o The amount of time required for a chemical rxn to come to completion can vary tremendously

• When you strike a match it seems flame up instantly

• Coal is made over millions of years from very slow chemical reactions

o Another topic we will explore in this lecture is the concept that some rxns tend to reverse upon themselves

Collision Theory

o The concept of rate is familiar• A fast sprinter may cover 100 m in 11.5 s

• A slower sprinter may take 15 s to run the same distance

o On average the 1st sprinter runs at a speed of 8.7m/s

o The 2nd runs at a speed of 6.7m/s• Both speeds are expressions of rates of travel

Collision Theory

o The word rate can be used as a synonym of speed• Rates measure the speed of

any change that occurs within an interval of time

• The interval of time may range from fractions of a second to centuries

o Rates of chemical change usually are expressed as the amount of reactant forming products per unit time.

Collision Theory

Collision Theory

o Rates of chem rxns are related to the properties of atoms, ions, and molecules through a model called collision theory

o According to collision theory, atoms, ions, and molecules can react to form products when they collide• provided that the particles

have enough kinetic energy

Collision Theory

Collision Theory

oThe minimum amount of energy that the particles or reactants must have in order to react is called the rxn’s activation energy.

• In a sense the activation energy is a barrier that reactants must get over to be converted to products

• The higher the barrier the larger the investment of energy in order to get the rxn to proceed

Collision Theory

o Collision theory explains why some naturally occurring rxns are immeasurably slow at room temp.

• Carbon and Oxygen react when charcoal burns, but this reaction has a high activation energy

• At room temp, the collisions of oxygen and carbon molecules aren’t energetic enough to react

• But the rxn can be helped along a number of ways

Collision Theory

o It is possible to vary the conditions of the rxn, the rate of almost any rxn can be modifiedo collision theory can help

explain why the rates can be modified

o Several strategies can be used:o Increase the temperatureo Increase the concentrationo Decrease the particle sizeo Employ a catalyst

Reaction Rates

o Increasing the temp speeds up the rxn, while lowering the temp slows down the rxn

o Increasing the temp increases the frequency of the collisions

• Collisions taking place more often more likely they are to stick

o And the extra energy increases the power of the collisions

• Also increasing the likelihood of a successful collision

Temperature

o Just sitting out, charcoal does not react at a measurable rate• However, when a starter flame touches the charcoal, atoms of reactants collide with higher energy and greater frequency

• Some of the collisions are high enough in energy that the product CO2 is formed

o The energy released by the rxn then supp-lies enough energy to get more C and O2 over the activation-energy barrier• Evidence of this would be if you remove the starter flame, the rxn will continue on its own.

o The more reacting particles you have in a given volume, the higher the rate of rxn.

o Cramming more particles into a fixed volume increases the concentration of reactants,

• Increasing the concentration, increases the frequency of the collisions, and therefore increasing the reaction rate.

Concentration

oThe smaller the particle size, the larger the surface area for a given mass of particles

oThe total surface area of a solid or liquid reactant has an important effect on the rate of reaction.

oAn increase in surface area increases the amount of the reactant exposed for collision to take place…

• Which increases the collision frequency and the reaction rate.

Particle Size

oOne way to increase the surface area of solid reactants is to dissolve them

• which separates the particles and makes them more accessible to other reactants.

oGrinding solids into a fine powder also increases the surface area of reactants

• Small dust-like particles can be very dangerous, can be highly explosive

Particle Size

o An increase in temp is not always the best way to increase the rate of rxn

• A catalyst is often better.o A catalyst is a substance that

increases the rate of a rxn without being changed during the rxn

• They permit rxns to proceed at lower energy than is normally required

o With a lower activation energy more reactants can form products in a given amount of time.

Catalyst

Catalyst

Catalyst

o Since catalysts are not consumed during a rxn, they do not appear as reactants or products in the chem eqn

• Often written above the yield arrow

o Catalysts are crucial for many life processes.

• Your body temp is only 37°C and cannot be raised significantly without danger

o Without catalysts, few rxns in the body would proceed fast enough at that temp

Catalyst

• Enzymes, biological catalysts, increase the rates of biological rxns

oWhen you eat a meal containing protein, enzymes in your digestive tract break down the protein molecules in a few hours..

• Without enzymes, the digestion of proteins at 37C takes yrs

oAn inhibitor is a substance that interferes with the action of a catalyst

• An inhibitor molecule could work by reacting with or “poisoning” the catalyst itself

Catalyst