Important AS Definitions Part II

Activation energy is the minimum energy required by colliding particles to start off a reaction.


A catalyst is a substance which changes the rate of a chemical reaction without itself undergoing any chemical change at the end of the reaction.

Homogenous catalyst is a catalyst in which the physical state is in the same phase as the reactants.

Heterogenous catalyst is a catalyst in which the physical state is in the different phase as the reactants.

Increase the concentration of reactants will increase the rate since there will be more molecules per unit volume resulting in an increase in the collision frequency between the reactants. This leads to higher number of effective collisions.

Increase in the temperature will increase the rate since at higher temperature the particles gain more kinetic energy resulting an increase in collision frequency at higher temperature. In addition, more molecules have kinetic energy greater than or equal to the activation energy. Consequently, the number of effective collisions increases.

A catalyst operates by providing an alternative route of lower activation energy for the reaction.

Geometric isomers are compounds with different spatial arrangement due to the restriction in the rotation about the double bond in an alkene. In geometric isomers, two different groups at each end of the double bond can either be on the same side (cis) or on opposite sides (trans)

Optical isomers are mirror images that have the same geometric and structural formula, but cannot be superimposed onto one another.

Cracking is a process to break long alkane into a shorter alkane and an alkene. Hydrogen may also be produced. Thermal cracking requires temperature between 400°C to 900°C and high pressure whereas catalytic cracking requires a zeolite catalyst and a temperature of about 450°C. The purpose of cracking is to form extra gasoline and to produce alkenes with higher commercial value

Homolytic fission is the breaking a covalent bond in which one electron goes to each atom to form free radicals.

Heterolytic fission is the breaking a covalent bond in which one atom takes both electrons forming negative and positive ions.

A free radical is a highly reactive atom or molecule with an unpaired electron.

An electrophile is an electron-deficient species that can form a new covalent bond by accepting an electron pair provided by the carbon compound. Example : positive ion such as hydrogen ion or nitronium ion and a polar molecule with a d+ charge

A nucleophile is an electron-rich species with a lone pair of electrons that can form a new covalent bond by donating a pair of electrons to the electron deficient carbon atom. Example : negative ions with at least a lone pair.