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  • 1. High School Chemistry Rapid Learning Series - 20 Rapid Learning Center Chemistry :: Biology :: Physics :: Math Rapid Learning Center Presents … p g Teach Yourself High School Chemistry in 24 Hours 1/54 http://www.RapidLearningCenter.com Reaction Rates & Equilibrium HS Ch i t R id Learning Series Chemistry Rapid L i S i Wayne Huang, PhD Kelly Deters, PhD Russell Dahl, PhD Elizabeth James, PhD Rapid Learning Center www.RapidLearningCenter.com/ © Rapid Learning Inc. All rights reserved. © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 1
  • 2. High School Chemistry Rapid Learning Series - 20 Learning Objectives By completing this tutorial you will learn… What Kinetics studies. Collision Th C lli i Theory. How factors affect rate. Dynamic equilibrium. Equilibrium constants. Reaction Quotients. Le Ch t li ’ P i i l L Chatelier’s Principle. 3/54 Concept Map Previous content Chemistry New content Kinetics studies Reaction Rate uses Collision Theory Studies Can be studied with Forward = reverse Matter Equilibrium Undergo Equation with Ratio of products : reactants Equilibrium Constant Expression Chemical Reactions When it’s disturbed, disturbed follow Le Chatelier’s Principle 4/54 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 2
  • 3. High School Chemistry Rapid Learning Series - 20 Kinetics and Reaction Coordinate Diagrams 5/54 Definition - Kinetics Kinetics – Study of the rates of reactions. Reactants Products Reaction Rate – Rate at which reactants produce products, i.e. how fast a reaction takes place. Rate = Δ[Product]/Δt = -Δ[Reactant]/Δt 6/54 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 3
  • 4. High School Chemistry Rapid Learning Series - 20 Definition - Collision Theory Collision Theory – Defines 3 circumstances to be met for a collision to occur. 1 Reactants must collide. 2 Collision must be at the correct orientation. 3 Collision must have minimum energy for reaction to occur. Only a small fraction of collisions meet the requirements and results in a successful reaction. 7/54 Collisions Must Occur In order for two molecules to react, they must come into contact with one another. F F O N O There’s no way they’ll ever react if they don’t run into one another! 8/54 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 4
  • 5. High School Chemistry Rapid Learning Series - 20 Collision with Correct Orientation For a collision to result in a chemical reaction, it must occur with the correct orientation. O F F F N F O N O O Side-to-Side: This is not the correct orientation. The reaction will not happen. Head-to-Head:This is the correct orientation. The reaction will happen. 9/54 Collision with Enough Energy For a collision to result in a chemical reaction, it must occur with the minimum energy for reaction. F F O F F N O O N N O O O Slow Speed: The collision does not have enough energy to produce a reaction. High Speed: This collision had more energy (faster moving molecules). A reaction will occur. 10/54 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 5
  • 6. High School Chemistry Rapid Learning Series - 20 Definition - Activation Energy Activation Energy – A ti ti E Energy that must be overcome for a reaction to occur. 11/54 Reaction Coordinate Diagram Reaction coordinate diagrams show the energy changes throughout the reaction. Activated complex (Also called the transition state) Energy Activation E A ti ti Energy Products Reactants Reaction proceeds Energy change for reaction 12/54 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 6
  • 7. High School Chemistry Rapid Learning Series - 20 Activated Complex What is an “activated complex”? Energ gy Reactants F2 + NO2 Activated Complex F2…NO2 Products F + FNO2 Reaction proceeds O F F F F F N F O N O O 13/54 Definition - Endo & Exothermic Endothermic Reaction – The reaction takes in energy…the energy the products have more energy than the reactants. heat in Exothermic Reaction – g The reaction gives off energy…the products have less energy than the reactants. 14/54 heat off Endothermic vs Exothermic Mnemonic: Endo- is to take in heat and Exo- is to give off heat = “Endo is to Enter (in) and Exo is to Exit (off)!” © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 7
