2. The amount of time required for a chemical rxn to come to completion can vary tremendouslyThe 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 Chemists find it useful, although difficult, to study a reactions progress over a period of time, which is called Kinetics.Chemists find it useful, although difficult, to study a reactions progress over a period of time, which is called Kinetics. Collision Theory

3. The concept of rate is familiarThe 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 1 st sprinter runs at a speed of 8.7m/sOn average the 1 st sprinter runs at a speed of 8.7m/s The 2 nd runs at a speed of 6.7m/sThe 2 nd runs at a speed of 6.7m/s –Both speeds are expressions of rates of travel Collision Theory

4. The word rate can be used as a synonym of speedThe 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.Rates of chemical change usually are expressed as the amount of reactant forming products per unit time. Collision Theory

6. Rates of chem rxns are related to the properties of atoms, ions, and molecules through a model called collision theoryRates 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 collideAccording 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

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

10. During a rxn, a particle that is neither reactant nor product forms momentarily, called an activated complexDuring a rxn, a particle that is neither reactant nor product forms momentarily, called an activated complex –if there is sufficient energy –and if the atoms are oriented properly An activated complex is a kind of transition molecule which has similarities to reactants & productsAn activated complex is a kind of transition molecule which has similarities to reactants & products –An activated complex is the arrangement of atoms at the peak of the activation-energy barrier. Collision Theory

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

12. It is possible to vary the conditions of the rxn, the rate of almost any rxn can be modifiedIt is possible to vary the conditions of the rxn, the rate of almost any rxn can be modified ocollision theory can help explain why the rates can be modified Several strategies can be used:Several strategies can be used: oIncrease the temperature oIncrease the concentration oDecrease the particle size oEmploy a catalyst Reaction Rates

13. Increasing the temp speeds up the rxn, while lowering the temp slows down the rxnIncreasing the temp speeds up the rxn, while lowering the temp slows down the rxn Increasing the temp increases the frequency of the collisionsIncreasing 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 collisionsAnd the extra energy increases the power of the collisions –Also increasing the likelihood of a successful collision Temperature

14. Just sitting out, charcoal does not react at a measurable rateJust 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 CO 2 is formed o The energy released by the rxn then supp- lies enough energy to get more C and O 2 over the activation-energy barrier Evidence of this would be if you remove the starter flame, the rxn will continue on its own. Evidence of this would be if you remove the starter flame, the rxn will continue on its own.

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

16. The smaller the particle size, the larger the surface area for a given mass of particlesThe 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.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…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. Particle Size

17. o 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. which separates the particles and makes them more accessible to other reactants. o 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 Small dust-like particles can be very dangerous, can be highly explosive Particle Size

18. o An increase in temp is not always the best way to increase the rate of rxn A catalyst is often better. 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 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

20. Catalyst

21. 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 rxn arrow(s) Often written above the rxn arrow(s) o Catalysts are crucial for many life processes. Your body temp is only 37°C and cannot be raised significantly without danger 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 Enzymes, biological catalysts, increase the rates of biological rxns

22. o When you eat a meal containing protein, enzymes in your digestive tract break down the protein molecules in a few hrs.. Without enzymes, the digestion of proteins at 37  C takes yrs Without enzymes, the digestion of proteins at 37  C takes yrs o An inhibitor is a substance that interferes with the action of a catalyst An inhibitor could work by reacting with or “poisoning” the catalyst itself An inhibitor could work by reacting with or “poisoning” the catalyst itself

23. The rate of a rxn depends in part on the concentration of the reactantsThe rate of a rxn depends in part on the concentration of the reactants –Concentration is a measure of how much stuff is available to react For a rxn in which reactant A reacts to form product B in 1 step, you can write a simple rxn eqn: A  BFor a rxn in which reactant A reacts to form product B in 1 step, you can write a simple rxn eqn: A  B The speed that A forms B is dependent on how the conc of A changes over timeThe speed that A forms B is dependent on how the conc of A changes over time –As the conc of A decreases the rate of the rxn generally decreases Rate Laws

24. Rate = AAAA AAAA tttt tttt You can express the rate as the change in A (  A) with respect to the change in time (  t) The rate of disappearance of A is proportional to the concentration or mol- arity (# of moles/Liter) of reactant AThe rate of disappearance of A is proportional to the concentration or mol- arity (# of moles/Liter) of reactant A This proportionality can be expressed as a constant (k) multiplied by the concentration of reactant AThis proportionality can be expressed as a constant (k) multiplied by the concentration of reactant A k [A] Rate Laws

25. This mathematical expression is an example of a rate lawThis mathematical expression is an example of a rate law –An expression which relates the rate of a rxn to the conc of reactants The magnitude of the rate constant (k) depends on the conditions at which the rxn is conductedThe magnitude of the rate constant (k) depends on the conditions at which the rxn is conducted –If reactant A reacts to form product B quickly, the value of k will be large –If reactant A reacts to form B slowly, the value of k will be small Rate Laws

26. Rxns are classified as either zero-order, first-order, second-order, or mixed order (higher order) rxns.Rxns are classified as either zero-order, first-order, second-order, or mixed order (higher order) rxns. –The rate of chemical rxns and the size of the rate constant (k) is dependent on the “order” of the rxn Zero-Order RxnsZero-Order Rxns –(Order = 0) have a constant rate. This rate is independent of the conc of the reactants. The rate law is: k, with k having the units of M/sec. Rate Laws

28. First-Order ReactionsFirst-Order Reactions –(order = 1) has a rate proportional to the conc of one of the reactants. A common example of a first-order rxn is the phenomenon of radioactive decay. The rate law is: k[A] 1 (or B instead of A), with k having the units of sec -1 Rate Laws

30. Second-Order ReactionsSecond-Order Reactions –(order = 2) has a rate proportional to the conc of the square of a single reactant or the product of the conc of two reactants. –Rate law =k[A] 2 (or substitute B for A or k multiplied by the concentration of A, [A], times the concentration of B, [B]), with the units of the rate constant M -1 sec -1 Rate Laws

32. Rate laws can only be determined experimentally.Rate laws can only be determined experimentally. –It is not an easy process to determine the order of the reaction or the rate constant –Unless you determine a class of rate laws called Integrated Rate Laws. Determining Rate Laws o Integrated Rate Laws are determined by graphing a series of rate data and analyzing the graph looking for a specific pattern.

