Unit 1 Kinetics and Equilibrium Chemistry 3202 6:47 AM Part 1: Reaction Kinetics (Chp. 12) Reaction Kinetics is the study of the rate of a chemical reaction Qualitative: Reactions may be described as being FAST or SLOW Fast – burning, explosions, precipitation Slow – rusting, fermentation 6:47 AM Reaction Kinetics POSSIBLE UNITS ?? Quantitative: The rate of a reaction measures how fast products are formed or how fast reactants are consumed Rate = Change in quantity Change in time 6:47 AM Measuring Reaction Rate The method used to determine reaction rate will depend on the reaction being studied. (p. 466) Methods: 1. monitor pH if there is an acid or base in the equation 2. record gas volume or changes in pressure if there is a gas in the reaction 6:47 AM Measuring Reaction Rate Methods: (cont’d) 3. record changes in mass if solids are present 4. monitor absorption of light if there is a color change 5. changes in electrical conductivity indicate changes in ion concentration 6:47 AM MC: What could we use to measure the rate of this reaction? Cu(s) + 2 AgNO3(aq) 2 Ag(s) + Cu(NO3)2(aq) a) pressure b) pH c) d) gas volume mass Answer: d) because a solid is present 6:47 AM MC: What could we use to measure the rate of this reaction? SO3(g) + H2O(l) H2SO4(aq) a) pressure c) gas volume b) pH d) mass Answers: a) and c) because a gas is present b) because an acid is being produced 6:47 AM 6:47 AM What determines RATE?? All chemical reactions are bond breaking/bond forming events The rate of a reaction depends on how quickly bonds are broken and how rapidly new bonds form. KMT and Collision Theory are used to explain reaction rates. 6:47 AM Kinetic Molecular Theory (KMT) Matter is made of particles (atoms, ions, or molecules) in continuous motion An increase in temperature: increases the speed of particles reduces the forces of attraction between particles 6:47 AM Kinetic Molecular Theory (KMT) KMT is supported by: Diffusion – particles of a gas spread to fill their container (‘perfume in a room’) - solids dissolve uniformly in liquids over time. Pressure – a balloon remains inflated because gas particles are continuously hitting the sides of the balloon 6:47 AM Energy Distribution of Particles 25 °C # of particles 200 °C Kinetic Energy 6:47 AM Collision Theory reactant particles must collide with one another for a chemical reaction to occur particles must collide with proper orientation collisions must have enough intensity to break old bonds and allow new bonds to form 6:47 AM Collision Theory to increase reaction rate you must increase the number of successful collisions between reactant molecules VIDEO (VHS): Reaction Rates LASERDISK: 3 VIDEO CLIPS 6:47 AM Factors Affecting Reaction Rate Concentration – an increase in the concentration of a reactant usually increases the rate of a chemical reaction - the rate increases because there are: - more particles resulting in - more collisions between particles & - more successful collisions. 1. 6:47 AM Factors Affectingmore Reaction Rate successful collisions Temperature - an increase in the temperature increases the rate of a chemical reaction - the higher temperature results in: - more collisions between particles faster rate - more intense collisions NOTE: A temperature increase of 10 ºC usually causes reaction rate to double. 2. 