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Topic 21 Topic 21 Topic 21: Reaction Rates Table of Contents Basic Concepts Additional Concepts Topic 21 Topic 21.

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Presentation on theme: "Topic 21 Topic 21 Topic 21: Reaction Rates Table of Contents Basic Concepts Additional Concepts Topic 21 Topic 21."— Presentation transcript:

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2 Topic 21 Topic 21

3 Topic 21: Reaction Rates Table of Contents Basic Concepts Additional Concepts Topic 21 Topic 21

4 An exothermic reaction releases heat, and an endothermic reaction absorbs heat. Exothermic and Endothermic Reactions Reaction Rates: Basic Concepts The exothermic reaction gives off heat because the products are at a lower energy level than the reactants. Topic 21 Topic 21

5 Exothermic and Endothermic Reactions Reaction Rates: Basic Concepts Topic 21 Topic 21 The endothermic reaction absorbs heat because the products are at a higher energy level than the reactants.

6 Scientists have observed that the energy released in the formation of a compound from its elements is always identical to the energy required to decompose that compound into its elements. Exothermic and Endothermic Reactions Reaction Rates: Basic Concepts Topic 21 Topic 21

7 Exothermic and Endothermic Reactions This observation is an illustration of an important scientific principle known as the law of conservation of energy. That law states that energy is neither created nor destroyed in a chemical change, but is simply changed from one form to another. Reaction Rates: Basic Concepts Topic 21 Topic 21

8 Exothermic and Endothermic Reactions In an exothermic reaction, the heat released comes from the change from reactants at higher energy to products at lower energy. In an endothermic reaction, the heat absorbed comes from the opposite change. Reaction Rates: Basic Concepts Topic 21 Topic 21

9 As you know, some chemical reactions are fast and others are slow; however, fast and slow are inexact, relative terms. Expressing Reaction Rates Chemists, engineers, medical researchers, and others often need to be more specific. Reaction Rates: Basic Concepts Topic 21 Topic 21

10 Recall from your study of math that the Greek letter delta () before a quantity indicates a change in the quantity. Expressing Reaction Rates In equation form, average rate or speed is written as We generally define the average rate of an action or process to be the change in a given quantity during a specific period of time. Reaction Rates: Basic Concepts Topic 21 Topic 21

11 For chemical reactions, this equation defines the average rate at which reactants produce products, which is the amount of change of a reactant in a given period of time. Expressing Reaction Rates Most often, chemists are concerned with changes in the molar concentration (mol/L, M) of a reactant or product during a reaction. Reaction Rates: Basic Concepts Topic 21 Topic 21

12 Therefore, the reaction rate of a chemical reaction is stated as the change in concentration of a reactant or product per unit time, expressed as mol/(Ls). Expressing Reaction Rates Brackets around the formula for a substance denote the molar concentration. For example, [NO 2 ] represents the molar concentration of NO 2. Reaction Rates: Basic Concepts Topic 21 Topic 21

13 Its important to understand that reaction rates are determined experimentally by measuring the concentrations of reactants and/or products in an actual chemical reaction. Expressing Reaction Rates Reaction rates cannot be calculated from balanced equations as stoichiometric amounts can. Reaction Rates: Basic Concepts Topic 21 Topic 21

14 Reaction rates must always be positive. Expressing Reaction Rates When the rate is measured by the consumption of a reactant, scientists commonly apply a negative sign to the calculation to get a positive reaction rate. Therefore, the following form of the average rate equation is used to calculate the rate of consumption: Reaction Rates: Basic Concepts Topic 21 Topic 21

15 Reaction data for the reaction between butyl chloride (C 4 H 9 Cl) and water (H 2 O) is given in the table. Calculating Average Reaction Rates Reaction Rates: Basic Concepts Topic 21 Topic 21 Calculate the average reaction rate over this time period expressed as moles of C 4 H 9 Cl consumed per liter per second.

