SHS General Chemistry 2 Quarter 3: Week 7 - Module 7 Reaction Rates, Factors Affecting Reaction Rates, and Collision Theory 0 0 STEM – GENERAL CHEMISTRY 2 Grade 11/12 Quarter 3: Week 7 - Module 7- Reaction Rates, Factors Affecting Reaction Rates, and Collision Theory First Edition, 2021 Copyright © 2021 La Union Schools Division Region I All rights reserved. No part of this module may be reproduced in any form without written permission from the copyright owners. Development Team of the Module Author: GLORIFINA R. DACANAY, T-III Editor: SDO La Union, Learning Resource Quality Assurance Team Illustrator: Ernesto F. Ramos Jr., P II Management Team: Atty. Donato D. Balderas, Jr. Schools Division Superintendent Vivian Luz S. Pagatpatan, Ph.D Assistant Schools Division Superintendent German E. Flora, Ph.D, CID Chief Virgilio C. Boado, Ph.D, EPS in Charge of LRMS Rominel S. Sobremonte, Ed.D, EPS in Charge of Science Michael Jason D. Morales, PDO II Claire P. Toluyen, Librarian II 0 0 General Chemistry 2 Quarter 3: Week 7 - Module 7 Reaction Rates, Factors Affecting Reaction Rates, and Collision Theory 0 0 Target Background Chemical kinetics is the area of chemistry concerned with the speeds, or rates, at which a chemical reaction occurs. REACTION RATES Figure 1: Reaction Rates The rates of chemical reactions span a range of time scales. For example, explosions are rapid, occurring in seconds or fractions of seconds; corrosion can take years; and the weathering of rocks can takes place over thousands or even millions of years. (images credit: Wikipedia)) After going through with this learning material, you are expected to: a. Describe how various factors influence the rate of a reaction STEM_GC11CKIIIi-j-130 b. Differentiate zero, first-, and second-order reactions STEM_GC11CKIIIi-j-132 c. Explain reactions qualitatively in terms of molecular collisions STEM_GC11CKIIIi-j-136 Specifically, you will be able to: 1. 2. 3. 4. 5. 6. define rate of reaction; identify factors that affect the rate of a reaction; describe how each factor can affect the rate of a reaction; determine the order of a reaction; state the premises of the Collision Theory; and explain the effect of each factor on the rate of a reaction using the Collision Theory. 0 0 Jumpstart Our goal in this module is to understand how to determine the rates at which reactions occur and to consider the factors that control these rates. Activity 1: Chemical Reactions in Everyday Life For each of the chemical reactions taking place from the picture, identify the occurrence of each reaction (fast or slow) and the factors that control the rate of the reaction. Figure 2: Examples of Chemical Reactions in Everyday Life (https://www.thoughtco.com/examples-of-chemical-reactions-in-everyday-life) Activity 2. Answer the following questions a. How long does it take an iron nail exposed to the rain to rust? ______________________________________________________________________________ 0 0 b. Compare the rusting of iron to how fast milk curdles when an acid like vinegar or calamansi juice is added to it. (Try doing this at home if you have milk, vinegar or calamasi juice.) ______________________________________________________________________________ c. Do you think you could light a log with a single matchstick? How about twigs or smaller pieces of wood? ______________________________________________________________________________ d. Why do we keep food in the refrigerator? ______________________________________________________________________________ e. How do particles move at high temperatures compared at low temperatures? ______________________________________________________________________________ Discover The Rate of a Chemical Reaction The study of the rate of a chemical reaction is called chemical kinetics. What is the “rate" of a reaction? The rate of a reaction is the speed of the reaction. It is not <how much= of a product is made, but instead <how quickly= a reaction takes place. Reaction Rate = Reactants → Products Time Reaction Rate is defined as the rate at which reactants change into products over time. How can we measure the rate? ➢ The rate of a chemical reaction can be studied by measuring either: • How quickly a reactant is used up, or • How quickly a product is formed. Consider the reaction: REACTANTS zinc + hydrochloric acid 