Chemistry 3

What is the cathode and anode sign? How is it different from galvanic cell?

Electrolytic cell: Cathode(-), Anode(+) Galvanic cell: Cathode(+), Anode(-)

Equiv. point vs. End point?

Equiv. point is where [titrant] = [analyte] End point: point when indicator causes color change There is no relationship between equiv. point and end point.

What is the concentration cell?

Everything same as Galvanic cell but with same species -> Cell potential is 0 -> what? E* = 0. but concentration cell is not in standard condition(1M of each solution). Instead it uses different concentration.

If cell potential is negative for electrolytic cell, there shouldn't be a voltage.. what makes positive cell potential?

External voltage(Vbattery) -> Cell potential = External voltage + electrolytic cell potential

How do you calculate F?

F(faraday) = charge on one mole of electrons So, one mole of e- = 6.022 x 10^23 electrons e- has charge of 1.6 x 10^-19 C so 9.6 x 10^4 C/mol

formation of rust on metal is an oxidation process involving water that is catalyzed by presence of salt. Which of the following would have highest reduction potential, E*? a) old rusted nail b) polyurethane pipe c) polymer coated wire d) stainless steel frying pan

FOrmation of rust = oxidation process, because metal gets oxidized. So highest reduction potential means not willing to get rusted. SO polyurethane pipe(no metal involved) has the highest reduction potential.

What is the unit of Farad? whats relationship between Faraday?

Faraday = charge on one mole of electrons Farad = capacitance = 1C^2s^2/(m^2 kg)

For which of the following titrations will the [OH-] = [H+] at equivalence point? For which titrations will [titrant] = [analyte] at equiv. point? a) SA w/ SB b) WB w/ SA c) WA w/ SB d) WA w/ WB

a) equiv. point of SA and SB [H+] = [OH-] / [titrant] = [analyte] b) equiv. point of WB w/ SA [H+] > [OH-] / [titrant] = [analyte] c) equiv. point of WA w/ SB [H+] < [OH-] / [titrant] = [analyte] d) equiv. point of WA w/ WB [H+] = [OH-] if same strength, otherwise, not same [titrant] = [analyte] everytime WA and WB is rarely used because not so useful

2) Strong base titrated with strong acid

Opposite to Strong acid with Strong base

Draw titration curves 1) Strong acid titrated with Strong base

Start at 1 or 2 as base is added, pH will stay low and slowly rise. Go vertical from around 6 (equiv. pH 7) After equiv. point, plot slowly go higher as more base is added

3) Weak acid titrated with strong base

Start below pH 7 usually 3 to 5. As strong base is added, pH will increase slowly through buffer region then go vertical Equiv. point won't cover as many pH units as it did for strong acid pH will more quickly rise Equiv. point will not be 7 but higher than 7

If 2 mole of Cu 2+ is used, what is the reduction potential of 2mole Cu 2+ ? (E* =0.52V)

Still 0.52V mole doesn't matter

"a strong base is titrated with strong acid". Which one is being added drop-wise and which one is in beaker? Which solution is referred to as titrant? Which is analyte?

Strong base is in beaker = analyte Base is "titrated with" the strong acid, so acid is being added = titrant

What is Cell Potential?

Sum of electrical potentials for two half-reactions!

Acid/conjugate acid and base/conjugate base

acid becomes conj. base after donation of H+ Base becomes conj. acid after accept of H+ HCl -> H+ Cl- HCl: Acid / Cl-: conj. base or Cl- base / HCl: Conj. acid

What is amphoteric substances?

act as acid or base H2O -> H+ + OH-

What is a salt of a weak acid?

conjugate base + cation to form a salt Na + HCO3- <-> CO3 2- + Na2CO3(salt) so salt = if acid, conj. base + cation if base, conj. acid + anion

What is a buffer solution?

contains a weak acid and weakbase, often conjugates there is an equilibrium between weak acid + conjugate base / weak base + conjugate acid

How about Ammonium hydroxide in ammonium chloride solution?

decreases

How does the percent dissociation of benzoic acid changes in sodium benzoate solution?

decreases

What is titration?

drop by drop mixing of an acid and base with indicator

What it means by large Ka and Kb? how it relates to pKa and pKb?

large Ka and Kb = equilibrium that has more products than reactants, so more dissociation. Ka = more H+ dissociated, so higher Ka = lower pKa (stronger acid) higher Kb = lower pKb(stronger base)

What is the Half-Equivalence point?

midpoint of nearly horizontal section of graph pH = pKa or [HA] = [A-] = [H+](if one equiv. is yielded for every one mole of HA) think as 50% dissociated -> 50% yield of [A-]

What is the Half-Equivalence point of SA/SB?

