Comprehensive CHEM II Laboratory Review

Thin Layer Chromatography (TLC)

  • PURPOSE
    • Separate tiny quantities of compounds; forensic use for dye ID.
    • Calculate R<em>fR<em>f (distance spot ÷ distance solvent front). • Relate R</em>fR</em>f to polarity & IM-forces.

  • HARDWARE / PHASES
    • Stationary phase: SiO<em>2\text{SiO}<em>2 or Al a on glass/plastic (≈0.25 mm). • Mobile phase: chosen solvent drawn upward by capillarity. • Greater attraction for solvent → larger R</em>fR</em>f; greater attraction for plate → smaller RfR_f.

  • PROCEDURE
    • Draw start line 1 cm from bottom, 5 spots (crime sample + 4 suspects).
    • Spot ≤ 2 mm; darken by re-spotting.
    • Place TLC into chamber with solvent below start line; saturate vapor with filter paper.
    • Develop until solvent ≈1 cm from top; mark solvent front immediately.
    • Visualize (visible, UV, or stain); measure all distances; compute RfR_f.

  • PRE-LAB CONCEPTS
    • Tight, small spot needed; size influences resolution.
    • Starting line & solvent level critical.
    • Do not let solvent run off plate before marking front.

Phase Diagram of t-Butyl Alcohol

  • OBJECTIVES
    • Locate triple, boiling & freezing points; sketch PP–TT diagram.
    • Reinforce concepts of vapor pressure, phase coexistence, T<em>bT<em>b, T</em>fT</em>f.

  • TRIPLE-POINT APPARATUS
    • Test tube w/ tt-BuOH + boiling chips, thermometer through #4 stopper, vacuum line via water aspirator.
    • Under ~42.4 mmHg42.4\,\text{mmHg} observe simultaneous boiling & freezing → record TtpT_{tp}.

  • BOILING POINT
    • Heat 600 mL water bath; immerse tube; read constant vapor-temperature (≈ normal TbT_b at lab PP).

  • FREEZING POINT
    • Cold-water (≈18 °C\mathrm C) bath; stir continually; record plateau temperature while solid forms.

  • DATA ANALYSIS
    • Plot ln⁡P\ln P vs 1/T1/T or simple sketch marking solid, liquid, gas fields; label all three key TT’s.

  • KEY EQUATIONS
    • Clausius–Clapeyron ln⁡P=−ΔHvapR 1T+C\displaystyle \ln P = -\frac{\Delta H_{vap}}{R}\,\frac1T + C

Boiling-Point Elevation & Molar-Mass Determination

  • THEORY
    • Colligative property: ΔT<em>b=K</em>bm\Delta T<em>b = K</em>b m (independent of solute identity).
    • mm = molality = nsolkg solvent\dfrac{n_{sol}}{\text{kg solvent}}.

  • PROCEDURE

    1. Find pure ethanol TbT_b in 90 °C water bath.
    2. Prepare 1 m, 2 m, 3 m ethylene-glycol / ethanol solutions (density 0.789 g mL−10.789\,\text{g mL}^{-1}).
    3. Record elevated T<em>bT<em>b; plot ΔT</em>b\Delta T</em>b vs mm → slope =Kb (EtOH)=K_b^{\,(EtOH)}.
    4. Dissolve unknown sample with mass equal to 1 m EG mass; measure ΔT<em>b\Delta T<em>b; compute molality then moles; M=m</em>unk/nunkM = m</em>{unk}/n_{unk}.

  • SAFETY: ethanol & ether are flammable; EG toxic.

Chemical Kinetics – Dye + Bleach

  • RATE LAW
    Rate=k[dye]a[OCl−]b\text{Rate}=k[\text{dye}]^{a}[\text{OCl}^-]^{b}
    • Pseudo-order: large excess NaOCl ⇒ k′=k[OCl−]bk' = k[\text{OCl}^-]^b.

  • EXPERIMENT
    • Microlab spectro-kinetics at 525 nm; follow fade of FD&C Red #3 or Blue #1.
    • Plot ln⁡[dye]\ln[\text{dye}] vs tt (1st-order) and 1/[dye]1/[\text{dye}] vs tt (2nd-order).
    • Best linear fit → order in dye; slope =−k′=-k'.
    • Repeat with new [OCl⁻]; compare k′k' values ⇒ determine bb & true kk.

Chemical Equilibrium – FeSCN²⁺ System

  • REACTION
    \ce{Fe^{3+} + SCN^-

  • EQUILIBRIUM CONSTANT
    • Measure absorbance at 468 nm; ε=7260 M−1cm−1\varepsilon=7260\,\text{M}^{-1}\text{cm}^{-1}.
    • [FeSCN2+]=A/(εb)[\text{FeSCN}^{2+}] = A/(\varepsilon b), use RICE to back-calculate reactant eq. conc.; compute KcK_c (triplicate trials, 𝑇 ≈ rt).

  • LE CHÂTELIER TESTS
    • Add various salts/temperature changes; note colour shift: darker → right (products), paler → left (reactants).

Redox Titration – Fe in Supplements

  • STOICHIOMETRY
    \ce{MnO4^- + 5Fe^{2+} + 8H^+ → Mn^{2+} + 5Fe^{3+} + 4H2O}
    • Endpoint: first persistent faint pink MnO4−\text{MnO}_4^- excess.