4> 0 PRODUCTS zinc chloride + hydrogen 0 There are two possible ways of measuring the rate: 1) measure how quickly one of the products (e.g. the hydrogen) is made 2) measure how quickly one of the reactants (e.g. the zinc) disappears Factors That Affect Rate of a Chemical Reaction Almost any reaction rate can be changed by varying the conditions under which the reaction takes place 1. Nature of the reactants • Substances vary in their chemical reactivity. It is a major factor that determines the rate of a reaction. • Chemists believe that the type, strength and number of chemical bonds or attractions between atoms determines with what speed the particles have to collide with each, to create an effective reaction. a. Reactions involving simple ions are most often instantaneous. This is due to the fact that the positive and negative charges attract each other and no bonds have to be broken in creating the new substances. IONS = FAST Cr 2+ (aq)+Fe 3+(aq)-->Cr 3+(aq)+Fe 2+(aq) b. Reactions between molecules are usually slower than ions. In molecules bonds have to be broken and new bonds reformed. This slows down reaction rates. MOLECULES = SLOW 2CO(g)+O2(g)-->2CO2(g) c. Gases tend to react faster than solids or liquids: ➢ It takes energy to separate particles from each other. In order to burn candle wax, the solid wax has to be melted and then vaporized before it reacts with oxygen. Methane gas is already in the gas state so it burns faster than wax. Fast CH4(g) + 2O2(g) → CO2(g) + 2H2O(g) Slow C25H52(s) + 38O2(g) → 25CO2(g) + 26H2O(g) d. Aqueous ions tend to react faster than species in other states of matter. ➢ Solid Pb(NO3)2 will react with solid KI, but the reaction is really, really slow. That's because the ionic bonding in each reactant is strong and the ions in each compound are hard to separate from each other. ➢ When aqueous solutions of these compounds are mixed, the formation of PbI2 iodide is rapid. In aqueous solutions, the ions of each compound are dissociated. When the two the solutions are mixed together, all that is required for a reaction to occur is contact between the lead(II) ions and the iodide ions. Fast Pb(NO3)2(aq) + 2KI(aq) → PbI2(s) + 2KNO3(aq) 0 0 Slow Pb(NO3)2(s) + 2KI(s) → PbI2(s) + 2KNO3(s) e. Reactions involving the breaking of fewer bonds per reactant proceed faster than those involving the breaking of a larger number of bonds per reactant. ➢ Kerosene burns more slowly than methane because there are more bonds to be broken per molecule of kerosene than there are per molecule of methane. Kerosene is a larger molecule. Fast CH4(g) + 2O2(g) → CO2(g) + 2H2O(g) methane Slow C13H28(g) + 20 O2(g) → 13 CO2(g) + 14 H2O(g) kerosene 2. Concentration of reactants • Concentration refers to how much solute is dissolved in a solution. • When the concentration of all the reactants increases, more molecules or ions interact to form new compounds, and the rate of reaction increases. • When the concentration of a reactant decreases, there are fewer of that molecule or ion present, and the rate of reaction decreases. In a less concentrated acid, the number of collisions is low, so the rate of the reaction is slower. 3M HCl 6M HCl With a more concentrated acid, the number of acid particles is greater, so the number of collisions is greater and the rate of the reaction is higher (faster). There are more particles of acid per unit volume in the 6M acid than there are in the 3M acid. 3. Temperature of reaction system • Generally speaking, an increase in temperature will increase the reaction rate. • Similarly, decreasing temperature will decrease the reaction rate. 4. Presence of a catalyst • A catalysts is a substance that affects the reaction rate without being used up in the reaction. • The presence of a catalyst usually increases reaction rate. 5. Surface Area • Surface area is the exposed matter of a solid substance. 