There is no Half-equiv. points. -> HA readily dissociates to H+ and A-(instantly 100% A-) , so there is no point where [HA] = [A+]

Aurora: colloid of air and tiny metallic elements interacting with upper atmosphere. How to explain?

Valence electrons from metals are excited, then release photons of various wavelengths as they relax

What are the differences between strong and weak?

Weak: start as not too low or not too high Equiv. region not too long Equiv point not 7

Is cell potential always positive?

YES but not electrolytic cells

So what is the pattern?

[H+] = [OH-] only for strong/strong [titrant] = [analyte] for all

Assign oxidation state to each atom in each a) (NH4)2SO4 b) FeCO3 c) H2O2 d) NaH e) SF6

a) (NH4)2SO4 NH4-> +1, N(-3), H(+1), S(+6) O(-2) b) FeCO3 Fe(+2) C(+4) O(-2) c) H2O2 H(+1) O(-1) d) NaH Na(+1) H(-1) e) SF6 S(+6) F(-1)

T/F? a) Species for which E* is negative can't be spontaneously reduced, but are often oxidized b) H half-cell has no affinity for electrons as demonstrated by its electrical potential, E*=0V c) based on half-reactions given in table above, the potential for Cu(s) to be reduced by one electron is -0.52V

a) F, it can if paired with even lower E* b) F, 0V is arbitrarily assigned c) F, Cu(s) solid!! doesn't get reduced

Hydrolysis of which of the following salts in solution will increase the pH of solution? a) NaNO2 b) NH4Cl c) NaF d) NaClO2 e) Ch3COONa f) NaCl

a) NaNO2 NO2 + H2O <-> HNO2 + OH- so increase pH b) NH4Cl NH4 + H2O <-> NH3 + H2O + H+ so decrease pH c) NaF F- + H2O <-> HF + OH- so increase pH d) NaClO2 ClO2- + H2O <-> HClO2 + OH- so increase pH e) Ch3COONa CH3COO- + H2O <-> CH3COOH + OH- so increase pH f) NaCl remain the same

What is the characteristics of strong acids and strong bases?

100% dissociation in water

How many equivalents of base can be neutralized by one equivalent of H2SO4?

2 equivalents of base.

Which of the following is not a difference between a galvanic cell and an electrolytic cell? A. Sign of anode and cathode B. sign of delta G C. external voltage source applied D. net gain or less or electrons

A. Sign of anode and cathode Sign is reversed B. sign of delta G gal: spontaneous, elec: non-spon C. external voltage source applied D. net gain or less or electrons for both, net is 0

Acid A has Kb of 1x10^-9 and Acid B has Kb of 1 x 10^-10. Which acid will create largest decrease in pH when added in equimolar amounts to pure water?

Acid B. Acid A with Kb of 1x10^-9 = Ka of 10^-5 so pH = 5 For B, pH = 4

Lewis Acid/Base?

Acid: Accept pair of electrons Base: Donate pair of electrons

Explain the equilibrium of water H2O + H2O <-> H3O+ + OH- the effect of addition of H+ and OH- with Le Chatelier's Principle

Adding more H+ will shift the eq. to left, by comsumption of OH-, so more H+ left, so increase in acidity and decrease in pH.

Which of the following atom will likely to get reduced when combined with other elements? a) Ag+2(E* = 0.8V) b) Cu+2(E* = 0.52V) c) Ni+2(E* = -0.23V) d) Zn+2(E* = -0.76V)

Ag+2

What is the equiv. point of strong acid + weak base or weak acid + strong base

Again, [titrant] = [analyte] Not 7 because initial [H+ ] not equal to [OH-] Weak acid/base will not dissociate 100% -> so equivalent point will be where during the titration, [H+] and [OH-] become equal.

AlCl3, BF3, NH3, OH- Lewis Acid/base?

