ba(oh) 2. 8h 2 o + 2nh 4 scn ba(scn) 2 + 2nh 3 + 10h 2 o what is the sign of g in this reaction?...
TRANSCRIPT
Ba(OH)2.8H2O + 2NH4SCN
Ba(SCN)2 + 2NH3 + 10H2O What is the sign of DG in this
reaction? What is the sign of DH in this
reaction? What is the driving force in this
reaction? How does the reaction proceed even
though both reactants are solids?
LeChatlier’s Principle
Any change to a system at equilibrium will alter so as to minimize the effects of the change. Reaction quotient: Ratio of
concentration of products to reactants at any time (not just at equilibrium) Q
If Q > K reaction not at equil. & will go toward reactants
If Q < K reaction not at equil. & will go toward products
If Q = K reaction is at equilibrium
Example problem
For reaction:H2O (l) === H+ + OH-
K = 1.0 x 10-14 @ 25oC. The [ ] of the system out of equilibrium = [H+] = 3.0 x 10-5M and [OH-] = 2.0 x 10-7M will reaction go toward the reactants or products to reach equilibrium?
How will reaction be affected if you remove product?
How will reaction be affected if you add product?
How will reaction be affected if you remove reactant?
How will reaction be affected if you add reactant?
Ba(OH)2.8H2O + 2NH4SCN
Ba(SCN)2 + 2NH3 + 10H2O If the temperature is raised, which
direction will equilibrium shift toward?
If the temperature is lowered? If the temperature of an exothermic
reaction is raised? If the temperature of an exothermic
reaction is lowered?
Remember! Just because a reaction is thermodynamically
favored K is big does not mean it is fast.
Kinetics and thermodynamics are
distinct!
Resources to find methods
Standard Methods for the Examination of Water and Wastewater
Journal of Chemical Education ASTM Methods Analytical Chemistry for Technicians Environmental Chemistry: Experiments
and Demonstrations Analytical Chemistry: Practice Methods for environmental trace analysis NIOSH manual of analytical methods / <
Print Book>
Steps in Experimental Design
1. Formulate the question2. Develop the hypothesis
Hypothesis must be testable3. Make predictions What will the
data look like if the hypothesis is supported? Not supported?
4. Plan methods for testing hypothesis What experimental methods will be used for testing the hypothesis?
Steps in Experimental Design
5. Collect data6. Evaluate data Does it support the
hypothesis? Do additional experiments need to be done?
Hypotheses Study of the effectiveness of laundry
detergents Taking aspirin reduces the length of
headache time versus taking Tylenol Lightbulbs: longevity vs. cost Application of fertilizer increases the
concentration of nitrate in nearby surface water
Null hypothesis: Increasing the size of the buffer zone between a farm field and surface water does not result in reduced nutrient levels.
Why can we ignore the concentration ratio of solvent (usually water) in equilibrium expressions? (Hint: consider the case of the dissociation of 0.1M acetic acid if you would like a concrete example.)
Why is it also permissible to omit the concentration of solids in equilibrium expressions? (If you want a concrete example use Mn, the density of Mn is 7.21g/mL.)
Solubility Product Example
Calculate the solubility product of Ag3PO4 if 500.mL of saturated solution contains 0.00325g of dissolved salt. Neglect any hydrolysis effects.
Common ion effect
Addition of a common ion displaces reaction toward reactants, less will dissolve. A salt will be less soluble if one of its constituent ions is already present in solution.
Solubility, like any other equilibria expression is affected by addition of common ions
Common Ion Effect Demo
The solubility of KCl at room temperature is 3.7M
Each test tube contains 20mL of saturated KCl initially
To test tube #1 20mL of 6M HCl is added To test tube #2 20mL of 12M HCl is added
Explain results which appear to contradict the principles of the common ion effect. (Hint: Calculate the concentrations of K+ and Cl- after the addition of HCl and calculate Q for each system.)
A waste bottle contains 0.01M Cu+1 and Sn+2 ions. For waste disposal, it is considerably cheaper if these two metals were precipitated separately rather than co-precipitated. Is such a separation possible? What would you precipitate the solutions with? (Use the Ksp values given at the back of your book.)
Solubility Rules When applying solubility rules focus mainly on
the anion portion of your molecule Always soluble:
Group I metals and ammonium salts are soluble Nitrates, acetates, and perchlorates are soluble Cl-, Br-, and I- are soluble except with Ag+, Pb2+, Cu+,
and Hg2+
Sulfates are soluble except with Ca2+, Sr2+, Ba2+, and Pb2+
Always insoluble: CO3
2- are insoluble except with Group I and NH4+
PO43- are insoluble except with Group I and NH4
+
S2- except Group I, II, and NH4+
OH- except with Group I, NH4+, Ca2+, Sr2+, and Ba2+
Acids and Bases Review
Bronsted – Lowry Acid: Proton donor Bronsted – Lowry Base: Proton
acceptor
Lewis acid: e- pair acceptor Lewis base: e- pair donor
Salt: ionic solid can be thought of as the product of an acid base reaction
Demonstration: pH of distilled water
Why is the pH of distilled water acidic? Write it down in your notebook.
How can this be minimized? How can the CO2 be removed?
Strong acids & bases completely dissociate
Strong Acids:
Everything else is weak
Strong Bases: Alkali earth metal OH R4NOH
R are all organic groups Alkaline earth metal OH (but not always
soluble)
Everything else is weak
Explain why, if benzoic acid is a weak acid with a Ka of 6.28 x 10-5, sodium benzoate is basic. Use equilibria.
If you have a 0.100M solution of sodium benzoate, what will be the pH of the solution?
Buffers
Buffer resists changes in pH How does a buffer work?
It contains significant amounts of both an acid and its conjugate base at equilibrium such that with the common ion effect/ L’Chatlier’s Principle it is able to minimize the effects of additions of acid or base
Buffer Example
What is the pH of 0.180 mole of NaNO2 mixed with 0.230 moles of HNO2 in 1.0 liter of solution?
Buffer capacity
How well a buffer resists changes to pH when a strong acid or strong base is added.
Buffers are most effective closer to their pKa
Do not want to generally make a buffer more than 1 pH unit from its pKa
Buffer example 2
How would you make a buffer at a pH of 7.80? (There is more than one correct answer.)
Buffer example 3
What would the pH be of a buffer solution containing 100.0mL of 0.15M NH3 and 4.25mL of 1.0M HCl?
Buffer example 4
How will the pH change when 10.0mL of 0.1M HCl is added to a buffer containing 0.15moles of Sodium acetate and 0.12 moles of acetic acid in 1.0 liter of solution?