Unit VI - Kinetics and Equilibrium

1. Unit VI - Kinetics and Equilibrium -study of I. Kinetics Reaction rate Velocity (speed) at which a reaction occurs. - Amt / time Reaction Mechanism Steps involved in producing the overall reaction CH3CH2OH(l) + 3 O2(g)  2 CO2(g) + 3 H2O(g) + 1236 kJ C12H22O11(s) + 8 KClO3(s) 12 CO2(g) + 11 H2O (g) + 8 KCl (s) C12H22O11(s)  12 C (s) + 11 H2O(g) + 429 kJ

2. Collision Theory and Rate of Reaction Collision Theory - Particles must collide in order for a reaction to occur and they must have “effective” collisions 1. Sufficient Energy – hard enough 2. Proper orientation ( direct hits )

3. A D B C AB + CD --> AD + CB A D A-D and B -C bonds form B C A-B and C-D bonds break Effective collision The collision must also have the correct orientation The bond between A and D cannot form. There is no reaction between molecules Ineffective collision = no reaction!

4. Factors Affecting Reaction Rate 1. Nature of Reactants - Covalent substances react slower than ionic substances - Fastest reactions happen with aqueous solutions

5. In AB + CD--> AD + CB, first AB and CD must break apart A B B B B B AB--> A+ + B- Now they can react with CD In ionic solutions, ionic substances will dissociate, bonds are already broken They have already completed the first step of the reaction before they are mixed A B B B B B C D D B D D

6. Therefore, ionic substances in solution react faster than in solid form

7. A A B A B A A A B B B A B B A B B B B A B A A 2. Concentration of Reactants [NaCl] “Concentration of NaCl” Usually molarity (M) “Concentration of chlorine ions” [Cl-] Higher concentration means more molecules are closer together As molecules move, more collisions will occur if they are closer together More collisions = faster reaction As concentrations increase, reaction rate increases

8. A A B B A A B B B A A B Pressure Only gases show a noticeable change in reaction rate as pressure is increased As pressure increases, we force the molecules closer together Closer together means more collisions and a faster reaction rate As pressure on gases increases, reaction rate increases

9. 3. Temperature Average kinetic energy As we increase temperature, we speed up the molecules Cold Hot Faster molecules means more collisions per second = Faster reaction As temperature increases, reaction rate increases Usually a 10oC temperature change will double the reaction rate

10. 4. Surface Area More surface area = more places for collisions to occur In this case, the liquid can only react at these places When we break the object, now we have more places for the liquid molecules to collide More surface area = Faster reaction rate Powder reacts faster than cube

11. Dragons Breath Describe what happens when the pile of lycopodium powder is lit with the lighter. Describe what happens when the lycopodium powder is ignited after being blown over a flame. Grain Silo Explosion Describe what happens when the lycopodium powder is ignited inside the coffee can. Why do you think what you observed happened?

12. A A D D D D D D D D A D D D D D D C C C C C C C C C B B B C C C C C B B B C C C 5. Catalyst Substance which lowers Ea (activation energy), but is not changed Catalyzed reactions occur faster Catalysts help molecules collide properly Allows for more effective collisions, Not more collisions, better collisions Different catalysts for different reactions Catalyst = Faster reaction rate

13. Catalytic acetone combustion O2(g) + C3H6O(g) H2O(g) + CO2(g) + heat Balance the above reaction. Hint start with the acetone. The above reaction can be described as a combustion reaction as it gives off water and carbon dioxide as products. What is another way to describe the above reaction. In the reaction above, the copper from the penny is not shown. Why do you suppose it is not written in the equation?

15. PE (kcals) Reaction coordinate PE of activated complex Ea H PE of reactants PE of products PE of products Reaction Coordinate - Represents progress of reaction Activation Energy -energy needed to make the activated complex - Needed to start reaction

16. PE (kcals) Reaction coordinate PE of activated complex Ea H PE of reactants PE of products PE of products Activated Complex - Occurs when reactants collide in proper orientatation -Occurs when bonds are in process of breaking and forming - Very unstable, only lasts moments Heat of Reaction (ΔH) - Energy released or absorbed during a chemical reaction - Measured in joules

17. ∆H = Hproducts - Hreactants H = Negative Products have less energy than reactants Energy is given off Exothermic - Product is more stable than the reactant - Enthalpy decreases