6:47 AM Factors Affecting Reaction Rate 3. Nature of Reactants – compounds with fewer bonds to break will react more rapidly than compounds with many bonds eg. propane (C3H8) burns faster than candlewax (C25H52) because it has fewer bonds 6:47 AM Factors Affecting Reaction Rate Nature of Reactants - compounds with weak bonds react more rapidly than compounds with strong bonds – ions will react more rapidly than atoms and molecules 3. 6:47 AM Factors Affecting Reaction Rate Surface Area - crushing a solid to produce a powder, or changing a substance to the gas phase, exposes more particles for collision if more particles are available for collision there will be: - more collisions - more successful collisions 4. 6:47 AM faster rate Factors Affecting Reaction Rate Catalysts - a catalyst increases the reaction rate by providing a different reaction pathway or mechanism with a lower activation energy - a catalyst IS NOT consumed by a chemical reaction. 5. 6:47 AM Ea with catalyst # of particles Ea without catalyst Kinetic Energy 6:47 AM Potential Energy Diagrams (p. 473) PE diagrams show changes in potential energy (stored chemical energy) during chemical reactions Exothermic reactions release more energy than they absorb (eg. burning) Endothermic reactions absorb more energy than they release (eg. photosynthesis) 6:47 AM NEW SLIDE ΔH written in the equation and outside the equation (Thermochemical Equation) 6:47 AM Potential Energy Diagrams ∆H represents the heat of reaction or enthalpy of reaction ∆H is the difference between the PE of the reactants and the PE of the products the minimum energy needed for a chemical reaction to occur is the activation energy 6:47 AM Potential Energy Diagrams the activated complex for a reaction is a temporary, unstable, intermediate species that quickly decomposes to products eg. H2 + I2 → H2I2 → 2 HI activated complex 6:47 AM ENDOTHERMIC site of the activated complex Products Eareverse PE ∆H (positive) Reactants activation energy (Ea forward) Reaction Progress 6:47 AM EXOTHERMIC site of AC Ea reverse PE Reactants ∆H (negative) Ea forward Products Reaction Progress 6:47 AM Formula: (OPTIONAL) Eaforward - Eareverse = ΔH This formula is NOT necessary if you prefer using the PE diagram. Animation 6:47 AM Ea fwd Ea rev 25 ΔH Endothermic or Exothermic -30 50 20 150 250 65 28 Sketch a PE diagram for each reaction Photosynthesis Earev C6H12O6 + O2 PE Eafwd CO2 + H2O Reaction Progress 6:47 AM ∆H Respiration Earev Eafwd PE C6H12O6 + O2 ∆H CO2 + H2O Reaction Progress 6:47 AM ∆H p. 474 6:47 AM ∆H 6:47 AM ∆H p. 475 6:47 AM Effect of a catalyst PE catalyzed no catalyst Reaction Progress 6:47 AM EXOTHERMIC no catalyst PE Reaction Progress 6:47 AM Sample problem: p. 475 Questions: p. 476; #’s 1 – 4 p. 484; #’s 1 – 4 p. 486; #’s 1,2, 4, 6, & 7 p. 468; # 4 6:47 AM Reaction Mechanisms (pp. 477 – 485) reaction mechanism – the steps that occur in a chemical reaction elementary reaction - each step in a reaction mechanism reaction intermediate – a molecule, atom or ion formed in one step and consumed in a later step NOTE: reaction intermediates are NOT included in the overall equation 6:47 AM Reaction Mechanisms eg. #1 Step #1 Step #2 NO(g) + O2(g) NO3(g) NO3(g) + NO(g) 2 NO2(g) Overall Equation: 2 NO(g) + O2(g) 2 NO2(g) 6:47 AM HBr + O2 → HOOBr fast HOOBr + HBr → 2 HOBr slow 2 HOBr + 2 HBr → 2 H2O + 2 Br2 fast p. 478 #’s 5 – 8 6:47 AM Reaction Mechanisms rate-determining step (RDS) - the