  • 8. High School Chemistry Rapid Learning Series - 20 Endo- and Exothermic Diagrams Energy Reaction coordinate diagrams show whether a reaction is endothermic or exothermic. Endothermic Products are higher energy than reactants. Energy Reaction proceeds Reaction proceeds 15/54 Exothermic Products are lower energy than reactants. Factors Affecting Reaction Rates 16/54 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 8
  • 9. High School Chemistry Rapid Learning Series - 20 Surface Area of Reactants How does the surface area of the reactants affect the reaction rate? Reactants must collide in order to react. Larger surface area means more particles can come in contact with each other at the same time. More reactants can collide at the same time and a fraction of those will result in reaction. As surface area increases, reaction rate increases. 17/54 Concentrations of Reactants How does the concentration of reactants affect the reaction rate? Only a small fraction of the collisions meet the requirements and result in a reaction. More reactants mean more collisions will occur. If more collisions occur, more will meet the requirements and result in a reaction. As the reactant concentration increases, the reaction rate increases. 18/54 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 9
  • 10. High School Chemistry Rapid Learning Series - 20 Temperature How does temperature affect the reaction rate? Reactants must collide with at least energy equal to the activation energy. If molecules are at a higher temperature, they have a higher average kinetic energy. With higher g energy molecules, collisions will have higher energy and more often result in reaction. For most reactions, as temperature increases, reaction rate increases. 19/54 Definition - Catalysts Catalysts – Substance that increases the rate of reaction without being used up up. A+B+C D+C “C” is the catalyst…it is present in the beginning and in the end. Energy Without Catalyst With Catalyst (lowering activation energy) Reaction proceeds 20/54 Ex: Enzymes are catalysts in our body. © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 10
  • 11. High School Chemistry Rapid Learning Series - 20 Catalysts How do catalysts help speed up the reaction without being used? They increase the chances that a collision will successfully produce a reaction. For example, catalysts hold one or more of the reactants in place to allow collisions to occur with the correct orientation. Once the reaction has occurred, the catalyst releases the molecule(s) and finds another one to help. 21/54 Equilibrium 22/54 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 11
  • 12. High School Chemistry Rapid Learning Series - 20 Definition - Equilibrium Reversible Reaction – A chemical reaction that can proceed in both directions (represented by a “ ”) b ”). Equilibrium – When the rate of the forward reaction equals the rate of the reverse reaction, i.e. Δ[Product]/Δt = -Δ[Reactant]/Δt 23/54 Establishing Equilibrium It takes time to establish equilibrium. At first, there are only reactants present. Only the forward reaction is p possible. Reactants 24/54 Products But once there are products as well, they can begin to reform p , y g reactants. The reverse reaction becomes possible. Forward rate slows and reverse rate increases until they are the same. Once the rate of the forward and reverse process are equal, it is at equilibrium. When equilibrium is established, the numbers of products and reactants doesn’t change…but the reaction keeps going. © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 12
  • 13. High School Chemistry Rapid Learning Series - 20 Definition - Dynamic Equilibrium Dynamic Equilibrium – The reaction continues to proceed in both directions, but at the same rate. The Th number of products and reactants no longer b f d t d t t l change, it may look as though the reaction has stopped… But the reaction continues! 25/54 Equilibrium Constants 26/54 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 13