18. Chlorination of acetylene pg 47 There are several reactions that take place in this demonstration. First…. CaC2(s) + H2O(l)  C2H2(g) + Ca(OH)2(aq) Balance the above and identify the compound that has both ionic and covalent bonds in it. Second…. NaClO(aq) + HCl(aq)  Cl2(g) + NaOH (aq) Balance the above and name NaClO _________________

19. Third… C2H2(g) + Cl2(g)  HCl (aq) + C (s) + heat Would ∆H be positive or negative for this equation? Fourth… C2H2(g) + O2(g)  4 CO2 + 2H2O(g) + heat Draw the electron dot diagram for oxygen, carbon dioxide and water. Draw the potential energy diagram for the above reaction. Put the reactants and products on the correct line and label the activation energy.

20. Products have more energy than reactants H = Positive Endothermic Energy is gained - Product is less stable than the reactant - Enthalpy increases

21. Example- What is the H of the following  H = +66.4 kJ a. N2 + 2 O2 + 66.4 kJ --> 2 NO2  H = -486.3 kJ b. H2(g)+ O2(g)  2 H2O (g) + 486.3 kJ Table I Heats of Reactions Shows the  H for various reactions - H is based on the MOLES of reactants and products used. Example A. How much energy is given off when carbon and oxygen gas make one mole of carbon dioxide? 393.5 kJ B. When 4.0 moles of HI is made from pure hydrogen and iodine, is energy released or absorbed? How much? H2 + I2 2 HI Since  H is +53.0 kJ, energy is absorbed. The written reaction is for 2 moles of HI, so for 4 moles, we absorb 106.0 kJ

22. Heating with sulfuric acid pg 155 When diluting an acid with water the little ditty “Do as you oughta, add acid to water” is often utilized. After watching the temperature change as the acid is added to the water why do you think that ditty is important to remember? Is the ∆H for this reaction positive or negative? Draw a simple energy diagram showing the reaction that happened.

23. Ea H PE of reactants PE (kcals) PE of products PE of products Reaction coordinate Ea with a catalyst Catalyst - Compound with lowers Ea Is not permanently changed in the reaction Can be reused Does not change the  H

24. PE (kcals) Reaction coordinate 3. Reversible Reaction Some reactions can run back to products A + B --> AB A + B <-- AB Eaf Ear A + B H AB RVS FWD Reactions are opposite One is exothermic, one is endothermic H is the same, but has opposite signs Eaf and Ear differ by H

27. Equilibrium - Forward and reverse reactions happen at the same time 1.) Takes place in a closed system 2.) Is dynamic - Overall appearance does not change 3.) Does not mean same amounts of products and reactants - Means that amts of products and reactants remain constant

28. Types of Equilibrium Phase Equilibrium -One compound in equilibrium in two phases - Amount turning to liquid = amt turning to solid - Amount turning to liquid = amt turning to gas Ex. H2O(l) <= => H2O(g) Sealed jar Vapor molecules Water molecules Again, the forward and reverse reaction still occur, but the total number of vapor and liquid molecules remains unchanged

29. Solution Equilibrium Solids in Liquids • Equilibrium is reached when number of particles • being dissolved = number of particles being crystalized Ex. C6H12O6(s)<==> C6H12O6(aq) Dissolve sugar Solid sugar - Forward and Reverse reaction are equal But since the forward and reverse reaction are equal, the total number of solid and dissolved molecules remains the same - Solutions in this condition are considered saturated

30. Na+ Cl- NaCl NaCl NaCl NaCl Electrolytes - With electrolytes in the same situation, the electrolyte dissociates as it goes into solution NaCl(s) ---> Na+(aq) + Cl-(aq) -And reassociates as it fall out of solution NaCl(s)<---- Na+(aq) + Cl-(aq) NaCl(s) <==> Na+(aq) + Cl-(aq) Na+ Na+ Cl- Cl- Cl- Na+

31. Nonelectrolytes (They don’t dissociate in water!) Ex. C6H12O6(s)<==> C6H12O6(aq)

32. Gases in Liquids - Equilibrium is reached when number of particles being dissolved = number becoming vapor - Only occurs in a closed system ( bottle of soda) CO2 (g) = CO2 (aq) - Soluabilty of gases are influenced by 1.) temperature 2.) pressure

