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Equilibrium and acids and bases

This text provides an overview of chemical equilibrium, including the dynamic nature of equilibrium, the application of Le Chatelier's Principle to predict shifts in equilibrium, and the effects of temperature, pressure, and concentration on equilibrium. The concept of acids and bases is also discussed, emphasizing their nature and behavior based on the Arrhenius concept.

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Equilibrium and acids and bases

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  1. Equilibrium and acids and bases • Chapter 14 and 15 in text

  2. Chemical Equilibrium • Concentrations of all reactants and products are constant with time. • That does not mean [reactant] = [product] Rather the rate of decomposition of [react] and formation of [prod] are constant.

  3. Equilibrium is a Dynamic Situation Demonstration B A

  4. When a car from island A moves to island B there is a stress placed on the equilibrium and the reaction (the members of the groups) must shift either left or right to relive the stress. • If a stress is placed on Island A then the equilibrium will shift to the right to compensate.

  5. What Just Happened? •  When a chemical reaction is at equilibrium, any disturbance of the system, such as a change in temperature, or addition or removal of one of the reaction components, will "shift" the composition of the reaction to a new equilibrium state.

  6. A Chemical Example of a Shift in Equilibrium Co(H2O)62+ + 4Cl CoCl42+ + 6H20 PinkBlue

  7. Demonstration of Equilibrium

  8. Question: Which direction would the equilibrium shift if HCl was added to the reaction? Co(H2O)62+ + 4Cl CoCl42+ + 6H20 PinkBlue HCl

  9. Answer The addition of HCl would cause an increase in Cl- and thus the reaction would react by shifting the reaction to the right (blue side) in order to “consume” the excess Cl- and return to a state of equilibrium. Co(H2O)62+ + 4Cl CoCl42+ + 6H20 PinkBlue

  10. The Law of Mass Action For any reversible reaction: jA + kB lC + mD The law of mass action is represented by the following equilibrium expression. (Prod /React) K = [C]l [D]m [A]j[B]k K = equilibrium constant Solids and liquids are not written in equilibrium expressions

  11. K > 1 Favors products • K<1 Favors reactants

  12. Graph of Equilibrium

  13. Question Write the equilibrium expression for the following equations: PCl5 (g) PCl3 (g) + Cl2 (g) Cl2O7(g) + 8H2(g) 2HCl(g) + 7H20(g) NOTE: Make sure the RXN is BALANCED!!!

  14. Answer K = [PCl3] [Cl2] [PCl5] K = [H2O]7 [HCl]2 [Cl2O7] [H2]8

  15. Calculating Equilibrium Example: Calculate the equilibrium constant K, for the following reaction at 25°C, H2 (g) + I2 (g) 2HI (g) if the equilibrium concentrations are [H2] = 0.106M [I2] = 0.022M [HI] = 1.29M

  16. Answer Equilibrium Expression: K = [HI]2 [H2] [I2] Equilibrium Constant: K = (1.29)2 = 7.1 x 102 (0.106) (0.022) Note the units cancel out! That’s one less thing to remember!

  17. Homework • Pg 633 1,8,9,14,16 • Equilibrium wks

  18. How do we predict which direction equilibrium will shift to? Le Chatelier’s Principle If we stress a reaction out the reaction will shift (either towards R or P) to reduce the stress. Stress = a change in: Concentration Pressure Volume Temperature

  19. Concentration • Ask yourself: • Is what is being added a solid or liq? (No effect) • a gas or aqueous reactant or a product? (effect!) • If it’s a reactant the reaction will shift toward the products to consume the extra reactant. • If it’s a product the reaction will shift toward the reactants to consume the extra product.

  20. Example • N2 g + 3H2 g 2NH3 g • Add NH3 • Remove N2 • Add H2

  21. Changes in V or P • P changes in L or aq have little effect • P changes of gases have huge effects (PV=nRT) (PV=PV) • Concentration is effected by pressure.

  22. An increase in Pressure (due to a decrease in V) will shift to decreases the total number of moles of gas. • If the number of moles on R and P side are the same then a change in P will have no effect on equilibrium.

  23. Example • Lets do Pg 626 example 14.12 in your text ↓P shift to side of reaction with greatest number of moles of gas ↑P shift to side of reaction with less moles of gas

  24. Temperature • ONLY a change in TEMPERATURE can change the value of K (equilibrium constant). • You must look at the H of the reaction to see how T will effect it • Temperature is our stress so the reaction will move in the direction that removes the stress.