16 You are given the initial and final concentrations of C 4 H 9 Cl and the initial and final times. Calculating Average Reaction Rates You can calculate the average reaction rate of the chemical reaction using the change in concentration of butyl chloride in four seconds. In this problem, the reactant butyl chloride is consumed. Reaction Rates: Basic Concepts Topic 21 Topic 21

17 Write the equation for the average reaction rate, insert the known quantities, and perform the calculation. Calculating Average Reaction Rates Reaction Rates: Basic Concepts Topic 21 Topic 21

18 Calculating Average Reaction Rates Reaction Rates: Basic Concepts Topic 21 Topic 21

19 According to the collision theory, atoms, ions, and molecules must collide with each other in order to react. Collision Theory The following three statements summarize the collision theory. 1. Particles must collide in order to react. 2. The particles must collide with the correct orientation. Reaction Rates: Basic Concepts Topic 21 Topic 21

20 3. The particles must collide with enough energy to form an unstable activated complex, also called a transition state, which is an intermediate particle made up of the joined reactants. Collision Theory Reaction Rates: Basic Concepts Topic 21 Topic 21

21 The minimum amount of energy that colliding particles must have in order to form an activated complex is called the activation energy of the reaction. Collision Theory Reaction Rates: Basic Concepts Topic 21 Topic 21 Particles that collide with less than the activation energy cannot form an activated complex. Click box to view movie clip.

22 Collision Theory Reaction Rates: Basic Concepts Topic 21 Topic 21 In an exothermic reaction, molecules collide with enough energy to overcome the activation energy barrier, form an activated complex, then release energy and form products at a lower energy level.

23 Collision Theory Reaction Rates: Basic Concepts Topic 21 Topic 21 In the reverse endothermic reaction, the reactant molecules lying at a low energy level must absorb energy to overcome the activation energy barrier and form high- energy products.

24 Reaction spontaneity is related to change in free energy, G. Collision Theory If G is negative, the reaction is spontaneous under the conditions specified. If G is positive, the reaction is not spontaneous. Lets now consider whether spontaneity has any effect on reaction rates. Reaction Rates: Basic Concepts Topic 21 Topic 21

25 To investigate the relationship between spontaneity and reaction rate, consider the following gas-phase reaction between hydrogen and oxygen. Collision Theory Here, G = –458 kJ at 298 K (25°C) and 1 atm pressure. Because G is negative, the reaction is spontaneous. Reaction Rates: Basic Concepts Topic 21 Topic 21

26 For the same reaction, G = 2484 kJ, which means that the reaction is highly exothermic. Collision Theory You can examine the speed of this reaction by filling a tape-wrapped soda bottle with stoichiometric quantities of the two gasestwo volumes hydrogen and one volume oxygen. Reaction Rates: Basic Concepts Topic 21 Topic 21

27 A thermometer in the stopper allows you to monitor the temperature inside the bottle. Collision Theory As you watch for evidence of a reaction, the temperature remains constant for hours. Have the gases escaped? Or have they simply failed to react? Reaction Rates: Basic Concepts Topic 21 Topic 21

28 If you remove the stopper and hold a burning splint to the mouth of the bottle, a reaction occurs explosively. Collision Theory Clearly, the hydrogen and oxygen gases have not escaped from the bottle. Yet they did not react noticeably until you supplied additional energy in the form of a lighted splint. Reaction Rates: Basic Concepts Topic 21 Topic 21

29 As these examples show, reaction spontaneity in the form of G implies absolutely nothing about the speed of the reaction; G merely indicates the natural tendency for a reaction or process to proceed. Collision Theory Factors other than spontaneity, however, do affect the rate of a chemical reaction. These factors are keys in controlling and using the power of chemistry. Reaction Rates: Basic Concepts Topic 21 Topic 21

30 The reaction rate for almost any chemical reaction can be modified by varying the conditions of the reaction. Factors Affecting Reaction Rates Reaction Rates: Basic Concepts Topic 21 Topic 21

31 An important factor that affects the rate of a chemical reaction is the reactive nature of the reactants. As you know, some substances react more readily than others. The Nature of Reactants The tendency of a substance to react influences the rate of a reaction involving the substance. The more reactive a substance is, the faster the reaction rate. Reaction Rates: Basic Concepts Topic 21 Topic 21

32 Reactions speed up when the concentrations of reacting particles are increased. Concentration One of the fundamental principles of the collision theory is that particles must collide to react. The number of particles in a reaction makes a difference in the rate at which the reaction takes place. Reaction Rates: Basic Concepts Topic 21 Topic 21

