Objective

The goal of this project is to measure the effect of reactant particle size on the rate of a chemical reaction.

Introduction

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 Research

To do this project, you should do research that enables you to understand the following terms and concepts:

• Molecules
• Temperature
• Reactants
• Products
• Reaction rate

Questions

• 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?