  • 14. High School Chemistry Rapid Learning Series - 20 Definition - Equilibrium Constant Equilibrium Constant Expression – Equation showing the ratio of the concentrations of prod cts to products reactants at equilibrium. Concentration is symbolized with brackets “[A]”. Equilibrium Constant (K) – The number calculated from the equilibrium constant expression. “K” is different for every reaction at every temperature! 27/54 Writing Equilibrium Constant Expressions To write an equilibrium constant expression: 1 Write the concentration of products on the top take each one to a power of the coefficient in the balanced equation. b l d ti 2 Write the concentration of reactants on the bottom also take each to the power of the balanced equation coefficient. Example: Write the equilibrium constant expression for the following: 2 H2 (g) + O2 (g) 2 H2O (g) K= [H2O] [H2] [O2] 28/54 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 14
  • 15. High School Chemistry Rapid Learning Series - 20 Definition: Homo- and Heterogeneous Equilibrium Homogeneous Equilibrium – All of the species are the same state of matter. matter 2 H2 (g) + O2 (g) 2 H2O (g) Heterogeneous Equilibrium – There are at least 2 states of matter. 2 H2 (g) + O2 (g) 2 H2O (l) 29/54 Concentrations of Solids and Liquids Pure solids and pure liquids have constant “concentrations”. If concentration (Molarity) = mole liters lit And Density = grams liters And Molar Mass = grams mole Then f Th for a pure solid or liquid, Molarity = grams / liters lid li id M l it grams / mole Or, Molarity = 30/54 Density . Molar Mass Both Density and Molar Mass are constants—they don’t change. Therefore, “concentration” of a pure solid or liquid is a constant. © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 15
  • 16. High School Chemistry Rapid Learning Series - 20 “K” Expressions with Solids or Liquids How does this affect the writing of Equilibrium Constant Expressions? If the “concentration” of a pure solid or liquid is constant, then th it will not change d i equilibrium and it is not written ill t h during ilib i d i t itt in the “K” expression. 2 H2 (g) + O2 (g) 2 H2O (g) K= [ H 2 O ]2 [ H 2]2 [O2 ] 2 H2 (g) + O2 (g) 2 H2O (l) K= 1 [ H 2]2 [O2 ] H2O is not included in this “K” expression because it’s a liquid. Only gases and solutions are included in “K” expressions! 31/54 Calculating “K” Example Example: Solve for equilibrium constant for Fe2O3 (s) + 3 H2 (g) 2 Fe (s) + 3 H2O (g) if the following are concentrations at equilibrium: [H2] = 0.45 M and [H2O] = 0.18 M [H2]eq = 0.45 M K= [ H 2O ]3 [ H 2 ]3 K= [H2O]eq = 0.18 M [0.18]3 [0.45]3 K=? Note that Fe2O3 and Fe were not included in the K expression as they are solids! K = 0.064 Most instructors and textbooks do not require units for “K” as each one would be different. 32/54 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 16
  • 17. High School Chemistry Rapid Learning Series - 20 Meaning of Equilibrium Constant What general meaning can you get from the magnitude of the equilibrium constant? [Products] If K is very large… [Reactants] There is a much larger ratio of products to reactants at equilibrium. The reaction is said to “lie to the right” (products are on the right). If K is very small small… [Products] [Reactants] There is a much smaller ratio of products to reactants at equilibrium. The reaction is said to “lie to the left”. 33/54 Using “K” to Find Equilibrium Concentration Example: Find the equilibrium concentration for NO if the equilibrium constant for N2 (g) + O2 (g) 2 NO (g) is 1.24×10-4, and the other equilibrium concentrations are [N2] = 0.166 M and [O2] = 0.145 M [N2]eq = 0.166 M K= [O2]eq = 0.145 M K = 1.24×10-4 [NO]eq = ? M 1.24 ×10 −4 = [ NO ]2 [ N 2 ][O2 ] [ NO ]2 (0.166 M )(0.145M ) (1.24 ×10 )(0.166M )(0.145M ) = [ NO] −4 [NO]eq = 0.00173 M 34/54 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 17
  • 18. High School Chemistry Rapid Learning Series - 20 Reaction Quotient 35/54 What is the Reaction Quotient? Reaction Quotient is “Q”. K Q Equilibrium Constant Reaction Quotient Expression is ratio of products to reactants with balanced equation coefficients as powers. Expression is ratio of products to reactants with balanced equation coefficients as powers. Only includes gases and solutions. Only includes gases and solutions. To solve for K, plug in concentrations at equilibrium. To solve for Q, plug in concentrations at any time. 36/54 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 18
