5B (Lab Activity)
Chemical Reactions and How You Know When
You’ve Made Something New
When you make a compound in the lab, like Percy Julian and Josef Pikl did, you have starting materials, you do something to those materials, and then you end up with something else. That may be a little oversimplified, but that’s basically how it works. Chemically speaking, what you start with are called reactants, what happens to those reactants is called a chemical reaction, and what you end up with are called products. To make physostigmine, Julian and Pikl started with some reactants, they brought these reactants together in a way that caused a chemical reaction, and they ended up with physostigmine as their product.
Another way to think about chemical reactions is in terms of the molecules of the reactants and products. Since molecules are made of atoms, we can take molecules apart and put their atoms together in new ways to make new molecules. Anytime we do this, we call it a chemical reaction. So, in a chemical reaction, “old” molecules (the molecules of the reactants) are broken apart and their atoms are put back together in different ways to make “new” molecules (the molecules of the products). It’s really not that different from taking apart your LEGO space cruiser and then using the same LEGO bricks to build a racecar.
How do you know if you’ve made something different from what you started with? That is the point, after all, isn’t it? You know your product is different from your reactants if it behaves differently from the reactants. Different molecules with different molecular structures will behave differently and have different properties. There are a number of tests that you can perform on your product to show for sure that it is different from what you started with. This activity will show you some of them.
Baking Soda + Hydrochloric Acid
The “old” molecules you’ll use to build your new molecules will be those in baking soda (a solid) and hydrochloric acid (a liquid). Hydrochloric acid is found in the fluid of your stomach. You have probably tasted it on occasion when your stomach fluid decides to go up to your mouth (yuck!).
After the acid and baking soda react, you will have to make sure you have something new, not the original molecules or chemicals you started with. The chemical reaction will be very apparent, as will at least one product, a gas. You will be expected to produce evidence that you have, in fact, produced new chemicals that are different from what you started with.
To show the formation of new substances and how chemists, such as Percy Julian, determine that new substances have been formed in the chemical reaction.
- Wear safety goggles at all times.
- Follow instructions and teacher guidelines for handling chemicals, particularly acids. If you spill any acid, report immediately to your teacher.
- Do not touch any chemicals with your bare hands. Always use scoops for transferring chemicals.
- Do not put chemicals back into their original containers, if you have any chemicals remaining.
- Be careful around hot surfaces.
Part I: The Chemical Reaction (Baking Soda + Hydrochloric Acid)
- Obtain a clean, dry ceramic dish or jar. Weigh the container and record the mass on the Data and Calculations Sheet.
- Keeping the container on the balance, measure out 2.5 grams of baking soda (sodium bicarbonate) into the container. (The sodium bicarbonate has the formula NaHCO3, which your teacher can explain later.)
- Place the container with the baking soda on your worktable. Obtain 5.0 mL of hydrochloric acid (HClthe stuff found in your stomach, but don’t go there for it!) from your teacher. It should be in a glass test tube. Caution: Handle this acid carefully. It can cause harmful burns if it touches your skin.
- Using a dropper, slowly add drops of the hydrochloric acid to your baking soda. It should bubble. Continue adding the acid until all bubbling stops. Stir the mixture with a stick or glass rod to make sure all of the baking soda has reacted. What is responsible for the bubbling?
- Take your container with all the chemicals in it to a spot designated by your teacher, where you will allow all the liquid in the container to evaporate overnight. The next day, when all liquid has evaporated, you should have a solid product. Weigh the dry container with solid product and record the mass on the Data and Calculations Sheet.
Part II: Testing Your Product to Show That You’ve Made Something New
Procedure and Discussion Questions
- The baking soda reacted with acid by bubbling. Now obtain some more acid and add a few drops to a SMALL sample of the solid product. Does it behave the same as the baking soda?
- Obtain a small amount of baking soda and add it to a test tube. Add water, drop by drop, counting the drops. Shake occasionally. Continue adding water (and counting) until the baking soda appears to have dissolved. Make a note of the number of drops you added. Now, obtain a similar amount of your solid product from the dish or jar, place it in a test tube, and add water drop by drop, counting as you go. Continue adding drops of water until you have added as much water as you added to the baking soda. Has all of the product dissolved? Do you think the product is the same as the original reactant, baking soda? Why or why not?
- Compare the mass of baking soda you started with and the mass of solid chemical you produced. Are they the same? Should they be? Are they different? Should they be? Discuss these questions with your partners.
- What happened to the liquid? What happened to the gas? Do they have mass? Explain.
- (Optional) Obtain a bottle of silver nitrate, if your teacher has some available. To the two test tubes containing baking soda and product, add several drops of the silver nitrate to each tube and observe. Are there differences in the results for the two test tubes? What can you conclude about the product, compared to the reactant, baking soda?