Kinetics and Equilibrium Chapter 18

KINETICS Deals with: Speed of chemical reactions RATE of reaction Way reactions occur MECHANISM of reaction

RATE A measure of the speed of any change that occurs within an interval of time IN CHEMISTRY: amount of reactant changing per unit of time Very fast to very slow

In Order For a Reaction to Occur Particles need to experience an EFFECTIVE COLLISION Collide with enough kinetic energy Collide with the proper orientation Particles lacking the necessary KE bounce apart unchanged Ex. Two balls of clay

When supplied with enough energy Substances decompose to simpler substances or reorganize themselves into new substances. The minimum energy that colliding particles must have in order to react is called the activation energy (E a ) (“barrier”)

Activated Complex Unstable arrangement of atoms that forms momentarily at the peak of the activation- energy barrier

Potential Energy Diagrams Represents the relationship between the activation energy and heat of rxn for a given chemical rxn

A little review Endothermic Exothermic

Heat of Reaction (ΔH) Measures if heat is absorbed or released Difference in heat content between the products and the reactants of a chemical reaction ΔH = H products -H reactants The difference in PE between the products and reactants

-∆H= exothermic (products have less PE) +∆H=endothermic (reactants have less PE) **TABLE I**

DOUBLE THE REACTANTS/PRO DUCTS, DOUBLE THE ∆H

6 Factors that affect the rate of chemical reactions: 1. NATURE OF THE REACTANTS due to the nature of the bonds Ionic faster than covalent- slight rearrangement of electrons usually occurs rapidly at room temp. Rxns in which many covalent bonds must be broken usually occur slowly at room temp.

2. TEMPERATURE Increase temp, increase rate of rxn WHY?  T,  KE (particles move faster),  collision energy and frequency For many rxns, a 10˚C rise in temp doubles the rate of rxn

3. CONCENTRATION Increase concentration, increase rate of rxn Increase frequency of effective collisions between reactants Any time the [ ] of one or more of the reactants is increased, the rate of rxn  Compressing a gas increases the [ ], therefore increasing the rxn rate

4. SURFACE AREA  S.A. of reactants  rxn rate Powdered zinc reacts faster than whole zinc

5. CATALYSTS A catalyst is a substance that speeds up a chemical rxn, the catalyst itself is not altered Lowers the activation energy The PE of activated complex  with a catalyst Products or reactants remain unaltered by the catalyst ∆H REMAINS UNCHANGED

Energy of Reactants and Products stays the same so ΔH stays the same!!!

6. REACTION MECHANISMS Reactions occur in steps The more steps involved, the slower the rxn rate Chemical equations don’t show us the mechanisms involved, just the start and end of the reaction N 2(g) + 3H 2(g) --> 2NH 3(g)

EQUILIBRIUM Must have a CLOSED system so reactants and products may not escape Rates of  and  reactions same, NOT amounts of reactants or products Reversible Reaction

Dynamic Equilibrium H 2 O (g) H 2 O (l) Rate of condensation = rate of evaporation Balance between two opposing rxns in which concentrations of both R. and P. remain CONSTANT But they are not necessarily the same as each other Changes in T, P and concentration affect the conditions of equilibrium

LeChatlier’s Principle Predicts the effect of stresses on a system at equilibrium Disturb a chemical system at equilibrium System changes to reduce the effects of the applied stress Equilibrium is reestablished under the new conditions by altering the [R] and [P]

Effect of Concentration N 2 + 3H 2 2NH 3 If the [N 2 ] , the rxn that reduces the amount of N 2 is favored (fwd rxn) If the [N 2 ] is decreased, the rxn that produces more N 2 is favored (reverse rxn) In either case, a new equilibrium point results

Effect of Pressure **R and P MUST BE GASES**  P pushes the rxn in the direction with the smaller volume (fewest moles) N 2(g) + 3H 2(g) 2NH 3(g) R= 4 moles P= 2 moles  P makes more NH 3 4  2: smaller volume

H 2(g) + Cl 2(g) 2HCl (g) Which is favored if pressure is lowered??

Effect of Temperature  the T affects the rxn that absorbs heat more Endothermic direction favored  the T favors the exo rxn

Effect of Catalysts Changes rate of both  and  rxns by the same amount Equilibrium is reached more quickly No net change in the [P] or [R] Less activation energy is required

Spontaneous Changes Changes that occur under a given set of conditions without the application of external work Heat will be absorbed from, or released to, the surroundings without any outside assistance

Examples Ice  25° C Liquid  -25° C Once activation energy is applied, match will combust rapidly under specific conditions

Factors that determine the direction of spontaneous reactions Two tendencies in nature that determine the direction of a spontaneous change: Change to a condition of less energy Change to a condition of greater randomness (disorder)  want higher entropy

The tendency toward lower energy Activation energy for exothermic rxns is less than endothermic rxns Most spontaneous rxns go in exo direction

Tendency toward Randomness Measured by entropy (S)- greater disorder, higher S Spontaneous rxns go in direction of greater disorder s  l  g Higher T, higher S A system tends to change from a state of great order to a state of less order

High T and Spontaneous Rxns Higher T, higher KE, higher S When the energy change favors one direction and the entropy change favors the reverse, the direction in which the change goes depends on the TEMPERATURE