  • 19. High School Chemistry Rapid Learning Series - 20 The Difference between K and Q What exactly is the difference? 2 H2 (g) + O2 (g) 2 H2O (g) K= [ H 2 O ]2 [ H 2]2 [O2 ] 2 H2 (g) + O2 (g) 2 H2O (g) Q= [ H 2 O ]2 [ H 2]2 [O2 ] The expressions for K and Q are the same. To solve for “K”, plug in concentrations at equilibrium only. To solve for “Q”, plug in concentrations at any time. 37/54 Using Reaction Quotient Reaction Quotient is used to determine if a system is at equilibrium…and if it’s not, which way does it need to go to get there. [products now] = Q K = [reactants now] Q=K Q>K Q<K 38/54 [products at equilibrium] [reactants at equilibrium] [now] = [equilibrium] [Products now] too large. [Reactants now] too small. [Products now] too small. [Reactants now] too large. © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com System is at equilibrium. System will make more S t ill k reactants to reach equilibrium. System will make more products to reach equilibrium. 19
  • 20. High School Chemistry Rapid Learning Series - 20 Reaction Quotient Example Example: For N2 (g) + O2 (g) 2 NO (g), if [N2] = 0.81 M, [O2] = 0.75 M and [NO] = 0.030 M, is the reaction at equilibrium if K = 0.0025? If not, which way will it go to reach equilibrium? [N2] = 0.81 M Q= [O2] = 0.75 M [ NO ]2 [ N 2 ][O2 ] Q= (0.030M ) 2 (0.81M )(0.75M ) [NO] = 0.030 M K = 0.0025 At equilibrium = ? Q = 0.0015 Q<K Reaction is not at equilibrium. More products will need to be made (and also thereby reducing reactants) to have Q = K. Reaction will go to the right (products) to reach equilibrium. 39/54 Le Chatelier’s Principle 40/54 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 20
  • 21. High School Chemistry Rapid Learning Series - 20 Definition - Le Chatelier’s Principle Le Chatelier’s Principle – If a system at equilibrium is disturbed, it will shift to reestablish the equilibrium equilibrium. A system will try to undo whatever you’ve done (de-stress or count-change). 41/54 Le Chatelier’s Principle Mnemonic: Le Chatelier is to lesser the change (stress) added = “Le Chatelier is Lessen (the) Change.” Increasing Concentrations How does adding a reactant or product affect a system at equilibrium? Reactants Products Adding a reactant. Adding a product. Q becomes too small. Q becomes too large. Reaction shifts R ti hift to right. (To get rid of extra reactants and make more products.) Reaction shifts to left. (To get rid of extra products and make more reactants.) 42/54 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 21
  • 22. High School Chemistry Rapid Learning Series - 20 Decreasing Concentrations How does removing a reactant or product affect a system at equilibrium? Reactants Products Removing a reactant. Q becomes too large. Removing a product. Q becomes too small. Reaction shifts to left. (make more reactants.) Reaction shifts to right. (make more products.) 43/54 Changes in Pressure How does changing the pressure affect a system at equilibrium? i.e. 2 Reactants 3 Products Decrease volume. (i.e. Backward reaction ). Increase volume. (i.e. Forward reaction ). Pressure increases. (i.e. Favors backward reaction). Pressure decreases. (i.e. Favors forward reaction). Reaction shifts to the side with less moles (or volume, i.e. left) of gas to decrease pressure. Reactions shifts to the side with the more moles (or volume, i.e. right ) of gas to increase pressure. 44/54 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 22
  • 23. High School Chemistry Rapid Learning Series - 20 Definition - Endo & Exothermic Reactions Endothermic Reaction – The reaction takes in heat energy…the products have more energy than the reactants. Energy is a reactant in the reaction. Reactants + heat Products Exothermic Reaction – The reaction gives off heat energy…the products have less energy than the reactants. 45/54 Energy is a product in the reaction. Reactants Products + heat Temperature and Endothermic For endothermic, think of temperature (or energy) as a “reactant”: Reactants + heat Products Reaction shifts R ti hift to right. Increase temperature of endothermic reaction. Increasing a “reactant”. Decrease temperature of endothermic reaction. Remove a “reactant”. (get rid of extra reactants and make more products.) Reaction shifts to left. (make more reactants.) 46/54 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 23
