Glossary
Activated Complex
An unstable, high-energy intermediate state formed at the peak of the energy diagram during a reaction, where old bonds are breaking and new bonds are forming.
Example:
The activated complex exists for only a fleeting moment before either reverting to reactants or proceeding to products.
Activation Energy (Ea)
The minimum amount of energy required for reactants to transform into products, representing an energy barrier that must be overcome for a reaction to occur.
Example:
Lighting a match requires a small amount of heat to overcome the activation energy and start the combustion reaction.
Condensation
An exothermic phase change where a gas transforms into a liquid by releasing heat energy.
Example:
Water droplets forming on the outside of a cold glass on a humid day is an example of condensation.
Deposition
An exothermic phase change where a gas transforms directly into a solid without passing through a liquid phase, by releasing heat energy.
Example:
Frost forming on a cold window pane on a winter morning is an example of deposition of water vapor.
Endothermic Reaction
A chemical reaction that absorbs heat energy from its surroundings, resulting in the products having higher potential energy than the reactants.
Example:
An instant cold pack uses an endothermic reaction to absorb heat from its surroundings, making the pack feel cold.
Energy Diagrams
Visual representations that illustrate the energy changes occurring during a chemical reaction or physical process, showing the relative energies of reactants, products, and the transition state.
Example:
An energy diagram for burning methane would show the reactants (methane and oxygen) at a higher energy level than the products (carbon dioxide and water), indicating an exothermic process.
Enthalpy (ΔH)
A thermodynamic property that represents the total heat content of a system at constant pressure, with ΔH measuring the heat exchanged during a reaction.
Example:
A negative ΔH value for a reaction indicates that it is exothermic, meaning heat is released.
Exothermic Reaction
A chemical reaction that releases heat energy to its surroundings, resulting in the products having lower potential energy than the reactants.
Example:
The combustion of propane in a grill is an exothermic reaction, releasing heat that cooks your food.
Freezing
An exothermic phase change where a liquid transforms into a solid by releasing heat energy.
Example:
When water turns into ice in a freezer, it undergoes freezing, releasing heat to the cold environment.
Latent Heat
The energy absorbed or released during a phase change at constant temperature, used to overcome intermolecular forces rather than increase kinetic energy.
Example:
When ice melts, it absorbs latent heat from the surroundings without its temperature rising above 0°C until all the ice has turned to water.
Melting
An endothermic phase change where a solid transforms into a liquid by absorbing heat energy.
Example:
Ice cubes undergo melting when left out on a warm day, absorbing heat from the air.
PEproducts
The potential energy stored within the chemical bonds and intermolecular forces of the substances formed at the end of a reaction.
Example:
If the PEproducts are lower than the PEreactants, the reaction is exothermic.
PEreactants
The potential energy stored within the chemical bonds and intermolecular forces of the starting materials in a reaction.
Example:
On an energy diagram, the initial energy level at the beginning of the reaction pathway represents the PEreactants.
Phase Changes
Physical processes where a substance transitions from one state of matter (solid, liquid, gas) to another, involving the absorption or release of energy.
Example:
Boiling water is a phase change from liquid to gas, requiring energy input.
Products
The new chemical substances that are formed as a result of a chemical reaction.
Example:
When baking soda and vinegar react, carbon dioxide gas is one of the products you observe bubbling.
Reactants
The starting chemical substances that undergo a transformation during a chemical reaction.
Example:
In the reaction 2H₂ + O₂ → 2H₂O, hydrogen and oxygen are the reactants that combine to form water.
Reaction Coordinate Diagrams
Graphs that illustrate the energy changes along the pathway of a chemical reaction, showing the progress from reactants to products through transition states.
Example:
Unlike simple energy diagrams, reaction coordinate diagrams can show multiple steps and intermediates in a complex reaction mechanism.
Sublimation
An endothermic phase change where a solid transforms directly into a gas without passing through a liquid phase, by absorbing heat energy.
Example:
Dry ice (solid CO₂) undergoes sublimation, turning directly into CO₂ gas at room temperature.
Vaporization/Boiling
An endothermic phase change where a liquid transforms into a gas by absorbing heat energy.
Example:
Heating water to 100°C causes vaporization or boiling, as liquid water turns into steam.