33 Think about a reaction where reactant A combines with reactant B. Concentration At a given concentration of A and B, the molecules of A collide with B to produce AB at a particular rate. Reaction Rates: Basic Concepts Topic 21 Topic 21

34 What happens if the amount of B is increased? Concentration Increasing the concentration of B makes more molecules available with which A can collide. Reactant A finds reactant B more easily because there are more B molecules in the area, which increases probability of collision, and ultimately increases the rate of reaction between A and B. Reaction Rates: Basic Concepts Topic 21 Topic 21

35 Surface Area Reaction Rates: Basic Concepts Topic 21 Topic 21 Increasing the surface area of reactants provides more opportunity for collisions with other reactants, thereby increasing the reaction rate.

36 Generally, increasing the temperature at which a reaction occurs increases the reaction rate. Temperature Reaction Rates: Basic Concepts Topic 21 Topic 21

37 Temperature Reaction Rates: Basic Concepts Topic 21 Topic 21 As you can see, increasing the temperature of the reactants increases the reaction rate because raising the kinetic energy of the reacting particles raises both the collision frequency and the collision energy.

38 Temperature Reaction Rates: Basic Concepts Topic 21 Topic 21 When the temperature is increased, more of the collisions at high temperatures have enough energy to overcome the activation energy barrier and react.

39 Youve seen that increasing the temperature and/or the concentration of reactants can dramatically increase the rate of a reaction. Catalysts However, an increase in temperature is not always the best (or most practical) thing to do. Reaction Rates: Basic Concepts Topic 21 Topic 21

40 Catalysts For example, suppose that you want to increase the rate of a reaction such as the decomposition of glucose in a living cell. Increasing the temperature and/or the concentration of reactants is not a viable alternative because it might harm or kill the cell. Reaction Rates: Basic Concepts Topic 21 Topic 21

41 Catalysts It is a fact that many chemical reactions in living organisms would not occur quickly enough to sustain life at normal living temperatures if it were not for the presence of enzymes. An enzyme is a type of catalyst, a substance that increases the rate of a chemical reaction without itself being consumed in the reaction. Reaction Rates: Basic Concepts Topic 21 Topic 21

42 Catalysts Although catalysts are important substances in a chemical reaction, a catalyst does not yield more product and is not included in the product(s) of the reaction. In fact, catalysts are not included in the chemical equation. Reaction Rates: Basic Concepts Topic 21 Topic 21

43 Catalysts Reaction Rates: Basic Concepts Topic 21 Topic 21 This energy diagram shows how the activation energy of the catalyzed reaction is lower and therefore the reaction produces the products at a faster rate than the uncatalyzed reaction does.

44 Catalysts Note that the activation energy for the catalyzed reaction is much lower than for the uncatalyzed reaction. Reaction Rates: Basic Concepts Topic 21 Topic 21 The lower activation energy for the catalyzed reaction means that more collisions have sufficient energy to initiate reaction.

45 Catalysts Another type of substance that affects reaction rates is called an inhibitor. Unlike a catalyst, which speeds up reaction rates, an inhibitor is a substance that slows down, or inhibits, reaction rates. Some inhibitors, in fact, actually prevent a reaction from happening at all. Reaction Rates: Basic Concepts Topic 21 Topic 21

46 Heterogeneous and homogeneous catalysts A heterogeneous catalyst exists in a physical state different than that of the reaction it catalyzes. Because the catalysts in a catalytic converter are solids and the reactions they catalyze are gaseous, the catalysts are called heterogeneous catalysts. Reaction Rates: Basic Concepts Topic 21 Topic 21

47 Heterogeneous and homogeneous catalysts A catalyst that exists in the same physical state as the reaction it catalyzes is called a homogeneous catalyst. For example, if both an enzyme and the reaction it catalyzes are in aqueous solution, the enzyme is a homogeneous catalyst. Reaction Rates: Basic Concepts Topic 21 Topic 21

48 Basic Assessment Questions Question 1 In aqueous solution, bromine reacts with formic acid (HCOOH) to produce hydrogen bromide and carbon dioxide gas. Calculate the average reaction rate over 150 s expressed in terms of the disappearance of Br 2. Topic 21 Topic 21

49 Basic Assessment Questions Answer Topic 21 Topic 21

50 Basic Assessment Questions Question 2 Use the factors discussed in this section to account for the following observation. Topic 21 Topic 21 When two gases react, compressing the gases generally increases the rate of reaction.

