Energy in Chemical Reaction Reaction Rates Chemical Equilibrium. Chapter Outline. Energy 6/29/2013

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1 Energy in Chemical Reaction Reaction Rates Chemical Equilibrium Chapter Outline Energy change in chemical reactions Bond dissociation energy Reaction rate Chemical equilibrium, Le Châtelier s principle 2 Energy Energy do not have mass and volume (<-> Matter) Energy is anything that has the capacity to do work Chemistry is the study of matter, Chemical process involve energy change it can cause physical and/or chemical changes in matter 3 1

2 Matter Possesses Energy when a piece of matter possesses energy, it can give some or all of it to another object all chemical and physical changes result in the matter changing energy 4 Kinds of Energy Kinetic and Potential Kinetic Energy is energy of motion, or energy that is being transferred from one object to another (running water) Potential Energy is energy that is stored (water above the dam) 5 Some Forms of Energy Electrical kinetic energy associated with the flow of electrical charge Heat or Thermal Energy kinetic energy associated with molecular motion Light or Radiant Energy kinetic energy associated with energy transitions in an atom Nuclear potential energy in the nucleus of atoms Chemical potential energy in the attachment of atoms or because of their position (example: Battery) 6 2

3 Units of Energy calorie (cal) is the amount of energy needed to raise one gram of water by 1 C kcal = energy needed to raise 1000 g of water 1 C food Calories = kcals Energy Conversion Factors 1 calorie (cal) = joules (J) 1 Calorie (Cal) = 1000 calories (cal) 1 kilowatt-hour (kwh) = 3.60 x 10 6 joules (J) 7 Unit Energy Use Energy Required to Raise Temperature of 1 g of Water by 1 C C Energy Required to Light 100-W Bulb for 1 hr Energy Used by Average U.S. Citizen in 1 day joule (J) x x 10 8 calorie (cal) x x 10 8 Calorie (Cal) ,000 kwh 1.1 x Convert 421 kj to kilocalories 1 calorie = Joule =101 kcal 9 3

4 Energy change in Chem. Rxn. Chemical bonds are stable: Breaking a bond requires energy. Cl-Cl + 58 kcal/mol 2Cl Energy released or absorbed in a reaction is called the Heat of Reaction, or Enthalpy change, ΔH. 10 Endothermic vs. Exothermic Reaction ΔH < 0: Product contains less energy than reactant. Exothermic reaction. Reaction releases heat to the environment. ΔH > 0: Product contains more energy. Endothermic reaction. Reaction absorbs heat from the environment. 11 Examples of Exothermic Reaction All Combustion reactions: Hydrogen gas burns in the air: O 2 + 2H 2 2H 2 O Acid-Base Neutralization Reaction: NaOH + HCl NaCl + 2H 2 O All explosive reactions 12 4

5 Examples of Endothermic Reaction Baking soda decomposes in the oven: 2NaHCO 3 Na 2 CO 3 + CO 2 + H 2 O Soda drink loses taste at room temperature: H 2 CO 3 CO 2 + H 2 O 13 Bond Dissociation Energy Breaking a stable chemical bond requires energy input: Endothermic reaction Bond Dissociation Energy: energy to equally breaking a covalent bond. Cl : Cl 2Cl ΔH = 58 kcal/mol Higher bond dissociation energy means stronger bond. 14 Calculate the Energy involved in Chemical Reaction For a certain chemical reaction, the energy absorbed or released depends on the amount of product, or the limiting reactant. The more product is generated, the more heat is involved. The energy involved (ΔH, in kcal/mol or kj/mol) can be considered as conversion factor. 15 5

6 Determine how much energy is released when 100. g of propane is completely reacted with oxygen. C3H 8(g) + 5O 2(g) 3CO 2(g) + 4H2O(l) ΔH = -531 kcal/mol Problem solving plan: Mass propane mol propane energy released 1.2x10 3 kcal 16 How Does Chemical Reaction Take Place? In order for a reaction to occur, the reacting molecules must collide with each other. Also two other factors: 1. Whether the collision has enough energy to start to break the bonds holding reactant molecules together." 2. Whether the reacting molecules collide in the proper orientation for new bonds to form. 17 Effective Collisions Effective Collisions: Collisions with sufficient energy and proper orientation (and therefore the reaction occurs). The higher the frequency of effective collisions, the faster the reaction rate. There is a minimum energy needed for a collision to be effective: Activation energy. 18 6

7 For a collision to lead to overcoming the energy barrier, the reacting molecules must have sufficient kinetic energy so that when they collide, it can form the activated complex. Effective Collisions: Kinetic Energy Factor 19 Effective Collisions: Orientation Effect 20 Activation Energy The energy barrier that prevents any collision between molecules from being an effective collision is called the Activation Energy. The larger the activation energy of a reaction, the slower it will be. At a given temperature. Comparable to the Hurdles: The taller hurdle is set, the harder for one to run overpass, fewer people will be able to clear. 21 7

