Kinetics, Collision Theory, and Maxwell-Boltzmann Distributions

Vocabulary flashcards covering chemical kinetics, collision theory, reaction rates, catalysts, and the Maxwell-Boltzmann energy distribution.

Activation energy ($E_A$)

The minimum energy which particles need to collide to start a reaction, usually needed to break the relevant bonds in one or either reactant molecules.

Maxwell-Boltzmann energy distribution

A continuous distribution curve showing the spread of energies possessed by molecules of a gas or liquid at a particular temperature.

Most probable energy ($E_{mp}$)

The energy possessed by the greatest fraction of molecules, represented by the peak of the Maxwell-Boltzmann distribution curve (not the same as mean energy).

Mean energy (Maxwell-Boltzmann distribution)

The average energy of all particles, located to the right of the peak ($E_{mp}$) because the distribution is asymmetrical.

Origin constraint of the Maxwell-Boltzmann curve

The requirement that the curve passes through the origin $(0,0)$ because there are no molecules with zero energy.

High-energy tail constraint

The condition that the curve never touches or crosses the x-axis because there is no theoretical maximum energy for molecules.

Area under the Maxwell-Boltzmann curve

The total area beneath the distribution curve, representing the total number of particles present.

Collision energy gain mechanism

The process by which particles with energy below $E_A$ can gain sufficient energy through collisions to react, allowing a reaction to go to completion.

Rate of reaction

The change in concentration of a substance per unit time, with the standard unit $\text{mol}\,\text{dm}^{-3}\,\text{s}^{-1}$.

Initial rate

The rate of reaction at the very start ($t = 0$), where the reaction is fastest.

Tangent method for reaction rate

Calculating the gradient of a tangent drawn to a concentration-time curve at a specific time to find the reaction rate at that instant.

Effect of concentration on reaction rate

Increasing concentration increases the number of particles per unit volume, causing particles to collide with greater frequency and resulting in a higher frequency of effective collisions.

Effect of concentration on the Maxwell-Boltzmann curve

The curve becomes taller with a greater area underneath, but the positions of $E_{mp}$ and mean energy remain unchanged because molecular energy distribution shape does not change.

Effect of temperature on the Maxwell-Boltzmann curve

The peak shifts to the right and flattens (higher $E_{mp}$ and mean energy, but fewer molecules at those values), widening the distribution and significantly increasing the proportion of particles with energy greater than $E_A$.

Effect of surface area on reaction rate

Increasing surface area causes successful collisions between reactant particles to occur more frequently, thereby increasing the rate of reaction.

Catalyst

A substance that increases reaction rates without being consumed by providing an alternative reaction route or mechanism with a lower activation energy.

Disappearing cross method

A technique using the reaction $\text{Na}_2\text{S}_2\text{O}_3 + 2\text{HCl} \rightarrow 2\text{NaCl} + \text{SO}_2 + \text{S} + \text{H}_2\text{O}$ to approximate reaction rate as $1/\text{time}$, based on sulfur precipitate clouding the solution until a cross disappears.