UNIT I Collision Theory COLLISION THEORY explains rates on the molecular level Basic Premise: before molecules can react, they must collide http://www.chem.iastate.edu/group/greenbowe/section s/projectfolder/animations/no+o3singlerxn.html COLLISION THEORY Collision Theory explains: Effect of Concentration COLLISION THEORY Collision Theory explains: Effect of Temperature Hebden Textbook Page 12 Questions # 20 22 1
ENTHALPY Enthalpy the heat content of a substance, the total KE and PE of a substance at constant pressure Chemists interested in enthalpy changes ( H ) Enthalpy changes are equivalent to changes in potential energy www.youtube.com Eureka! Episode 10 Potential Energy EXOTHERMIC REACTIONS H 2 + S > H 2S H = 20 KJ negative H means exothermic S(g) + O 2(g) > SO 2(g) + 296 kj heat term shown on right side of arrow means exothermic ( it gives off heat like a product ) ENDOTHERMIC REACTIONS 6C + 3H2 > C6H6 H = + 83 KJ positive H means endothermic CH3OH + 201KJ C(s) + 2H2(g) + ½ O2(g) heat term shown on left side of arrow means endothermic ( it uses up heat like a reactant ) Hebden Textbook Page 16 Questions 24 28 Sep 21 1:51 PM 2
KINETIC ENERGY DISTRIBUTIONS http://www.bclearningnetwork.com/moodle/mod/resource/vi ew.php?id=577 KINETIC ENERGY DISTRIBUTIONS Curve at Lower Temperature Curve at Higher Temperature 3
KINETIC ENERGY DISTRIBUTIONS NOTICE: At the higher temperature, there are less slow (low KE) molecules and more fast (high KE) molecules The curve is more spread out at the higher temperature. The TOTAL AREA UNDER THE CURVE is the same for the high temperature as for the low temperature. The minimum energy needed in a collision before a reaction take place. It can also be defined as the minimum energy colliding particles must have in order to have a successful collision. A collision in which the molecules have sufficient energy for a reaction to take place is called a SUCCESSFUL COLLISION. 4
Rule of Thumb If the activation energy (threshold) is near the tail of the curve: If the temperature is increased by 10 o C, the reaction rate will about double. (ie. about twice the number of molecules have sufficient KE for a successful collision.) Note: If Activation Energy or ME is near the middle of the curve (or left side) the reaction is already fast, so an increase in temperature has a less drastic effect on the reaction rate. Hebden Textbook Pages 19 20 Questions #29 32 http://www.wwnorton.com/college/chemistry/gilbert2/tutorials/inter face.asp?chapter=chapter_14&folder=collision_theory Back to Collision Theory... Potential and Kinetic Energy During a Collision: REPULSIVE FORCE http://www.bclearningnetwork.com/moodle/mod/resour ce/view.php?id=577 As colliding molecules approach, the repulsion slows them down so kinetic energy decreases. As they push against the repulsive force potential energy increases (like compressing a spring). Kinetic Energy is converted to Potential Energy KE + PE = Total E (stays constant) 5
As molecules approach each other, KE is converted to PE. Molecules form a temporary unstable species called the ACTIVATED COMPLEX. Activated Complex rearranges to form the PRODUCT molecules. Product Molecules move apart and speed up. PE is converted to KE. http://chem.salve.edu/chemistry/temp2a.asp Recall, Activation Energy: The minimum energy required for a successful collision. OR The minimum energy reacting molecules must have in order to form the Activated Complex. The Activated Complex can be defined as a very short lived, unstable combination of reactant atoms that exists before products are formed. NOTE: The Activation Energy (Ea) is fixed by the nature of the reactants (# s and strengths of bonds in reactants). Ea is NOT affected by temperature or concentration! Increasing the temperature increases the fraction of molecules which have sufficient energy to form the Activated Complex (ie. sufficient energy to make it over the activation energy barrier). This is one reason that increasing the temperature will INCREASE the rate of reaction. Also, note that a change in temperature does NOT change the Potential Energy diagram at all. Temperature does NOT affect the Activation Energy or the H!! 6
Review the difference between Activated Complex and Activation Energy on the top of page 21 of your textbook. Review The 3 Cases on page 21 of your textbook. Also study the diagram at the bottom of page 21, where it compares the KE distribution and the PE diagram. 7
COLLISION GEOMETRY http://chem.salve.edu/chemistry/temp2a.asp 8
COLLISION GEOMETRY If a collision has an unfavourable alignment, the molecules require higher energy in order for the collision to be effective. If a collision has a favourable alignment, the molecules require lower energy in order for the collision to be effective. COLLISION GEOMETRY Route with Unfavourable Geometry Route with Favourable Geometry SUMMARY OF COLLISION THEORY For any successful collision (one resulting in a reaction) Three Requirements: 1.) Particles must collide. 2.) They must collide with sufficient energy > Ea. 3.) They need to have correct alignment (collision geometry) to keep Ea as low as possible. 9
EA, H AND BOND STRENGTHS FOR FORWARD AND REVERSE REACTIONS EA, H AND BOND STRENGTHS FOR FORWARD AND REVERSE REACTIONS Ea(f) = Ea(r) + H Using the graph above, find: Ea (forward rxn) = kj H (forward rxn ) = kj This forward reaction is thermic Using the graph above, find: Ea (reverse rxn) = kj H (reverse rxn ) = kj This reverse reaction is thermic CHECK YOUR UNDERSTANDING Given the following Potential Energy Diagram for the reaction: A 2 + B 2 2AB 45 kj 25 kj CHECK YOUR UNDERSTANDING a) Ea (forward) = kj b) Energy needed to break bonds in A 2 & B 2 kj c) Ea (reverse) = kj d) Energy needed to break bonds in AB kj e) Which has the stronger bonds A 2 & B 2 or 2AB? 5 kj 10
CHECK YOUR UNDERSTANDING CHECK YOUR UNDERSTANDING f) On a PE diagram, species with stronger bonds (more stable) are (low/high) er on the graph. g) Which set of species (A 2 & B 2, A 2B 2, or 2AB) have the weakest bonds? This species is the most stable. It is called the. h) Which set of species has the highest PE? i. Which set of species has the highest KE? Draw a graph of KE vs. Reaction Proceeds for the same forward reaction. Hebden Textbook Pages 23 25 Questions #33 45 11