51 Basic Assessment Questions Answer Topic 21 Topic 21

52 Basic Assessment Questions Question 3 The rate of gaseous reactions can also be increased by pumping more gas into the reaction container. Topic 21 Topic 21 Use the factors discussed in this section to account for the following observation.

53 Basic Assessment Questions Answer Topic 21 Topic 21 Adding gas increases concentration by adding particles to a given space.

54 Reaction Rates: Additional Concepts Topic 21 Topic 21 Additional Concepts

55 Reaction Rates: Additional Concepts Reaction Rate Laws The equation that expresses the mathematical relationship between the rate of a chemical reaction and the concentration of reactants is called a rate law. For example, the reaction A B, which is a one-step reaction, has only one activated complex between reactants and products. Topic 21 Topic 21

56 Reaction Rate Laws The rate law for this reaction is expressed as, Rate = k [A] where k is the specific rate constant, or a numerical value that relates reaction rate and concentration of reactants at a given temperature. Reaction Rates: Additional Concepts Topic 21 Topic 21

57 Reaction Rate Laws Units for the rate constant include L/(mols), L 2 /(mol 2 s), and s –1. Depending on the reaction conditions, especially temperature, k is unique for every reaction. The rate law means that the reaction rate is directly proportional to the molar concentration of A. Thus, doubling the concentration of A will double the reaction rate. Reaction Rates: Additional Concepts Topic 21 Topic 21

58 Reaction Rate Laws Increasing the concentration of A by a factor of 5 will increase the reaction rate by a factor of 5. The specific rate constant, k, does not change with concentration; however, k does change with temperature. A large value of k means that A reacts rapidly to form B. What does a small value of k mean? Reaction Rates: Additional Concepts Topic 21 Topic 21

59 Reaction order In the expression Rate = k[A], it is understood that the notation [A] means the same as [A] 1. In other words, for reactant A, the understood exponent 1 is called the reaction order. Reaction Rates: Additional Concepts Topic 21 Topic 21

60 Reaction order The reaction order for a reactant defines how the rate is affected by the concentration of that reactant. For example, the rate law for the decomposition of H 2 O 2 is expressed by the following equation. Reaction Rates: Additional Concepts Topic 21 Topic 21

61 Reaction order Because the reaction rate is directly proportional to the concentration of H 2 O 2 raised to the first power, [H 2 O 2 ] 1, the decomposition of H 2 O 2 is said to be first order in H 2 O 2. Because the reaction is first order in H 2 O 2, the reaction rate changes in the same proportion that the concentration of H 2 O 2 changes. Reaction Rates: Additional Concepts Topic 21 Topic 21

62 Reaction order So if the H 2 O 2 concentration decreases to one-half its original value, the reaction rate is halved as well. Reaction Rates: Additional Concepts Topic 21 Topic 21

63 Reaction order Because reaction order is based on reaction rates, it follows that reaction order also is determined experimentally. Finally because the rate constant describes the reaction rate, k, too, must be determined experimentally. Reaction Rates: Additional Concepts Topic 21 Topic 21

64 Determining Rate Laws Consider the following reaction and its experimentally determined rate law. This rate equation shows that the rate depends on the concentrations of both NO 2 and F 2, each to the first power. Reaction Rates: Additional Concepts Topic 21 Topic 21

65 Determining Rate Laws In other words, the reaction is first order in NO 2 and first order in F 2. So, if [NO 2 ] is doubled while [F 2 ] remains the same, the rate doubles. Also, if [F 2 ] is doubled while [NO 2 ] remains the same, the reaction rate doubles. Reaction Rates: Additional Concepts Topic 21 Topic 21

66 Determining Rate Laws Now, examine the following reaction and its rate law. Because the rate depends on the square of the concentration of NO, doubling [NO] while leaving [O 2 ] the same will increase the reaction rate by a factor of 2 2, or 4. Reaction Rates: Additional Concepts Topic 21 Topic 21

67 Determining Rate Laws Use the data in the table below to determine the form of the rate law for the following reaction. Reaction Rates: Additional Concepts Topic 21 Topic 21

