Energetics of a Reaction

Energetics of a Reaction Basics

• The term energetics refers to the study of energy changes in chemical reactions.
• Chemical reactions involve the breaking and forming of bonds, which involve energy changes because energy is required to break bonds and energy is released when bonds are formed.
• Exothermic reactions are those in which heat energy is released, while endothermic reactions are those where heat energy is absorbed from the surroundings.
• The term enthalpy is used to represent the total heat content of a system, denoted as H.

Types of Enthalpy Change

• Standard enthalpy change of formation (∆Hf°): The enthalpy change when one mole of a substance is formed from its elements in their standard states.
• Standard enthalpy change of combustion (∆Hc°): The enthalpy change when one mole of a substance completely reacts with oxygen under standard conditions.
• Standard enthalpy change of reaction (∆Hr°): The enthalpy change when a reaction occurs in the molar quantities shown in the chemical equation, under standard conditions.

Calculating Enthalpy Change

• Hess’ Law states that the total enthalpy change for a reaction is independent of the route by which the chemical change takes place.
• Hess’ Law can be applied to calculate the enthalpy change for a reaction by forming a Hess cycle.
• Heat energy can be determined experimentally through calorimetry, where heat absorbed or released by a chemical reaction is determined by observing the temperature change in the surroundings.

Bond Enthalpies

• A bond enthalpy is the energy required to break one mole of a specific type of bond in a gaseous molecule.
• When a bond is broken, energy is absorbed (endothermic), and when a bond is formed, energy is released (exothermic).
• The total energy change in a reaction depends on the balance between the energy taken in to break bonds and the energy released when new bonds are formed. This is called the mean bond enthalpy.

Spontaneous Reactions and Gibbs Free Energy

• The Gibbs free energy change (∆G) is a measure of the spontaneity of a process at constant pressure and temperature.
• If ∆G is negative, the reaction is spontaneous, and if ∆G is positive, the reaction is non-spontaneous.
• The concept of entropy (∆S), a measure of disorder or randomness, also contributes to the spontaneity of a reaction. Higher entropy favours reaction spontaneity.
• The relationship between Gibbs free energy, enthalpy and entropy is given by the equation ∆G = ∆H - T∆S.
• This equation combines both the enthalpic and entropic trends to give an overall indication of the feasibility of a reaction.