UNIT 9: KINETICS & EQUILIBRIUM. Essential Question: What mechanisms affect the rates of reactions and equilibrium?

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1 UNIT 9: KINETICS & EQUILIBRIUM Essential Question: What mechanisms affect the rates of reactions and equilibrium?

2 What is Kinetics? Kinetics is the branch of chemistry that explains the rates of chemical reactions Collision Theory: in order for a reaction to occur, reactant particles MUST collide based on n spatial orientation n energy of colliding particles

3 Factors Affecting Rates of Reaction Nature of the Reactants Since reactions involve the breaking of existing bonds and formation of new ones IONIC bonds are faster to react than COVALENT bonds (since covalent require more energy to break the higher # of bonds) Conclusion: The more IONIC the bond, the faster the reaction rate

4 Factors Affecting Rates of Reaction Concentration Ideally, if there s MORE of a reactant available/at our disposal, then. based on the kinetic molecular theory (phys. behavior of matter...) the MORE reactant available, the MORE product able to form Conclusion: The higher the CONCENTRATION, the faster the reaction rate

5 Factors Affecting Rates of Reaction Surface Area What will react faster à a finely divided powder or a lump of the same mass *sugar cube v. sugar powder Since the powder has a larger surface area exposed, there are MORE chances for the reactant particles to collide Conclusion: The more SURFACE AREA, the faster the reaction rate

6 Factors Affecting Rates of Reaction Pressure (of gases) When pressure increases, what happens to it s solubility, and therefore it s concentration? (think back to solutions) à the higher the pressure, the more soluble the gas, the higher the gaseous concentration Conclusion: The higher the PRESSURE of a gas, the faster the reaction rate

7 Factors Affecting Rates of Reaction Catalyst Catalysts are substances that increase the rate of reaction by providing a different & EASIER pathway for a reaction THEY REMAIN UNCHANGED BY THE REACTION after completion *think baking pan Conclusion: In the presence of a CATALYST, the reaction rate increases

8 Factors Affecting Rates of Reaction Temperature Temperature increases energy of particles (Kinetic Molecular Theory) Temperature increases motion of particles (KMT) Conclusion: The higher the TEMPERATURE, the faster the reaction rate

9 CONCLUSIONS: Rates of Reaction Factor: 1) Nature of reactants 2) Concentration 3) Surface Area 4) Pressure 5) Catalyst 6) Temperature Increases Rate: à ionic MORE v. covalent à with ñ concentration à with ñ Surface Area à with ñ Pressure (GAS) à presence of Catalyst à with ñ Temperature

10 Potential Energy Diagrams Illustrate the changes in potential energy that occur during a chemical reaction. REACTION COORDINATE is the horizontal axis shows DIRECTION or progress of reaction Potential because as reactant particles approach each other, KINETIC energy is converted into POTENTIAL energy

11 Potential Energy Diagrams 1 à 2 à 3 à A à B à C à D à E à Reactants Activated Complex Products PE of Reactants Activation Energy PE of the Activated Complex Heat of Reaction PE of Products

12 Potential Energy Diagrams Activated Complex temporary, intermediate; highest PE of system before reaction COMPLETES Activation Energy Amount of energy needed to form the activated complex from the reactants Heat of Reaction (ΔH) Difference between PE of Reactants & Products

13 Potential Energy Diagrams 1 à 2 à 3 à PE of Reactants PE of Activated Complex PE of Products

14 Potential Energy 4 à 5 à 6 à Activation Energy (Forward) HEAT of Reaction (ΔH) Activation Energy (Reverse) Diagrams

15 P. Energy Diagrams with CATALYSTS 4 à 7 à FORWARD REACTION Act. En. NO catalyst Act. En. WITH catalyst REVERSE REACTION 6 à Act. En. NO catalyst 8 à Act. En. WITH catalyst

16 P. Energy Diagrams with CATALYSTS Comparing Catalyzed & Un-Catalyzed Reactions Major difference in both curves?? Activation Energy needed Major constant in both curves?? Heat of Reaction (ΔH) = #5

17 Endothermic V. Exothermic *ENDOTHERMIC Reaction* Characteristics Heat of Reaction (ΔH) = + (POSITIVE) curve starts at a LOWER P.E. (reactants) & ends at a HIGHER P.E. (products) ABOSORBED energy, aka reactant + heat (kj) see table I

18 Endothermic V. Exothermic *EXOTHERMIC Reaction* Characteristics Heat of Reaction (ΔH) = - (NEGATIVE) curve starts at a HIGHER P.E. (reactants) & ends at a LOWER P.E. (products) RELEASED energy, aka product + heat (kj) see table I

19 What is EQUILIBRIUM? A chemical reaction in a state of equilibrium is said to have both the forward and the reverse reactions occurring at the same time RATES are equal, not reactant/product quantities! CAN ONLY OCCUR in a system in which neither the reactants nor the products can leave the system à CLOSED SYSTEM

20 Physical Equilibrium ~ Phases Occurs during PHYSICAL processes à dissolving, change of state solid/liquid: water & ice exist at same time! H 2 O (s) H 2 O (l) rate of melting equal to rate of freezing liquid/gas: water & vapor exist at same time! H 2 O (l) H 2 O (g) evaporation rate equal to rate of condensation

21 Physical Equilibrium ~ Solutions Occurs when a solution is saturated! (Gas or Liquid) saturated: no more solute can dissolve C 12 H 22 O 11 (s) C 12 H 22 O 11 (aq) process of dissolving STILL taking place while recrystallization occurs