68 Determining Rate Laws The general rate law for this type of reaction is as follows. To start compare the data from trial 1 and 2. Notice that [NO] in trial 1 is 4.0 x 10 –3, mo1/L and [NO] in trial 2 is 8.0 x 10 –3 mo1/L, double that in trial 1. Reaction Rates: Additional Concepts Topic 21 Topic 21

69 Determining Rate Laws Now see how the rate changes from trial 1 to trial 2. The rate in trial 2, 4.8 x 10 –5 mo1/L s, is four times the rate in trial 1, which is 1.2 x 10 –5 mo1/Ls. Reaction Rates: Additional Concepts Topic 21 Topic 21

70 Determining Rate Laws When [NO] doubles, the initial reaction rate quadruples. Therefore, it is likely that the reaction rate depends on the square of the concentration of NO. Reaction Rates: Additional Concepts Topic 21 Topic 21

71 Determining Rate Laws Next, determine how the rate depends on the change in [H 2 ]. When [H 2 ] doubles, the initial rate doubles. This result indicates that the rate is directly proportional to the concentration of H 2. Reaction Rates: Additional Concepts Topic 21 Topic 21

72 Determining Rate Laws Now you can write the rate law based on your comparisons. The rate law means that the reaction is second order in [NO] and first order in [H 2 ]. Reaction Rates: Additional Concepts Topic 21 Topic 21

73 In a complex reaction, a certain series of steps must occur with correct fit and in an exact sequence in order to yield the reaction products. This series of steps is called a reaction mechanism. Instantaneous Reaction Rates and Reaction Mechanisms Many chemical reactions are complex. Reaction Rates: Additional Concepts Topic 21 Topic 21

74 Instantaneous Reaction Rates and Reaction Mechanisms As an example, consider that hydrogen peroxide (H 2 O 2 ) decomposes when iodide ions (I – ) are present. Experiments have shown that the reaction takes place in two steps as follows. Reaction Rates: Additional Concepts Topic 21 Topic 21

75 Instantaneous Reaction Rates and Reaction Mechanisms The IO – ion is called an intermediate in the reaction. An intermediate is an atom, an ion, or a molecule produced in one step of a reaction and consumed in a later step. Reaction Rates: Additional Concepts Topic 21 Topic 21

76 Instantaneous Reaction Rates and Reaction Mechanisms In a complex reaction, one step is always slower than the others. This step is called the rate-determining step because it will determine how fast the reaction forms products, no matter how fast the other steps are. Reaction Rates: Additional Concepts Topic 21 Topic 21 In the reaction, the first step is the slower step, therefore, it is the rate-determining step for the reaction

77 Additional Assessment Questions Question 1 Topic 21 Topic 21 The rate law for a reaction is rate = k[A][B]. What happens to the rate if the concentration of B is tripled while [A] remains the same? If the concentration of A is halved while [B] remains the same?

78 Answer rate triples; rate halves Additional Assessment Questions Topic 21 Topic 21

79 Question 2 Additional Assessment Questions Topic 21 Topic 21 The rate law for a reaction is rate = k[X][Y] 2. What happens to the rate if the concentration of X is doubled while [Y] remains the same?

80 Answer rate doubles Additional Assessment Questions Topic 21 Topic 21

81 Question 3 Discuss two possible ways to determine the average rate of the following reaction in the laboratory. What quantities would you measure in each case? Additional Assessment Questions Topic 21 Topic 21

82 Answer There are three possibilities. Measure the reduction in concentration of H 2, the reduction in concentration of I 2, or the increase in concentration of HI. In all three cases, time is the second quantity that must be measured. Additional Assessment Questions Topic 21 Topic 21

83 Question 4 The collision theory says that particles must collide in order to react. What other two requirements must be met in order for particles to react? Additional Assessment Questions Topic 21 Topic 21

84 Answer The particles must collide in the correct orientation and with sufficient energy to form an activated complex. Additional Assessment Questions Topic 21 Topic 21

85 To advance to the next item or next page click on any of the following keys: mouse, space bar, enter, down or forward arrow. Click on this icon to return to the table of contents Click on this icon to return to the previous slide Click on this icon to move to the next slide Click on this icon to open the resources file. Help Click on this icon to go to the end of the presentation.

86 End of Topic Summary File


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