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1 Equilibrium Section 3 2 Mass of solute dissolved When solute is first added to water it begins to rapidly dissolve time 3 Mass of solute dissolved As solute dissolves, some begins to undissolve… time 4 Mass of solute dissolved As long as more solute is dissolving than undissolving, the concentration of the solution continues to rise. time 5 Mass of solute dissolved The “undissolving” starts out slow, and speeds up as the solution becomes more concentrated time 6 Mass of solute dissolved At some point the rate of un-dissolving and rate of dissolving become the same. time 7 Mass of solute dissolved At this point the solution has become saturated. Maximum = Saturated time 8 Notice that the mass of dissolved solute has reached a maximum Mass of solute dissolved The solution’s concentration remains constant over time since dissolving is balanced by the undissolving time 9 Saturated solution CuSO4 (s) Cu2+ (aq) + SO42- (aq) Notice the double arrow in the equation? This means that the dissolving is in equilibrium with the undissolving. 10 11 Equilibrium Balance between opposing changes 12 Another example: In a closed soda bottle the dissolved CO2 gas is in equilibrium with the gas above the liquid. Soda Dissolved CO2 CO2 (g) CO2 (aq) Though there are many examples of physical equilibrium, we are also concerned with many chemical reaction equilibria. 13 What changes are in equilibrium here? 14 Chemical equilibrium In Reversible reactions the products of the reaction will also react to reform the starting reactants. 15 Look carefully: In this reaction H2 molecules react with I2 molecules to form HI molecules. 16 HI molecules can then break apart and reform H2‘s and I2‘s 17 The reaction could be written: H2 + I2 2HI OR 2HI H2 + I2 ? 18 Notice that the products once formed can go back to reactants. 19 Graphing equilibrium H2 + I2 HI HI increases…to a point H2 and I2 decrease…to a point Starting with H2 and I2, once equilibrium is reached… 20 Graphing equilibrium H2 + I2 HI HI increases…to a point H2 and I2 decrease…to a point …concentrations of H2 I2 and HI remain constant over time 21 Also, starting with the product; HI the same equilibrium is reached - H2 I2 and HI remain constant over time We could of course write the equation as the reverse: HI H2 + I2 22 H2 + I2 HI HI H2 + I2 23 The Haber process A chemistry classic N2 + 3H2 NH3 As nitrogen and hydrogen react ammonia is formed 24 Ammonia is produced during the Haber Process N2 + 3H2 NH3 This reaction starts off fast …but slows as reactants are used up If you recall, reaction speed depends on effective collisions As concentration decreases the reaction slows down. 25 Ammonia is produced during the Haber Process N2 + 3H2 NH3 As ammonia molecules collect …they can decompose to form the reactants The reverse reaction doesn’t start until some NH3 is formed The frequency of collisions between NH3 is fairly low in the beginning 26 Ammonia is produced during the Haber Process N2 + 3H2 NH3 The reverse reaction starts slow …and speeds up as ammonia collects The frequency of collisions between NH3 increases as more NH3 molecules are formed 27 Ammonia is produced during the Haber Process N2 + 3H2 NH3 Because the reaction is reversible eventually an equilibrium is reached The forward reaction and reverse reaction are happening at the same speed The concentration of reactants N2 and H2; and the concentration of product NH3 will remain constant over time 28 Equilibrium: When rate of forward reaction equals RATE of reverse reaction concentrations of both reactant and product will remain constant ..when concentration remains constant over time 29 Equilibrium: Notice: concentration of reactant IS NOT equal to concentration of product The process is affected by temperature and pressure ..when concentration remains constant over time 30 Reversible reactions can reach equilibrium Imagine fast moving molecules collide breaking bonds and rearranging to form products? N2 + 3H2 2NH3 Molecules of product can also collide, break apart and rearrange to form the reactant. N2 + 3H2 2NH3 Which of the two reactions do you expect to predominate? The forward reaction (At equilibrium there will usually be a lot more NH3) For A B At what time does the system reach equilibrium? How do you know? Once concentrations stop changing (are constant) we know that the forward and reverse reactions are in equilibrium with each other. 31 32 33 LeChatier’s Principle Section B Upsetting an Equilibrium Dissolved CO2 CO2 (g) CO2 (aq) Visualize in your mind a bottle of soda which is closed 34 Upsetting an Equilibrium 35 Dissolved CO2 CO2 (g) CO2 (aq) The dissolving and un-dissolving of CO2 are in equilibrium. Upsetting an Equilibrium 36 What happens to the balance when the bottle is opened? Upsetting an Equilibrium Most of the CO2 gas escapes 37 Upsetting an Equilibrium CO2 (g) 38 CO2 (aq) Now the un-dissolving is the only change that can occur 39 Upsetting an Equilibrium CO2 (g) CO2 (aq) If the bottle is closed again, the CO2 gas can collect. 40 Upsetting an Equilibrium CO2 (g) And the equilibrium can be re-established CO2 (aq) 41 Upsetting an Equilibrium CO2 (g) So a system thrown out of balance, will always try to return to equilibrium CO2 (aq) 42 Upsetting an Equilibrium CO2 (g) This is called LeChatlier’s principle. CO2 (aq) 43 Upsetting an Equilibrium CO2 (g) Lechatlier (Le – shat – lee - aye) CO2 (aq) 44 Upsetting an Equilibrium CO2 (g) CO2 (aq) Lechatlier says that a system at equilibrium will “shift” to counteract the effect of a “stress” 45 Upsetting an Equilibrium CO2 (g) CO2 (aq) The stress: allowing CO2 gas to escape stops the reverse reaction. 