Collision Theory and the Rate of Reaction

Collision Theory and the Rate of Reaction 6.4

A chemical system consists of particles that are in constant, random motion at various speeds • A chemical reaction must involve collisions of particles with each other or the walls of the container • An effective collision is one that has sufficient energy and correct alignment/positioning of the colliding particles so that bonds can be broken and new bonds formed Collision Theory

Will a Reaction Occur?

Nitrogen and oxygen in the air could react to form the colourful, toxic gas, nitrogen dioxide. Why is it that we do not experience the formation of nitrogen dioxide even if there are an estimated 1030 molecular collisions every second? • The explanation to this question involves the concept of Activation Energy! • Activation energy  is the minimum energy that reactant particles must have in order to react, causing effective collisions Example

Activation Energy The activated complex is the arrangement of atoms at the peak activation energy; the orientation of colliding particles resulting in reaction.

The Role of a Catalyst A catalyst increases the rate of reaction without being used up itself. It acts as a site for successful collisions and decreases the activation energy needed.

ΔH +ΔH

-ΔH favours spontaneity • E.g. a campfire or fireworks is a spontaneous reaction that generates a tremendous amount of heat energy, and have a large –ΔH • ΔH is measured in kJ/mol Enthalpy

The second factor that determines spontaneity is entropy • Entropy is known as disorder or chaos in a system • Entropy is used to describe the way energy is distributed in a system. Like enthalpy, only a change in entropy S, can be measured. • An increase in entropy results in particles having “freedom of movement” or “greater randomness”; the molecules of carbon dioxide gas have greater entropy than molecules of solid carbon dioxide (dry ice). • The natural tendency is for things to get more random or chaotic, creating a +ΔS • +ΔS favours spontaneity Entropy

It generally takes energy input to make entropy decrease (-ΔS) • There is very little tendency for a pile of bricks to organize itself into a house! • It takes the energy of a bricklayer Entropy

Gibbs Free Energy is the maximum possible work that can be obtained from a chemical system. It is also measured as a change in energy, G. • The relationship between Enthalpy, Entropy and Gibb’s Free Energy is where T is temperature in degrees Kelvin. • G = H - TS • A spontaneous chemical reaction is exothermic (-H) and results in an increase in entropy (+S) • This results in a negative value for Gibbs Free Energy (G = -H - TS) Gibb’s Free Energy

Spontaneity?

The balanced chemical equation for a chemical reaction does not (in general) indicate how the reaction occurs • The collision of more than three molecules at a single place and time is very unlikely • Reactions such as this usually occur in more than one step Reaction Mechanisms

The reaction 4HBr (g) + O2(g) 2H2O (g) + 2Br2(g) takes place in a series of steps: • (1) HBr(g) + O2(g)HOOBr(g) slow (high activation energy) • (2) HOOBr(g) + HBr(g) 2 HOBr(g) fast • (3) 2[HOBr(g) + HBr(g)  H2O (g) + Br2(g)] fast Reaction Mechanism for the Production of Bromine gas from Hydrogen Bromide

The sequence of reactions that produce bromine and water is called the reaction mechanismfor the overall reaction. • The slowest step in the reaction mechanism, in this case, the first step, is called the rate determining step. • Substances that are formed during the reaction but immediately react again and are not present at the completion of the reaction are called reaction intermediates

A traditional clock reaction can be used to show how reaction mechanisms work, and how factors like concentration affect the rate of the reaction • Here are two examples: • Fun Clock Reactions • Iodine Clock Reaction (explanation) • Halloween Clock Reaction The Clock Reaction

Collision Theory and the Rate of Reaction