33. The Zero order integrated rate law shows that its rate is independent of the [A]The Zero order integrated rate law shows that its rate is independent of the [A] –Where [A] vs. t is a straight line with a slope of - k Integrated Rate Law: Zero Order

34. Zero Order Rate Law k Rate Constant Slope = - k Integrated Rate Law [A] = -kt + [A] 0 Graph [A] versus t ½ Life t ½ =[A] 0 /2k

35. The first order integrated rate law can be used to determine the concentration of [A] at any time.The first order integrated rate law can be used to determine the concentration of [A] at any time. –It can be determined graphically WhereWhere –y = ln[A] –x = time Integrated Rate Law: First Order m = -k m = -k b = ln[A] 0 b = ln[A] 0

36. First Order Rate Law k[A] Rate Constant Slope = - k Integrated Rate Law ln[A] = -kt + ln[A] 0 Graph ln[A] versus t ½ Life t ½ =0.693/k

37. The second order integrated rate law can be used to determine the concentration of [A] at any time.The second order integrated rate law can be used to determine the concentration of [A] at any time. –It can be determined graphically WhereWhere –y = 1/[A] –x = time Integrated Rate Law: Second Order m = k m = k b = 1/[A] 0 b = 1/[A] 0

38. Second Order Rate Law k[A] 2 Rate Constant Slope = k Integrated Rate Law Graph 1/[A] versus t ½ Life t ½ =1/k[A] 0

39. In some kinds of rxns, such as double replacement, 2 substances react to give productsIn some kinds of rxns, such as double replacement, 2 substances react to give products The coefficients in the eqn for such a rxn can be represented by lower-case letters: aA + bB  cC + dDThe coefficients in the eqn for such a rxn can be represented by lower-case letters: aA + bB  cC + dD For a 1 step rxn of A+B, the rate of rxn is dependent on the concentrations of reactants A & BFor a 1 step rxn of A+B, the rate of rxn is dependent on the concentrations of reactants A & B It’s rate law would follow the eqn:It’s rate law would follow the eqn: Rate = k[A] a [B] b Rate Laws

40. o When each of the exponents a & b in the rate law equals 1, the rxn is said to be 1 st order in A, 1 st order in B, & 2 nd order overall o The overall order of a rxn is the sum of the exponents for the individual reactants o If enough info were available, you could graph all the energy changes that occur as the reactants are converted to products in a chem rxn Rate Laws

41. o Such a graph would be called a rxn progress curve o The simplest would be a one-step, elementary rxn Reactants form products in a single step Reactants form products in a single step 1 activated complex 1 activated complex 1 energy peak 1 energy peak Rate Laws

42. For a more complex rxn, or a higher order rxn, the rxn progress curve resembles a series of hills & valleysFor a more complex rxn, or a higher order rxn, the rxn progress curve resembles a series of hills & valleys –The peaks correspond to the energies of the activated complexes –Each valley represents an intermediate product which becomes a react of the next stage of the rxn Intermediates have a significant lifetime compared with an activated complexIntermediates have a significant lifetime compared with an activated complex –They have real ionic or molecular structures and some stability Reaction Mechanism

43. Intermediates do not appear in the overall eqn for a rxnIntermediates do not appear in the overall eqn for a rxn For example in the following overall rxn:For example in the following overall rxn: H 2 (g) + 2ICl(g) I 2 (g) + 2HCl(g) Reaction Mechanism o This reaction is not exactly accurate There is an intermediate reaction in between the reactants and products. There is an intermediate reaction in between the reactants and products. 1) H 2 (g) + 2ICl(g)  ICl(g) + HCl(g) + HI(g) 2) ICl(g) + HCl(g) + HI(g)  I 2 (g) +2HCl(g)

44. Reaction Mechanism

45. o If a chem rxn proceeds in a sequence of steps, the rate law is determined by the slowest step because it has the lowest rate. The slowest-rate step is called the rate- determining step The slowest-rate step is called the rate- determining step o Consider this rxn: NO 2 + CO  NO + CO 2 the rxn is believed to be a 2 step process following this mechanism the rxn is believed to be a 2 step process following this mechanism Reaction Mechanism Step 1: NO 2 + NO 2  NO + NO 3 Step 2: NO 3 + CO  NO 2 + CO 2 SLOW FAST

46. o In the 1 st step 2 molecules of NO 2 collide, forming the intermediate NO 3. 2 molecules of NO 2 collide, forming the intermediate NO 3. The NO 3 species collides with a molecule of CO and reacts quickly to produce 1 molecules each of NO 2 and CO 2 The NO 3 species collides with a molecule of CO and reacts quickly to produce 1 molecules each of NO 2 and CO 2 o The 1 st step is the slower of the 2 steps and is therefore the rate-determining step Reaction Mechanism Its rate law: R=k[NO 2 ] 2 The rate determining step and the rate law are both determined experimentally