Explaining Chemical Changes 12.1 Activation Energy

Reaction Progress Chemical equations indicate whether a reaction will occur but do not say anything about how fast or the details of the molecular changes that take place. For a chemical reaction to occur, reactant particles must collide, some chemical bonds have to break, and new bonds have to form. Not all reactions occur spontaneously.

Reaction Progress Why do some chemicals react faster than others, when all other variables are controlled, except for the type of chemicals? e.g. Mg(s) reacts much faster with HCl(aq) than Zn(s). Why do some reactions need an initial input of external energy to start? e.g. A match is needed to start the combustion of a hydrocarbon.

Collision-Reaction Theory HI A chemical sample consists of entities (atom, ions, or molecules) that are in constant random motion at various speeds, rebounding elastically from collisions with each other. A chemical reaction must involve collisions of reactant entities.

An effective collision requires sufficient energy An effective collision requires sufficient energy. Collisions with the required energy have the potential to react. An effective collision also requires the correct orientation (positioning) of the colliding entities so that bonds can be broken and new bonds can be formed.

Ineffective collisions involve entities that rebound elastically from the collision.

E.g. Consider a simple reaction involving a collision between two molecules - ethene, CH2=CH2, & hydrogen chloride, HCl, for example. These react to give chloroethane.

The reaction can only happen if the hydrogen end of the H-Cl bond approaches the carbon-carbon double bond. Any other collision between the two molecules doesn't work. The two simply bounce off each other.

Of the collisions shown in the diagram, only collision 1 may possibly lead on to a reaction.

Activation Energy of a Reaction Activation energy is the minimum energy that colliding entities must have in order to react. This initial input energy may be in the form of heat, light, or electricity.

CO(g) + NO2(g) → CO2(g) + NO(g) ΔrH° = – 224.9 kJ

CO(g) + NO2(g) → CO2(g) + NO(g) ΔrH° = – 224.9 kJ The diagram on the right just tells us about the “before and after.”

H2(g) + I2(g) → 2 HI(g) ΔrH° = + 53.0 kJ

The activated complex is the chemical entity containing the collided reactants. The activated complex occurs at the maximum potential energy point in the change along the energy pathway. If a large quantity of energy is needed to start a reaction and if the reaction progresses relatively slowly, then the activation energy is large. A spontaneous reaction at room temperature and a higher rate of reaction is interpreted as a relatively small activation energy.

Activation Energy & Ozone The biosphere, the animals on our planet &certainly human life became possible due to the formation of ozone in the stratosphere. Ozone protects us from the UVC and UVB radiation of the sun (light with wavelength of less than 320 nm), which damages the biomolecules. UV light drives ozone formation & destruction.

In the stratosphere there are two forms of oxygen: normal oxygen O2 (consisting of two O atoms) & ozone O3 (consisting of three O atoms). In order to transform one form into the other intensive UV-light is necessary. The O-O bond of an oxygen molecule is broken by UVC. The formed O atom reacts with oxygen (and for energetic reasons a collision partner M) & forms ozone. This UV-light (yellow wave in the image on the left) comes from the sun.

1. Ozone formation - light splitting oxygen molecules leads to ozone formation (UVC)

In an analogous way, ozone is destroyed by photolysis (UVB radiation), if the O-O bond in an ozone molecules is split by sunlight. In this case the formed O atom reacts with another ozone molecule and forms two oxygen molecules O2, which then act to replenish the ozone.

2. Ozone photolysis -degradation by sunlight (UVB)

Homework: Read pgs. 524 – 530 Practice #’s 1 – 3 (p. 526) Section 12.1 Questions #’s 1 – 7 (p. 531)