Unit 2: Equilibrium, Acids & Bases, Buffers

Question 1

What do the different relationships between Q (reaction quotient) and K (equilibrium constant) mean?

(<, >, and =)

Answer 1

• If Q > K, the reaction proceeds leftward toward reactants because product concentration is higher than at equilibrium.
• If Q < K, the reaction proceeds rightward toward products because product concentrations are lower than at equilibrium.
• If Q = K, equilibrium has been reached and the reaction remains stable.

Question 2

What is chemical equilibrium?

Answer 2

• A state that occurs when the forward and reverse rates of a reaction are equal and concentrations of reactants and products are no longer changing with time

Question 3

Quadratic Formula

Answer 3

Question 4

Homogeneous Equilibrium

Answer 4

• all species are in the same phase

Question 5

What is K_p?

Answer 5

• the equilibrium constant with respect to pressure of gases
• it is only expressed in terms of gases, SOLIDS AND LIQUIDS DO NOT COUNT towards K_p
• it equals the partial pressures of products over the partial pressures of reactants, at equilibrium and raised to their stoichiometric coefficients

Question 6

What is the equation for the relationship between K_p and K_c?

Answer 6

• Kp is equal to Kc times the product of the ideal gas constant and temperature (Kelvin) raised to the power of the difference in moles between products and reactants

the picture shows the inverse:

Question 7

Heterogeneous Equilibrium

Answer 7

• a reversible reaction in which the chemical species of reactants and prodcuts are in different phases
• pure solids and liquids DO NOT COUNT in the equilibrium expression (their values remain constant at a given temperature)

Question 8

Le Châtelier’s Principle

Answer 8

If external stress is applied to a system at equilibrium, the system adjusts such that the stress is offset and equilibrium is re-established.

Question 9

Factors that affect equilibrium

Answer 9

• temperature
• pressure
• volume
• concentration

Question 10

How does temperature affect equilibrium?

Answer 10

• Depending on the ΔH of the reaction, heat is either a reactant or a product
• ΔH > 0, reaction is endothermic, heat is reactant, raised temperature drives reaction productsward
• ΔH < 0, reaction is exothermic, heat is a product, raised temperature drives reaction reactantsward

Question 11

How does pressure affect equilibrium?

Answer 11

• An increase in pressure drives a reaction towards whichever side has fewer moles

Question 12

How do catalysts affect equilibrium?

Answer 12

They DO NOT. They only affect the rate at which it is reached.

Question 13

What are standard state conditions?

Answer 13

1 atm pressure and 1 M concentration

Question 14

How is ΔG related to the equilibrium constant?

Answer 14

• By a proportionality constant R x T, with R being the constant 8.314 J/(mol K) and T being temperature (Kelvin)

ΔG (standard state) = - RT x ln K

or for any conditions

ΔG (any conditions) = RT x ln Q + ΔG (standard state)

Question 15

What’s a Bronsted-Lowry Acid/Base?

Answer 15

• acid is capable of yielding an H⁺ ion and is therefore a “proton donor”
• base yields OH^- ion and is a “proton acceptor”

Question 16

What are conjugate acids/bases?

Answer 16

A conjugate acid or base is produced from the gain or loss of H+ ions, respectively.

Question 17

What are the 7 strong acids?

And what makes them “strong”?

Answer 17

• HBr (Hydrobromic Acid)
• HI (Hydroiodic Acid)
• HCl (Hydrochloric Acid)
• HClO₃ (Chloric Acid)
• HClO₂ (Chlorous Acid)
• H₂SO₄ (Sulfuric Acid)
• HNO₃ (Nitric Acid)
• They completely dissociate in solution.

Question 18

What are the strong bases? And what makes them strong?

Answer 18

• Group I and II Metal Hydroxides ( ex: NaOH, Ca(OH)₂ )
• Group I and II Metal Oxides ( ex: MgO, CaO )
• They dissociate completely in solution to produce H⁺ accepting compounds.

