2Collision Theory Used to Explain Reaction Rates Atoms, ions, and molecules can form a chemical bond when they collide, provided the particles have enough kinetic energy.Particles lacking the necessary kinetic energy to react still collide, but simply bounce apart.Activation energy - the minimum energy colliding particles must have in order to react.
3Chemical Reactionsordinarily occur as a result of collisions between reacting particles. Consider the reaction:CO(g) + NO2(g) ----> CO2(g) + NO(g) rate = k(conc CO)(conc NO2(g))doubling the [CO], holding [NO2] constant, the number of collisions in a given time doubles.doubling the [NO2] , holding CO constant, has the same effect.the number of collisions per unit time is directly proportional to the concentration of CO or NO2.The fact that the rate is directly proportional to these concentrations indicates that reaction occurs as a direct result of collisions between CO and NO2 molecules.
4NOT EVERY COLLISION LEADS TO REACTION!!!!! It is possible to calculate the rate at which molecules collide with each other by using the kinetic theory. Consider a mixture of CO and NO2 at 700 K and a concentration of 0.10 mol/Levery molecule would collide with about a billion other molecules in one second!if every collision resulted in a reaction, then the whole mixture would be reacted in a fraction of a second.the actual reaction takes about 20 seconds.
5Effective CollisionsIn order for collisions to be effective, there must be considerable force in the collisions. The slower moving molecules do not have enough kinetic energy to react when they collide...they bounce off one another and retain their identity.Only those molecules moving at high speed have enough energy for collisions to result in a reaction.Every reaction requires a certain minimum energy for the reaction to occur--it is called activation energy, Ea, and is expressed in kJ.
6Activated Complex-an unstable, high energy species forward reaction exothermic (∆H<0), Ea is smaller than Ea1forward reaction endothermic (∆ H>0), Ea is larger than Ea1if ∆ H = +200 kJ, then Ea = Ea1 + DH = Ea kJ
7Factors that Affect Reaction Rates Collision Theory 1. Temperature2. Concentration3. Particle size4. Catalyst
8Factors that Affect Reaction Rates Collision Theory TemperatureIncreasing temperature increases the number of particles that have enough kinetic energy to react when they collide.Concentration changes (amt per vol)Cramming more particles into a fixed volume increases the collision frequency.
9Factors that Affect Reaction Rates Collision Theory Particle Sizethe smaller the particle size, the larger the surface area for a given mass of particles. Decreasing particle size will increase the rate of reaction.CatalystA catalyst is a substance that increases the rate of a reaction without being used up itself in the reaction.
10Effects of Catalyst on Activation Energy Enzymes are biological catalyst usually made of proteins.Speed reactions by lowering the activation energy of the reaction.
11Chemical Equilibrium Dynamic (in constant motion) Reversible Chemical equilibrium occurs when the forward and reverse reaction are taking place at the same rate.There is no net change in the actual amounts of the components of the system.
13Rate vs Equilibrium At Equilibrium: RATES ARE EQUAL the concentrations of reactants and products are constant.D [ ]’s = 0The forward and reverse reactions continue after equilibrium is attained.
14Kinetics Reaction Rate Orders the order of reaction with respect to a certain reactant, is defined as the power to which its concentration term in the rate equation is raised.For example, 2A + B → Cr = k[A]2[B]1 the reaction order with respect to A would be 2 and with respect to B would be 1, the total reaction order would be 2 + 1 = 3.Reaction orders can be determined only by experiment. The reaction order is not necessarily related to the stoichiometry of the reaction, unless the reaction is elementary. Complex reactions may or may not have reaction orders equal to their stoichiometric coefficients
15The Reaction Quotient, Q In general, all reacting chemical systems are characterized by their REACTION QUOTIENT, Q.When the system is at equilibrium, Q = K
16Reactions Review“Systems”: two reactions that differ only in directionAny reversible reaction H2 + I2 ↔ 2HInoted by the double arrow; ↔Two reactions: only difference is the DirectionH2 + I2 ↔ 2HIReactant products2HI ↔ H2 + I2Left Right
17Reversible Reactions H2 + I2 ↔ 2HI the products may react back to original reactants.“closed system”: ONLY if all reactant are presentIf one piece is completely gone it has ”gone to competition” and no longer reversibleExamples: Reversible Reactions.Unopened SodaBreathingRechargeable batteriesColor changing shirt
18Blue to pink Co(H2O)4Cl2 + 2 H2O Co(H2O)6Cl2 The product is hexa-coordinated with water and the reactant is tetra-coordinated with water. Notice that the net charge on the left and right sides of the first equation is zero.Inclusion of 4[H2O] assumes additional information that the reactant is hexa-coordinated with water and the product is tetra-coordinated with water. Notice that the net charge on the left and right sides of the first equation is zero.Blue to pink Co(H2O)4Cl2 + 2 H2O Co(H2O)6Cl2
19Properties of an Equilibrium Pink to blueCo(H2O)6Cl2 → Co(H2O)4Cl H2OBlue to pinkCo(H2O)4Cl H2O → Co(H2O)6Cl2Equilibrium systems areDYNAMIC (in constant motion)REVERSIBLEcan be approached from either direction
20Reaction at Equilibrium A and B are _________. C and D are _______.