slowest step in a reaction mechanism - to increase the rate of a reaction you must speed up the RDS - increasing the concentration of a reactant will increase the rate ONLY IF the reactant is in the RDS 6:47 AM Reaction Mechanisms PE diagrams - every step in a reaction mechanism has an activation energy which can be drawn on a PE diagram 6:47 AM Reaction Mechanisms 3-step mechanism #2 PE #1 RDS ?? #3 Reaction Progress 6:47 AM Reaction Mechanisms eg: Step #1 H2CO2 + H+ H2CO2H+ fast Step#2 H2CO2H+ HCO+ + H2O slow Step #3 HCO+ CO + H+ fast 6:47 AM Reaction Mechanisms eg: Overall H2CO2 H2O + CO Omit H+ - catalyst Omit H2CO2H+ & HCO+ - reaction intermediates 6:47 AM Reaction Mechanisms PE H2CO2 + H+ Reaction Progress 6:47 AM CO + H+ p. 476; #’s 1 – 4 p. 484; #’s 1 – 4 p. 486; #’s 1,2, 4, 6, & 7 p. 468; # 4 p. 478; #’s 5 - 8 p. 484 #’s 5 – 9 p. 485 #’s 10, 12 p. 486 #’s 8, 10, 11 p. 487 #’s 14, 17 p. 829 #’s 128,129, 131, 132 6:47 AM p. 829 # 128 Step 1 H2(g) + NO(g) → H2O(g) + N(g) Step 2 Step 3 H2(g) + O(g) → H2O(g) 2H2(g) + 2NO(g) → N2(g) + 2H2O(g) Part 2: Chemical Equilibrium Equilibrium A balancing Act! Text Ch 13: p 488 - 541 Part 2: Chemical Equilibrium All reactions we have done have shown reactants being converted 100% to products Many reactions are reversible with some products being converted back to reactants 6:47 AM Part 2: Chemical Equilibrium Dynamic equilibrium occurs when 2 opposing processes occur at the same rate A chemical equilibrium occurs when two opposing chemical reactions occur at equal rates. Demo: p. 491 6:47 AM Types of Equilibria 1. Phase Equilibria An equilibrium may be established between different phases of a compound in a sealed container eg. H2O(l) in a sealed container 6:47 AM Types of Equilibria Initially: H2O(l) changes to H2O(g) H2O(l) → H2O(g) Gradually: H2O(g) changes to H2O(l) H2O(l) ← H2O(g) 6:47 AM Types of Equilibria Using equilibrium notation: H2O(l) ⇌ H2O(g) Temperature changes? Closed vs. open system? 6:47 AM Types of Equilibria 2. Solubility Equilibria occur in saturated solutions when NaCl(s) is placed in water, the initial rate of dissolving is fast NaCl(s) 6:47 AM NaCl(aq) Types of Equilibria as more solid dissolves, the rate of dissolving slows and recrystallization begins. eg. NaCl(s) NaCl(aq) 6:47 AM Types of Equilibria when the solution is saturated there are NO VISIBLE CHANGES At equilibrium, the RATE of dissolving and the RATE of recrystallization are EQUAL. equilibrium eg. NaCl(s) 6:47 AM ⇌ NaCl(aq) Types of Equilibria Temperature change?? Open vs. Closed ?? 6:47 AM Types of Equilibria 3. Chemical Equilibrium Chemical reactions that are reversible usually result in chemical equilibrium eg. 6:47 AM NO2 gas changing to N2O4 Types of Equilibria Initially the forward rate is high eg. 2 NO2(g) N2O4(g) as more product forms, the reverse reaction begins and increases in rate. eg. 2 NO2(g) N2O4(g) 6:47 AM Types of Equilibria eventually the forward rate slows and the reverse rate increases such that the FORWARD AND REVERSE RATES ARE EQUAL eg. 2 NO2(g) ⇌ N2O4(g) http://www.chm.davidson.edu/ronutt/che115/EquKin/EquKin.htm 6:47 AM Conditions for Equilibrium (p. 492) 1. 2. 