Reaction Rates & Collision Theory PDF

6.3 Explaining Reaction Rates Collision Theory Chemical reactions can occur only if reactant atoms, molecules, or ions ​collide. Two Stipulations: 1) They must collide in a specific orientation. 2) They must collide with enough kinetic energy that any bonds in the reactants will break and new bo...

6.3 Explaining Reaction Rates Collision Theory Chemical reactions can occur only if reactant atoms, molecules, or ions ​collide. Two Stipulations: 1) They must collide in a specific orientation. 2) They must collide with enough kinetic energy that any bonds in the reactants will break and new bonds will form, making the products. rate = frequency of collisions x fraction of effective collisions ORIENTATION The way that reactants collide determines ​whether or not they will react. Consider the decomposition reaction of nitrosyl bromide Only some of ​ 2BrNO(g) --> 2NO(g) + Br2(g) these collisions ​are effective. (a) ​and (b) allow Br to ​ contact one ​ another and react while the ​ orientation of (c) ​collision does not. ​(c) is an ​ineffective ​ collision. ACTIVATION ENERGY For a reaction to occur, the reactants must have ​enough kinetic energy for the collision to be ​ effective (Activation Energy- Ea) Activation Energy: a) used to overcome the ​ electrostatic repulsive forces between colliding entities b) used to weaken the bonds ​of the reactants. In a Chemical Reaction -Potential energy- energy stored in the bonds within and among the entities of the reactants - Kinetic energy is their movement. When entities collide in an ​ appropriate orientation, a chemical reaction can only proceed if the ​kinetic energy is enough to break these bonds. It takes considerable energy (243 kJ/mol) to break a Br–N bond. 2BrNO(g) --> 2NO(g) + Br2(g) If 2 nitrosyl bromide molecules do not have enough kinetic energy to ​get over this potential energy hill, or barrier, the reaction will not take ​ place.  The Ea represents the energy difference between ​the reactants and the ​ activated complex. Notice: 1) exothermic H =(-)ve 2) endothermic H= (+)ve How can we manipulate the Collision Theory to increase or decrease reaction rate? 1) Temperature - ↑ T= ↑R ↓T = ↓R 2) Chemical Nature of Reactants 3) Concentration - ↑[reactant] = ↑ R ↓[reactant]= ↓ R 4) Surface area - ↑SA = ↑R ↓SA = ↓R 5) Catalyst - addition of catalyst = ↑R Temperature of the Reaction System Temperature increases the movement of the molecules (the thermal energy breaks intermolecular forces holding entities ​together). Therefore: Temp = ↑ average kinetic energy Temp Increases, then... - number of collisions having enough kinetic ​ energy to form activated complex increases. - number of collisions between reactants increases so the probability of colliding in proper orientation ​increases. (studies based on reactants in gas state) Only entities with a ​kinetic energy greater than or ​equal to the ​activation energy ​(required energy to break bonds) will be ​involved in an ​effective reaction.  Even a small temperature rise can cause a very large increase in the number ​of entities that have energy exceeding the activation ​energy. * For any given activation energy, Ea, a much larger amount of a reactant will have the required ​ kinetic energy at a higher temperature than at a lower ​temperature. This graph will ​allow us to ​ represent the ​ same reaction ​occurring at two ​separate ​ temperatures T2 is higher than T1 showing that a greater number of ​entities have enough kinetic energy to reach the ​ activation energy and collide successfully.  Chemical Nature of Reactants: Reactions involving the ​breaking of fewer bonds per reactant proceed faster than those ​involving the breaking of a larger number of bonds per reactant. has a higher reaction rate than--> Weaker bonds are broken at a faster rate than stronger bonds. (Less energy to break a single C–C bond than a double C=C bond. higher reacti on rate  Reactions between molecules are usually slower than reactions between ions. This is because, in molecules, covalent bonds have to be broken and new bonds reformed. Faster rate Slower rate The size and shape of a molecule or ion can also affect reaction ​rate. (Less complicated molecules react more easily) Faster rate Slower rate Concentration Increase the concentration of a reactant, the probability of collisions between reactant molecules increases. Surface Area Increasing the surface area of the solid reactant increases the reaction rate. Catalyst A catalyst provides an alternative pathway for the reaction, (has a lower activation energy). Any given temperature, more reactant(s) will have kinetic energy equal to or greater than this lower activation energy therefore more effective collisions = increased rxn rate Although a catalyst lowers the activation energy, Ea, for a reaction, it does not affect the energy difference between products and reactants. ​(ie. enthalpy change stays the same) Pg 372 # 1 - 6 Interest: Read Pg 373 on Biocatalysts