Rates of Reactions

1. Rates of Reactions

2. Collision Theory • 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 • Another topic we will explore in this lecture is the concept that some rxns tend to reverse upon themselves

3. Collision Theory • 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 • On average the 1st sprinter runs at a speed of 8.7m/s • The 2nd runs at a speed of 6.7m/s • Both speeds are expressions of rates of travel

4. Collision Theory • 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 • Rates of chemical change usually are expressed as the amount of reactant forming products per unit time.

5. Collision Theory

6. Collision Theory • Rates of chem rxns are related to the properties of atoms, ions, and molecules through a model called collision theory • According to collision theory, atoms, ions, and molecules can react to form products when they collide • provided that the particles have enough kinetic energy

7. Collision Theory

8. Collision Theory • The 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

10. Collision Theory • 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

11. Reaction Rates • It is possible to vary the conditions of the rxn, the rate of almost any rxn can be modified • collision theory can help explain why the rates can be modified • Several strategies can be used: • Increase the temperature • Increase the concentration • Decrease the particle size • Employ a catalyst

12. Temperature • Increasing the temp speeds up the rxn, while lowering the temp slows down the rxn • Increasing the temp increases the frequency of the collisions • Collisions taking place more often more likely they are to stick • And the extra energy increases the power of the collisions • Also increasing the likelihood of a successful collision

13. 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 • 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.

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

15. Particle Size • The smaller the particle size, the larger the surface area for a given mass of particles • The total surface area of a solid or liquid reactant has an important effect on the rate of reaction. • An 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.

16. Particle Size • One 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. • Grinding solids into a fine powder also increases the surface area of reactants • Small dust-like particles can be very dangerous, can be highly explosive

17. Catalyst • An increase in temp is not always the best way to increase the rate of rxn • A catalystis often better. • 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 • With a lower activation energy more reactants can form products in a given amount of time.

18. Catalyst A B B B Using a Catalyst A A in a chem rxn Catalyst Catalyst Catalyst +

19. Catalyst

20. Catalyst • 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 • Catalysts are crucial for many life processes. • Your body temp is only 37°C and cannot be raised significantly without danger • Without catalysts, few rxns in the body would proceed fast enough at that temp • Enzymes, biological catalysts, increase the rates of biological rxns

21. Catalyst • When 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 37C takes yrs • An 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