3. Macroscopic properties are constant ie. NO OBSERVABLE CHANGE Forward and reverse rates must be equal A CLOSED SYSTEM is required for equilibrium 6:47 AM Conditions for Equilibrium 4. Equilibrium may occur from either direction eg. 2 NO2(g) ⇌ N2O4(g) OR N2O4(g) ⇌ 2 NO2(g) p. 493; #’s 1 – 6 Kinetics & Equilibrium #4 6:47 AM Shifts in Equilibrium Equilibrium occurs when the forward rate equals the reverse rate. Changes in concentration, temperature and pressure/volume can cause the forward or reverse reaction rate to change. 6:47 AM Shifts in Equilibrium Eventually, a new equilibrium will be established with different reactant and product concentrations Le Châtelier’s Principle is used to predict changes in concentrations when a stress is applied to a system at equilibrium. 6:47 AM Le Châtelier’s Principle (p.520) When a stress is applied to a system at equilibrium, the system will adjust or shift WHAT??? to relieve the stress. A change(stress) is applied to a system at equilibrium Forward or reverse rate will change New equilibrium established 6:47 AM Le Châtelier’s Principle Changes in concentration An increase in concentration on one side of an equation favors or drives the reaction to the opposite side. eg. What will happen if CO is added to this system at equilibrium? 1. CO(g) + 2 H2(g) 6:47 AM ⇌ CH3OH(g) forward rate increases; reverse rate catches up Le Châtelier’s Principle - the system will ‘shift’ to the right to use the CO and produce more CH3OH - some H2 will be used [CH3OH] will increase [H2] will decrease [CO] ‘spikes’ and then drops to a value higher than it was before the change 6:47 AM ?? possible graph ?? CO mol/L CH3OH H2 time 6:47 AM Le Châtelier’s Principle An equilibrium shifts away from a substance that increases in concentration or toward a substance that decreases in concentration. 6:47 AM Le Châtelier’s Principle WHY??? - rates at equilibrium ??? - increasing the [CO] & forward rate ?? - reverse rate?? 6:47 AM Le Châtelier’s Principle - [H2] ?? - new equilibrium concentrations ?? ** NO CHANGE in Keq ** 6:47 AM Le Châtelier’s Principle CO(g) + 2 H2(g) ⇌ What happens if we: decrease [CO] decrease [CH3OH] increase [H2] 6:47 AM CH3OH(g) Le Châtelier’s Principle IMPORTANT NOTE: - adding a solid or liquid does not change molar concentration - changing the amount of a solid or liquid in an equilibrium will NOT cause a shift 6:47 AM Le Châtelier’s Principle eg. CaCO3(s) ⇌ CaO(s) + CO2(g) What happens if we: add CaCO3(s) ?? add CO2(g) ?? 6:47 AM Le Châtelier’s Principle 2. (ΔH & equations) Temperature - Raising the temperature of an exothermic equilibrium favors the formation of reactants. eg. CO(g) + 2 H2(g) ⇌ CH3OH(g) + 65 kJ What happens if we increase temperature in this equilibrium? 6:47 AM Le Châtelier’s Principle - Raising the temperature of an endothermic equilibrium favors formation of products. eg. CaCO3(s) + heat ⇌ CaO(s) + CO2(g) What happens if we increase temperature in this equilibrium? 6:47 AM Le Châtelier’s Principle OR - Raising the temperature shifts the equilibrium away from the energy term. - Decreasing the temperature shifts the equilibrium toward the energy term. CO(g) + 2 H2(g) ⇌ CH3OH(g) + 65 kJ CaCO3(s) + heat ⇌ CaO(s) + CO2(g) 6:47 AM Le Châtelier’s Principle eg. How would an increase in temperature affect these equilibria? N2(g) + 3 H2(g) ⇌ 2 NH3(g) + heat 2 SO3(g) ⇌ 2 SO2(g) + O2(g) ΔH = +197 kJ 2 SO3(g) + 197 kJ ⇌ 2 SO2(g) + O2(g) **A change in temperature changes Keq** 6:47 AM Le Châtelier’s Principle 3. Pressure/Volume An increase in pressure of a system at equilibrium has the same effect as a decrease in the