Rates, Temperature and Potential Energy Diagrams Worksheet

Transcription

1 SCH4U1 ER10 Name: Date: Rates, Temperature and Potential Energy Diagrams Worksheet Part 1: 1. Use the potential energy diagram shown to the right to answer the following: a. Label the axis. y axis is potential energy (kj or kj/mol) x axis is reaction progress b. What does each curve represent? Each curve represents a different reaction pathway. Likely the red curve represents a pathway that uses a catalyst which means a lower is needed. c. What type of reaction is occurring in terms of energy flow to or from the surroundings? Energy is flowing from the surrounding into the reaction i.e. endothermic reaction. d. What does each number (i, ii, iii) represent? i) for the reaction ii) E a for the catalyzed reaction iii) ΔH for the reaction 2. Use the multi step reaction shown to the right to answer the following: a. What is the overall activation energy for the reaction? The overall activation energy for the reaction is 60kJ. (Hint: look for the difference from the highest peak to the reactants) b. What is the reaction enthalpy (ΔH) for the reaction? ΔH = 35kJ c. What is the rate determining step for the reaction? Likely the first step because it involve the largest individual activation energy. d. Is the reaction exothermic or endothermic? Exothermic e. Which letters represent activated complexes? V, X and Z f. Which letters represent reaction intermediates? W and Y

2 Part 2: 1. Which diagram is described by each of these statements? a) an exothermic reaction that is unlikely to occur at room temperature. Diagram 3 since high E a b) an endothermic reaction. Diagram 1 c) the activation energy ( ) is greater than the energy released ( H) Diagram 3 d) a spontaneous exothermic reaction. Diagram 2 depending on the values but looks quite small. To be spontaneous needs to be 20kJ 2. Draw a potential energy (E p ) diagram for a reaction in which H = 80 kj/mol and = +28kJ/mol. Label the axes, activation energy, H, site of the activated complex, reactants and products. Your starting value for the reactants might be different that is ok, as long as you show the proper E a and H values.

3 3. Using the potential energy diagrams for an endothermic and exothermic reaction shown, choose the letter that best fits each statement. Reaction I Reaction 2 a) E P of the reactants C R b) E P of the products E Z c) H of the reaction B S d) activation energy of the D T forward reaction e) site of activated complex A W f) E P of the activated G X complex g) activation energy of the reverse reaction F Y 4. In the following reaction, the enthalpy change of the forward reaction is H = 36 kj/mol and the activation energy for the forward reaction is 73 kj/mol. A + B C + 36 kj a) Draw a potential energy diagram for the the reaction. Your starting value for the reactants might be different that is ok, as long as you show the proper E a and H values. b) What is the activation energy of the reverse reaction? of reverse reaction is 109 kj/mol

4 5. The activation energy of a forward and reverse reaction are as follows: i) C 2 (g) + H 2 (g) C 2 H 6 (g) = 180 kj/mol ii) C 2 H 6 (g) C 2 (g) + H 2 (g) = 317 kj/mol a) Draw a potential energy diagram for this reversible reaction. Your starting value for the reactants might be different that is ok, as long as you show the proper E a and H values. b) Calculate the enthalpy change ( H) for each reaction. i) ΔH forward = 137 kj/mol i) ΔH reverse = 137 kj/mol 6. Compare these reactions: i) C 2 H 5 Cl ( l ) C 2 (g) + HCl (g) = 254 kj/mol ii) C 2 H 5 Br ( l ) C 2 (g) + HBr (g) = 219 kj/mol Which of these two reactants would decompose more rapidly under the same reaction conditions and temperature? Explain your response. The reaction of C 2 H 5 Br because it has the lower activation energy meaning more molecules will have the required energy to react when they collide.

5 7. A reaction consists of two steps as follows: (kj/mol) H (kj/mol) Step 1: A + B C + E Step 2: A + E 2D a) Write the overall reaction equation. 2 A + B C + 2 D b) What is meant by the rate determining step? The rate determining step is the slowest step in a reaction mechanism. c) Which of these steps is the rate determining step? Why? Step 2 is likely the slowest step since it has the highest E a d) What is the effect on the overall rate of increasing the concentration of A? What is the effect on the overall rate of increasing the concentration of B? Explain. If you increase the concentration of A then the reaction rate should increase as reactant A is involved in the rate determining step. If you increase the concentration of B there would be no effect on the rate since B is not part of the rate determining step. 8. Examine the two Boltzmann distributions showing the distribution of kinetic energy possessed by the reactant molecules in two different reactions and the activation energy ( ) for each reaction. a) Which reaction will be fastest at room temperature? Explain Reaction 2 would be the fastest at room temperature because it has a much lower E a b) When the temperature is increased to 60 o C, what will happen to the rate of (i) Reaction 1 and (ii) Reaction 2? Why? The rate of both reactions will increase because more collisions will take place and more of the particles will have the energy required to overcome the activation energy barrier. c) In which case does the activation energy requirement change when the temperature is increased? Changing the temperature will have no impact on the activation energy requirement. 9. For the reaction: H 2 + Cl 2 2 HCl kj the activation energy for the process is 156 kj/mol. What is the activation energy for the decomposition of HCl to produce H 2 and Cl 2? for the decomposition of HCl would be 340 kj/mol of H 2 produced

6 10. Using the concepts of kinetic energy and surface area, explain why sugar cubes can be used in hot coffee but granulated sugar is preferable for making iced tea. In hot coffee there is a rapid dissolving rate because of the high temperature so using sugar cubes is no problem. With iced tea, the dissolving rate is much slower so granulated sugar is a better option because there is more surface area to help speed up the reaction. 11.Given the kinetic energy distribution curves and threshold energy ( ) for reactions A and B: a) Which reaction will be faster at room temperature? Reaction A since most of the particles are above the activation energy requirement. b) Which reaction will show the greatest increase in the rate of reaction if the temperature is increased? Reaction B since increasing the temp will put a lot more particles over the activation energy requirement.