Glossary
Activation Energy (Ea)
The minimum amount of energy required for reactants to transform into products, representing the energy barrier that must be overcome for a reaction to occur.
Example:
For a match to ignite, the friction provides enough heat to overcome the activation energy of the combustion reaction.
Elementary Reactions
Individual steps that make up a larger, more complex chemical reaction. Each step represents a single molecular event.
Example:
The decomposition of ozone (O₃) often involves an initial elementary reaction where O₃ breaks down into O₂ and an oxygen atom (O).
Endothermic
A chemical reaction that absorbs energy from its surroundings, typically in the form of heat, causing the surroundings to cool down.
Example:
An instant cold pack uses an endothermic reaction, like the dissolution of ammonium nitrate in water, to absorb heat and create a cooling sensation.
Exothermic
A chemical reaction that releases energy, typically in the form of heat, causing the surroundings to warm up.
Example:
The burning of natural gas is an exothermic reaction, releasing heat that can be used for cooking or heating homes.
Intermediates
Species that are formed in one step of a reaction mechanism and then consumed in a subsequent step, so they do not appear in the overall balanced equation.
Example:
In the decomposition of hydrogen peroxide, a hydroxyl radical (•OH) might be an intermediate that is quickly formed and then reacts further.
Overall Energy Change (ΔE)
The difference in potential energy between the products and the reactants in a chemical reaction, indicating whether energy is released or absorbed.
Example:
If the products of a reaction have lower potential energy than the reactants, the overall energy change will be negative, signifying an exothermic process.
Overall Reaction
The net chemical change that occurs from start to finish, representing the sum of all elementary reactions in a mechanism.
Example:
When hydrogen gas reacts with oxygen gas to form water, the balanced equation 2H₂ + O₂ → 2H₂O represents the overall reaction.
Products
The new chemical substances that are formed as a result of a chemical reaction.
Example:
When baking soda and vinegar react, the bubbles you see are carbon dioxide gas, one of the products of the reaction.
Reactants
The starting chemical substances that undergo a transformation during a chemical reaction.
Example:
In the photosynthesis equation, carbon dioxide and water are the reactants that plants use to produce glucose and oxygen.
Reaction Energy Profile
A graph that illustrates the potential energy changes of reactants and products as a reaction progresses, showing activation energies and overall energy changes.
Example:
A chemist might draw a reaction energy profile to visualize how a catalyst lowers the energy barrier for a specific synthesis reaction.
State Symbols
Letters (g, l, s, aq) placed in parentheses after each chemical formula in an equation to indicate the physical state of the substance.
Example:
When writing the combustion of methane, CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l), the (g) and (l) are state symbols indicating gas and liquid phases, respectively.
Stoichiometric Coefficients
The numbers placed in front of reactants and products in a balanced chemical equation, indicating the relative number of moles or molecules involved.
Example:
In the reaction N₂ + 3H₂ → 2NH₃, the stoichiometric coefficient for hydrogen is 3, meaning three moles of H₂ react for every one mole of N₂.
Transition State
A high-energy, unstable arrangement of atoms that exists momentarily at the peak of the activation energy barrier during a chemical reaction.
Example:
During an SN2 reaction, the carbon atom undergoing substitution forms a five-coordinate transition state where it is partially bonded to both the incoming nucleophile and the leaving group.