volume of the system. (inverse relationship) 6:47 AM Le Châtelier’s Principle Increasing the pressure of a system at equilibrium by reducing volume causes the equilibrium to shift in the direction that reduces pressure ie. shift to the side with fewer molecules of GAS!! 6:47 AM Le Châtelier’s Principle eg. How would an increase in pressure caused by a decrease in volume - affect these equilibria? N2(g) + 3 H2(g) ⇌ 2 NH3(g) 4 NH3(g) + 5 O2(g) ⇌ 4 NO(g) + 6 H2O(g) 6:47 AM Le Châtelier’s Principle eg. Why would a change in pressure NOT affect the following equilibria? H2(g) + I2(g) ⇌ 2 HI(g) 2 Ag(s) + Zn2+(aq) ⇌ 2 Ag+(aq) + Zn(s) 6:47 AM Le Châtelier’s Principle pp. 529, 530 #’s 33 – 37 (omit 36 for now) p. 533 #’s 1 – 3, 5 Answers on p. 537 Animation Lab: Perturbing Equilibrium 6:47 AM Le Châtelier’s Principle 4. Catalyst - Does NOT cause a shift in equilibrium - Increases BOTH rates equally so equilibrium is reached faster 5. Surface Area - same as a catalyst 6:47 AM Equilibrium Constant (Keq) For any system at equilibrium, there is a mathematical relationship between reactant and product concentrations (Guldberg and Waage, 1864) P. 494 6:47 AM Equilibrium Constant (K) See p. 495 6:47 AM eg. N2O4(g) ⇌ 2 NO2(g) Equilibrium Constant (K) For the general equilibrium below: aP + bQ ⇌ cR + dS Keq = c [R] d [S] a b [P] [Q] 6:47 AM Equilibrium Constant (K) - - - a, b, c, & d are coefficients used to balance the equation P, Q, R, & S are the reactants and products Kc is sometimes used instead of Keq when units are molar concentration 6:47 AM Equilibrium Constant (K) eg. Write the expression for Keq for: 2 SO2(g) + O2(g) ⇌ 2 SO3(g) 2 HCl(g) ⇌ 6:47 AM H2(g) + Cl2(g) Equilibrium Constant (K) NOTE!! Solids or liquids ARE NOT included in the Keq or Kc expression because their concentration is constant 6:47 AM Equilibrium Constant (K) eg. Write the expression for Kc for: 2 NaCl(s) + H2SO4(aq) ⇌ 2 HCl(g) + Na2SO4(aq) CaCO3(s) ⇌ p. 497; 6:47 AM CaO(s) + CO2(g) #’s 1 - 5 Interpreting K Large vs small K values? K much larger than 1 - products favoured K much smaller than 1 - reactants favoured 6:47 AM Changing K (p. 497) For a given system at equilibrium, the value of the equilibrium constant depends only on temperature ie. For any equilibrium, the only way to change the actual value of K is to change the temperature 6:47 AM Calculations with K Types of K calculations: 1. Given equilibrium concentrations, find K 2. Find a missing concentration given K and other concentrations. 3. Given initial concentrations and equilibrium data, find K (ICE tables) 6:47 AM 1.Given equilibrium concentrations, find K eg. Calculate Kc for this equilibrium using the equilibrium concentrations given: H2(g) + I2(g) ⇌ 2 HI(g) [H2] = 0.22 mol/L [I2] = 0.30 mol/L [HI] = 1.56 mol/L 6:47 AM Given the equilibrium concentrations; [CO(g)] = 0.105 mol/L, [H2(g)] = 0.250 mol/L [CH3OH(g)] = 0.00261 mol/L, What is the value of Keq for the equilibrium below? CO(g) + 2 H2(g) ⇌ CH3OH(g) (A) 0.0994 (B) 0.398 (C) 2.51 (D) 10.0 6:47 AM Be Careful!! The other answers are possible if you mess up the calculation Given the equilibrium concentrations below, what is the value of Keq for N2(g) + O2(g) ⇌ 2 NO(g) ? [N2(g)] = 0.10 mol/L [O2(g)] = 0.20 mol/L [NO(g)] = 0.0030 mol/L (A) (B) (C) (D) 6:47 AM 2.2×10−4 4.5×10−4 1.5×10−1 3.0×10−1 Find Keq for the equilibrium below: 4 NH3(g) + 5 O2(g) ⇌ 4 NO(g) + 6 H2O(g) [NH3(g) ] = 0.100 mol/L [O2(g) ] = 0.200 mol/L [ NO(g) ] = 0.300 mol/L [ H2O(g) ] = 0.250 mol/L 6:47 AM 2. Find a missing concentration . . . eg. Find the [HI] in the equilibrium below if Kc = 36.9, [H2] = 0.125 mol/L and [I2] = 2.56 mol/L. H2(g) + I2(g) ⇌ 2 HI(g) p. 499; #’s 6 – 9 p. 538; # 6 6:47 AM Which reaction is faster - forward or reverse? 