0 0 • • An increase in surface area will result in an increase of the exposure of reactants to one another. The greater the exposure, the greater the reaction rate. 8Inner9 particles are protected and cannot collide with other particles until they become 8exposed9. The surface particles are 8exposed9 and can react. There is now a greater surface area with more exposed particles so more collisions can occur, hence faster reaction. If we break up this 8lump9 into smaller pieces the number of particles has not changed but there are now more 8surface9 particles. The Rate Law ➢ The rate of a reaction often depends on the concentration of one or more of the reactants. when A is a single reactant Rate Law A → products r = k[A]n Where k is a constant of proportionality called the rate constant and n is the reaction order Example: Reaction Rate Law 2H2O2 (a) → 2H2O(l) + O2(g) r = k[H2O2] • when A and B are reactants and C is the product Rate Law A + bB → C r = k[A]x[B]y Where [A] and [B] express the concentrations of A and B, in units of moles/liter. • The exponents x and y reaction orders • k is known as the rate constant of the reaction. Example: Reaction 2NO(g) + O2(g) → 2NO2(g) Rate Law r = k[NO]2 [O2] Note that the reaction order is unrelated to the stoichiometric coefficients of the reaction; it must be determined experimentally. 0 0 Reaction order From the rate law of given concentration of reactants, r = k[A]n and r = k[A]x[B]y, the exponents n, x and y are not from the balanced equation, they must be determined experimentally. Their values determine how the rate depends on the concentration of the reactants. • • • If the exponent n, and x and y = 0, the reaction is zero order and the rate is independent of the concentration of the reactants. If exponent n, and x and y = 1, the reaction is first order and the rate is directly proportional to the concentration of the reactants. If exponent n, and x and y = 2, the reaction is second order and the rate is proportional to the square of the concentration of A. Figure 3: Reaction concentration as a function of time for different reaction orders (Image credit: Chemistry: A Molecular Approach) A. Zero-Order Reaction In a zero-order reaction, the rate of the reaction is independent of the concentration of the reactant. r = k[A]0 • • • For a zero-order reaction, increasing the concentration of the reacting species will not speed up the rate of the reaction. A reaction is zero-order if concentration data is plotted versus time and the result is a straight line. The concentration of the reactant decreases linearly with time.(See Figure 3) The slope of the line is constant indicating a constant rate, because the reaction does not slow down as the concentration of the reactant decreases. B. First-Order Reaction In a first-order reaction, the rate of the reaction is directly proportional to the concentration of the reactant. r = k[A]1 0 0 • • A reaction that depends on the concentration of only one reactant (a unimolecular reaction). Other reactants can be present, but each will be zero-order. For a first-order reaction the rate slows down as the reaction proceeds because the concentration of the reactant decreases. C. Second-Order Reaction In the second-order reaction, the rate of the reaction is proportional to the square of the concentration of the reactant. r = k[A]2 or r = k[B]2 • A second-order reaction will depend on the concentration (s) of one second-order reactant or two first-order reactants. How to determine the order of a reaction experimentally? • The method of determining the order of a reaction is known as the method of initial rates. Method of Initial Rates is measured by running the reaction several times with different initial reactant concentrations to determine the effect of the concentration on the rate. Example 1: First-Order Reaction Let9s say that at 25 °C, we observe that the rate of decomposition of N 2O5 is 1.4×10-3 M/s when the initial concentration of N 2O5 is 0.020 M. Then, let9s say that we run the experiment again at the same temperature, but this time we begin with a different concentration of N 2O5, which is 0.010 M. On this second trial, we observe that the rate of decomposition of N2O5 is 7.0×10-4 M/s. The balanced chemical equation for the decomposition of dinitrogen pentoxide is 2N2O5(g) → 4NO2(g) + O2(g) Since there is only one reactant, the rate law for this reaction has the general form: r = k[A]n substituting the given reactant for A: r = k[N2O5]n Step 1: Tabulate the given data [N2O5] (M) Initial Rate (M/s) 0.020 1.4×10-3 0.010 7.0×10-4 Step 2: Set up a ratio of the first rate to the second rate Rate 1 = k[N2O5]n1 Rate 2 k[N2O5]n2 Step 3: Substitute and simplify the equation. 