UNIT I Collision Theory

C OLLISION T HEORY explains rates on the molecular level Basic Premise: before molecules can react, they must collide ctions/projectfolder/animations/NO+O3singlerxn. html

C OLLISION T HEORY Collision Theory explains: 1. Effect of Concentration

C OLLISION T HEORY Collision Theory explains: 2. Effect of Temperature Hebden Textbook Page 12 Questions # 20-22

E NTHALPY 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

E XOTHERMIC R EACTIONS H 2 + S ---> H 2 S  H = - 20 KJ negative  H means exothermic S(g) + O 2 (g) ---> SO 2 (g) kJ “heat term” shown on right side of arrow means exo thermic ( “it gives off heat like a product”)

E NDOTHERMIC R EACTIONS 6C + 3H 2 ---> C 6 H 6  H = + 83 KJ positive  H means endothermic CH 3 OH + 201KJ  C(s) + 2H 2 (g) + ½ O 2 (g) “heat term” shown on left side of arrow means endo thermic (“it uses up heat like a reactant”) Hebden Textbook Page 16 Questions 24-28

P ROVINCIAL E XAM Q UESTIONS

K INETIC E NERGY D ISTRIBUTIONS ce/view.php?id=577

K INETIC E NERGY D ISTRIBUTIONS Curve at Lower Temperature Curve at Higher Temperature

K INETIC E NERGY D ISTRIBUTIONS 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.

A CTIVATION E NERGY 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.

A CTIVATION E NERGY

Rule of Thumb If the activation energy (threshold) is near the tail of the curve: I f 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.)

A CTIVATION E NERGY 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 Questions #29-32

A CTIVATION E NERGY /interface.asp?chapter=chapter_14&folder=collision_theor /interface.asp?chapter=chapter_14&folder=collision_theor Back to Collision Theory... Potential and Kinetic Energy During a Collision: REPULSIVE FORCE

P OTENTIAL E NERGY D IAGRAMS esource/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 )

P OTENTIAL E NERGY D IAGRAMS

As molecules approach each other, Molecules form a temporary KE is converted to PE. 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.

P OTENTIAL E NERGY D IAGRAMS 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!

P OTENTIAL E NERGY D IAGRAMS 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 !!

P OTENTIAL E NERGY D IAGRAMS 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.

P OTENTIAL E NERGY D IAGRAMS

P ROVINCIAL E XAM Q UESTIONS

C OLLISION G EOMETRY

C OLLISION G EOMETRY 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.

C OLLISION G EOMETRY Route with Unfavourable Geometry Route with Favourable Geometry

S UMMARY OF C OLLISION T HEORY 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.

P ROVINCIAL E XAM Q UESTIONS

E A,  H AND B OND S TRENGTHS FOR F ORWARD AND R EVERSE R EACTIONS Using the graph above, find: Ea (forward rxn) = _________kJ  H (forward rxn ) = _________kJ This forward reaction is ______thermic

E A,  H AND B OND S TRENGTHS FOR F ORWARD AND R EVERSE R EACTIONS Using the graph above, find: Ea (reverse rxn) = _________kJ  H (reverse rxn ) = _________kJ This reverse reaction is ______thermic

C HECK Y OUR U NDERSTANDING Given the following Potential Energy Diagram for the reaction: A 2 + B 2  2AB 45 kJ 25 kJ 5 kJ

C HECK Y OUR U NDERSTANDING 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? ___________

C HECK Y OUR U NDERSTANDING 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 2 B 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? __________

C HECK Y OUR U NDERSTANDING i. Which set of species has the highest KE?_________ j) Draw a graph of KE vs. Reaction Proceeds for the same forward reaction. Hebden Textbook Pages Questions #33-45