What it shows
When baking soda reacts with vinegar, the carbon dioxide escapes and the flask gets lighter. Following the mass on a balance every 30 seconds gives a rate curve, and repeating with diluted vinegar shows that the rate falls as the acid concentration falls, while the total gas depends only on how much acid was present.
The science
Acetic acid reacts with sodium bicarbonate to give sodium acetate, water and carbon dioxide gas. As the gas leaves the flask the mass falls, so the rate of mass loss measures the rate of reaction. Using an excess of baking soda makes the acid the limiting reactant. More concentrated acid has more acid particles per unit volume, so collisions with the bicarbonate happen more often and the mass falls faster at the start. Diluting the acid also halves the total gas produced, because half the acid means half the reaction.
Materials
Cost: under Rs 50. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A digital kitchen or lab balance reading 0.1 g
- A small conical flask or bottle
- Vinegar, 200 ml
- Baking soda, 20 g
- Measuring cylinder and water for dilutions
- A loose cotton wool plug for the flask neck
- A stopwatch and graph paper
Safety
For this project: Vinegar is a dilute acid; avoid eyes and wash hands. Never cap the flask tightly while it fizzes; the cotton plug lets gas out but stops spray.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Prepare four vinegar strengths: undiluted, 1 part vinegar to 1 water, 1 to 3, and 1 to 7, each 50 ml.
Put 5 g of baking soda in the flask, place it on the balance with the cotton plug beside it, and note the mass.
Add 50 ml of undiluted vinegar quickly, put the plug in the neck and start the stopwatch at once.
Record the balance reading every 30 seconds until it stops changing, usually within 5 to 8 minutes.
Rinse and dry the flask, then repeat with each dilution using fresh baking soda.
Plot mass lost against time for all four runs on one graph.
Measure the initial slope of each curve, the mass lost in the first minute, as the initial rate.
Plot initial rate against vinegar concentration and comment on the shape; also compare the final mass lost for each run.
What you should see
The undiluted vinegar curve is steepest and loses the most mass, each dilution gives a shallower curve and a smaller total loss, and initial rate rises roughly in proportion to concentration.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why does the mass fall?
Carbon dioxide gas escapes from the flask.
2Why use excess baking soda?
So the acid is the limiting reactant and the gas produced depends only on the vinegar.
3Why does concentrated vinegar react faster?
There are more acid particles per unit volume, so collisions with bicarbonate are more frequent.
4What is the purpose of the cotton plug?
It lets gas out while stopping spray droplets from leaving and spoiling the mass reading.
Take it further
Keep the concentration fixed and run the reaction at three temperatures to compare the effect of temperature with that of concentration.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
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