Chemical Equilibrium and the Equilibrium Constant Lab 3

Purpose Students will determine the equilibrium constant of [Fe(SCN)] 2+ complex by reacting Fe 3+ 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 Fe 3+, 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 Fe 3+ (aq) + SCN - (aq)  [Fe(SCN)] 2+ (aq) iron thiocyanate bright orange complex thenK = = From [[Fe(SCN)] 2+ ] eq and [Fe 3+ ] 0 and [SCN - ] 0 we can calculate [Fe 3+ ] eq and [SCN - ] eq and K.

Calculations [ ] 0, M[ ] change, M[ ] eq, M [Fe 3+ ]1.89 x x(1.89 x ) – x [SCN - ]2.17 x x(2.17 x ) – x [[Fe(SCN)] 2+ ]0+ x0 + 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 -1 cm -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 Light Source Wavelength Selector SampleDetector I0I0 I b Incident Light Transmitted Light 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!

Equipment Setup

Safety Concerns Reagents: Ferric Nitrate (Fe(NO 3 ) 3 HNO 3 (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.