Chemical Reactions and Mass

Objective

The student should already have a good notion of what is meant by the word ‘mass’. That is, that mass is a measure of the amount of matter in an object and can be understood to relate to the number and kinds of atoms in that object. This activity is designed to make students familiar with some of the terminology of chemical reactions and especially to make clear that mass is a quantity that is conserved in chemical reactions. In chemical reactions atoms and molecules are rearranged but no matter is created or destroyed. That is why scientists say that mass is conserved. The origin of this idea can be traced back to Antoine Lavoisier and his wife Marie Anne who performed the first experiments which proved the principle.

Overview

In this activity students will perform a chemical reaction in a closed system: a zip-top plastic bag. They will observe the course of the reaction and make observations about it which will help them to define chemical change. They will find the mass of the materials both before and after the reaction takes place in order to determine whether the principle of conservation of mass is correct.

Students may contrast the conclusions reached in this lab with the notion of the equivalence of matter and energy that is embodied in the equation E = mc².

Materials

Safety

• Wear goggles or risk sitting out the lab
• Do not allow chemicals to touch bare skin: wash well with water immediately if you touch anything accidentally
• When mixing the chemicals make sure the bag is sealed and pointed away from fellow students
• Feeling the bag while the reaction proceeds is OK…
  but do not squeeze the bag!

Procedure

Remember to record your observations in your lab notebook or on a piece of paper in your binder before you leave class. When making observations be sure to use all of your senses except taste. Never taste anything in the chemistry lab.

1. Take a moment to describe on paper the two chemicals you will be reacting.
2. Place the ¼ tsp baking soda and ¼ tsp citric acid into the zip-top bag. Shake the bag to mix them together. Does anything happen? Make some observations at this point.
3. Put about 20 mL of water from the tap into the 50 mL beaker.
4. Add a few drops of the indicator solution provided by your teacher. Describe the color of the solution in your notes.
5. Put the bag with the chemicals and the beaker with water onto the lab balance and record the mass to the nearest 0.01 g.

6. Take the temperature of the materials before the reaction by putting the bulb of the thermometer in the colored water for about 1 minute. Record the temperature.
7. Seal the zip-top bag while pushing as much air out of it as possible. Once you are sure it is sealed, mix the water with the dry chemicals.
8. Hold the bag so that your hand is near the reaction: do not squeeze the bag. What do you observe? Record what your senses tell you, writing down as many observations as you can. Allow others in your group to feel it as well.
9. Take the temperature of the substances in the bag while the reaction is going on.
10. Place the closed bag back on the balance and find its mass again when it looks like the reaction has finished. Record the mass to the nearest 0.1 g. If you can, record the mass to ±0.01 g.

Questions

Write your answers on a separate piece of paper. Ask your teacher whether this will be a formal or informal lab write-up.

1. Summarize the procedure you followed in two or three paragraphs and include your observations. Be detailed and thorough: don’t leave out anything important. Do this before answering the following questions.
2. Background: A key sign that a process is a chemical reaction is that it absorbs or gives off heat while producing one or more new substances. When a reaction gives off heat (that is, it makes its surroundings warmer than before) it is called exothermic. When a reaction absorbs heat (that is, it makes its surroundings colder than before) it is called endothermic.
   Question: Is the reaction you observed exothermic or endothermic? Justify your answer using your observations.
3. Which substances in this lab could be called reactants? What does reactant mean?
4. Is water a reactant in this lab? Why or why not?
5. Which substances in this lab could be called products? What does product mean?
6. How did the mass of the reactants compare to the mass of the products? It is possible to quantify any difference as a quantity called percent difference using the formula:
   

   |Massproducts – MassReactants|         (the lines are absolute value signs)
    ————————————————————————— × 100%
          MassReactants
   

   If the percent difference is small then there is less difference between the two. If it is large, then there is a large difference between them. What is the percent difference?
7. What might have caused any difference in mass that you found? In particular, what would explain the difference. Design an experiment to test your idea.
   (Above & Beyond: Plan such an experiment and present it in writing to your teacher for approval. If approved and carried out and reported correctly in a formal lab write-up with the rest of this lab you can earn a 5 for this assignment.)
8. Your teacher claims that you have observed a chemical reaction. What evidence, if any, can you give from your observations that your teacher is correct? If you cannot give evidence to support your teacher’s claim then provide evidence that the claim is false.
9. Your teacher claims that mass is conserved in this reaction. What evidence, if any, can you give from your observations and your calculation of the percent difference of the masses that your teacher is correct? If you cannot give evidence to support your teacher’s claim then provide evidence that the claim is false.