Lab · Rate Law by the Iodine Clock · Unit 5 · AP Chemistry

Some reactions take hours; some are over before you blink. How fast a reaction goes depends on the concentration of its reactants, and the rate law tells you exactly how. Today you measure that dependence directly by timing a colour change: the famous iodine ("Landolt") clock. You vary one reactant at a time, time how long until the flask flashes blue, and from those times deduce the order in each reactant and the rate constant k.

CED 5.1–5.2 · SP 5 (Mathematical Routines) & SP 6 (Argumentation) · College Board Recommended Lab #12 (rate & order of a reaction) · 1 block

Background CED 5.1–5.2

The reaction you study is iodate ion oxidising bisulphite ion in acid:

Clock reaction: IO₃⁻(aq) + 3 HSO₃⁻(aq) → I⁻(aq) + 3 SO₄²⁻(aq) + 3 H⁺(aq)

This first step is slow and produces iodide, I⁻. Bisulphite is the limiting reactant. As long as any bisulphite remains, it instantly destroys any iodine that forms, so nothing happens visibly. The instant the bisulphite is used up, the leftover iodate reacts with the iodide to make molecular iodine, which clamps onto the starch already dissolved in the mixture and the whole flask snaps dark blue:

The "clock": 5 I⁻ + IO₃⁻ + 6 H⁺ → 3 I₂ + 3 H₂O then I₂ + starch → blue complex

Why timing works: the blue flash always marks the same small amount of reaction (all the bisulphite consumed). So the time t to turn blue is a stopwatch on the reaction rate: a faster reaction → shorter t. We use rate ∝ 1/t as our measured initial rate.

Rate law: we expect rate = k[IO₃⁻]ᵐ[HSO₃⁻]ⁿ. The method of initial rates finds the exponents (orders) m and n by changing one concentration while holding the other fixed and watching how 1/t responds.

Safety read before you start

Goggles on the whole time; aprons on. Two solutions are mildly acidic and the bisulphite can release a little sulphur-dioxide odour.

Sodium bisulphite (NaHSO₃): irritant; may release small amounts of SO₂ gas. Work in a well-ventilated room; tell Dr. Mulinta if you have asthma and you will be seated away from the prep area.
Potassium iodate (KIO₃): oxidiser and irritant; do not let it contact skin, eyes, or combustible material. Acidified solution is a mild irritant.
No food or drink. The blue product is not toxic in these amounts but nothing here goes near your mouth.
Spills: wipe small spills with a damp paper towel and rinse with water; report anything larger. Wash hands when finished.
Disposal: all mixtures go down the drain with plenty of running water (dilute, non-hazardous) unless told otherwise.

PPE: splash goggles, apron, closed-toe shoes. Long hair tied back.

Materials & Equipment low-gear · everyday glassware

Lab tech Kathy will pre-set two labelled stock bottles per bench: Solution A = 0.020 M KIO₃ · Solution B = 0.0020 M NaHSO₃ with starch indicator already dissolved. She will also pour the acid into the bisulphite stock so you never handle concentrated acid.

10 small test tubes in a rack (or 10 small clear cups), labelled
two 10-mL graduated cylinders (one for A, one for B), do not mix them up
one small graduated cylinder or plastic dropper-pipette for adding distilled water
wash bottle of distilled water
stopwatch or phone timer (one per pair)
white paper or a white tile to set tubes on (so the blue is easy to see)
fine-tip marker for labelling; paper towel

No water bath, no spectrophotometer, no hot plate needed, this whole lab runs at room temperature with plastic cups and a phone timer if glassware is short.

Procedure work in pairs · keep every time

One person mixes, one person times and records. Start the timer the instant the solutions touch; stop it at the first hint of blue. Record to 0.1 s.