Question 19

What is a weak acid?

Answer 19

• Any acid not within the 7 strong acids which dissociates only partially in solution

Question 20

What are the weak bases?

Answer 20

Amines - contain N with a lone electron pair

Carbonates - CO₃^2- + H₂0 <—> HCO₃^- + OH^-

Question 21

What determines the strength of an acid bond?

Answer 21

• smaller atoms bond more tightly to their H⁺s and therefore produce weaker acids (because they give off their H⁺ ions less readily)
• within a group, elements closer to the top of the table (smaller atoms) produce weaker acids, whereas elements further down the table (bigger atoms) produce stronger acids

Question 22

Determining strength of oxy acids with different central atoms, but same # of oxygens.

Answer 22

• the more electronegative central atom inductively draws electron density from H-O bond, weakening it and therefore strengthening the acid
• ex: HClO₄ is stronger than HBrO₄

Question 23

Determining strength of oxy acids with same central atoms but different #s of oxygens.

Answer 23

• more oxygens inductively weaken the O-H bond, strengthening the acid
  ex: HClO₃ is stronger than HClO₂

Question 24

Lewis Acids and Bases

Answer 24

• Acid accepts a pair of electrons
• Base donates a pair of electrons

Question 25

What is K_w?

Answer 25

• it is the ion product of water at a given temperature

K_w = [H⁺][OH^-] = 1.0 x 10^-14

Question 26

What is pH?

what is pOH?

Answer 26

• log [H⁺]
• log [OH^-]

Question 27

What should one do to find the pH of extremely dilute acids?

Answer 27

• Add back in the H⁺ ions from water
  ex: pH of 1.0x10^-8 M HCl is equal to:
• log (1.0x10^-8 + 1.0x10^-7)

Question 28

Polyprotic Acids

Answer 28

• some acids have more than one acidic proton and more than one K_a value
• most H⁺ concentration comes from the 1st dissocation so usually K_a1 is used

Question 29

What are the acidic metals?

Answer 29

Small cations (above group 4) with large charges (> +1)

ex: Al, B

Question 30

What is the relationship between K_a and K_b?

Answer 30

K_a x K_b = K_w (1.0 x 10^-14)

Question 31

What is the composition of a salt that forms a neutral solution?

Answer 31

An anion of a strong acid and a cation of a strong base

Question 32

What is the composition of a salt that forms an acidic solution?

Answer 32

• An anion of a strong acid and a cation of a weak base

Question 33

What is the composition of a salt that forms a basic solution?

Answer 33

Anion of a weak acid and cation of a strong base

Question 34

How can you determine the acidity or basicness of a salt formed from a weak base and weak acid?

Answer 34

Compare the K_a of the cation with the K_b of the anion.

Whichever is larger determines pH.

Question 35

Amphoteric Salts

Answer 35

• can act as acids or bases, compare Ka and Kb values of reacting species
• the higher K value determines acidity/basicity

Question 36

Henderson-Hasselbach Equation

Answer 36

• used for buffer systems only

Question 37

Equivalence Point of a Titration Curve

(and how to calculate its pH)

Answer 37

• the point at which the moles of acid and moles of conjugate base are equal
• usually found where pH is changing rapidly

calculation:

1. Write out ICF table, acid and base react completely, leaving only conjugate and water
2. Write out ICE table for conjugate and water reaction (resulting in acid and OH^-
3. Use K_b and ICE to find [OH^-]
4. Use [OH^-] to find pH

Question 38

Calculating pH before an equivalence point on a titration curve.

Answer 38

• use the Henderson-Hasselbalch Equation

Question 39

Calculating pH at 1/2 of the equivalence point of a titration curve.

Answer 39

pH = pK_a

Question 40

Calculating pH beyond the equivalence point of a titration curve (acid titrated with base).

Answer 40

• Base is now in excess of acid, so use the unreacted base to determine the pH