Lab 4 — Interactive Learning Guide

What Are We Doing?

Acetone reacts with bromine (Br₂) in acid solution. Bromine is orange-brown; the product is colorless. You'll watch the color fade in a spectrophotometer and use the data to figure out how each ingredient affects the reaction speed.

CH₃COCH₃ + Br₂ → CH₃COCH₂Br + HBr

The big question: write a rate law — rate = k[acetone]ⁿ[H⁺]ᵐ[Br₂]ᵖ — and find n, m, and p experimentally.

Week 1 Goal

Build a standard curve (Beer's Law)
Run 3 reactions, collect absorbance data every 15 s
~65 minutes total

Week 2 Goal

Plot [Br₂] vs time in Excel
Extract slopes → find n, m, and k
~45 minutes total

The 3 Runs at a Glance

Run	What changes	[Br₂]₀ (M)	[HCl]₀ (M)	[Acetone]₀ (M)
Run 1 Baseline	—	2.0 × 10⁻³	0.20	0.80
Run 2	[HCl] doubled	2.0 × 10⁻³	0.40	0.80
Run 3	[Acetone] doubled	2.0 × 10⁻³	0.20	1.60

Br₂ is the only reagent you track by color. Acetone and HCl are always in large excess so their concentrations barely change during a run.

Your Progress Through This Guide

Tab completion1 / 7

Work through each tab in order. The quiz tab unlocks the answer key when you submit.

Why Does the Reaction Even Go?

Acetone reacts through a two-step mechanism. The slow step determines the rate — and Br₂ is not in it.

Step 1 — Slow (Rate-Determining)

CH₃COCH₃ + H⁺ ⇌ CH₂=C(OH)CH₃ + H⁺

Acid (H⁺) pulls a hydrogen off acetone to form an enol. This is slow because a C–H bond breaks — that takes energy.

Key point

No Br₂ appears in this step. The rate of the whole reaction is set here, before Br₂ ever shows up.

Step 2 — Fast

CH₂=C(OH)CH₃ + Br₂ → CH₂BrCOCH₃ + HBr

The enol reacts instantly with Br₂. This step is so fast that it doesn't control the pace of the reaction.

∴ Rate = k [acetone][H⁺]

Br₂ does NOT appear in the rate law. The reaction is zero order in Br₂.

What "Zero Order in Br₂" Means for Your Graph

If zero order:

[Br₂] vs time is a straight line (constant slope = −k'). It doesn't matter how much Br₂ you start with — the line goes down at the same rate.

In the lab:

All 3 runs have the same [Br₂]₀, so we can't directly prove zero order from a concentration comparison. Instead, the straight line itself is the proof.

If your plot of [Br₂] vs time is linear (R² > 0.98), the reaction is zero order in Br₂. That's the check.

Interactive — Watch the Mechanism

The slow enolization step controls the clock. Br₂ just waits.

Beer's Law — Your Br₂ Ruler

You can't weigh Br₂ as it disappears. But you can shine light through the solution: the more Br₂ present, the more orange color, the more light absorbed.

A = ε · l · c

A = absorbance (dimensionless, what SPEC 20 reads)
ε = molar absorptivity (L mol⁻¹ cm⁻¹) — fixed for Br₂ at 395 nm
l = path length (1.0 cm in your cuvette)
c = concentration of Br₂ (mol/L) — this is what you want

Bottom line

A is proportional to [Br₂]. Plot A vs [Br₂] for known standards → get a straight line → use it to convert every absorbance reading into a [Br₂] value during your runs.

Interactive — Build Your Standard Curve

Adjust the Br₂ concentration of each standard below. The graph updates live.

Std 1 — [Br₂] (M) 0.0005

Std 2 — [Br₂] (M) 0.0010

Std 3 — [Br₂] (M) 0.0015

Std 4 — [Br₂] (M) 0.0020

Std 5 — [Br₂] (M) 0.0025

How to Use the Standard Curve

Prepare 5 Br₂ standards by diluting 10⁻² M stock: volumes 0.5, 1.0, 1.5, 2.0, 2.5 mL diluted to 5.0 mL each with DI water.

Zero the SPEC 20 with DI water at 395 nm (wavelength dial to 395).

Read absorbance of each standard. Record in your template.

In Week 2, plot A vs [Br₂] in Excel, add trendline, get slope (ε·l) and intercept. Use these to convert all your kinetics absorbance readings to concentration.

What is a Rate Law?

The rate law tells you how the speed of a reaction depends on concentration:

rate = k [acetone]ⁿ [H⁺]ᵐ [Br₂]ᵖ

You don't know n, m, and p ahead of time — you measure them. By changing one ingredient at a time across runs and watching how the slope changes, you can solve for each exponent.

Pseudo-Zero-Order Conditions

Acetone and HCl are in huge excess compared to Br₂. Even after Br₂ is completely consumed, [acetone] and [H⁺] have barely changed. So for a single run:

rate ≈ k' [Br₂]ᵖ where k' = k [acetone]ⁿ [H⁺]ᵐ = constant

This makes the math much simpler: within each run, you're really just studying the [Br₂] dependence.

