Energy cycles

Energy Cycles

An energy cycle presents a visual representation of the energy changes occurring in a chemical reaction.
Within an energy cycle, reactions are represented as pathways, and each step in the pathway corresponds to a particular change in energy (enthalpy change).
Energy cycles are a practical application of Hess’s Law, as they provide an alternative method to calculate enthalpy changes for reactions that are difficult to measure directly.
Energy cycles are typically depicted as diagrams with arrows indicating the direction of the reactions and energy changes.

There are two common types of energy cycles: Combustion cycles and Formation cycles.
Combustion cycles demonstrate the enthalpy changes in reactions involving the complete combustion of a substance.
Formation cycles involve the formation of a substance from its elements in their standard states.
The choice between a combustion and formation cycle depends on knowledge of the enthalpies of reaction, combustion, or formation.

The direction of the arrows in an energy cycle is specially chosen. Clockwise arrows describe exothermic processes (negative enthalpy changes), while anticlockwise arrows represent endothermic processes (positive enthalpy changes).
Energy levels of reactants and products are taken into account. Higher energy levels indicate that the substance is less stable and more reactive.
The enthalpy of a reaction can be calculated by adding up the enthalpies along the arrows in the cycle leading from reactants to products, following Hess’s Law.

Energy cycles assume that all reactions occur under standard conditions of temperature (298 K) and pressure (1 atm).
If a substance in the energy cycle is in a different state (solid, liquid, gas) than its standard state, this must be accounted for by including the corresponding enthalpy of transition (fusion, vaporisation, etc.).
There are limitations in representing multi-step reactions in a simple energy cycle. It doesn’t provide information about reaction intermediates.
Energy cycles provide no information about the rate of reaction; they only show the overall energy change for a reaction.
All energy values used in the energy cycle must be under the same standard conditions to maintain consistency.