1.4×10-3 = k[0.020]n 7.0×10-4 k[0.010]n 0 0 Notice that the left side of the equation is simply equal to 2, and that the rate constants cancel on the right side of the equation. Everything simplifies to: 2 = 2n Clearly, then, n =1, and the decomposition is a first-order reaction. rate = k[N2O5]1 Determining the Rate Constant k Once we have determined the order of the reaction, we can go back and plug in one set of our initial values and solve for k. We find that: rate = k[N2O5]1 = k[N2O5] Substituting in our first set of values from our table, we have 1.4×10-3 M/s = k[0.020 M] 0.020 M 0.020 M k = 0.07 /s Example 2: Second-Order Reaction Consider the following set of data: Trial Reactant A (M) Reactant B (M) 1 0.200 0.200 2 0.300 0.200 3 0.200 0.400 Initial Rate (M/s) 5.46 12.28 5.42 Determining Reaction Order in A In order to determine the reaction order for A, we need to choose two experimental trials in which the initial concentration of A changes, but the initial concentration of B is constant, so that the concentration of B cancels. We can set up our first equation by picking Trials 1 and 2 as follows: r1 = k[A]x1[B]y1 r2 k[A]x2[B]y2 5.46 = k[0.200]x [0.200]y 12.28 k[0.300]x [0.200]y 5.46 = k[0.200]x 12.28 k[0.300]x 0.444 = (²∕3)x log (0.444) = log (²∕3)x log (0.444) = x log (²∕3) log (0.444) = x log (0.67) log (0.67) log (0.67) x≈2 Note: log Ax = x log A Use scientific calculator to get the log value. Therefore, the reaction is second-order in A. Rate Law is r = k[A]2[B]y 0 0 Determining Reaction Order in B To determine the reaction order for B we have to pick two trials in which the concentration of B changes, but the concentration of A does not. Trials 1 and 3 will do this for us, and we set up our ratios as follows: r1 = k[A]x1[B]y1 r3 k[A]x3[B]y3 5.46 = k[0.200]x [0.200]y 5.42 k[0.200]x [0.400]y 5.46 = [0.200]y 5.42 [0.400]y 1 = (½)y Any non-zero number to the If y = 0 then zero power equals one. 1=1 Therefore, the reaction is zero-order in B. Rate law is r = k[A]2[B]0 Overall Reaction Order • The overall order of a reaction is the sum of all the exponents of the concentration terms in the rate equation. • We have determined that the reaction is second-order in A, and zero-order in B. Therefore, the overall order for the reaction is second-order (2+0 = 2) , and the rate law will be: rate = k[A]2 The Collision Theory Collision theory states that for a chemical reaction to occur, the reacting particles must collide with one another. • In a chemical reaction, products are formed due to the collision between the reactant molecules. • The conditions for the collisions to form products are: Effective Collisions Collision Theory Right orientation Sufficient energy A. Collisions should be effective. ✓ The reactant particles must collide with each other. Collisions between reactant molecules convert the reactants into the products of the reaction. 0 0 But remember… NOT EVERY COLLISION LEADS TO REACTION!!!!! In order for collisions to be effective, there must be considerable FORCE in the collisions. The slower moving molecules do not have enough kinetic energy to react when they collide...they bounce off one another and retain their identity. Only those molecules moving at high speed have enough energy for collisions to result in a reaction. Head-on collision: More energetic – reaction takes place Glancing blow: Less energetic – no reaction Consider the reaction: NO3(g) + CO(g) -----> NO2(g) + CO2(g) O O N O O N O O C O C C O B. The right orientation of reactant molecules towards each other. Consider another reaction between ClNO2 and NO. ClNO2(g) + NO(g) 9------: NO2(g) + ClNO(g) Reaction won't occur if the oxygen end of the NO molecule collides with the chlorine atom on ClNO2 Nor will it occur if one of the oxygen atoms on ClNO2 collides with the nitrogen atom on 0 0 In the course of this reaction, a chlorine atom is transferred from one nitrogen atom to another. In order for the reaction to occur, the nitrogen atom in NO must collide with the chlorine atom in ClNO2. Based on Collision Theory, reactions occur when two reactant molecules effectively collide, each having minimum energy and correct orientation. O O N O O N O C O C O NO3(g) + CO(g) 9-----> NO2(g) + CO2(g) c. All molecules should possess a minimum amount of energy to form product molecules. ➢ The minimum amount of energy required to initiate a chemical reaction is called activation energy, Ea (expressed