Exam 2

Chemical Equilibria and Acid / Base Equilibria



What the Student Brings to Exams
  • official UT ID card (with your picture and name on it)
  • a simple scientific calculator (not a graphing calculator)
  • a pencil(s) and eraser
  • memorized formulas in your head - not on paper or anything else
  • nothing else is allowed
What we provide for the Exams
  • A printed copy of the exam (every exam has a unique version number on it).
  • An answer sheet for the exam. This is a bubblesheet for your answers.
  • An exam cover page that has ALL needed conversion factors and data. No formulas will be given.
  • A periodic table of the elements with symbols, atomic number, and atomic weights.

What Formulas & Concepts the Student Should Memorize
Chemical Equilibrium

\(a\,{\rm A} + b\,{\rm B} \rightleftharpoons c\,{\rm C} + d\,{\rm D}\)

mass action:  \( \displaystyle{ {a_{\rm C}^c \cdot a_{\rm D}^d} \over {a_{\rm A}^a \cdot a_{\rm B}^b} }\)  (activities, \(K\))

mass action:  \(\displaystyle{[{\rm C}]^c[{\rm D}]^d\over[{\rm A}]^a[{\rm B}]^b}\)  (concentrations, \(K_{\rm c}\))

mass action:  \(\displaystyle{P_{\rm C}^{\,c}P_{\rm D}^{\,d}\over P_{\rm A}^{\,a}P_{\rm B}^{\,b}}\)  (gases, \(K_{\rm p}\))

\(\Delta G = \Delta G^\circ + RT\ln Q\)

\(\Delta G^\circ = -RT\ln K\)

\(K = e^{-\Delta G^\circ\over RT}\)

if Q < K    shift right →
if Q > K    ← shift left
if Q = K    no change ⇌

Van't Hoff Equation:   \(\ln\left({K_2\over K_1}\right) = {\Delta H\over R}\left({1\over T_1} - {1\over T_2}\right)\)

some equilibrium conditions

  1. all concentrations and pressures are NOT changing
  2. kinetics: rateforward = ratereverse
  3. thermo: \(\Delta G = 0\)
  4. mass action: \(Q = K\)
  5. free energy of the system is at a minimum for the stated conditions

Acid / Base Theory

acid = a proton donor

base = a proton acceptor


water

\(K_{\rm w} = {\rm [H^+][OH^-]}\)

\(\rm pH = -log[H^+] \hskip24pt [H^+] = 10^{-pH}\)

\(\rm pOH = -log[OH^-] \hskip24pt [OH^-] = 10^{-pOH}\)


weak acids / weak bases

\(\displaystyle{K_{\rm a} = {\rm {[H^+][A^-]}\over [HA]}}\)

\(\displaystyle{K_{\rm b} = {\rm {[OH^-][BH^+]}\over [B]}}\)

conjugate pairs:   \(K_{\rm w} = K_{\rm a}K_{\rm b}\)


Henderson-Hasselbach Equation

\(\rm pH = p{\it K}_{\rm a} + \log\left({[A^-]\over [HA]}\right)\)


What we provide on the exam cover page or additional handout page

Periodic Table

Conversion factors

  • Constants
    • Kw = 1.0 × 10-14
    • R = 8.314 J/mol K
    • R = 0.08206 L atm/mol K
  • Tables of Data
    • Table for Weak Acids (name, formula, \(K_{\rm a}\))
    • Table for Weak Bases (name, formula, \(K_{\rm b}\))
    • Table for Polyprotic Acids (name, formula, \(K_{\rm a}\)'s)

Learning Outcomes for Chemical Equilibrium

Students will be able too...

  1. Describe the relationship between free energy and equilibrium
  2. Convert ΔG to Q, as well as ΔG° to K and vice versa.
  3. Know the importance of the activity of a species and how it relates to concentration, pressure, and equilibrium.
  4. Write the mass action expression for homogeneous and heterogeneous equilibria.
  5. Determine new values for K when combining multiple reactions.
  6. Determine if a system is at equilibrium and if not which direction the reaction will shift to achieve equilibrium.
  7. Know the difference between Kp and Kc and be able to convert between the two.
  8. Set up and solve a RICE table for a multitude of various reaction types.
  9. Calculate the concentration/pressure of all species at equilibrium.
  10. Show a complete understanding of Le Chatelier's principle.
  11. Predict the response of a reaction to an applied stress (concentration, pressure, volume, temperature) both qualitatively and quantitatively.
  12. Calculate the new value of K when the temperature changes to a new value.

Learning Outcomes for Acid/Base Equilibria

Students will be able too...

  1. Understand the strength of an acid (or base) as determined by the percent of ionization in solution.
  2. Identify strong and weak acids and bases.
  3. Identify acid/base conjugate pairs and their relative strengths.
  4. Understand the process of auto-ionization of water and what is meant by acidic, basic, and neutral.
  5. Know the value of Kw at 25°C, and the relationship between Ka and Kb for a conjugate pair.
  6. Convert between hydronium ion concentration, hydroxide ion concentration, pH and pOH for a given solution.
  7. Determine the pH of a strong acid or base solution.
  8. Determine the pH of a weak acid or weak base solution.
  9. Determine the pH of the solution made from the salt of a weak acid or the salt of a weak base.
  10. Recognize and predict the components of a buffer solution.
  11. Calculate the pH of a buffer solution, and a buffer solution after the addition of strong acid or strong base.
  12. Determine the majority species for acid/base solutions as well as the pH following neutralization.
  13. Interpret a titration curve plot including calculating the concentration and Ka or Kb for the analyte.
  14. Understand the concept of an acid/base indicator, and determine which indicators are appropriate for a given titration.
  15. Determine the protonation state (or overall charge) for a polyprotic species at a particular pH.
  16. Apply concepts from equilibria to acid/base problems