AlCl3, BF3: electrophile so acid NH3, OH- : nucleophile so base

Draw a galvanic cell and label following: Anode, cathode, terminals, salt bridge, electron flow, current flow, load(resistor, or voltmeter), electrodes, oxidation half-reaction, and reduction half-reaction

Anode = oxidation Cathode = reduction

What is Arrehenius acid and base?

Arrhenius Acid: produce H+ Arrhenius Base: produce OH-

Explain the purpose of salt bridge in Galvanic cell. Why would galvanic cell quickly cease to function w/o a salt bridge?

As Cu2+ is used up, negative salts accumulate, and Zn2+ is given off, positive salts will accumulate. Polarity ceases electron flow. SO salt bridge allows Na+ and NO3- and metal cations to move

Color-change of the indicator during the titration is due to what?

Dissociation of the indicator Ex) Litmus changes to red(acid) and blue(base) HLit(red) <-> H+ + Lit-(blue) so, during the titration when buffered region is passed, OH- should be dominating ions and Lit- floats so solution looks blue

How to fine E in concentration cell?(Nernst Equation)

E = E* - (0.06/n) x log[lower]/[higher] n = moles of electrons transferred( Fe 3+ -> Fe(s) = 3, Ag+ -> Ag(s) = 1)

If reduction potential of Ag is 0.8V, what is the oxidation potential?

-0.8V

What about pH or pOH for weak acid?

1) HA <-> H+ + A- 2) use ICE 3) then we get Ka = [x][x] / [HA - x] assuming initial conc. of H+ and A- is 0 4) for pKa, -log[Ka] and for pH, just -log[H+] and this case -log[x]

What are the equivalent points for 1) WB w/ SA 2) WA w/ SB 3) SA w/ SB 4) WA w/ WB?

1) WB w/ SA : pH <7 2) WA w/ SB : pH >7 3) SA w/ SB : about 7 4) WA w/ WB? : about 7

How do you recognize the buffer solution?

1) Watch for equimolar amounts: maximum buffering strength when [HA] = [A-] 2) equimolar of conjugates of each other like 1) 3) If Weak acids or bases: SA, SB never buffers 4) Resistance to pH change: not change "very much", "changes slightly" 5) Half-equivalence point: WA, WB!! 6) Watch for pH = pKa 7) ratio of [A-] / [HA]

What must we assume to make this calculation?

1) We assume that either acid or base concentration is bigger(in magnitude) than 10^-7 when both acid and base are present(10^-7M) in water. So, we are ignoring that 10^-7M because acid will be something like 10^-2 or 10^-4M conc. and even if we add 10^-2 and 10^-7 = 10^-2. But if the added acid concentration is smaller than 10^-7, we can't use -log system. 2) assume 100% dissociation rate. In real situation, ion-pairing will reduce effective concentration of ions.

Why is HF a not a good acid?

Because F is so small and has strong electronegativity that it forms a strong bond between H and F. OR F- is far smaller than Cl-, so when dissociated, F- has to bear full formal charge, it experiences greater charge density so more unstable. Larger atom like Cl- can spread out this charge over greater area.

What is the pKa and Ka of weak acids and pKb and Kb or weak base?

Both Ka or Kb <1 Both pKa or pKb >0

Bronsted-Lowry Acid/Base?

Bronsted-Lowry Acid: Donate H+ Bronsted-Lowry Base: Accetp H+

What is the difference between Galvanic cell and Electrolytic cell?

Galvanic cell is a natural cell(+ cell potential) Electrolytic cell -> external voltage is applied, forcing electrons to flow in opposite direction(- cell potential)

What are strong bases?

Group Ia hydroxides(NaOH, KOH,,,), NH2-, H-, Ca(OH)2, Sr(OH)2, Ba(OH)2, Ba2O, CaO

Examples of weak acids?

H2O, H2S, NH4+ , HF, HCN, H2CO3, H3PO4, Acetic acid, Benzoic acid, etc

Examples of weak bases?

H2O, NH3, R3N, pyridine, Mg(OH)2...

What are the strong acids?

HI, HBr, HCl, HNO3, HClO4, HClO3, H2SO4, H3O+ Note: H3O+ is in the borderline(pKa = -1.7)

Acid HX dissociates 80% in water. Would you expect its Ka to be greater than, less than, or equal to one?

