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

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.

Another Ksp example

Calculate the solubility of Ag2CrO4 @ 25oC in moles/liter. Ksp = 2.4 x 10-12.

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 example

Calculate the solubility of AgCl @ 25oC in 0.010M AgNO3 solution.

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?

Ladder Diagrams