# Chemical Equilibrium and the Equilibrium Constant

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Chemical Equilibrium and the Equilibrium Constant
Lab 3

Outline Purpose Chemical Equilibrium Reaction Calculations Beer’s Law
Spectrophotometers Buret Reminders Safety Concerns Precautions Next Lab Reminder Did you…

Purpose Students will determine the equilibrium constant of [Fe(SCN)]2+ complex by reacting Fe3+ and SCN-. Students will become experienced using a spectrophotometer and making solutions of the required concentrations. The MicroLab spreadsheet will be used to calculate the initial and equilibrium concentrations of Fe3+, SCN-, and [Fe(SCN)]2+. Once the data is entered into the spreadsheet, the calculation of the equilibrium constant can be completed.

Chemical equilibrium Not all reagents are always converted to product.
In most cases, dynamic equilibrium is established once most of the limiting reagent has been used. At equilibrium, the rates of the forward and reverse reactions are equal.

Dynamic Equilibrium Initially [R] ↔ [P] forward rxn = fast
reverse rxn = slow Later [R] ↔ [P] forward rxn = slow reverse rxn = fast Equilibrium [R] ↔ [P] rates are equal

Equilibrium Conditions
[R]eq and [P]eq are related through K aA + bB  cC + dD K =

Equilibrium Conditions
K tells us if the equilibrium favors the products or reagents. If K >> 1, the equilibrium favors the products If K << 1, the equilibrium favors the reagents This follows Le Chatelier’s Principle that states that the system opposes applied change to reduce the effect of the change.

Reaction Fe3+(aq) + SCN-(aq)  [Fe(SCN)]2+(aq) iron thiocyanate bright orange complex then K = = From [[Fe(SCN)]2+]eq and [Fe3+]0 and [SCN-]0 we can calculate [Fe3+]eq and [SCN-]eq and K.

Calculations Change in x because of 1:1 ratio of reagents.
[ ]0, M [ ] change, M [ ]eq, M [Fe3+] 1.89 x 10-3 - x (1.89 x 10-3) – x [SCN-] 2.17 x 10-4 (2.17 x 10-4) – x [[Fe(SCN)]2+] + x 0 + x Change in x because of 1:1 ratio of reagents. We obtain x from spectrophotometric measurements and Beer’s Law. By measuring A, we measure the absorbance of the bright orange complex [Fe(SCN)]2+.

Beer’s Law A = ε b c; where: A= absorbance (no units)
ε = molar absorptivity (M-1cm-1) b = pathlength (cm) c = concentration (M) To solve for the concentration of the complex in each sample, we rearrange Beer’s Law: [[Fe(SCN)]2+]eq = c = x =

Spectrophotometry Transmitted Light Incident Light I0 I Light Source
Wavelength Selector Sample Detector b b = mm or cm

Spectrophotometers Spectrophotometers use the following equation to determine %Transmittance: We can convert from %Trans to Absorbance using the following equation:

Blank Solution A blank solution is used before the first sample is inserted into the spectrophotometer. The blank solution is most always the solvent of your solutions. The solvent may contain species that can absorb light at the same wavelength as the analytical wavelength for your analyte. The spectrophotometer is zeroed out with the blank solution (Abs = 0 or %Trans = 100%) The blank solution therefore corrects for the matrix effects of the solvent.

Spectrophotometer function
Your instructor will remind you of how the MicroLAB™ spectrophotometers work. NEVER pour solution directly into the sample chamber. Samples are poured into a cuvet first. The cuvet is inserted into the sample chamber. Use a KimWipe to wipe your fingerprints off the cuvet before insertion into the sample chamber. Ask your instructor if you need help!

Buret Reminders Disassemble and wash your buret and stopcock before AND after you use your buret. Rinse the buret with the solution you want to fill it with BEFORE you fill it with the solution. Hang the buret using a buret clamp. Release all air from the buret tip before making your initial volume reading. Determine the volume dispensed “by difference.” The smallest graduation on the buret is 0.1 mL. To what level of precision can you therefore report your buret volume readings?

Safety Concerns Reagents: Eye Contact: Skin Contact: Inhalation:
Ferric Nitrate (Fe(NO3)3 HNO3 (1.0N) Potassium Thiocyanate (0.1N) Sulfamic Acid Eye Contact: Blurry vision. Severe irritation, redness, pain, burns, conjunctivitis and permanent corneal damage. Skin Contact: Severe irritation, burns, redness, pain, stains and ulcers. Inhalation: Destructive to mucosa and upper respiratory tract. May cause burning, coughing, choking, wheezing, laryngitis, shortness of breath, headache, nausea, vomiting, methemoglobinemia, cyanosis, convulsions, tachycardia, dyspnea, pneumonia, pulmonary edema, asphyxia, chemical pneumonitis and death. Ingestion: Pain and burns of the mouth, throat, esophagus and gastrointestinal tract. Gastrointestinal irritation with nausea, vomiting, diarrhea, methemoglobinemia, cyanosis, convulsions, systemic toxic effects on the heart, liver, and kidneys and death.

Precautions Conserve chemicals and distilled water.
If you spill, clean it up. All solutions in this experiment are acidic. Dispose of all excess and waste solutions in provided acid waste containers in the fume hood(s). Please DO NOT unplug the hotplates!

Lab 4 Reminder Read the required reading sections in your textbook as you prepare for the next experiment. Read the write-up for Lab 4 and complete and submit the pre-lab questions. Study for the quiz. Submit your Lab 3 Report at the start of next week’s lab.

Did you… Disassemble the stopcock of your burets to clean the individual pieces? Clean up your station and place glassware and CLAMPS back in their appropriate drawers? Wipe down your counter?