Chemistry Equilibrium Notes

Transcription

1 Chemistry Equilibrium Notes

2 Fe 3+ (aq) + SCN - (aq) FeSCN 2+ (aq) Here s the equation for the reaction from last week s experiment. Based on the color changes that we saw, we drew three major conclusions from the results.

3 Fe 3+ (aq) + SCN - (aq) FeSCN 2+ (aq) 1. Thermal (physical) systems, like hot and cold water, can reach a place of rest called thermal equilibrium. Similarly, chemical systems can reach a place of rest called chemical equilibrium.

4 Fe 3+ (aq) + SCN - (aq) FeSCN 2+ (aq) 2. When a system like the above reaches chemical equilibrium, portions of all of the reacting materials might be present all of the reactants along with all of the products. Not all of the reactants are used up.

5 Fe 3+ (aq) + SCN - (aq) FeSCN 2+ (aq) 3. When a system reaches equilibrium, it s possible that the reaction could be reversed.

6 Fe 3+ (aq) + SCN - (aq) FeSCN 2+ (aq) Is there a mathematical regularity or pattern that we see when a system reaches equilibrium?

7 H 2(g) + I 2(g) 2 HI (g) Partial pressure (concentration) data taken at equilibrium for this system: H 2 conc. at equil. I 2 conc. at equil. HI conc. at equil

8 Does the pattern work on another system? N 2(g) + 3 H 2(g) 2 NH 3(g) N 2 conc. at equil. H 2 conc. at equil. NH 3 conc. at equil

9 Here is how we write an equilibrium constant expression: aa + bb cc + dd KK eeee = [CC]cc [DD] dd [AA] aa [BB] bb

10 Write an equilibrium constant expression for our friend from last week: Fe 3+ (aq) + SCN - (aq) FeSCN 2+ (aq) KK = [FFFFFFFFFF2+ ] eeee [FFFF 3+ ] [SSSSSS ]

11 Fe 3+ (aq) + SCN - (aq) FeSCN 2+ (aq) KK eeee = [FFFFFFFFFF2+ ] [FFFF 3+ ] [SSSSSS ] Le Chatelier s principle: If we disturb a reaction that has reached equilibrium, the reaction will respond in a way that compensates for the disturbance.

12 PbO 2(s) + Pb (s) + 2H 2 SO 4(aq) 2PbSO 4(s) + 2H 2 O (l) E = 2.04V A car battery brings together these ideas about equilibrium. If the forward reaction proceeds far enough, the reaction will reach equilibrium. The battery will be dead.

13 PbO 2(s) + Pb (s) + 2H 2 SO 4(aq) 2PbSO 4(s) + 2H 2 O (l) E = +2.04V Now apply an opposite voltage to the system that exceeds V (The car s alternator provides ~(-2.3 V per cell)). The reaction is forced to run in reverse the battery becomes recharged.

14 Thermal equilibrium occurs when materials in contact reach a uniform temperature. Chemical equilibrium occurs when a system has delivered all of the that it can for the doing of.

15 Note: Heat can t be completely converted to work (2 nd law of thermodynamics). There must be another measure of a system s ability to provide energy for work besides the H value for the reaction.

16 Another note: All of these arguments about equilibrium only apply to closed systems. Any system in which matter/energy is coming in or going out will not reach equilibrium in the way that we ve described.

17 Here s an equilibrium system. We ve written the equation in the reverse manner, which is fine. H 2 O (l) H + (aq) + OH - (aq) KK eeee = [HH+ ] [OOOO ] [HH 2 OO] =

18 The molarity of water essentially is constant, even when acting as a solvent, so we can simplify the previous expression to this: KK ww = [HH + ] [OOOO ] =

19 At the special equilibrium position of this system where moles of acid = moles of base (the position called neutral, where phenolphthalein changes color), then [H + ] = [OH - ] = 10-7

20 ph, then, is defined this way: ph = (-)log[h + ]

21 Calculate the ph of these HCl concentrations: a) 1.0 M HCl b) 0.10 M HCl c) M HCl d) M HCl (10-10 M) e) M HCl (10-14 M)

22 Calculate the ph of these HCl concentrations: a) M HCl b) M HCl c) M HCl d) M HCl (7.6 x 10-7 M) e) M HCl (2.5 x M)

23 An acid concentration of < 10-7 M, which corresponds to a ph > 7, means that the solution is basic, not acidic. The concentration of OH - is greater than the concentration of H +.

24 H 2(g) + I 2(g) 2 HI (g) We know that at equilibrium, it is possible that all three of these materials could be present. We also know that it is possible to make the reaction go in reverse.

25 Le Chatelier s principle is a verbal statement of what we can see mathematically in the equilibrium constant expression. Fe 3+ (aq) + SCN - (aq) FeSCN 2+ (aq) KK = [FFFFFFFFFF2+ ] eeee [FFFF 3+ ] [SSSSSS ]

26 Here s something cool: The equilibrium constant for a reaction and the G⁰ for the reaction are directly related by this equation: G⁰ = -RT ln(k eq ) R = kj/mol/k K is the system s Kelvin temperature