UNIT #10: Reaction Rates Heat/Energy in Chemical Reactions Le Chatlier s Principle Potential Energy Diagrams

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1 UNIT #10: Reaction Rates Heat/Energy in Chemical Reactions Le Chatlier s Principle Potential Energy Diagrams NAME: 1. REACTION RATES a) The speed of a chemical reaction determined by the change in concentration of a reactant or product per unit time, expressed as mol/(l s). b) Reaction rates are determined experimentally by measuring the concentrations of reactants and/or products in an actual chemical reaction. c) Factors Effecting Reaction Rates 1. Nature of Reactants: The more reactive a substance, the faster the reaction rate. 2. Concentrations: Increased concentration of reactants increases reaction rate by increasing the chance of particle collisions; likewise, decreasing concentration of reactants decreases reaction rate. 3. Surface Area: Increased surface area increases reaction rate due to more exposed particles to react. Ex. Pulverizing (grinding) a solid reactant will increase its reaction rate; granulated sugar will dissolve faster in water than a sugar cube. 4. Temperature: Generally, increasing temperature increases reaction rate by increasing the average kinetic energy of reactants; thus, collisions between particles are more frequent and occur with more energy, increasing the chance that product will be formed. 5. Catalysts: Catalysts are compounds introduced to a reaction that are not consumed in the reaction. The activation energy of a reaction is the minimum amount of energy required to start a reaction. Catalysts increase reaction rates by decreasing activation energy and therefore, more particle collisions have sufficient energy to initiate product formation. 2. HEAT/ENERGY IN CHEMICAL REACTION a) The Law of Conservation of Energy states that in any chemical or physical process, energy is neither created or destroyed but instead, is conserved. b) In a chemical reaction, the bonds between atoms of the reactants are broken. Breaking bonds requires an input of energy. Energy contained by reactants is expressed as the H reactants. c) Atoms rearrange and form new bonds, resulting in the product. When bonds are formed, energy is released. Energy contained by products is expressed as the H products. d) The heat of reaction is an expression of the net energy involved in a chemical reaction and is expressed as ΔH reaction = H products - H reactants

2 e) An endothermic reaction is one that requires a net input of energy, meaning it takes more energy to break bonds of reactants than is released in the formation of product. Since energy is transferred and conserved, the resulting products contain more energy. Since the ΔH reaction = H products - H reactants, the ΔH reaction for an endothermic reaction is always positive. f) An exothermic reaction is one that has a net release of energy, meaning more energy is released in the formation of products than is required to break the bonds of the reactants. Since energy is transferred and conserved, the resulting products contain less energy. Since the ΔH reaction = H products - H reactants, the ΔH reaction for an exothermic reaction is always negative. g) Summary: Endothermic reaction Reactants + Energy Products H products is greater than H reactants and energy appears on the reactant side of the equation. ΔH reaction is positive. Exothermic reaction Reactants Products + Energy H products is less than H reactants and energy appears on the product side of the equation. ΔH reaction is negative. 3. LE CHATELIER S PRINCIPLE a) Many chemical reactions occur in both a forward and a reverse direction. This is depicted as Reactants Products. b) Chemical equilibrium is a state in which the rate of the forward reaction is equal to the rate of the reverse reaction. c) Le Chatelier s principle states that if a stress is applied to a system at equilibrium, the system shifts in the direction that relieves the stress. 1. Stress can be a change in concentration, volume and pressure or temperature. 2. Change in Concentration a. Addition of reactants: system compensates by using up additional reactants to form product. System shifts right towards products. b. Removal of reactants: system compensates by reversing reaction to replace reactants. System shifts left towards reactants. c. Addition of products: system compensates by reversing reaction to use up additional products. System shifts left towards reactants. d. Removal of products: system compensates by producing more product. System shifts right towards products. 3. Change in Volume and Pressure a. Changes in volume and pressure effect only a system that contains reactants and products in a gaseous state. b. Changes in volume and pressure affect a system in equilibrium only if the total number of moles of gaseous reactants is different than the total number of moles of gaseous products. Coefficients of the balanced equation are used to determine moles of reactants and products.

3 c. Changes in volume and pressure are interrelated because decreasing the volume of a reaction vessel at constant temperature increases the pressure inside. Conversely, increasing the volume decreases the pressure. d. Decreasing volume/increasing pressure will shift a gaseous reaction towards the side of the equation with fewer moles. e. Increasing volume/decreasing pressure will shift a gaseous reaction toward the side of the equation with more moles. 4. Changes in Temperature a. In an endothermic reaction, energy is on the reactant side of the equation and predictions of equilibrium shift are equivalent to changes in concentration of reactants. 1. Increase in temperature: system compensates by using up additional energy. System shifts right towards products. 2. Decrease in temperature: system compensates by producing more energy. System shifts left towards reactants. b. In an exothermic reaction, energy is on the product side of the equation and predictions of equilibrium shift are equivalent to changes in concentration of products. 1. Increase in temperature: system compensates by using up additional energy. System shifts left towards reactants. 2. Decrease in temperature: system compensates by producing more energy. System shifts right towards products. 4. POTENTIAL ENERGY DIAGRAMS a) A potential-energy diagram is used to depict the shift in energy as a chemical reaction progresses. Specifically, it shows the changing potential energy between the reactants and products. b) A potential energy diagram shows: 1. The potential energy of the reactant. 2. The potential energy of the activated complex. The activated complex is an intermediate structure of atoms that exists as bonds are breaking and new bonds are forming. It occurs temporarily before the final product is formed. 3. The potential energy of the product. 4. The activation energy of the forward reaction, which is the minimum energy required to form an activated complex. On the diagram, activation energy is the difference between the PE of the reactants and the PE of the activated complex. 5. The heat of reaction, which is the difference between the PE of the product and the PE of the reactant. c) In an endothermic reaction, the H product is greater than the H reactants due to an input of energy into the reaction. Therefore, the potential energy of the products will be higher than the reactants on the diagram.

4 d) In an exothermic reaction, the H product is less than the H reactant due to a release of energy. Therefore, the potential energy of the products will be lower than the reactants on the diagram. e) Potential energy diagrams can also depict the energy shift of a catalyzed reaction in which the activated energy is reduced and consequently, the reaction is accelerated.

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16 Unit I Note Quiz Questions Unit 10.1: Collision Theory 1. a 3. a 2. a 4. a

17 5. a 6. Which shows the activation energy of the reaction? 7. Which letter is the enthalpy of the reaction? 8. Which letter shows the potential energy of CH 4 and O 2? 9. Is the reaction endo/exothermic? 10. a

18 Unit 10.2: Le Châtelier's principle 1. a 2. A 3. A 4. A 6. A 5. A 7. A 8. A

19 9. A 10. A