Experiment 12: Reaction Rates — The Fizzing Tablet Race

The tablet hits the water and the race is on: a storm of bubbles, a swirling cloud, and then — gone. Effervescent tablets are a chemical reaction with a built-in stopwatch, because the fizz *is* the reaction happening. This experiment times it under four conditions and pulls apart the two great rate levers you first met in the dissolving race: temperature and surface area.

The chemistry behind it

Inside the dry tablet, citric acid and sodium bicarbonate sit harmlessly side by side. Dissolved in water they react at last:

citric acid + sodium bicarbonate -> sodium citrate + water + carbon dioxide

— the same acid + bicarbonate → CO₂ chemistry as Experiment 1, with a different acid. The bubbles are CO₂ escaping. Chemists explain the rate with collision theory: reaction happens when particles collide with enough energy to clear a barrier called the activation energy. Heat makes collisions faster, harder and more frequent. Crushing exposes more of the tablet's interior, so water meets far more reactant surface at once. To compare rates fairly when every run starts with the same tablet mass, use rate ∝ 1/time: a tablet gone in 30 s reacted twice as fast as one gone in 60 s.

Variables. Independent variables (one at a time): water temperature (cold vs warm) and tablet form (whole vs crushed). Dependent variable: time until fizzing stops and the tablet has disappeared. Controlled variables: same brand and mass of tablet, same water volume, same cup, the same endpoint rule.

Safety first

• An adult handles the warm water. Warm, never boiling — and not only for safety: boiling would also wreck the comparison.
• Do not taste the tablets or the solution, even though they are medicines — dose and hygiene are not controlled here.
• Never seal a fizzing tablet inside a closed bottle or jar. CO₂ pressure builds fast and can burst the container. Fizzing always happens in an open cup.

Doing the investigation

Hands-on. Fill two cups with cold water and two with warm, all with equal volumes. Drop a whole tablet into a cold cup and time it to the endpoint; repeat in warm. Crush a tablet inside its wrapper with a spoon and repeat both temperatures. Run every condition twice — the pair of trials is your scatter check.

In the virtual lab below (Reaction Rates — The Fizzing Tablet Race) set the Water temperature, Tablet form (Whole / Half tablet / Crushed) and Water volume, press Drop tablet, and read the time to dissolve, the reaction rate for the 3.2 g tablet, and the fizz intensity. Follow its guided procedure: whole at 25 °C, crushed at 25 °C, whole at 60 °C — then change the water volume to 100 mL and see whether volume alone moves the time. Log the fastest combination in your observation log.

Worked investigation: separating the two factors

One student's timings (two trials per condition):

Step-by-step processing:

Temperature factor (whole):   120 / 45 = 2.7x faster when warm
Crushing factor (cold):       120 / 48 = 2.5x faster when crushed
Both together, predicted:     2.7 x 2.5 = 6.7x
Both together, measured:      120 / 18 = 6.7x

Conclusion. The two factors are independent multipliers: warming multiplied the rate by about 2.7 and crushing by about 2.5, and using both gave almost exactly the product, 6.7×. That multiplicative behaviour is the signature of two separate mechanisms — collision energy (temperature) and collision opportunity (surface area) — each scaling the rate on its own. Of the pair, here temperature was the slightly stronger lever; the virtual lab's chart shows its lead growing as temperature climbs toward 60 °C.

Uncertainty. Timing carries ±1 s of human reaction error — trivial on 120 s (±0.8%) but ±5.6% on the 18 s warm-crushed run, which is exactly why that condition most needs its repeat trials. Trial pairs agreed within 2–4 s everywhere (about ±3%), the honest scatter to quote.

Evaluation: how good is this evidence?

• Crushing is not standardised. "Crushed with a spoon" gives different particle sizes each time, and particle size *is* the variable. Improvement: crush in the wrapper with a fixed routine (e.g. 10 firm presses of a rolling pin) or sieve the powder and use the same fraction.
• Warm water cools during the run — by several degrees over 45 s, more for a slow run. Improvement: pre-warm the cup, use a larger water volume as a heat reservoir, record start and end temperatures.
• The endpoint is a judgement. "Tablet gone" vs "fizzing stopped" can differ by seconds as trapped fragments dissolve silently. Improvement: pick one definition — no visible solid — and use it for every run.
• Tablet masses vary slightly between tablets (±0.1 g on 3.2 g ≈ ±3%). Improvement: weigh each tablet and divide rate by its actual mass.