33. Review the rate of the forward reaction equals the rate of the reverse reaction Equilibrium only occurs when At equilibrium the amount of reactants do not equal the amount of products At equilibrium, the concentration of reactants and products do not change The reverse reaction only begins when enough product has been created

34. Chemical Equilibrium - A dynamic equilibrium is reached when both the forward and reverse reactions are equal N2 + 3 H2 2NH3 Three indications that a reaction is at equilibrium: No change in: 1.) color 2.) temperature 3.) pressure

36. LeChatelier’s Principle If a stress is applied to a system at equilibrium, the equilibrium is shifted in a way that tends to reduce the effects of the stress. Stress Change in temperature, pressure, concentration of chemicals and other factors What you kids give me! A + B C + D If we apply a stress, the equilibrium will shift to the left to create more products, or to the right to create more reactants

37. A + B C + D C + D A + B A + B A + B C + D C + D Types of Stress **** A catalyst is NOT a stress and has no effect on equilibrium A. Concentration - By adding / removing product or reactant Reaction shifts to 1.) Replace the loss 2.) Consume the addition - Increase [reactants], equilibrium is pushed to products A -Increase the [products] , equilibrium is pushed to reactants C

38. If a reactant or product is removed, the reaction will shift to replace the missing substance Example - For the equation 3 H2 + N2 2 NH3, List three ways to increase the concentration of the products and three ways to increase the concentration of the reactants. A(s) + B(s) C(s) + D(g) D(g) D(g) D(g) D(g) D(g) D(g) D(g) D(g) D(g) D(g) D(g) D(g) C(s) + D(g) A(aq) + B(aq ) C(s) + D(g) A(s) + B(s) In an open container, gas D floats away. Equilibrium is constantly pushed to products More products More reactants Add more H2 Remove some H2 Add more N2 Remove some N2 Remove some NH3 Add more NH3

40. b. Temperature Consider heat as either a product or a reactant 3 H2 + N2 2 NH3 + 91.8 kJ In this case, heat is a product. If we increase the temperature, it would be the same as adding a product

41. 3 H2 + N2 2 NH3 + 91.8 kJ A + B + 43 kJ D + C H2 H2 N2 H2 H2 NH3 H2 NH3 NH3 H2 N2 N2 H2 If heat is on the reactant side then adding temperature is adding a reactant. The equilibrium will shift to the products

42. Example – In the reaction XY + B XB + Y + 2 kJ , what will happen when For endothermic reactions, as temperature increases [products] increases For exothermic reactions, as temperature increases [reactants] increases a. The container was heated? Equilibrium shifts to reactants b. The container was cooled? Equilibrium shifts to products

43. LeChatliers principle demo pg 236 • Balance the above equation. • Find the ionic charge of the reactant side and product side. • Predict what you will observe when: • Water is added to the cobalt(II)chloride solution. • Use arrows to show shift and changes of concentration.

44. Predict what you will observe when: • HCl is added to the cobalt(II)water complex solution. • Use arrows to show shift and changes of concentration. • Using table F what will Silver Chloride look like in solution. • Using table F what will Silver Nitrate look like in solution.

45. What happened when AgNO3 was added? • Explain why the observed results happened. • Use arrows to show shift and changes of concentration. • Predict what you will observe when: • Heat is added to the cobalt(II)water complex solution. • Use arrows to show shift and changes of concentration.

46. Predict what you will observe when: • Ice is applied to the cobalt(II)chloride solution. • Use arrows to show shift and changes of concentration.

48. C. Pressure - Only effects reactions with gases - Increasing pressure is like increasing concentration of gas - Shifts toward side without gas CO2(g) CO2(aq) What if gases are found on BOTH sides of the reaction? N2(g) + 3 H2(g) = 2 NH3(g) - Equilibrium will shift to side with least number of moles of gas ( takes up less space) - If same number of moles on each side then no effect on equilibrium In carbonated soft drinks the cap on the soda keeps a high pressure on solution so more gas is dissolved. When soda is opened pressure is released and according to LC Principle carbon dioxide gas leaves solution to relieve stress.

50. Enthalpy and Entropy A. Enthalpy All matter has energy stored in it. Either as a chemical bond, a phase, or kinetic energy Enthalpy Total energy of a substance Very difficult to measure, but we can determine the change in enthalpy Called H aka - the heat of reaction Nature favors lower energy - H