  • 24. High School Chemistry Rapid Learning Series - 20 Temperature and Exothermic For exothermic, think of temperature (or energy) as a “product”: Reactants Products + heat Reaction shifts to left. Increase temperature of exothermic reaction. Increasing a “product”. Decrease temperature of exothermic reaction. Remove a “product”. (get rid of extra products and make more reactants.) Reaction shifts to right. (make more products.) 47/54 Changes that Have No Effect Some changes have no effect because they do not affect the value of “Q”. Adding a pure solid or liquid reactant or g p q product. Increasing pressure by adding an inert gas. Changing the volume of a reaction with an equal number of moles of gas on each side of the reaction reaction. Adding a catalyst. A catalyst will speed up how fast equilibrium is established—but not the number of reactants and products once it’s at equilibrium. 48/54 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 24
  • 25. High School Chemistry Rapid Learning Series - 20 Le Chatelier’s Examples Example: Which way will the reaction shift for each of the following changes: NH4Cl (s) NH3 (g) + HCl (g) Removing NH4Cl No change (it s a solid) (it’s Adding HCl (Adding a product) Adding Ne (g) No change (it’s an inert gas) (Goes to side with least gas moles) Decreasing volume Example: Which way will the reaction shift for each of the following changes: 2 SO2 (g) + O2 (g) 2 SO3 (g) + Heat … an exothermic reaction Increasing volume (Goes to side with most gas moles) Raising temperature (Energy is a product) Adding O2 (Adding a reactant) Removing SO2 (Removing a reactant) 49/54 Le Chatelier in Industry When companies need to make large amounts of product, a reaction with a very small K is a problem. Small K Small ratio of products to reactants. Lots of reactants left over (wasting money) and few products made (not making money). They can push the reaction towards the products. e.g. Remove the products as they’re made, adjust pressure or temperature as needed to push it to the right. 50/54 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 25
  • 26. High School Chemistry Rapid Learning Series - 20 Haber Process The reaction to produce ammonia, NH3, is very important to manufacturing. N2 + 3 H2 2 NH3 (an exothermic reaction: ΔH = -92 kJ/mol) 92 Rate = Δ[NH3]/Δt = -(1/2)Δ[N2]/Δt = -(3/2)Δ[H2]/Δt In order for the reaction to occur at a reasonable rate, the temperature must be very high. But when the temperature is high, the equilibrium constant is very low. A compromise is made and a catalyst is added to increase the rate at the lower temperature. The reaction yields 20%...the leftover reactants are recycled and put back into the reaction again. 51/54 Learning Summary Le Chatelier’s Principle governs h how a reaction at ti t equilibrium will change when disturbed. Kinetics is the study of the rates of reaction, which are affected by surface area, concentration, temperature and catalysts. Dynamic equilibrium is established when the rates of the forward and reverse reactions are equal. The Th equilibrium constant ilib i t t give the ratio of product : reactants with the stoichiometric ratios as the powers. 52/54 © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 26
  • 27. High School Chemistry Rapid Learning Series - 20 Congratulations You have successfully completed the core tutorial Reaction Rates & Equilibrium Rapid Learning Center Rapid Learning Center Chemistry :: Biology :: Physics :: Math What’s N t Wh t’ Next … Step 1: Concepts – Core Tutorial (Just Completed) Step 2: Practice – Interactive Problem Drill Step 3: Recap – Super Review Cheat Sheet Go for it! 54/54 http://www.RapidLearningCenter.com © Rapid Learning Inc. All rights reserved. :: http://www.RapidLearningCenter.com 27