Equilibrium Constant of a Reaction

Interactive Study Guide — Fe³⁺ + SCN⁻ ⇌ FeSCN²⁺

CHM 152 · Lab 5 6 Learning Modules Interactive

What Is Chemical Equilibrium?

In a reversible reaction, equilibrium is reached when the rate of the forward reaction equals the rate of the reverse reaction. At this point, concentrations stop changing — but both reactions are still occurring.

Fe³⁺(aq) + SCN⁻(aq) ⇌ FeSCN²⁺(aq)
iron(III) + thiocyanate ⇌ iron(III) thiocyanate (blood-red)
Key insight: Equilibrium does NOT mean the concentrations are equal! It means they are constant. The ratio of products to reactants is described by KC.

The Equilibrium Constant (KC)

For the reaction above:

KC = [FeSCN²⁺] / ([Fe³⁺] × [SCN⁻])
All concentrations measured at equilibrium
For this reaction, KC ≈ 100–150 at room temperature. Since KC > 1, products are favored — that's why the solution turns red!

Le Chatelier's Principle & Temperature

When a system at equilibrium is disturbed, it shifts to partially counteract the disturbance.

For an Endothermic Reaction (absorbs heat):

Reactants + Heat → Products

↑ Temperature → shifts right → KC increases

For an Exothermic Reaction (releases heat):

Reactants → Products + Heat

↑ Temperature → shifts left → KC decreases

Remember: Only temperature changes KC. Adding more reactant or product shifts the equilibrium but does NOT change KC.

Beer's Law (A = εlc)

Beer's Law states that absorbance is directly proportional to concentration (when path length and molar absorptivity are constant).

A = ε × l × c
A = absorbance  |  ε = molar absorptivity  |  l = path length  |  c = concentration

In this lab, we measure absorbance at 447 nm because FeSCN²⁺ absorbs light most strongly at this wavelength.

Interactive: See Beer's Law in Action

Adjust the concentration slider to see how the solution color and absorbance change.

6.0 × 10⁻⁵ M
Cuvette at 447 nm
A = 0.135
Spec 20 Reading
Notice: As concentration increases, the solution appears darker red and absorbance increases proportionally. This is exactly Beer's Law!

Why Do We Make a Standard Curve?

  1. Prepare known solutions — We use excess Fe³⁺ to ensure all SCN⁻ → FeSCN²⁺, so we know the exact [FeSCN²⁺].
  2. Measure absorbance — Use the Spec 20 at 447 nm for each standard.
  3. Plot the data — [FeSCN²⁺] on the x-axis, Absorbance on the y-axis.
  4. Draw line of best fit — The slope = ε × l (molar absorptivity × path length).
  5. Use it! — For unknown solutions, measure absorbance → read concentration from the graph.

What Is an ICE Table?

ICE stands for Initial, Change, Equilibrium — a systematic way to track concentrations through a reaction.

Fe³⁺SCN⁻FeSCN²⁺
I (Initial)[Fe³⁺]₀[SCN⁻]₀0
C (Change)− x− x+ x
E (Equilibrium)[Fe³⁺]₀ − x[SCN⁻]₀ − xx
In this lab, x = [FeSCN²⁺]eq, which you determine from your standard curve. Once you know x, you can calculate all equilibrium concentrations!

Practice: Fill in the ICE Table

Given: [Fe³⁺]₀ = 1.00 × 10⁻³ M, [SCN⁻]₀ = 2.00 × 10⁻⁴ M, [FeSCN²⁺]eq = 3.50 × 10⁻⁵ M

Enter values in scientific notation (e.g., 9.65e-4)

Fe³⁺ (M)SCN⁻ (M)FeSCN²⁺ (M)
Initial
Change
Equilibrium

Now calculate KC:

Using your equilibrium values above:

KC = [FeSCN²⁺]eq / ([Fe³⁺]eq × [SCN⁻]eq) =  

Lab Procedure Walkthrough

Step through the lab procedure with explanations at each stage.