Part 1: vary the iodate (Solution A)

Label five test tubes A1–A5. Using the A graduated cylinder, measure Solution A into them: 10.0, 8.0, 6.0, 4.0, 2.0 mL respectively.
To each tube add distilled water to bring the total volume to exactly 10.0 mL (so add 0, 2.0, 4.0, 6.0, 8.0 mL of water). Now every A-tube holds 10.0 mL but a different amount of iodate.
Label five more tubes B1–B5 and, using the B cylinder, put 10.0 mL of Solution B into each.
Take tube A1 and tube B1. Pour A1 into B1, then pour back and forth between the two tubes 3 times to mix fully. Start timing the moment they first meet.
Stop timing at the first sign of blue. Record the time in your Part 1 table. Rinse the tubes.
Repeat steps 4–5 for A2+B2, A3+B3, A4+B4, A5+B5.

Part 2: vary the bisulphite (Solution B)

Now do the mirror image. Label five tubes B1–B5 and measure Solution B: 10.0, 8.0, 6.0, 4.0, 2.0 mL; add distilled water to bring each to 10.0 mL.
Label five tubes A1–A5 and put 10.0 mL of Solution A in each.
Mix A1+B1 (back and forth 3 times), timing from first contact to first blue. Record. Repeat for the remaining four pairs.

Tip: view each tube against the white paper and watch from the side, the blue appears suddenly. If a run is much slower or faster than its neighbours, re-run it.

Data Tables record as you go · units & sig figs

In each part, total mixed volume is 20.0 mL (10.0 mL of the diluted reactant + 10.0 mL of the other). Compute the concentration in the mixed flask later, in Analysis Q1.

Part 1: [HSO₃⁻] held constant, [IO₃⁻] varied

Run	mL Sol. A (0.020 M)	mL water	mL Sol. B (0.0020 M)
A1	10.0	0	10.0
A2	8.0	2.0	10.0
A3	6.0	4.0	10.0
A4	4.0	6.0	10.0
A5	2.0	8.0	10.0

Part 2: [IO₃⁻] held constant, [HSO₃⁻] varied

Run	mL Sol. B (0.0020 M)	mL water	mL Sol. A (0.020 M)
B1	10.0	0	10.0
B2	8.0	2.0	10.0
B3	6.0	4.0	10.0
B4	4.0	6.0	10.0
B5	2.0	8.0	10.0

Analysis Questions show every calculation · units & sig figs

Concentrations in the flask. For each run, find the initial [IO₃⁻] and [HSO₃⁻] after mixing. Remember two dilutions happen: first the reactant is diluted to 10.0 mL, then the two 10.0-mL portions combine into 20.0 mL total. Show your formula C₁V₁ = C₂V₂ once, then fill a column for each table. (Check: Run A1 should give [IO₃⁻]₀ = 0.0100 M and [HSO₃⁻]₀ = 0.00100 M.)
Order in iodate (m). Using Part 1, pick two runs where [IO₃⁻] differs by a clean factor (e.g. A1 vs A5, a factor of 5). Treat 1/t as the rate. Set up (rate₁)/(rate₂) = ([IO₃⁻]₁/[IO₃⁻]₂)ᵐ and solve for m. Round to the nearest whole number.
Order in bisulphite (n). Repeat with Part 2 (e.g. B1 vs B5) to find n.
Write the rate law using your rounded m and n, and state the overall order.
Graphical check. For Part 1, plot 1/t (y) versus [IO₃⁻]₀ (x). For a first-order dependence the points fall on a straight line through the origin. Does your graph agree with your answer to Q2? What would a curve that bends upward tell you about the order?
Rate constant k. Using the rate law and one run (say A1, with rate = 1/t), solve k = rate / ([IO₃⁻]ᵐ[HSO₃⁻]ⁿ). State the units of kderive them from the overall order. Then compute k for two more runs and report the average.
Collision reasoning. In one or two sentences, explain in terms of particle collisions why lowering a reactant's concentration makes the flask take longer to turn blue.
Error. Identify one source of timing error and state whether it would tend to make your measured t too long or too short, and how that would shift your k.
Error analysis, reads too HIGH. Identify one source of error that would make your measured rate (and therefore your rate constant k) come out too high. Trace the direction through rate ≡ 1/t and k = rate/([IO₃⁻]ᵐ[HSO₃⁻]ⁿ).
Error analysis, reads too LOW. Now identify one source of error that would make your measured rate (and k) come out too low, and trace its direction through the same two relationships.