Glossary
BARF mnemonic
A memory aid (Breaking Absorbs, Releasing Forms) used to remember that breaking bonds requires energy (absorbs) and forming bonds releases energy.
Example:
Using the BARF mnemonic helps quickly recall that energy is absorbed when breaking the bonds in reactants.
Bond Dissociation Energy (BDE)
The specific amount of energy required to break one mole of a particular type of bond in the gaseous state. It is a measure of bond strength.
Example:
The high Bond Dissociation Energy of the N≡N triple bond explains why nitrogen gas is so unreactive.
Bond breaking
The process where chemical bonds between atoms are severed, requiring an input of energy.
Example:
When you ignite methane, the C-H bonds in the bond breaking process absorb energy before new bonds can form.
Bond energetics
The study of energy changes associated with the breaking and forming of chemical bonds during a reaction.
Example:
Understanding bond energetics helps predict if a chemical reaction will release heat or absorb it from the surroundings.
Bond forming
The process where new chemical bonds are established between atoms, leading to the release of energy.
Example:
The creation of water from hydrogen and oxygen involves significant bond forming, releasing a large amount of energy as heat.
Bond length
The average distance between the nuclei of two bonded atoms, which is inversely related to bond strength.
Example:
A shorter bond length generally indicates a stronger bond, like in a C≡C bond compared to a C-C bond.
Double bonds
Covalent bonds formed by the sharing of two pairs of electrons between two atoms, stronger and shorter than single bonds but weaker than triple bonds.
Example:
The carbon-carbon double bond in ethene (C₂H₄) is rigid and prevents rotation, influencing molecular geometry.
Endothermic
A process or reaction that absorbs energy from its surroundings, resulting in a positive enthalpy change (ΔH > 0).
Example:
Melting ice is an endothermic process because it requires heat energy from the environment to break the intermolecular forces.
Enthalpy of Reaction (ΔH)
The change in enthalpy during a chemical reaction, representing the total heat absorbed or released at constant pressure.
Example:
A negative Enthalpy of Reaction for combustion indicates that the reaction releases heat, making it exothermic.
Exothermic
A process or reaction that releases energy, typically as heat or light, into its surroundings, resulting in a negative enthalpy change (ΔH < 0).
Example:
The combustion of propane in a grill is an exothermic reaction, releasing heat that cooks your food.
Products
The substances that are formed as a result of a chemical reaction.
Example:
In the reaction 2H₂ + O₂ → 2H₂O, water is the product.
Reactants
The starting materials in a chemical reaction that undergo transformation to form new substances.
Example:
In the reaction 2H₂ + O₂ → 2H₂O, hydrogen and oxygen are the reactants.
Single bonds
Covalent bonds formed by the sharing of one pair of electrons between two atoms, typically the longest and weakest type of covalent bond.
Example:
The C-C single bond in ethane (C₂H₆) allows for free rotation around the bond axis.
Triple bonds
Covalent bonds formed by the sharing of three pairs of electrons between two atoms, making them very strong and short.
Example:
The carbon-carbon triple bond in acetylene (C₂H₂) is incredibly strong, requiring a lot of energy to break.
ΣH(broken)
The sum of the bond dissociation energies for all bonds that are broken in the reactant molecules during a chemical reaction.
Example:
In the reaction of H₂ with Cl₂, the ΣH(broken) would include the energy to break the H-H and Cl-Cl bonds.
ΣH(formed)
The sum of the bond dissociation energies for all bonds that are formed in the product molecules during a chemical reaction.
Example:
For the formation of water, the ΣH(formed) would account for the energy released when O-H bonds are created.