Common mistakes

• Testing crushed-in-warm against whole-in-cold and announcing a winner — two variables changed, nothing isolated.
• Dropping the tablet before the water has settled at its temperature, or starting the watch late.
• Crushing the tablet *into* the water (it starts reacting mid-pour, unmeasured).
• Assuming the factors add (2.7 + 2.5 = 5.2×) rather than multiply — check against the data.

Exam Experience

Question 1 — analysis [4 marks]. At 30 °C a whole tablet takes 78 s and a crushed tablet 31 s. A whole tablet at 50 °C takes 40 s. (a) Calculate the crushing factor and the temperature factor. (b) Predict the time for a crushed tablet at 50 °C, showing your reasoning.

Scoring guide:

• Crushing factor: 78 ÷ 31 ≈ 2.5 [1]
• Temperature factor: 78 ÷ 40 ≈ 1.95 ≈ 2.0 [1]
• Combined factor ≈ 2.5 × 1.95 ≈ 4.9 [1]
• Predicted time: 78 ÷ 4.9 ≈ 16 s [1]

Question 2 — design [4 marks]. The virtual lab lets you change water volume as well as temperature and tablet form. Design an investigation to find out whether water volume affects the rate, keeping the test fair.

Scoring guide:

• Independent variable: water volume (e.g. 100, 250, 500 mL); temperature and tablet form fixed [1]
• Same tablet brand/mass, same cup shape as far as possible, same endpoint rule [1]
• At least two trials per volume, means compared against the trial scatter [1]
• A mechanism-based prediction stated (e.g. little effect while the tablet stays covered, slower if the solution around the tablet saturates with products) [1]

Multiple choice 1. The fizz produced by an effervescent tablet in water is:

• A. Oxygen from the air dissolving
• B. Carbon dioxide from the acid + bicarbonate reaction
• C. Water vapour from the reaction's heat
• D. Hydrogen from the citric acid

Answer: B. Citric acid + sodium bicarbonate → CO₂ — the same gas, and the same reaction family, as vinegar + baking soda in Experiment 1.

Multiple choice 2. Heating speeds the reaction up mainly because:

• A. Hot water dissolves the tablet's coating faster, and that is all
• B. Particles move faster, colliding more often and with more energy, so more collisions clear the activation-energy barrier
• C. Heat creates extra reactant inside the tablet
• D. Bubbles rise faster in hot water, making it *look* faster

Answer: B. Collision theory: temperature raises collision frequency a little and the fraction of collisions energetic enough to react a lot.

Going deeper: the two levers run the world

Your tablet race used the two mildest rate levers in chemistry, but industry and safety engineering pull them much harder. Food spoilage is a bundle of slow reactions and microbial growth with a strong temperature factor — the fridge's roughly 2× slowdown per 10 °C (the Q₁₀ rule of thumb) is the same staircase you spotted in Experiment 6's dissolving times, which is why a fridge at 4 °C buys days that a 24 °C counter cannot. Powdered medicines and vitamin tablets are the crushing lever as therapy: grinding a dose multiplies its surface area so it dissolves and absorbs in minutes rather than an hour.

The dark side of surface area is the dust explosion. A lump of flour will not burn in a candle flame; the same flour dispersed as a dust cloud in air has such an enormous total surface that oxidation runs through the cloud in milliseconds — grain silos and custard factories have been destroyed by exactly your "crushed tablet" effect, uncontained. Safety rules in those industries (no dust build-up, no ignition sources, venting panels) are collision theory written as law. And the catalytic converter in a car exhaust is both levers at once: a honeycomb with the surface area of a football field, wash-coated with catalyst, that must reach ~250 °C ("light-off") before its collisions are energetic enough to clean the exhaust — which is why short cold trips pollute most. Temperature and surface area: two levers, everywhere.

A note on the half-tablet option

The virtual lab's third form — the half tablet — teaches a distinction this lesson depends on: finishing time is not rate. A half tablet (1.6 g) disappears sooner than a whole one mainly because there is less of it; its *rate in grams per second* at the same temperature is close to the whole tablet's, since the dissolving surface chemistry is unchanged. That is why the worked investigation converts times into g/s before comparing — and why pharmacists test real medicines in standardised dissolution apparatus (a paddle stirring at a fixed speed in a fixed volume at 37 °C), where a tablet must release a stated percentage of its dose within a stated time. Your cups, stopwatch and 1/time arithmetic are the same quality-control idea, run on a kitchen budget.