CHEM 1212 Unit 4

What is the common-ion effect?

• The shift in equilibrium upon the addition of more of an ion that is already involved in the equilibrium

What is a buffer solution? How does it work?

• A buffer is a solution that resists change in pH with the addition of either an acid or a base

• They contain either lots of a weak acid and its conjugate base or lots of weak base and its conjugate acid

• The acid or base component of a buffer will meutralize any H3O+ or OH- ions

• The effectiveness of a buffer depends on pK values and the original pH

What are the 3 main ways to make a buffer?

Mixing a weak acid or a weak base and its conjugate salt

• Ex: CH3COOH and NaCH3COO

Mixing two salts that provide a conjugate acid-base pair

• Ex: NaH2PO4 and Na2HPO4

Reacting some (not all) of a weak base with a strong acid or some of a weak acid with a strong base

How do you calculate the pH of a buffer solution?

• Identify the major species in the solution, look at which species will be reacting

• Identify the reaction that will be the source of H+ or OH-

• Make a RICE table

• Calculate

What is the Henderson-Hasselbalch equation?

• ONLY WHEN THE RULE OF 100 APPLIES

• We can use this equation to find pH instead of using a RICE table

What is buffer capacity?

Buffer capacity

• Refers to the amount of acid or base that a buffer can neutralize before the solution’s pH changes appreciably

• Essentially it is how much H30+ or OH- the buffer can “soak up”

• This is where [HA] and [A-] are approx equal to each other

What is buffer range?

• This is the pH range over which a buffer can effectively neutralize and acid or base

• You want a buffer with a pKa or pKb closest to the desired pH

• Effective pH range of a buffer is pKa ± 1

How can the structure of a compound determine the relative strength of an acid?

• If it is easy to pull electrons away from a compound, then it is a strong acid

• If it is hard to pull electrons away, then it is a weak acid

What factors affect the acid strength of binary acids?

Binary acids (H-X)

• Bond polarity

• Bond strength

• Conjugate base stability

How does bond polarity affect binary acid strength?

• When the bond between H and the other element is polar AND hydrogen has the partial positive charge, the compound is acidic

• Acid strength increases as the electronegativity difference increases, so up and to the right

How does bond strength affect binary acid strength?

• The stronger the bond is between hydrogen and the other element, the harder it is to dissociate the hydrogen, meaning a weaker acid

• Larger atoms will be further from the H atom, so the bond will be weaker

How does conjugate base stability affect binary acid strength?

• The greater the stability of an acid’s conjugate base, the stronger that acid will be

• Binary acid strength increase right and down

• Oxyacid strength increase up and right

What factors can affect the acid strength of oxyacids?

Acid strength is related to the strength and polarity of the H—O bond, which depends on

• Electronegativity of the central atom

• The number of oxygens bonded to the central atom

• The oxidation state of the central atom

How does central atom electronegativity affect acid strength of oxyacids?

The more electronegative the central atom

• The stronger the oxyacid

• The weaker the H—O bond

• Acidity of oxyacids decreases down a group (atom size)

How does the number of oxygens in oxyacids affect acid strength?

• The more oxygens attached to the central atom, the stronger the oxyacid

• More oxygen further weakens the H—O bond

• They draw away electron denseity from the halogen and hydrogen

• Increasing the number of oxygens also helps stabilize the conjugate base by spreading out the negative charge

How does oxidation state affect oxyacid strength?

• The larger the oxidation state of the central atom, the stronger the oxyacid

• This is essentially saying the same thing as more oxygen = more acidic

How does charge affect acid strength?

• A compound becomes more acidic as the positive charge increases because it is easier to accept an electron pair

• A compound becomes more basic as the negative charge increases because it is easier to donate an electron pair

What is important to note about the Ka values of polyphonic acids and pH?

Ka1 is always much larger than Ka2 or Ka3

• Therefore, the amount of H3O+ ion formed by the first ionization step is much larger

• We can consider the first step the only step that contributes to the H3O+ concentration

What is a titration?

• A procedure for determining the concentration of a unknown solution by adding to that solution a precisely measured volume of an acid or base with a known concentration

• The acid or base being added is the titrant

• The solution with an unknown concentration is called the titrand

What is the equivalence point on a titration curve? What is the useful formula for titrations?

• The equivalence point is the point at which the quantities of acid and base in solution are stoichiometrically equivalent

• Moles H30+ = Moles OH-

• M(acid) x V(acid) = M(base) x V(base)

What does equivalence point location tell us about a solution?

If the equivalence point has a pH

• < 7, then the salt is acidic

• = 7, then the salt is neutral

• > 7, then the salt is basic

What is the end point of a titration curve?

• This is the point where a pH indicator will change color, indicating that a certain pH has been reached

What happens when a strong acid and a strong base are titrated together?

• The equivalence point has a pH of 7 and the salt produced will be neutral

What happens when a weak acid and strong base are titrated together?

• The equivalence point is above 7 and the salt produced will be basic

What is the half-equivalence point in a titration?

• If a weak acid/base is titrated with a strong acid/base, this is the point where half of the weak substance has been converted to its conjugate

• [A-] = [HA]

• In the Henderson-Hasselbalch equation, pH =pKa

What happens when a weak acid and a weak base are titrated together?

• When this happens, the pH at the equivalence point depends on the Ka of the weak acid and the Kb of the weak base

• Ka > Kb : acidic

• Ka < Kb : Basic

• Ka = Kb : neutral

What happens if you titrate a polyprotic acid?

• Each Ka value represents a separate proton dissociation and an individual equivalence point

• The further the Ka values are from each other, the easier it is to distinguish the equivalence points

What is the solubility product constant?

• It is the equilibrium constant for the dissociation of a solid salt in aqueous solution

• The magnitude of Ksp is a measure of how much of the solid dissolves in solution

What is molar solubility?

• This is the number of moles of solute that dissolves in one liter of solution

• Its the molarity

What is the rule about relative solubility?

• In order for Ksp values to be compared, the compounds being compared must have the same dissociation stoichiometry

How can you predict if a precipitate will form using the solubility constant?

• We can also find Qsp (the solubility quotient)

• If Qsp > Ksp, a precipitate will form because the actual solubility is above the equilibrium solubility (supersaturated)

• Qsp < Ksp means the solution is unstaturated

• Qsp = Ksp means the solution is saturated

How does adding a common ion effect solubility?

• The addition of a solute with a common ion as another solute will decrease the solubility of that solute

• This is due to Le Chatlier’s principle

What is the relationship between solubility and pH?

• If the anion of a salt solute is basic, then the solubility of the salt increases in acidic solutions

• The pH of a solution very directly affects the solubility of hydroxides because of the common ion effect

• Higher pH = more OH- = lower solubility of ionic hydroxide

What is the one thing all REDOX reactions have in common?

• Electrons moving from one element to another

What are the oxidation states in order of priority

• Atoms in elemental form: 0

• A monoatomic cation: Same as its charge

• Fluorine: -1

• Oxygen: -2, UNLESS it is peroxide (-1)

• Hydrogen: +1, UNLESS it is bonded to a metal (-1)

• Halogens: -1

What are the steps to balancing a REDOX reaction?

What are the steps to balancing REDOX reactions in acidic solution?

What are the steps to balancing REDOX reactions in basic solutions?