Step 1: Prepare Standard Solutions

You'll prepare 5 standard solutions in 50 mL volumetric flasks. Each contains:

  • 5.00 mL of 0.2 M Fe(NO₃)₃ — a huge excess of Fe³⁺
  • A measured volume of 2.00 × 10⁻³ M KSCN (varies: 2.00, 3.00, 4.00, 5.00 mL)
  • 0.10 M HNO₃ — filled to the 50.0 mL mark
Why excess Fe³⁺? When Fe³⁺ is in huge excess, the equilibrium shifts far to the right. Nearly ALL the SCN⁻ converts to FeSCN²⁺. This lets us assume [FeSCN²⁺] ≈ [SCN⁻]initial after dilution.

Step 2: Calculate Standard Concentrations

Use the dilution formula to find [FeSCN²⁺] in each standard:

[FeSCN²⁺] = [KSCN]stock × (VKSCN / Vtotal)

Example — Standard C3:

[FeSCN²⁺] = 2.00 × 10⁻³ M × (3.00 mL / 50.0 mL) = 1.20 × 10⁻⁴ M

Step 3: Measure Absorbance (Standard Curve)

Using the Spec 20 spectrophotometer at 447 nm:

  1. Set wavelength to 447 nm
  2. Zero with C1 (the blank — no SCN⁻ added)
  3. Measure absorbance of C2 through C5
  4. Record all values
Tip: Always wipe cuvettes with Kimwipes! Fingerprints scatter light and give false readings.

Step 4: Plot the Standard Curve

Plot your data with:

  • X-axis: [FeSCN²⁺] in M
  • Y-axis: Absorbance

Draw the line of best fit. Determine:

  • Slope — this equals ε × l (molar absorptivity × path length)
  • Y-intercept — should be close to zero for a good curve
  • R² — should be ≥ 0.99

The equation of your line: A = slope × [FeSCN²⁺]

Step 5: Prepare Equilibrium Solutions

Now you use dilute Fe(NO₃)₃ (2.00 × 10⁻³ M) in 10 mL volumetric flasks.

Each test tube has different volumes of Fe³⁺ and SCN⁻ — but same total volume (10 mL).

Critical difference: Unlike the standards, neither reactant is in excess. The reaction reaches TRUE equilibrium — not all SCN⁻ converts to product.

Step 6: Measure and Calculate

  1. Measure absorbance of each equilibrium solution
  2. Use your standard curve to find [FeSCN²⁺]eq
  3. Calculate initial concentrations using dilution (C₁V₁ = C₂V₂)
  4. Find equilibrium concentrations: [X]eq = [X]init − [FeSCN²⁺]eq
  5. Calculate KC for each run

Step 7: Analyze Results

  • All 5 KC values should be similar (within ±20–30%)
  • Calculate the average KC
  • Calculate the standard deviation
  • Compare to literature value (~100–150 at room temperature)
If your values are wildly different, check for: wrong Fe(NO₃)₃ solution, pipetting errors, or misreading the graph.
Step 1 of 7

Key Term Flashcards

Click each card to flip it. Review until you can answer without peeking!

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Knowledge Check — 10 Questions

Score: 0 / 10

1. What does it mean when a system is at chemical equilibrium?

2. In this lab, why do we use a large excess of Fe³⁺ for the standard solutions?

3. What wavelength does the Spec 20 use in this experiment?

4. Beer's Law states that absorbance is directly proportional to:

5. If [Fe³⁺]₀ = 1.00 × 10⁻³ M and [FeSCN²⁺]eq = 5.0 × 10⁻⁵ M, what is [Fe³⁺]eq?

6. For the reaction Fe³⁺ + SCN⁻ ⇌ FeSCN²⁺, the KC expression is:

7. If an endothermic reaction is heated, KC will:

8. Adding more Fe³⁺ to an equilibrium mixture will:

9. When 3.00 mL of 2.00 × 10⁻³ M KSCN is diluted to 50.0 mL, the final [SCN⁻] is:

10. A student's KC value is only 15 instead of the expected ~100. The most likely error is:

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