Interactive — See How Slope Changes with p

Order in Br₂ (p) 0 (zero order)

Finding n and m from Slopes

Because all 3 runs have the same [Br₂]₀, the slope of each [Br₂] vs time plot equals −k'. Comparing k' values between runs isolates the effect of one reactant at a time:

Comparison	Formula	Tells you
Run 2 / Run 1	k'₂/k'₁ = 2ᵐ	[H⁺] doubled → find m
Run 3 / Run 1	k'₃/k'₁ = 2ⁿ	[acetone] doubled → find n

If the ratio = 1, exponent = 0. If ratio ≈ 2, exponent = 1. If ratio ≈ 4, exponent = 2. Round to nearest integer.

Then Get k

k = k' / ([acetone]ⁿ × [H⁺]ᵐ)

Calculate k for each run and average them. Units are M⁻ⁿ⁻ᵐ s⁻¹ — but if n=m=1, units are M⁻¹ s⁻¹.

Week 1 — What to Actually Do

Total time: ~65 minutes. You'll do Part A (standard curve) and Part B (3 kinetics runs).

Part A — Standard Curve (~20 min)

Get ~15 mL of 10⁻² M Br₂ stock in a clean beaker. Keep it in the hood — Br₂ vapor is irritating.

Set SPEC 20 wavelength to 395 nm. Zero with DI water (blank).

Prepare 5 standards in 5.0 mL total volume using DI water as diluent:

Std	mL of 10⁻² M Br₂	mL DI water	[Br₂] (M)
1	0.50	4.50	1.0 × 10⁻³
2	1.00	4.00	2.0 × 10⁻³
3	1.50	3.50	3.0 × 10⁻³
4	2.00	3.00	4.0 × 10⁻³
5	2.50	2.50	5.0 × 10⁻³

Read absorbance for each, lowest to highest concentration. Record immediately.

Part B — 3 Kinetics Runs (~40 min)

Each run uses a 10.0 mL total volume. Prepare the mixture table volumes below. Mix acetone + HCl + water in one beaker; Br₂ in another. Add Br₂ last and start the timer.

Run	mL 10⁻² M Br₂	mL 4.0 M HCl	mL 4.0 M Acetone	mL DI Water
Run 1	2.0	1.0	1.0	6.0
Run 2	2.0	2.0	1.0	5.0
Run 3	2.0	1.0	2.0	5.0

Mix the non-Br₂ components first. Pour into a labeled beaker. Have the SPEC 20 zeroed and ready.

Add the 2.0 mL Br₂. Swirl briefly. Transfer to cuvette. Start timer. First reading at t = 0 s.

Record absorbance every 15 seconds for 300 seconds (5 minutes) = 21 readings per run.

If absorbance drops below 0.02 (solution is essentially colorless), stop — Br₂ is depleted. Note the time.

Clean cuvette thoroughly between runs. Repeat for all 3 runs.

Safety Reminders

Wear gloves and goggles — Br₂ stains skin and is a lung irritant
Work in the fume hood when dispensing Br₂ stock
Dispose of all Br₂ solutions in the designated waste container — not the sink
Acetone is flammable — no open flames nearby

Simulator — Preview Run 1

Click Start to see a simulated Run 1. Absorbance should decline roughly linearly.

t = 0 s

Week 2 — Excel Analysis

You'll convert absorbance readings to concentration, plot [Br₂] vs time, extract slopes, compute k', find n and m, then calculate k. ~45 minutes.

Step 1 — Standard Curve First

Enter your standard curve data (5 standards: [Br₂] in col A, Absorbance in col B).

Select both columns → Insert → Chart → Scatter. Add trendline (linear). Check "Display equation" and "R² value".

Note slope (call it ε·l) and intercept. You'll need: [Br₂] = (A − intercept) / slope

Step 2 — Convert Absorbance → [Br₂]

For each run, make a new worksheet:

Col A: Time (s) — 0, 15, 30, … 300
Col B: Absorbance (raw readings)
Col C: [Br₂] = =(B2 - intercept)/slope — drag down for all rows

Step 3 — Plot [Br₂] vs Time

Select Col A (time) and Col C ([Br₂]) → Scatter plot.

Add trendline → Linear. Check Display equation + R².

The slope = −k' (negative because Br₂ is decreasing). So k' = |slope|.

R² should be ≥ 0.97. If much lower, check your data for outliers (pipetting error, etc.).

Repeat for all 3 runs. Put all 3 trend lines on one chart for comparison (right-click → add data series).

All 3 lines should be parallel (nearly equal slopes) if Br₂ is zero order. The slopes differ only because [acetone] or [H⁺] changes between runs.

Step 4 — Find n, m, and k

Step	Calculation	Expected result
k'₁	\|slope of Run 1\|	~6 × 10⁻⁶ M/s
k'₂	\|slope of Run 2\|	~12 × 10⁻⁶ M/s
k'₃	\|slope of Run 3\|	~12 × 10⁻⁶ M/s
m	log(k'₂/k'₁) / log(2)	≈ 1
n	log(k'₃/k'₁) / log(2)	≈ 1
k	k'₁ / ([ace]₁ × [H⁺]₁)	~3.75 × 10⁻⁵ M⁻¹s⁻¹

Calculate k from each run separately, then average. If one run gives a very different k, recheck your calculations before assuming it's experimental error.

rate = k [acetone][H⁺]

The final rate law — first order each in acetone and H⁺, zero order in Br₂ (p = 0, n = 1, m = 1)

Lab 4 — Rate of a Reaction