  • ANALYSIS STEPS

    1. Dissolve crushed tablet in \ce{H2SO4}.
    2. Titrate 10 mL aliquots w/ standard 0.0020 MKMnO40.0020\,\text M KMnO_4 (triplicate).
    3. Moles \ce{Fe^{2+}} = \dfrac{1}{5} n_{\ce{MnO4^-}}.
    4. Convert to mg Fe; %Fe = (mg Fe / tablet mass)×100.

Thermodynamics via CoCl₂ Complexation

  • EQUILIBRIUM
    \ce{[Co(H2O)6]^{2+} + 4Cl^-

  • DATA COLLECTION
    • Record absorbance at λ<em>max\lambda<em>{max} (≈ 650 nm for blue species) at 10° C steps (10–40 °C). • K=[CoCl</em>42−][Co(H<em>2O)</em>62+][Cl−]4K = \dfrac{[\text{CoCl}</em>4^{2-}]}{[\text{Co(H}<em>2\text O)</em>6^{2+}][Cl^-]^4} (Cl⁻ large excess ⇒ constant).
    • Plot ln⁡K\ln K vs 1/T1/T : slope =−ΔH∘/R=-\Delta H^\circ/R; intercept =ΔS∘/R= \Delta S^\circ/R.
    • ΔG∘=−RTln⁡K\Delta G^\circ = -RT\ln K at each T.

Acid–Base & Titration Concepts

  • WEAK-ACID K<em>aK<em>aK</em>a=[H3O+][A−][HA]K</em>a = \frac{[H_3O^+][A^-]}{[HA]} ;
    % ionization increases on dilution.

  • STRONG ACID–STRONG BASE CURVE
    • S-shaped; equivalence pH=7pH=7.
    • Half-equivalence of weak acid: pH=pKapH = pK_a.

  • BUFFER
    pH=pK<em>a+log⁡[base][acid]\text{pH}=pK<em>a+\log\frac{[\text{base}]}{[\text{acid}]} (Henderson-Hasselbalch). • Effective range pK</em>a±1pK</em>a ±1.

Polyprotic Acid Titration

  • Diprotic example: \ce{H2A}
    • Two equivalence points; volumes ratio 1 : 1.
    • K<em>a1K<em>{a1} from pH at 1st half-eq; K</em>a2K</em>{a2} from 2nd.

Solubility Product of Ca(OH)2\text{Ca(OH)}_2

  • EQUILIBRIUM
    \ce{Ca(OH)2(s)

  • TITRIMETRIC DETERMINATION

    1. Filter saturated Ca(OH)₂ (discard first 5 mL).
    2. Titrate 10.00 mL aliquots with 0.0500 MHCl0.0500\,\text M HCl using phenolphthalein.
    3. n<em>OH−=n</em>HCln<em>{\text{OH}^-}=n</em>{HCl}; [OH−]=n/V[OH^-]=n/V; [Ca2+]=0.5[OH−][Ca^{2+}] = 0.5[OH^-].
    4. Calculate KspK_{sp} and molar solubility s=[Ca2+]s=[Ca^{2+}]; average over 3 trials.

  • COMMON-ION TEST
    • Add drops of \ce{NaOH}, \ce{CaCl2}, \ce{HCl}, etc. to aliquots; monitor pH shifts:
    – Add OH⁻ or Ca²⁺ → ↓solubility (shift left).
    – Add H⁺ or precipitating \ce{Ag^+} (removes OH⁻) → ↑solubility (shift right).

Electrochemistry

  • STANDARD CELL POTENTIAL
    E∘<em>cell=E∘</em>cath−Eanode∘E^\circ<em>{cell}=E^\circ</em>{\text{cath}}-E^\circ_{\text{anode}}
    • Positive ⇒ spontaneous (voltaic).

  • LINE NOTATION
    Zn∣Zn2+∣∣Cu2+∣Cu\text{Zn}|\text{Zn}^{2+}||\text{Cu}^{2+}|\text{Cu}

  • ELECTROLYTIC CALCULATIONS (Faraday’s Law)
    • Charge Q=ItQ=It, 1 F=96485 C mol−1 e−1\,\text F = 96485\,\text C\,\text{mol}^{-1}\,e^-.
    • Mass plated m=ItMnFm = \dfrac{ItM}{nF}.

  • ELECTROPLATING EXAMPLE
    • Ni²⁺ + 2e⁻ → Ni(s) on Cu; at 5 V, 600 s, measured 1.02×10−3 mol e−1.02\times10^{-3}\,\text{mol }e^- ⇒ QQ ≈ 98 C, II ≈ 0.16 A, theoretical Ni mass ≈\approx 0.030 g (close to observed 0.029 g).

General Lab & Data Quality

  • Discard initial filtrate when filtering saturated solutions (filter paper adsorption).
  • Deviations: DEV<em>i=∣x</em>i−xˉ∣DEV<em>i = |x</em>i - \bar x|; AD=∑DEVnAD = \frac{\sum DEV}{n}.
  • Maintains CO₂-free basic solutions; quick titration reduces error.
  • Safety: acids corrosive, bases caustic, KMnO₄ strong oxidizer, Cr⁶⁺ toxic, heavy-metal waste collected.