8 Many Chemical Reactions Need Initial Push to Start Rubbing a match head against a rough surface provides the activation energy needed for the match to ignite. 22 Reaction Energy Diagram Transition state (or Activated complex) 23 Relative potential energy Exothermic Reaction Activation energy, large Reactants ΔH reaction Activation energy, small Products Progress of reaction 24 8

9 Endothermic Reaction Relative potential energy Reactants Activation energy Products ΔH reaction Progress of reaction 25 Reaction Rates Some chemical reactions proceed rapidly. Like the precipitation, acid-base, and gas-evolving reactions (Chapter 7). Other reactions proceed slowly. Like the decomposition of dye molecules of a sofa placed in front of a window. The rate of a reaction is measured in the amount of reactant that changes into product in a given period of time. Generally molarity change of reactant per second. Like miles per hour. Chemists study ways of controlling reaction rates. 26 Factors Effecting Reaction Rate: Reactant Concentration The higher the concentration of reactant molecules, the faster the reaction will generally go. Increases the frequency of reactant molecule collisions High concentration (full strength) of CLORAX might overbleach or even damage fiber due to high reaction rate When using Drano (NaOH-based solution), avoid running water to keep high concentration of NaOH. Since reactants are consumed as the reaction proceeds, the speed of a reaction generally slows over time. 27 9

10 Factors Effecting Reaction Rate: Temperature Increasing the temperature, more molecules in the sample with enough energy to collide and overcome the activation energy. Increasing the temperature also increases the frequency of collisions. So the rate increases because the frequency of effective collisions increases. Warm temperature quicken reagents such as CLORAX or DRANO in cleaning. Both these mean that increasing temperature increases the reaction rate. 28 Catalysts A catalyst is a substance that increases the rate of a reaction, but is not consumed in the reaction. Catalysts lower the activation energy of a reaction. Catalysts work by providing an easier pathway for the reaction. 29 Catalyst Effect on Activation Energy: Lower the Hurdles 30 10

11 Catalyst Effect on Activation Energy, Continued 31 Enzymes: Proteins as Biochemical Catalysts Enzymes are protein molecules produced by living organisms that catalyze chemical reactions. The enzyme molecules have an active site to which organic molecules bind. When the organic molecule is bound to the active site, certain bonds are weakened. This allows a particular chemical change to occur with greater ease and speed. i.e., the activation energy is lowered. 32 Amylase: An Enzyme Breaking down starch molecules 33 11

12 Overview of Factors Affecting Reaction Rates 34 Many Chemical Reactions are Reversible If the products of a reaction are removed from the system as they are made, then a chemical reaction will proceed until the limiting reactants are used up. However, if the products are allowed to accumulate, they will start reacting together to form the original reactants. This is called the Reverse Reaction. Reactions that can proceed in both the forward and reverse directions are called Reversible Reactions. 35 Examples of Reversible Reactions Photochromic sunglass: Transparent + Light Dark Temperature dependent coloration: N 2 O 4 + heat 2NO 2 Rechargeable battery: Charged Depleted + electricity 36 12

13 Reaction Dynamics, Continued The forward reaction slows down as the amounts of reactants decreases. At the same time, the reverse reaction speeds up as the concentration of the products increases. Eventually, the forward reaction is using reactants and making products as fast as the reverse reaction is using products and making reactants. This is called Chemical Equilibrium. Dynamic equilibrium is reached when the rates of two opposite processes are the same. 37 Equilibrium Equal The rates of the forward and reverse reactions are equal at equilibrium. But that does not mean the concentrations of reactants and products are equal. Some reactions reach equilibrium only after almost all the reactant molecules are consumed: position of equilibrium favors the products. Other reactions reach equilibrium when only a small percentage of the reactant molecules are consumed: the position of equilibrium favors the reactants. 38 An Analogy: Population Changes When Narnians feel overcrowded, some will emigrate to Middle Earth. The population of Middle Earth is so low that no one emigrates to Narnia. As time passes, emigration will occur in both directions at the same rate, leading to populations in Narnia and Middle Earth that are constant, though not necessarily equal

14 Disturbing and Re-Establishing Equilibrium Once a reaction is at equilibrium, the concentrations of all the reactants and products remain the same. However, if the conditions are changed, the concentrations of all the chemicals will change until equilibrium is re-established. The new concentrations will be different, but the equilibrium constant will be the same. Unless you change the temperature. 40 Equilibrium Constant Even though the concentrations of reactants and products are not equal at equilibrium, there is a relationship between them. For the reaction H 2 (g) + I 2 (g) 2HI(g) at equilibrium, the ratio of the concentrations raised to the power of their coefficients is constant. 2 [ HI] K eq = H I [ ] [ ] Equilibrium Constant, Continued For the general equation aa + bb cc + dd, the relationship is given below: The lowercase letters represent the coefficients of the balanced chemical equation. Always products over reactants. The constant is called the equilibrium constant, K eq. c d [ C] [ D] K eq = A a B [ ] [ ] b 42 14

