Aqueous Equilibria Chapter 17 Additional Aspects of Aqueous Equilibria You love Chemistry You might...

AqueousEquilibria

Chapter 17Additional Aspects of Aqueous

Equilibria

You love Chemistry

You might have doubts, but deep, deep, deep down you

know there is a little love for the central science!

AqueousEquilibria

The Common ion effect How do they effect dissociation?

AqueousEquilibria

Compare:

• Calculate the pH of a 0.25 M propionic acid solution (Ka=1.3 x 10-5)

• Calculate the pH of a 0.25 M propionic acid solution that also has 0.10 M sodium propionate add.

AqueousEquilibria

Common Ion Effect

• How does LeChatelier support the previous calculations?

AqueousEquilibria

Common Ion Effect

• Summarize the Common Ion Effect:

AqueousEquilibria

Buffers:

• Solutions of a weak conjugate acid-base pair.

• They are particularly resistant to pH changes, even when strong acid or base is added.

AqueousEquilibria

How do Buffers Resist pH Changes?

• Consider a buffer composed of equal concentrations of nitrous acid and nitrite ion.

• 1) Does this meet the criteria for a buffer? • 2) What would happen if a volume of HCl was added to

the buffer?

• 3) What would happen if a volume of NaOH was added to the buffer

AqueousEquilibria

How do Buffers Resist pH Changes?

• The pH of a buffer will change somewhat, but not significantly. The Balance between the conjugate acid/base pair is

disrupted Either the conjugate acid or the conjugate base will be

present in a higher concentration after the addition This will cause a minor change to the pH

AqueousEquilibria

Calculating the pH of a Buffer Solution

• In order to calculate the pH of a buffer solution you will need to use the Henderson-Hasselbalch Equation:

pH = pKa + log[base][acid]

AqueousEquilibria

Henderson–Hasselbalch Equation

What is the pH of a buffer that is 0.12 M in benzoic acid and 0.20 M in sodium benzoate? Ka for benzoic acid is 6.3 10−5.

AqueousEquilibria

Another Application of the Henderson-Hasselbalch Equation

• How many moles of NH4Cl must be added to 2.0 L of 0.10 M NH3 to form a buffer whose pH is 9.00? (Assume that the addition of NH4Cl does not change the volume of the solution.) Kb for NH3 is 1.8 x 10-5.

AqueousEquilibria

pH Range and Buffer Capacity

• The pH range is the range of pH values over which a buffer system works effectively.

• It is best to choose an acid with a pKa close to the desired pH.

• Buffer Capacity is the amount of acid or base a buffer can neutralize before significant pH changes The higher the molarity or volume of the conjugate

pairs, the greater the capacity of the buffer.

AqueousEquilibria

Calculating pH Changes in Buffers

A buffer is made by adding 0.300 mol HC2H3O2 and 0.300 mol NaC2H3O2 to enough water to make 1.00 L of solution. Calculate the original pH of the buffer and the pH after 0.020 mol of NaOH is added. Ka of acetic acid is 1.8 x 10-5

AqueousEquilibria

Homework

• Ch. 17: 15, 17, 21, 23, 25

AqueousEquilibria

Solubility Product Constant

Using Equilibrium to Determine the dissociation of a solid in solution.

AqueousEquilibria

Solubility Products

Consider the equilibrium that exists in a saturated solution of BaSO4 in water:

Write the Equilibrium Expression for this reaction.

BaSO4(s) Ba2+(aq) + SO42−(aq)

AqueousEquilibria

Solubility Products

The equilibrium constant expression for this equilibrium is

Ksp = [Ba2+] [SO42−]

where the equilibrium constant, Ksp, is called the solubility product.

The solubility product defines the dissociation of the solid in solution

AqueousEquilibria

Practice Problem

• Write separate expressions for the solubility product constant for CaF2 and Silver Sulfate

AqueousEquilibria

Practice Problem

• Determine the concentration of each ion in a saturated solution of zinc hydroxide. Zinc hydroxide has a Ksp=3.0 x 10-16

AqueousEquilibria

Solubility Products

• Ksp is not the same as solubility.

