Presentation on theme: "Chapter 8 Activity Goodbye Freshman Chemistry Hello Real World."— Presentation transcript:
Chapter 8 Activity Goodbye Freshman Chemistry Hello Real World
Activity Equilibrium up to this point has been based on a very simple model. Basically that ions (a species with a high density of charge) do not interact other than the interaction of interest.
Activity Real Solutions are rather complex in their nature. Ion interact with all ions in solution and other polar species. Ions will interact with water since water is a polar substance. Ions will have these water molecules in their ‘waters of hydration’.
Just how much water? CompoundTightly bound water CH 3 CH 2 CH 3 0 CH 3 OCH 3 1 CH 3 CN3 CH 3 COO - 5 NH 3 9 NH 4 + 12
Activity How do we quantify this ionic atmosphere. We use a term called ionic strength. It can be calculated
Activity Let’s do an example. What is the ionic strength of a solution that is 0.010 M Na 2 SO 4 and 0.050 M NaCl We will prepare a Table and ask our first question. What ions are present?
Activity IonConcChargeCharge^2Product Na + SO 4 2- Na + Cl - Sum 0.5*Sum
0.010 M Na 2 SO 4 and 0.050 M NaCl What are the concentrations?
Activity IonConcChargeCharge^2Product Na + 0.02 SO 4 2- 0.01 Na + 0.05 Cl - 0.05 Sum 0.5*Sum
0.010 M Na 2 SO 4 and 0.050 M NaCl What are the charges and charges squared?
Activity IonConcChargeCharge^2Product Na + 0.02+11 SO 4 2- 0.01-24 Na + 0.05+11 Cl - 0.051 Sum 0.5*Sum
0.010 M Na 2 SO 4 and 0.050 M NaCl Let’s Finish the Math
Activity IonConcChargeCharge^2Product Na + 0.02+110.02 SO 4 2- 0.01-240.04 Na + 0.05+110.05 Cl - 0.0510.05 Sum0.16 0.5*Sum0.08 M
Activity It gets even more complex. Not all soluble salts will dissociate into the individual species. We get another form that will exist in solution. This is called and Ion Pair MgSO 4 (s) + H 2 O = Mg 2+ (aq) + SO 4 2- (aq) + MgSO 4 (aq) The MgSO 4 (aq) is the ion pair.
Activity You can see from Appendix J in the book the equilibrium constants for this pair formation. Mg 2+ (aq) + SO 4 2- (aq) = MgSO 4 (aq) Log K = 2.23 Which means that about ~93% will be in the ion pair form if the formal concentration of MgSO 4 is 1M.
Activity Ion Pairing has important implications in areas such as drug transport and delivery. Any drug species must be in the proper form to give its activity.
Activity So how do we find the “Activity of an ion in solution”. Where A is the activity of species C is “activity coefficient” [C] is the concentration in moles per liter
Activity Let’s revisit our equilibrium expression. aA + bB = cC + dD Which we write
Activity Lets revisit an equilibrium we have seen several times. The solubility of silver sulfide.
Activity How do we find the value for the “activity coefficient” ? For ionic strengths from 0.1M and down we can look up the value on a Table or we can calculate from the Extended Debye-Huckel equation.
Activity Where z is the ion charge, is the ionic strength and is the hydrated radius
Activity Effect of Ionic Strength, Ion Charge and Ion Size on Activity Coefficient (0 to 0.1M) increases decreases z increases the faster the departure from from unity smaller then the greater activity effects
Activity What is the pH of very, very pure water? K w = A H A OH = 1.0 x 10 -14 A H = 1.0 x 10 -7 Thus pH = 7.00 What is the pH of 0.10 M NaCl? Would it still be 7.00?
Activity K w = H [H] OH [OH] Look up the for H + and OH - on Table 8-1. H = 0.83 and OH = 0.76 [H + ] = [OH - ] So Solving for [H + ] = 1.26 x 10 -7 pH = - log A H = -log (0.83)(1.26 x 10 -7 ) = 6.98