Section D: ions

Question 1

what is implied about the direction of reaction when Q is less than K?

Answer 1

ΔG is negative; products are favoured

Question 2

what is implied about the direction of reaction when Q > K?

Answer 2

ΔG is positive; reactants are favoured

Question 3

what is Arrhenius’ definition of acids and bases?

Answer 3

acid = increases the concentration of H+ ions
base = increases the concentration of OH- ions

Question 4

what is the Brønsted-Lowry definition of acids and bases?

Answer 4

acid = releases H+ ions (proton donor)
base = accepts H+ ions (proton acceptor)

Question 5

what is the Lewis definition of acids and bases?

Answer 5

acid = accepts a pair of electrons
base = donates a pair of electrons

Question 6

what is meant by a large Ka?

Answer 6

large Ka = small pKa = strong acid

Question 7

how do buffers work?

Answer 7

• buffers resist changes in pH by providing a source or sink for protons
• a buffer solution contains roughly equal concentrations of a weak acid and its conjugate base

Question 8

what is the Henderson-Hasselbalch equation?

Answer 8

pH = pKa + log (base [A-] / acid [HA] )

Question 9

what are the steps for designing a buffer?

Answer 9

• choose an acid/conjugate base with the pKa ≈ pH required
• calculate the ratio cA/cB using the H-H equation
• check with the pH meter and adjust as necessary

Question 10

what is the common ion effect?

Answer 10

• when a sparingly soluble electrolyte is dissolved in a solution containing one of the ions, the solubility is markedly decreased
• due to le Chatelier’s principle - an increased ion concentration causes the equilibrium to shift to the left to counter the change, resulting in a decreased solubility
• only applies to sparingly soluble salts

Question 11

what does electrochemistry study?

Answer 11

only reactions involving the transfer of electrons or redox reactions that are thermodynamically stable

Question 12

what are the main variables of electrochemistry?

Answer 12

• voltage = driving force of a reaction (thermodynamics)

- current = quantity of electricity (extent of reaction)

Question 13

in what other units can 1 V be expressed?

Answer 13

1 V = 1 J.A^-1.s^-1

Question 14

in what other units can 1 A be expressed?

Answer 14

1 A = 1 C.s^-1

Question 15

how can an electrochemical cell be represented for a mixture of solids and liquids?

Answer 15

red (s) | ox (l) || ox (l) | red (s)

Question 16

how can an electrochemical cell be represented for a mixture of liquids?

Answer 16

Pt | red (l) , ox (l) || ox (l) , red (l) | Pt

Question 17

how is the cell reaction written using the representation of the electrochemical cell?

Answer 17

left (red) + right (ox) —> left (ox) + right (red)

Question 18

represent a calomel electrode as an electrochemical cell

Answer 18

Cl- (aq) | Hg2Cl2 (s) , Hg (s) | Pt

Question 19

what is the electrode reaction of a calomel electrode?

Answer 19

1/2 Hg2Cl2 (s) + e- —> Hg (s) + Cl- (aq)

Question 20

what is the potential of a calomel electrode?

Answer 20

+ 0.2676 V at 25C

Question 21

what is the Nernst equation?

Answer 21

E = E0 - RT/nf lnQ

Question 22

how are ΔG0 and E0 related?

Answer 22

ΔG0 = - nfE0

Question 23

what is the difference between a galvanic cell and an electrolytic cell?

Answer 23

galvanic cell - produces electricity; the reaction will proceed until equilibrium
electrolytic cell - drive a non-spontaneous reaction using an external source of current

Question 24

how are C and F related?

Answer 24

1 F = 96485 C

Question 25

what is 1 F in terms of electrons?

Answer 25

1 F = 1 mole of electrons

Question 26

in what other units can 1 C be expressed?

Answer 26

1 C = 1 A.s

Question 27

what is the Gibbs free energy?

Answer 27

the amount of energy available to a system that can be harnessed to do useful work