46 Upsetting an Equilibrium CO2 (g) CO2 (aq) The shift: dissolved CO2 comes out of solution to replace the lost gas. 47 Upsetting an Equilibrium CO2 (g) CO2 (aq) And now the system can return to equilibrium Upsetting an Equilibrium Equilbria are stressed by changes in… Concentration Temperature and Pressure (…if gases are involved of course…) 48 Factors Affecting Equilibrium Changes (stresses) to a system cause equilibrium to "Shift" left or right Lets look again at the Haber process: N2 + 3 H2 2 NH3 1. Concentration: an increase in concentration speeds up a reaction ex: Adding N2 [increasing its concentration]: the Forward reaction speeds up - (more collisions) 49 50 Concentration N2 + 3 H2 2 NH3 1. Concentration: increase in concentration speeds up a reaction The forward reaction is now faster than the reverse and the Result: H2 decreases, while NH3 increases 51 Concentration N2 + 3 H2 2 NH3 1. Concentration: increase in concentration speeds up a reaction A trick: Draw a “pile” over the N2 and shift right to get rid of the excess. 52 N2 + 3 H2 2 NH3 Another example: Removing NH3 the reverse reaction slows down - (fewer collisions) (the forward reaction is now the faster one) Reaction SHIFTS RIGHT (to replace the lost NH3 ) Result: N2 and H2 decrease, NH3 increases 53 N2 + 3 H2 2 NH3 Another example: Removing NH3 A trick: Draw a hole under the NH3 and shift right to fill it. 54 N2 + 3 H2 2 NH3 LeCHATELIER'S PRINCIPLE: "When a system at equilibrium is subjected to a stress, the equilibrium will shift in the direction which tends to counteract the effect of the stress.“ In other words: When you add something, the reaction shifts to get rid of it, when you take something away, it shifts to replace it.] 55 2. Temperature N2 + 3 H2 2 NH3 + 92 KJ Increasing temperature speeds up BOTH forward and reverse reactions. But… favors reactions which require energy …endothermic 56 2. Temperature N2 + 3 H2 2 NH3 + 92 KJ Increasing temperature - reaction SHIFTS to the LEFT - to use up extra heat Result: N2 and H2 increase, NH3 decreases 57 N2 + 3 H2 2 NH3 + 92 KJ Decrease temperature? reaction SHIFTS to the RIGHT - to replace lost heat Reverse reaction slows down more (since there’s less heat available) 58 3. Pressure N2(g) + 3 H2(g) 2 NH3(g) 3. Pressure: - only affects gases increasing pressure speeds up both forward and reverse reactions by increasing concentration 59 4 1 N2(g) + 3 H2(g) 2 NH3(g) Pressure: depends on moles (coefficients) of reactants and products 4 moles of reactants vs. 2 moles of products Increase pressure: reaction SHIFTS to the RIGHT to reduce # of moles - reduces pressure 60 1 N2(g) + 3 H2(g) 2 NH3(g) Pressure: depends on moles (coefficients) of reactants and products 4 moles of reactants vs. 2 moles of products Increase pressure: reaction SHIFTS to the RIGHT to reduce # of moles - reduces pressure reducing the number of moles reduces pressure! 61 N2(g) + 3 H2(g) 2 NH3(g) Pressure: depends on moles (coefficients) of reactants and products 4 moles of reactants vs. 2 moles of products Increase pressure: reaction SHIFTS to the RIGHT to reduce # of moles - reduces pressure Result: N2 and H2 decrease, NH3 increases 62 What effect would a Catalyst have? It Speeds up both reactions equally, favors both forward and reverse reaction Doesn’t affect the equilibrium / No shift occurs For each reaction, identify the shift and the change in concentrations of reactants and products that will occur: PCl5(g) + heat PCl3(g) + Cl2 (g) Increase [PCl5] PCl5(g) + heat PCl3(g) + Cl2 (g) increase in temp PCl5(g) + heat PCl3(g) + Cl2 (g) Increase press PCl5(g) + heat PCl3(g) + Cl2 (g) 1 mole 2 moles 63 C(s) + H2O(g) + energy CO(g) + H2(g) decrease [CO] C(s) + H2O(g) + energy CO(g) + H2(g) decrease temp C(s) + H2O(g) + energy CO(g) + H2(g) decrease press C(s) + H2O(g) + energy CO(g) + H2(g) grind up the C(s) into powder C(s) + H2O(g) + energy CO(g) + H2(g) 64 65 66 67 1. Define these in your own words: Reversible reaction – Dynamic equilibrium – 2. What is the significance of double arrows in an equation? 3. How is the term equilibrium used to describe an aqueous solution which is saturated? 4. How do the rates of forward and reverse reactions compare at a state of dynamic equilibrium? 5. How do the concentrations (or amounts) of reactants and products change once equilibrium is reached? 6. State LeChatlier’s principle in simple English. 7. How is equilibrium position of this reaction affected by the following changes? Explain each (shift left or right: hint – draw piles and move them, or holes and fill them) C(s) + H2O (g) + energy CO (g) + H2 (g) a. increasing temp b. increasing gas pressure c. adding H2 d. removing H2 8. What effect does each change have on the concentration of NH3 ? Explain in terms of shift N2 (g) + 3H2 (g) 2NH3 (g) a. adding heat b. Increasing pressure c. adding a catalyst 68 69 9. Can a change in pressure affect any reaction at equilibrium? Explain. 10. Use LeChatlier’s principle to explain why pressure in a bottle of soda increases again after it is capped. The equation for the dissolving of Carbon dioxide gas in soda is shown below: CO2 (g) CO2 (aq) 11. Given the reaction at equilibrium: N2 (g) + 3H2 (g) 2NH3 (g) + energy What effect does raising the temperature have on the rate of the forward reaction? Explain