HX <-> H+ + X- Ka = [H+][X-] / [HX] 80% dissociation means more products than reactants, so Ka would be bigger than 1 and pKa would be smaller than 1 for strong acid

Use that equation to demonstrate that pH = pKa at half-equivalence point

Half-equivalence point = where [A-] = [HA] so pH = pKa + log[A-] / [HA] log 1 = 0, so pH = pKA at Half-equivalence point.

Would you expect a strong oxidizing agent to have a high or low reduction potential?

High

What is electrical potential in lay man's term?

How much it wants to get reduced?

What is the pH of HA <-> H- + A- assuming HA is a weak acid?

If strong acid, its 100% dissociation so pH = p[H+] = -log[H+] But if weak acid, like buffer solution, its not 100% dissociation, so pH should reflect rate of dissociation of [HA] Ka = [H+][A-] / [HA] -log Ka = -log [H+][A-] / [HA] pKA = -log[H+] -log[A-] / [HA] pH = pKA + log[A-] / [HA]

T/F. An aqueous solution with pH of 8 is basic and therefore by definition it does not contain any unreacted H+ ions

It does NOT mean that it has no H+. pH = 8 = -log[H+], so [H+]=10^-8 moles/L It is classified as base because fewer [H+] is found in neutral H2O and more [OH-]

HA + H2O <-> H3O+ + A- whats Ka?

Ka = [H3O+][A-] / [HA]

What is Ka x Kb?

Ka x Kb = Kw = 10^-14

정리! what is Kw? Ka? Kw? pKw? pKa + pKb? Ka x Kb?

Ka: [H+][A-]/[HA] Kb: [OH-][AH]/[A-] so Kw: Ka x Kb = [H+]/[OH-] = 10^-14 pKw = pKa + pKb = 14 Ka x Kb = Kw = 10^-14

A- + H2O <-> HA + OH- what is Kb?

Kb = [HA][OH-]/[A-]

What is Equivalence point or Stoichiometric point of both Strong acid and strong base?

Midpoint of nearly vertical section of graph where [titrant] = [analyte]

Mixture of 1.5 moles of NH4Cl and 1.5 moles of NH3 is added to 2L of water and thoroughly stirred. If 2.0 mL of Ca(OH)2 is added to solution, what will be observed? And how do you chracterize the solution prior to Ca(OH)2?

NH4+ and NH3 are weak acid and conjugate base. meaning it is a buffer solution. And 2mL of OH won't be much even though it's a strong base, small increase of pH will be observed. BUFFER SOLUTION!!!

Where is the buffer region?

Nearly horizontal area at this region, putting large amount of titrant has small effect on pH -> reason why flat

What is the Free Energy of redox reaction?

n = moles of electrons transferred so negative E* = spontaneous reaction

4) Weak base titrated with strong acid

opposite to weak acid titrated with strong base

what is the relationship between pH and pOH

pH + pOH = 14

What is pH formula?

pH = -log[H+] scale: 1 ~ 14 (7=neutral)

How does hydrogen ion concentration differ for two solutions, one with a pH = 2 and the other with pH = 4?

pH = 2 : [H+] = 10^-2 pH = 4 : [H+] = 10^-4 so pH 2 solution is x100 [H+] of solution of pH 4

How to calculate pH or pOH for strong acid?

pH/pOH = -log[pH or pOH]

when at 25C, Kw = [H3O+][OH-] = 10^-14, whats pKw?

pKw = -log[Kw] = 14 note 25C that temperature is the only thing that affects K.

What is pOH?

pOH = -log[OH-]

3 common indicators are methyl orange, litmus, and phenylphthalein. These indicators change color at oH of 3.7, 6.5 and 9.3. Which is used to identify equiv. point for WA/ SB?

phenylphthalein. Because WA/SB will have equiv. point >7

What do you omit when thinking K?

pure liquids and pure solids

What happens when weak acids or weak bases dissolve in water?

reform the weak acid and weak base by hydrolysis

What is general rule for strong acid and strong base?

strong acid: stronger than H3O+ strong base: stronger than OH-

What is the indicator?

weak acids that change color as they dissociate from HA into H+ and A- amount of indicator is so small to titrant and analyte that it has no impact on pH

What is each axis of titration curves?

x-axis = mL of titrant y-axis = pH