U1 S1 L3 Factors affecting Reaction Rates Textbook Readings page 466: Method for Measuring Reaction Rates page 467: Factors That Affect Reaction Rate pages : Collision Theory and... Concentration, Surface Area, Nature of Reactants, and Temperature

Upon completion of this lesson, you will be able to: explain using collision theory how each of the following affect the rate of reaction: 1.temperature 2.concentration/pressure 3.surface area 4.nature of reactants describe the effect of a catalyst on the rate of a reaction. draw and label a one-step potential energy diagram to show the effect of a catalyst on the reaction.

Chemical reactions proceed at different rates. The factors that affect reaction rates are: 1.surface area of a solid reactantsurface area 2.concentration or pressure of a reactantconcentrationpressure 3.temperaturetemperature 4.nature of the reactantsnature of the reactants 5.presence/absence of a catalyst.catalyst A change in any one or more of these factors may alter the rate of a reaction. In this lesson, you will define these factors, and describe and predict their effects on reaction rates.

In general, anything that can increase the frequency (number) of collisions and/or the intensity (energy) of collisions will speed up the reaction. The reverse is true too, anything that decreases the frequency and/or intensity of collisions will slow down a reaction. Recall: the collision theory.

1. Surface area Surface area is the exposed matter of a solid substance. What has more surface area: –A junk of wood or splits of wood? –A 5.0 g chunk of coal or 5.0 g of coal dust?

The rate of reaction is directly proportional to surface area. ↑ surface area ↑ rate of reaction The collision theory says: –the greater the chance of collisions the greater the chance of a reaction occurring. With increased surface area we have more molecules that are now in contact with each other which would increase the frequency (the number) of collisions. –Analogy: a busy highway in a city with increased number of cars will potentially result in more accidents compared to traffic loads in the country. –Grain elevators / coal mines / carburators (fuel injectors)

2. Concentration of reactants In terms of the collision theory, –increasing the concentration of a reactant increases in the number of collisions between the reacting species per second and therefore increases the reaction rate. ↑ Concentration ↑ Reaction rate –Only true for gases and aqueous solutions. Pure solids are already 100%. Ie: you can not increase the concentration of Zn (s) Consider: Which one will react faster?

Gas pressure The concentration of a gas is a function of the pressure on the gas. Increasing the pressure of a gas is exactly the same as increasing its concentration. If you have a certain number of gas molecules, you can increase the pressure by forcing them into a smaller volume. Under higher pressure or at a higher concentration, gas molecules collide more frequently and react at a faster rate. Conversely, increasing the volume of a gas decreases pressure which in turn decreases the collision frequency and thus reduces the reaction rate. Analogy: –Traffic converging from two lanes down to one!

3. Temperature Temperature (in Kelvin degrees) is proportional to the kinetic energy of the particles in a substance. –For example, if the Kelvin temperature of a substance is doubled, then the average kinetic energy of the particles in that substance is doubled. At higher temperatures, particles collide more frequently and with greater intensity, thus increasing the reaction rate. ↑ temperature ↑ reaction rate

Thus temperature has a duel effect: –Increased number of collisions –Collisions are more intense (more energetic)

4. Nature of reactants The nature of the reactants refers to the properties of the reactants themselves. –State of matter (s, l, g, aq), size, shape, strength of bonds, number of bonds to be broken… –For this we will state generalizations only a) State of Matter –Gases tend to react faster than solids or liquids: –Aqueous ions tend to react faster than species in other states of matter. b) Bond Type –Reactions involving ionic species tend to proceed faster than reactions involving molecular compounds.

c) Bond Strength –The weaker the bonds to be broken the faster the reaction. d) Number of Bonds/Molecular Size –The fewer the bonds to be broken the faster the reaction.

5. Catalyst A catalyst is a species that speeds up a chemical reaction without being chemically changed upon completion of the reaction. –In other words, the mass of a catalyst is the same before and after a reaction occurs. –Common examples of catalysts include: MnO 2 in the decomposition of H 2 O 2 Fe in the manufacture of NH 3 Pt in the conversion of NO and CO to N 2 and CO 2 Recall that collisions only result in reactions if the particles collide with enough energy to get the reactions started (i.e. to overcome the activation energy barrier). Also recall that activation energy corresponds to threshold energy.

Textbook Readings page 466: Method for Measuring Reaction Rates page 467: Factors That Affect Reaction Rate pages : Collision Theory and... Concentration, Surface Area, Nature of Reactants, and Temperature. Textbook Practice Items pages : items 1, 2, 4, 5, and 6 pages : items 1 and 2 pages : items 1, 2, 3, 4, 5, 6, 9, 12, and 13 pages : items 12, 13, 28, 32, 34 and 35