in kJ). (Activation energy will be further discuss in the next module.) Explore Determining Reactant Orders from Actual Data 1. Given the following data, determine the (a) order of reaction of reactant A, (b) order of reaction of reactant B, and (c) total reaction order for the equation. Experiment [NO] (M) [Cl2] (M) 1 0.0300 0.0100 2 0.0150 0.0100 3 0.0150 0.0400 Rate (M/s) 3.4 × 1034 8.5 × 1035 3.4 × 1034 2. The initial rate data for the reaction 2N2O5(g) → 4NO2(g) + O2(g) is shown in the following table. Experiment [N2O5](M) Rate (M/s) 1 1.28 × 102 22.5 2 2 2.56 x 10 45.0 a. Determine the order of reaction of the given reactant. b. Determine the value of the rate constant for this reaction. 0 0 Deepen Use the equation to answer the questions. C6H12O6 (s) + 6 O2(g) → 6 H2O (g) + 6 CO2 (g) 1. What happens to the concentrations of: a. C6H12O6 and O2 as the reaction proceeds →? _____________________________________________________________________________ b. H2O + CO2 as the reaction proceeds →? ______________________________________________________________________________ 2. According to the collision theory, what 3 circumstances are needed for C6H12O6 & O2 to react? ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ 3. Complete the table below. Does this increase or Explain why using Change in condition: decrease the rate Collision Theory. of reaction? a. Increasing the temperature Ex: Increases (speeds up) b. Increasing the concentration of C6H12O6 c. Decreasing the concentration of O2 d. Increase the surface area by chewing up food in your mouth e. Decreasing the temperature f. Increasing the pressure in the container g. Decreasing the concentration of H2O h. Increasing the volume of the container the reaction occurs in i. Increasing the concentration of CO2 j. Using a catalyst (like salivary amylase) 0 0 Ex: Molecules move faster & collide more = Increased reaction rate Gauge Answer the following. Use a separate sheet for your answer. 1. For a reaction 2A + B → 2C, with the rate equation: Rate = k[A]2[B]. What is the correct reaction order? A. the order with respect to A is 1 and the order overall is 1 B. the order with respect to A is 2 and the order overall is 2 C. the order with respect to A is 2 and the order overall is 3 D. the order with respect to B is 2 and the order overall is 2 2. Which one of the following is NOT a key concept of the collision theory? A. particles must collide in order to react B. particles must move slowly when they collide, otherwise they simply <bounce off= one another C. particles must collide with the proper orientation D. particles must collide with sufficient energy to reach the activated complex in order to react 3. Which of the following statements is correct? A. The rate of a reaction decreases with passage of time as the concentration of reactants decreases. B. The rate of a reaction is same at any time during the reaction. C. The rate of a reaction is independent of temperature change. D. The rate of a reaction decreases with increase in concentration of reactant(s). 4. What will you do if you wanted to create a more space for particles to collide? A. Raise the pressure B. Raise the temperature C. Increase the concentration D. Increase the surface area 5. If the rate law for the reaction 2A + 3B → products is first order in A and second order in B, then what is the correct rate law equation? A. k[A]2[B]3 B. k[A]2[B]2 C. k[A]2[B] D. k[A][B]2 6. Which is a factor that would not affect the rate of a reaction? A. Temperature B. Adding a catalyst C. Concentration D. The weather 7. The rate law for a reaction is k[A][B]2. Which one of the following statements is false? A. The reaction is first order in A. B. The reaction is second order in B. C. The reaction is second order overall. D. k is the reaction rate constant 0 0 8. As the temperature of a reaction is increased, the rate of the reaction increases. Why? A. activation energy is lowered B. reactant molecules collide less frequently C. reactant molecules collide more frequently and with greater energy per collision D. reactant molecules collide less frequently and with greater energy per Collision 9. A reaction was found to be zero order in A. If we increase the concentration of A by a factor of 3, what will happen to the reaction rate? A. remain constant B. increase by a factor of 9 