How size determines rate

Summary

Key Concepts

Reaction rate, surface area

Credits

Svenja Lohner, PhD, Science Buddies

A whole Alka-seltzer tablet, a second cut in half, a third cut in quarters, and a fourth ground into a powder.

Introduction

Did you know that flour can explode? Luckily, this does not happen spontaneously on your kitchen counter, but only if the conditions are right. The trick is that you need a very fine powder of flour to make an explosion happen. In fact, any solid flammable material that is dispersed in the air as a dust cloud will explode once it is ignited. Why is that? It has to do with the particle size of the solid material, which determines how rapidly a chemical reaction takes place. In this activity, you can test this for yourself! While you will not create a dangerous explosion, there will be a lot of bubbles and plenty of fizzy action.

This activity is not recommended for use as a science fair project. Good science fair projects have a stronger focus on controlling variables, taking accurate measurements, and analyzing data. To find a science fair project that is just right for you, browse our library of over 1,200 Science Fair Project Ideas or use the Topic Selection Wizard to get a personalized project recommendation.

Background

A chemical reaction normally involves liquids, gases or solids. Some reactions happen very fast, whereas others seem to take ages. In heterogeneous chemical reactions, which means whenever a solid is one of the reactants, you can actually change the reaction rate by varying the size of the solid that reacts with the liquid or the gas. How does this work? For a chemical reaction to happen, the molecules or atoms of all the reactants need to collide with each other. This can only happen at the surface of the solid, as all the molecules trapped within the body of the solid cannot react. However, if you take the same material and break it into smaller pieces, there is much more surface area exposed that can interact with the other reactants.

Take a flammable material (such as flour) as an example: If you heat up a lump of flour, it will not burn at all or start burning away slowly in a controlled and harmless manner because there is only a limited surface area available that can react with the oxygen in the air. However, if you take the same flammable material and disperse it in the air as a fine powder, the surface area exposed to the air is much greater, and after ignition, the reaction takes place rapidly and results in an explosion. Increasing the surface area of a reactant does not only increase the quantity of material available to react, but will also increase the rate of the reaction as well. In this activity, you will demonstrate this effect by measuring the rate of a different kind of chemical reaction: the dissolution of sodium bicarbonate from an effervescent antacid tablet in water.

Materials

Effervescent antacid tablets (at least 4)
Sheet of paper
Hammer or metal spoon
4 clear 12 ounce (or larger) drinking glasses
Measuring cup
Teaspoon
Tap water
Stopwatch
Pen

Preparation

Take four antacid tablets out of their packages. Take care not to break them, as the tablets are very brittle.
Put one whole antacid tablet aside for now.
Take the second tablet and break it into half and set both halves aside.
Take the third tablet and break it evenly into quarters. First, break it into halves and then break the halves again into two parts. Set all four pieces aside.
Take the fourth tablet and ground it into a powder. To do this, put the tablet to be ground inside a clean, folded piece of paper. Place the folded paper on a solid surface and use a hammer or metal spoon to carefully crush the tablet into a powder. Keep the powder folded into the paper and set it aside.
Use a measuring cup to add about 250 mL (about 8 ounces) of tap water to each of the four glasses. The temperature of the water should be the same in each.

Instructions

Take one of the glasses with tap water and the whole antacid tablet and put them in front of you.
Pick up the whole tablet and hold it above the water surface.
Get your stopwatch ready.
Drop the whole tablet into the water and at the same time start the stopwatch. What happens once the tablet hits the water? Can you see a chemical reaction happening?
Stir the water gently and steadily with the teaspoon. Observe the tablet closely in the water. What do you notice about the tablet? What reaction do you think is taking place?
Once all the solid material of the tablet has dissolved in the water and the chemical reaction is completed, stop the stopwatch and write down the reaction time on a sheet of paper. How long did the reaction take? Do you think this reaction is fast or slow?
Get a fresh glass of water and this time take the antacid tablet that you broke in half.
Take both pieces of the tablet and hold them above the water surface. What do you think will change once you put the two pieces of the tablet into the water compared to the whole tablet?
Reset your stopwatch and get it ready.
Drop both pieces of the tablet into the water and start the timer again. Compared to the whole tablet, do you see the same reaction happening in the water?
Again, stir the water gently and observe how the two tablet pieces dissolve in the water. Do you see more or fewer bubbles forming? Do you think this reaction will be complete faster or more slowly than with the whole tablet?
Once all the solid tablet material has completely disappeared and the bubbles have stopped forming, stop the stopwatch and record the reaction time. Did the reaction time change compared to the whole tablet? Was this reaction faster or slower? Why do you think this is the case?
With the two remaining glasses, repeat steps 7 to 12 with the antacid tablet that you broke into four pieces and the tablet that you crushed into a powder. Do you observe any changes in the chemical reaction happening in the water? How fast or slow are these tablets dissolving compared to the other tablets? Do you notice any correlation between the reaction time and the size of the tablet pieces?

Extra: Can you think of other chemical reactions that you could use to test how the surface area of one of the reactants affects the reaction rate? Think of other ingredients in your kitchen that come in various sizes and forms, such as sugar crystals, cubes or powder. Will the same effect be observable for these substances?

Extra: What other factors can change the rate of a chemical reaction? Repeat this experiment, but only use whole antacid tablets, and this time, vary the temperature of the water in which you dissolve the tablets. How do you think the temperature will influence the reaction rate? Will the tablet dissolve faster or more slowly in hot water compared to cold?

Observations and Results

Did you find that the tablet powder dissolved much faster than the whole tablet? What you probably observed in all of your experiments was some vigorous bubbling once you dropped the antacid tablet into the water. Effervescent antacid tablets are made from aspirin, citric acid and sodium bicarbonate. When sodium bicarbonate dissolves in water, it reacts with hydrogen ions from the citric acid and forms carbon dioxide. Because carbon dioxide is a gas, it forms bubbles inside the water that you can see as foam on the surface.

The fizzing and bubbling was probably more pronounced the smaller the tablet pieces were that you dropped into the water. At the same time, you probably noticed that the whole tablet took the longest to dissolve, whereas the tablet powder dissolved really quickly. This is because with smaller tablet pieces, there is more surface area of the tablet available that can react with the water, which results in a faster disintegration of the antacid tablet, as you observed.