15 What Does the Value of K eq Imply? When the value of K eq > > 1, we know that when the reaction reaches equilibrium, there will be many more product molecules present than reactant molecules. The position of equilibrium favors products. When the value of K eq < < 1, we know that when the reaction reaches equilibrium, there will be many more reactant molecules present than product molecules. The position of equilibrium favors reactants. 43 A Large Equilibrium Constant 44 Le Châtelier s Principle Le Châtelier s principle guides us in predicting the effect on the position of equilibrium when conditions change. When a chemical system at equilibrium is disturbed, the system shifts in a direction that will minimize the disturbance

16 Le Chatelier: Concentration Effect Scenario A: [Reactant], Other reactants, products (shift toward product) That has the same K eq. Scenario B: [Product], Other products, reactants (shift toward reactant) You can keep removing product in an equilibrium to drive a reaction to completion! Remember: Adding more of a solid or liquid does not change its concentration and, therefore, has no 46 effect on the equilibrium. An Analogy: Population Changes At equilibrium, rate forward = rate reverse ; so the populations stay constant. What if an influx of population enters Middle Earth because of a war? Since the population of Middle Earth increases, rate forward < rate reverse, so the populations between Narnia and Middle Earth is no long at equilibrium, more people moving to Narnia. a new equilibrium between the populations is established. However, the new populations will have different numbers of people than the old ones. 47 Chemical Reactions cont d Concentration changes that result when H 2 is added to an equilibrium mixture

17 Practice Predict the Effect on the Equilibrium When the Underlined Substance Is Added to the Following Systems: 2 CO 2 (g) 2 CO(g) + O 2 (g) BaSO 4 (s) Ba 2+ (aq) + SO 4 2- (aq) CH 4 (g) + 2 O 2 (g) CO 2 (g) + 2 H 2 O(l) 49 Practice Predict the Effect on the Equilibrium When the Underlined Substance Is Added to the Following Systems, Continued: 2 CO 2 (g) 2 CO(g) + O 2 (g) Shift right, removing some of the added CO 2 and increasing the concentrations of CO and O 2. BaSO 4 (s) Ba 2+ (aq) + SO 4 2- (aq) Shift left, removing some of the added Ba 2+ and reducing the concentration of SO CH 4 (g) + 2 O 2 (g) CO 2 (g) + 2 H 2 O(l) Shift right, removing some of the added CO 2 and decreasing the O 2, while increasing the concentration of CO Effect of Volume Change on Equilibrium For solids, liquids, or solutions, changing the size of the container has no effect on the concentration. Changing the volume of a container changes the concentration of a gas. Same number of moles, but different number of liters, resulting in a different molarity

18 Effect of Volume Change for Gas on Equilibrium Decreasing the size of the container increases the concentration of all the gases in the container. According to Le Châtelier s principle, the equilibrium should shift to remove that pressure. The way to reduce the pressure is to reduce the number of molecules in the container. When the volume decreases, the equilibrium shifts to the side with fewer molecules. 52 The Effect of Volume Change on Equilibrium, Continued When Since the there pressure are more is decreased gas molecules by increasing on the reactants volume, the side position of the of reaction, equilibrium when shifts the toward pressure the is side increased with the the greater position number of of equilibrium molecules the shifts toward reactant the products. side. 53 Volume Changes on Equilibrium Volume decrease: dark brown color fades away (2NO 2 N 2 O 4 ) Volume increase: brown color turns darker (N 2 O 4 2NO 2 ) 54 18

19 Practice Predict the Effect on the Equilibrium When the Volume Is Reduced. 2 CO 2 (g) 2 CO(g) + O 2 (g) BaSO 4 (s) Ba 2+ (aq) + SO 4 2- (aq) CH 4 (g) + 2 O 2 (g) CO 2 (g) + 2 H 2 O(l) 55 Practice Predict the Effect on the Equilibrium When the Volume Is Reduced, Continued. 2 CO 2 (g) 2 CO(g) + O 2 (g) Shift left because there are fewer gas molecules on the reactant side than on the product side. BaSO 4 (s) Ba 2+ (aq) + SO 4 2- (aq) No effect because none of the substances are gases. CH 4 (g) + 2 O 2 (g) CO 2 (g) + 2 H 2 O(l) Shift right because there are fewer gas molecules on the product side than on the reactant side. 56 The Effect of Temperature Changes on Equilibrium Exothermic reactions release energy and endothermic reactions absorb energy. If we write heat as a product in an exothermic reaction or as a reactant in an endothermic reaction, it will help us use Le Châtelier s principle to predict the effect of temperature changes. However, heat is not matter and not written in a proper equation

20 Temperature on Equilibrium Online video: N 2 O 4 (colorless) 2NO 2 (brown): endothermic Lower temperature favors N 2 O 4 58 Temperature affects the Equilibrium CoCl H 2 O Co(H 2 O) Cl - : exothermic Lower temperature favors Co(H 2 O)