which the reaction proceeds, thus indirectly affecting the timeframe in which the energy change occurs.