• Solubility is generally expressed as the mass of solute dissolved in 1 L (g/L) or 100 mL (g/mL) of solution, or in mol/L (M).

AqueousEquilibria

Ksp from Solubility Data

• Solid silver chromate is added to pure water at 25°C. Some of the solid remains undissolved at the bottom of the flask. Analysis of the equilibrated solution shows that its silver ion concentration is 1.3 10–4 M. Assuming that Ag2CrO4 dissociates completely in water and that there are no other important equilibria involving the Ag+ or CrO4

2– ions in the solution, calculate Ksp for this compound.

AqueousEquilibria

Solubility from Ksp

• The Ksp for CaF2 is 3.9 10–11 at 25°C. Assuming that CaF2 dissociates completely upon dissolving and that there are no other important equilibria affecting its solubility, calculate the solubility of CaF2 in grams per liter.

AqueousEquilibria

Refresher Problems

Getting our mind back into Chemistry Mode.

AqueousEquilibria

Equilibrium Expressions

• Write equilibrium expressions for the following reactions:

• 1) Ni(OH)2(s) Ni2+(aq) + 2 OH-(aq)

• 2) 2NOBr(g) 2 NO(g) + Br2(g)

• 3) HClO3(ag) H+(aq) + ClO3-(aq)

• 4) NH3(aq) + H2O(l) NH4+(aq) + OH-(aq)

AqueousEquilibria

pH

• What is the pH of a solution that consists of 250 ml of 0.75 M hydrofluoric acid and 2.1 grams of sodium fluoride? (Ka=6.8 x 10-4)

AqueousEquilibria

Solubility

• What’s the molar concentration of each ion in an equilibrated solution of lead (II) fluoride. Ksp of lead (II) fluoride= 3.6 x 10-8

• Can we figure out the pH of this solution?

AqueousEquilibria

Buffer Solutions

• What is the pH of a 500.0 mL buffer solution that consists of 1.25 M acetic acid and 1.00 M sodium acetate if 15.0 mL of 0.750 M nitric acid is added to it? Ka of acetic acid is 1.80 x 10-5

AqueousEquilibria

Homework

Read and Take Notes on section 17.3 Acid-Base Titrations

AqueousEquilibria

Factors that Affect Solubility

AqueousEquilibria

Factors that Affect Solubility

• From your understanding of equilibrium and how it relates to acids/bases, what would be some factors that either increase or decrease the dissociation of a solid in solution?

AqueousEquilibria

Factors Affecting Solubility

1) The Common-Ion Effect If one of the ions in a solution equilibrium

is already dissolved in the solution, the equilibrium will shift to the left and the solubility of the salt will decrease.

BaSO4(s) Ba2+(aq) + SO42−(aq)

AqueousEquilibria

Factors Affecting Solubility

2) pH If a substance has a

basic anion, it will be more soluble in an acidic solution.

Substances with acidic cations are more soluble in basic solutions.

AqueousEquilibria

Factors Affecting Solubility

• pH Explain how the

solubility of Mg(OH)2 would be affected by the presence of an acid.

AqueousEquilibria

Practice Problem

• Which of the following substances will be more soluble in acidic solution than in basic solution:

(a)Ni(OH)2(s)

(b) CaCO3(s)

(c) AgCl(s)

(d) BaF2(s)

AqueousEquilibria

Evaluating a Solution

Will more solid dissolve or will a precipitate form?

AqueousEquilibria

Will a Precipitate Form?

• In a solution, If Q = Ksp, the system is at equilibrium

and the solution is saturated. If Q < Ksp, more solid will dissolve until

Q = Ksp.

If Q > Ksp, the salt will precipitate until Q = Ksp.

AqueousEquilibria

Practice Problem

• Will a precipitate form when 0.10 L of 8.0 10–3 M Pb(NO3)2 is added to 0.40 L of 5.0 10–3 M Na2SO4? (Ksp for PbSO4=6.3 x 10-9)