22 Chemical Equilibrium Reactants are mixed to FORM products (which DON T exist yet) THEN, the concentrations of reactants DECREASE while producing (or INCREASING) products Ex) CH 4 (g) + H 2 O (g) à 3H 2 (g) + CO (g) decreasing à increasing THEN: rate of REVERSE reaction will now increase, until RATES of BOTH reactions become EQUAL = EQUILIBRIUM

23 Le Châtelier s Principle ANY change in temp, concentration, or pressure on an equilibrium system is called a stress Le Châtelier s Principle explains how a system at equilibrium responds to relieve any stress on it! Concentration Changes: (of rctnts/pdts) STAYS constant CH 4 (g) + H 2 O (g) 3H 2 (g) + CO (g) What will happen if we INCREASE CH 4?? reaction will go TO THE RIGHT (forward), USE the higher concentration of CH 4 available, and create more Products (H 2 & CO) THEN: reaction will keep oscillating direction (forward & backward) until it reaches Equilibrium!

24 4NH 3 (g) + 5O 2 (g) STRESS Le Châtelier s Principle 4NO (g) + 6H 2 O (g) + Heat **VARYING CONCENTRATIONS** EFFECT SYSTEM SHIFT EFFECT EFFECT EFFECT + NH 3 - O 2 à + NO + H 2 O + heat Increase Decrease AWAY from stress Increase Increase Increase - NH 3 + O 2 ß - NO - H 2 O - heat Decrease Increase TOWARD the stress Decrease Decrease Decrease

25 Le Châtelier s Principle Temperature Changes: N 2 (g) + 3H 2 (g) 2HN 3 (g) + heat What will happen if we INCREASE heat?? reaction will go TO THE LEFT (reverse), because HEAT is a product of the reaction & a change in temp. is essentially a change in the concentration of that product RESULT: the ENDOTHERMIC reverse reaction is favored (in this example) over the EXOTHERMIC forward reaction

26 4NH 3 (g) + 5O 2 (g) EFFECT Le Châtelier s Principle EFFECT 4NO (g) + 6H 2 O (g) + Heat **VARYING TEMPERATURES** SYSTEM SHIFT EFFECT EFFECT STRESS + NH 3 + O 2 ß - NO - H 2 O + heat Increase Increase AWAY from stress Decrease Decrease Increase - NH 3 - O 2 à + NO + H 2 O - heat Decrease Decrease TOWARD the stress Increase Increase Decrease

27 Le Châtelier s Principle Pressure Changes: ONLY affect gases-not liquids/solids CO 2 (g) CO 2 (aq) What will happen if we INCREASE pressure? concentration of gaseous CO 2 increases *think solubility rules/curve* MOVE AWAY from added stress (aka to the RIGHT) What will happen if we DECREASE pressure?? reaction shifts TO THE LEFT (toward gaseous CO 2 ) to make MORE gas **Think SODA BOTTLE: decrease pressure (when open bottle), dissolved gas becomes BUBBLES of gaseous CO 2

28 Le Châtelier s Principle N 2 (g) + 3H 2 (g) 2NH 3 (g) What will happen if we INCREASE pressure NOW? concentration of ALL gases increases Reaction direction will be favored TOWARD the side with FEWER # of gas molecules Conclusions left side (reactants) = 4 gas molecules right side (products) = 2 gas molecules Therefore: increase in pressure will favor reaction towards products, or > amount of NH 3 formed

29 Le Châtelier s Principle N 2 (g) + 3H 2 (g) 2NH 3 (g) What will happen if we DECREASE pressure? Reaction direction will be favored TOWARD the side with GREATER # of gas molecules Conclusions: decrease in pressure will favor reaction towards reactants, or > amount of N 2 & H 2 formed and reduce amount of NH 3 What if a rxn has the same # gas molecules on both sides??? NO EFFECT! What if a rxn has a catalyst??? changes rate of both forward/reverse rxns EQUALLY

30 Enthalpy The tendency in nature to change to a state of LOWER energy Exothermic reactions move toward lower energy state energy contained in the reactants is RELEASED the products have less P.E. than the reactants Exothermic reactions (lower enthalpy) are more likely than endothermic because less activation energy necessary

31 Entropy The tendency in nature to change to a state of greater CHAOS DISORDER RANDOMNESS the greater the disorder, the higher the Entropy systems will often go from conditions of >order (low entropy) to conditions of > disorder (high entropy) phase changes (solid à liquid à gas) compounds v. elements (High # of molecules = greater entropy) Low Slight High

32 The Equilibrium Expression Mathematical expression that shows the relationship of reactants and products in a system at equilibrium K eq = equilibrium constant How to write the FORMULA: Write the equilibrium expression for the equilibrium system of 0.5M nitrogen (N 2 ), 0.3M hydrogen (H 2 ), and 1.5M ammonia (NH 3 ). 1. Write a balanced equation for the system à N 2 (g) + 3H 2 (g) 2NH 3 (g) + heat

33 2. Place products as factors in numerator and reactants as factors in denominator of a fraction à NH 3 H 2 x N 2 The Equilibrium Expression 3. Place a square bracket [ ] around each formula. à [NH 3 ] = this means molar concentration (M) [H 2 ] [N 2 ] 4. Write the coefficient of each substance as a POWER of its concentration, then label K eq K eq = [NH 3 ] 2 = [0.5] 2 K [H 2 ] 3 [N 2 ] [0.3] 3 [1.5] eq = 6.17

34 The Equilibrium Expression SPECIFIC for a specific temperature therefore: changes in concentration & catalysts will NOT change the value of K eq K eq is LARGE when numerator > denominator MORE products than reactants = products favored K eq is SMALL when denominator > numerator LESS products than reactants = reactants favored