The goal of this project is to measure the effect of reactant particle size
on the rate of a chemical reaction.
You may have seen a television commercial for Alka-Seltzer tablets, or heard
one of their advertising slogans: "Plop, plop, fizz, fizz, oh what a relief
it is!®" When you drop the tablets in water, they make a lot of bubbles,
like an extra-fizzy soda. And like a soda, the bubbles are carbon dioxide gas
(CO2). However, with Alka-Seltzer®, the CO2 is produced
by a chemical reaction that occurs when the tablets dissolve in water.
The main ingredients of Alka-Seltzer tablets are aspirin, citric acid, and
sodium bicarbonate (NaHCO3). When sodium bicarbonate dissolves in
water, it dissociates (splits apart) into sodium (Na+) and
bicarbonate (HCO3−) ions. The bicarbonate reacts
with hydrogen ions (H+) from the citric acid to form carbon dioxide
and water. The reaction is described by the following chemical equation:
So how does particle size come into this? In order for the reaction shown
above to take place, the ingredients in the tablet first have to dissolve. The
table has a large surface area, so this step should be pretty fast, right? What
effect do you think particle size will have on the speed of the
bicarbonate reaction? You can find out for yourself by plopping prepared
Alka-Seltzer® tablets (whole tablets, halved tablets, quartered tablets, and
powdered tablets) into water at the same temperature, and timing how long it
takes for the chemical reaction to go to completion.
Terms, Concepts and Questions to Start Background
To do this project, you should do research that enables you to understand
the following terms and concepts:
- Reaction rate
- Do you think changing the particle size will have a measurable effect on
the chemical reaction rate?
- Will smaller particles speed up or slow down the reaction?