3. ICE tables Keq may be calculated given initial concentrations and at least one equilibrium concentration Using the Initial concentration and the Change in concentration we can find the missing Equilibrium concentrations and calculate Keq 6:47 AM ICE tables eg. 4.30 mol of NH3 was placed in a 1.00 L closed container to establish this equilibrium: 2 NH3(g) ⇌ N2(g) + 3 H2(g) Calculate Kc if the equilibrium concentration of H2(g) = 0.500 mol/L 6:47 AM ICE tables 2 [NH3] I C E 4.30 mol/L 2x -2x 4.30 - 2x [N2] 3 [H2] 0 0 x +x 3x +3x x 3x 0.500 mol/L 6:47 AM [H2] = 0.500 mol/L 3x = 0.500 x = 0.167 [N2] = x = 0.167 mol/L [NH3] = 4.30 - 2x = 3.97 mol/L 6:47 AM K = (0.500)3 x (0.167) (3.97)2 = 0.00132 ICE tables eg. 2.00 mol of N2, 4.00 mol of H2 and 3.00 mol of NH3 were allowed to come to equilibrium in a 1.00 L container 2 NH3(g) ⇌ N2(g) + 3 H2(g) Calculate Kc if the equilibrium concentration of NH3(g) = 3.50 mol/L 6:47 AM ICE tables I 2 [NH3] [N2] 3 [H2] 3.00 2.00 4.00 2x +2x -x x 3x -3x C E 3.00 + 2x = 3.50 6:47 AM 2.00 - x 4.00 - 3x [NH3] = 3.50 mol/L 3.00 + 2x = 3.50 x = 0.25 [H2] = 4.00 – 3x = 3.25 [N2] = 2.00 - x = 1.75 mol/L 6:47 AM K = (3.25)3 x (1.75) (3.50)2 = 4.90 ICE tables eg. The oxidation of ammonia occurs according to the following expression: 4 NH3(g) + 5 O2(g) ⇌ 4 NO(g) + 6 H2O(g) 0.800 mol of each chemical were placed in a 1.00 L container and there was 0.450 mol of NH3 at equilibrium. Calculate the equilibrium concentrations and Kc 6:47 AM 4 NH3(g) H2O(g) 6:47 AM 5 O2(g) 4 NO(g) 6 [NH3] = 0.450 mol/L 0.800 – 4x = 0.450 x = 0.0875 [O2] = 0.3625 mol/L [NO] = 1.15 mol/L [H2O] = 1.325 mol/L 6:47 AM K = 37000 ICE tables eg. 2.30 mol of NH3 was placed in a 2.00 L closed container to establish this equilibrium: 2 NH3(g) ⇌ N2(g) + 3 H2(g) Calculate Kc if 25 % of the NH3(g) reacts. 6:47 AM [NH3] = 2.30 mol/2.00 L = 1.15 mol/L 2 [NH3] [N2] 3 [H2] I 1.15 0 0 C 2x -2x E 1.15 - 2x +xx 3x +3x x = (0.75 x 1.15) = 0.8625 mol/L 6:47 AM 3x [NH3] = 0.8625 1.15 - 2x = 0.8625 x = 0.1438 [N2] = x = 0.1438 mol/L [H2] = 3x = 0.4314 mol/L 6:47 AM K = 0.0155 ICE tables eg. 0.800 mol of NH3 was placed in a 1.00 L closed container to establish this equilibrium: 2 NH3(g) ⇌ N2(g) + 3 H2(g) Calculate Kc if 70% of the NH3 reacts. 6:47 AM Le Châtelier’s Principle Chapter Review p. 535 p. 536 p. 538 p. 539 6:47 AM #’s 2, 3, 5-7 #’s 13, 15 #’s 7, 8, & 10 (MC) #’s 15, 17, 19, 20, 22 ICE tables Assignment test 6:47 AM Energy PRODUCED ΔH & isequations EXOTHERMIC The energy term may be included in a chemical equation, or written as ΔH to the right of the equation. eg. CO(g) + 2 H2(g) ⇌ CH3OH(g) + 65 kJ OR CO(g) + 2 H2(g) ⇌ CH3OH(g) 6:47 AM ΔH = - 65 kJ ENDOTHERMIC Energy is REQUIRED ΔH & equations eg. N2(g) + O2(g) + 90 kJ ⇌ 2 NO(g) OR N2(g) + O2(g) ⇌ 2 NO(g) ΔH = + 90 kJ Back to Temperature 6:47 AM ΔH & equations The energy term may be included in a chemical equation, or written as ΔH to the right of the equation. eg. CO(g) + 2 H2(g) ⇌ CH3OH(g) + 65 kJ OR CO(g) + 2 H2(g) ⇌ CH3OH(g) 6:47 AM ΔH = - 65 kJ