C. increase by a factor of 27 D. triple 10. The rate law of the overall reaction is k[A][B]0. Which of the following will not increase the rate of the reaction? A. increasing the concentration of reactant A B. increasing the concentration of reactant B C. increasing the temperature of the reaction D. adding a catalyst for the reaction 11. Which of the following would NOT increase the rate of reaction? A. raising the temperature B. adding catalyst C. increasing the volume of the container D. increasing the concentration of the reactants 12. What is the correct expression of the rate of a chemical reaction? A. grams per mole B. energy consumed per mole C. molarity per second D. volume of gas per unit time 13. Why an increase in temperature increases a reaction rate? A. only because there are more collisions B. only because the particles collide with greater energy C. by providing an alternative pathway for the reaction to follow D. because the particles collide more frequently and with greater energy 14. Which condition will increase the rate of a chemical reaction? A. Increased temperature and decreased concentration of reactants B. Increased temperature and increased concentration of reactants C. Decreased temperature and decreased concentration of reactants D. Decreased temperature and increased concentration of reactants 15. In terms of collision theory, When is a chemical reaction occurs? A. chemical bonds are broken by a catalyst. B. two reactant molecules collide with each other. C. reactants collide with sufficient energy to form new bonds. D. reactants collide with sufficient energy to break bonds in the reactants 0 0 0 1. C 2. B 3. A 4. D 5. D 6. D 7. C 8. C 9. A 10. B 11. C 12. C 13. D 14. B 15. D Change in condition: a. Increasing the temperature b. Increasing the concentration of C6H12O6 c. Decreasing the concentration of O2 d. Increase the surface area by chewing up food in your mouth e. Decreasing the temperature f. Increasing the pressure in the container g. Decreasing the concentration of H2O h. Increasing the volume of the container the reaction occurs in i. Increasing the concentration of CO2 j. Using a catalyst (like salivary amylase) Does this increase or decrease the rate of reaction? Ex: Increases (speeds up) Increases Decreases Increases Decreases Increases Decreases Decreases Increases increases GAUGE: 0 Explain why using Collision Theory. Ex: Molecules move faster & collide more = Increased reaction rate With more concentration, the number of C6H12O6 particles is greater, so the number of collisions is greater and the rate of the reaction is higher (faster). Less particles, less chances of collision. Increasing the surface area increases the number of collisions per second and therefore increases the reaction rate. Molecules move slower decreasing the chance of effective collision. When you increase the pressure, the molecules have less space in which they can move. That greater density of molecules increases the number of collisions. No effect, only reactants are involved in the reaction. Increasing the volume increases the space. Wider or larger space decreases the chance of collision. No effect, only reactants are involved in the reaction. A catalyst speeds up the reaction because it supplies the energy needed in the reaction. 3. a. 2nd-order b. 1st-order c. 2 + 1 = 3 EXPLORE 1. a. decreases b. increases 1. c. sufficient energy 2. 2. a. effective collision b. correct orientation a. 1st-order b. k = 0.18 DEEPEN Answer Key References: Chang, Raymond. (2010). Chemistry.-10th edition. USA. McGraw-Hill. pp. 556 3 562 Tro, Nivaldo J. (2011). Chemistry: a molecular approach. USA. Pearson Prentice Hall. pp. 562 - 569. <Factors that Affect Reaction Rate=, accessed December 23, 2020, https://courses.lumenlearning.com/introchem/chapter/factors-that-affectreaction <Factors Affecting Rate of Reaction=, https://www.bcsoh.org/cms/lib3 accessed December 26, 2020, <Rates of reaction=, accessed December 31, 2020, https://whs.rocklinusd.org <Rate of Reaction=, accessed https://byjus.com/chemistry/rate-of-reaction December 25, 2020, <The Collision Theory=, accessed December 28, 2020, https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry _Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry )/Kinetics/_Modeling_Reaction_Kinetics/_Collision_Theory/_The_Collision_The ory 0 0