21Factors Affecting Equilibrium Changes in temperature, pressure, and concentration affect equilibrium.The outcome is governed by LE CHÂTELIER’S PRINCIPLE“...if a system at equilibrium is disturbed, the system tends to shift its equilibrium position to counter the effect of the disturbance.”
22Writing and Manipulating Keq Solids NEVER appear in equilibrium expressions.S(s) + O2(g) ---> SO2(g)Liquids NEVER appear in equilibrium expressions.NH3(aq) + H2O(liq) ---> NH4+(aq) + OH-(aq)
23Henri Le Châtelier Henri Le Châtelier 1850-1936 Studied mining engineering.Interested in glass and ceramics.
24A + B C + D Change in Concentration What happens when there is an increase in reactant or product?What happens when there is a decrease in reactant or product?
25Change in Concentration A + B C + DChange in ConcentrationStress ShiftIncrease in A or B forwardIncrease in C or D reverseDecrease in A or B reverseDecrease in C or D forward
26Changes in PressureOnly affects equilibrium system with an unequal number of moles of gaseous reactants and productsDecrease in pressure shifts the reaction in the direction that produces the larger number of moles of gas.An increase in pressure shifts the reaction in the direction that produces the smaller number of moles of gas.
27Product or Reactant Favored? Keq greater than 1 Products favoredKeq less than 1 Reactants favored
28Endothermic vs Exothermic Exothermic - Heat is a product. (-ΔH)Endothermic - Heat is a reactant. (+ΔH)
29Changes in Temperature An increase in temperature favors endothermic reactions.A decrease in temperature favors exothermic reactions.
30Changes in Temperature +ΔH = Endothermic↑ temp favors the forward rxn.-ΔH = Exothermic↑ temp favors the reverse rxn.
31Temperature Effects on Equilibrium N2O4 (colorless) + heat NO2 (brown)
32Equilibrium and Catalysts Add catalyst = no change equilibrium concentrationA catalyst only affects the RATE of approach to equilibriumCatalytic exhaust system.
33Applying a Stress to a System at Equilibrium N2 + 3H2 2NH3 + HeatΔH = -92 kJ/mol rxnIncrease temperatureIncrease pressureAdd a catalyst No ChangeAdding H2Removing NH3
38Magnitude of Keq Varies only with temperature Is constant at a given temperatureIs independent of the initial concentrations
39Magnitude of Keq A + B ↔ C + D Keq > 1 mostly products Keq < 1 mostly reactantsKeq ~ 1 equal amounts of products and reactants
40The Meaning of Keq For N2(g) + 3 H2(g) 2 NH3(g) Keq is used to tell if a reaction will favor products or reactants.For N2(g) + 3 H2(g) 2 NH3(g)Concentration of products is much greater than that of reactants at equilibrium.The reaction strongly favors products
41The Meaning of Keq Concentration of products is much less AgCl(s) ↔ Ag+(aq) + Cl (aq)Keq = 1.8 x 10-5Concentration ofproducts is much lessthan that of reactants atequilibrium.The reaction stronglyfavors reactants
42The Meaning of KeqAgCl(s) ↔ Ag+(aq) + Cl (aq) Keq = 1.8 x 10-5 Reactant Favored The Reverse reaction Ag+(aq) + Cl-(aq) ↔ AgCl(s) Keq = 1.8 x 105 is product-favored.
43Calculation of Keq – Learning Check For the reaction2HI(g) ↔ H2(g) +I2(g), at 448•C,The equilibrium concentrations areHI = M,H2 = M, andI2 = M.Calculate the equilibrium constant at this temperature.Write the equilibrium expression (Prod/React)Fill in the values and solve
44Calculation of Keq 2HI(g) ↔ H2(g) + I2(g), Equil
46Calculation of Keq Learning Check N2 + 3H2 2NH3When equilibrium is established in a L container the equilibrium concentrations are: N2, 3.01 mol; H2, mol; NH3, mol.Calculate KeqWrite the equation for KeqCalculate molar concentration (M)Fill in and solve.