  25. ↑ temperature the rxn moves in the endothermic direction (+ΔH) ENDO  Right ( K ↑ ) ↓ temperature the rxn moves in the exothermic direction (-ΔH) EXO  Left ( K ↓ ) **K only depends on temperature, catalysts have NO EFFECT on K** K>>1 favors products K<< 1 favors reactants

  26. Think about heat like a R or P • Endothermic +ΔH Heat + A  B H = 400 kJ Favors ↑ in T therefore K ↑ when it is heated and K ↓ when it is cooled. Exothermic -ΔH A  Heat + B H = - 400 kJ Favors ↓ in T therefore K ↑ when it is cooled and K ↓ when it is heated

  27. Example • Examples 14.13 on pg 628-629 in text

  28. Homework • Pg 636 pg 49,51,53-55,62

  29. Acids: Sour Taste If a solution has a high [H+] = acidic Base: Bitter Taste / Slippery feel If a solution has a high [OH-] = base Nature of Acids and Bases

  30. About the scale • P = quantity • So … • pH = quantity of H ion • pOH = quantity of OH ion • pH + pOH = 14

  31. Concentrations • [H+] = [OH-] = Neutral • [H+] > [OH-] = Acid • [H+] < [OH-] = Base

  32. Arrhenius Concept • Focuses on what ions were formed when acids and bases dissolved in water. • Acids dissociate in water give hydrogen ions (H+ or H3O+ hydronium ion) • Bases dissociate in water give hydroxide ions (OH- hydroxide ion).

  33. Arrhenius acid - Any substance that ionizes when it dissolves in water to give the H+ ion.  e.g.    Arrhenius base - Any substance that ionizes when it dissolves in water to give the OH- ion.  e.g.    

  34. The theory can only classify substances when they are dissolved in water since the definitions are based upon the dissociation of compounds in water. • It does not explain why some compounds containing hydrogen such as HCl dissolve in water to give acidic solutions and why others such as CH4 do not. • The theory can only classify substances as bases if they contain the OH- ion and cannot explain why some compounds that don't contain the OH- such as Na2CO3 have base-like characteristics.

  35. Acid:substance that can donate a proton (+) Base: substance that accepts a proton (+) (aka they have a lone pair of e- to accept a proton) Unlike Arrhenius concept this is applicable in both aqueous and non-aqueous states. Bronsted Lowery Acid Base Concept

  36. Equilibrium • Reaction produces reactants and products at the same rate, but not necessarily in the same amounts • Bathroom theory

  37. Example • NH3+ (aq) + H20 (l) NH4 (aq) + OH- (aq) • Equilibrium will favor the formation of the weaker acid and the weaker base. • In this rxn the [NH4] and [OH- ] will be low because they are the stronger acid and base.

  38. In the above reaction, the H+ from HCl is donated to H2O which accepts the H+ to form H3O+, leaving a Cl- ion. 

  39. Conjugate Acid and Base Pairs • The part of the acid remaining when an acid donates a H+ ion is called the conjugate base.  • The acid formed when a base accepts a H+ ion is called the conjugate acid. 

  40. For the generic acid HA: Formed when a proton Is transferred to the base Everything that is left after a proton to the base. NOTE Strong acids have weak conjugate bases. Strong bases have weak conjugate acids.

  41. Question If H20 is an acid what would its conjugate base be? • What is the conjugate acid of HPO42- ? • What is the conjugate base of HS-

  42. Answer • H20 take away a proton OH- • HPO42- add an H+H2PO4- ( we added a proton and that needs to be reflected in the molecular charge) • HS-S2-

  43. Amphoteric • The ability of a substance to act as an acid or a base. • Ex: H2PO4- and H2O can act as both acids and bases.

  44. Back to Old Faithful In this equation the stronger base will win the competition for H+. If H2O is a stronger base than A-, then it will have a greater affinity for the protons and the equilibrium will lie to the right favoring the formation of H3O+. If A- is stronger then equilibrium will fall to the left and acid in the form HA will form.

  45. HCl HBr HI HNO3 HClO4 HClO3 H2SO4 Cl- Br - I- NO3- ClO4- ClO3- HSO4- Strong AcidsConj. Bases You must memorize all of these

  46. Common Strong Bases

  47. Example • Identify the CA and CB for each reaction • HNO3 + H2O ↔ • NH3 + H2O ↔

  48. Acid-dissociation equilibrium constant (Ka) • The relative strength of an acid is described as an acid-dissociation equilibrium constant. • The acid-dissociation equilibrium constant is the mathematical product of the equilibrium concentrations of the products of this reaction divided by the equilibrium concentration of the original acid

  49. Think Products over reactants

  50. Question • Write an ionization equation for the following and then write the acid dissociation constant for both. (all occur in water) • Hydrochloric acid • Acetic acid HC2H3O2

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