Modeling Conservation of Matter

Transcription

1 Modeling Conservation of Matter Imagine that you and two of your classmates want to make a strawberry banana smoothie. You lay out the ingredients: one banana, five strawberries and two scoops of ice cream. Do you think the mass of the smoothie will be different from the masses of the ingredients combined? The Law of Conservation of Matter states that matter and mass cannot be created or destroyed. Therefore, the mass of the beginning ingredients should equal the mass of the final smoothie. In the same way that you cannot make your strawberry banana smoothie turn into a raspberry lemonade smoothie, chemical reactions must consist of the same number of atoms it started with. Moreover, just like the ingredients weigh a certain amount, so do atoms in a chemical reaction. Chemical formulas describe the atoms held together by chemical bonds. A compound is a group of atoms of different elements joined together by sharing or transferring electrons. The atoms are then held together by a chemical attraction, called a bond. A covalent compound forms when two or more atoms combine by sharing electrons. The smallest unit of a covalent compound is called a molecule. You may be familiar with a molecule of water or carbon dioxide. An example of atoms held together by transferring electrons is sodium chloride. You may also have seen the chemical formulas for these compounds. A chemical formula is a representation of the smallest unit of a compound using elemental symbols to show the type of elements in the unit. Subscripts are used in the chemical formula to show the number of each type of element. Each element is represented in the chemical formula. The number of each type of element is represented by a subscript after the symbol. 1

2 Let s look at how to write the chemical formulas of some common substances. Some substances contain only two types of atoms. An example is sodium chloride, which you may also know as salt. Its chemical formula uses the symbols for sodium (Na) and chloride (Cl): NaCl. Na-Cl salt has one Na and one Cl Water also contains two different atoms: hydrogen (H) and oxygen (O). It has the chemical formula H 2 O.The subscript after the H in the formula is written to identify the number of hydrogen atoms in the molecule. Therefore, a molecule of water contains two hydrogen atoms. When the smallest unit of a compound contains only one atom of an element, such as oxygen in water, or sodium and chloride in salt, a subscript is not needed. H O H water has two H and one O Chemical equations obey the Law of Conservation of Matter Each chemical reaction starts with the reactants on the left side of the equation and ends with the products on the right side of the equation. à The reactants and products must be balanced; therefore, the number and type of atoms on the reactant side must equal the number and type of atoms on the product side. The only difference is that the atoms are rearranged to form new products. For example, look at the equation below: H 2 + N 2 à NH 3 H = 2 H = 3 N = 2 N = 1 If you count each type of element on the reactant side, there are two H (hydrogen) and two N (nitrogen). However, if you count each type of element on the product side, there are three H (hydrogen) and only one N (nitrogen). Is this a balanced equation? No, an extra hydrogen cannot just appear, and one nitrogen cannot be lost or destroyed. Now let s look at the mass of the reactants and the mass of the products. 2

3 The mass of hydrogen on the Periodic Table is amu and the mass of nitrogen on the Periodic Table is amu. 3

4 Let s look at the unbalanced equation again. In order to calculate the mass of the reactants and products: 1. Count the number of each atoms on each side. 2. Then find the mass of each element on the Periodic Table. 3. Now multiply the total number of atoms on the reactant side by its mass. 4. Do this for each atom in the equation. 5. Now add the mass of all the reactants together to get the total mass. 6. Repeat this procedure on the product side. *If the number of elements are not balanced, neither will the total mass balance. H 2 + N 2 à NH 3 # of Hydrogen 2 3 # of Nitrogen 2 1 Mass of Hydrogen 2 * = * = Mass of Nitrogen 2 * = * = Total Mass = = When balancing equations, do not confuse the coefficient with the subscript of a formula. Remember that the coefficient describes the number of molecules of each substance that are present during a reaction. You can change this number to balance a chemical reaction. The subscript in a chemical formula determines the number of atoms that are present during a chemical reaction. This number cannot be changed to balance the chemical reaction. In general, when balancing a chemical reaction, save any hydrogen or oxygen for the last atoms to balance. It will limit the amount of trial and error when balancing. If there are two nitrogen atoms on the reactant side and only one on the product side, you need to add a coefficient of two in front of NH 3. This now gives you two nitrogen atoms, but now there are six hydrogen atoms (multiply the coefficients by the subscript to get the total number of atoms). Since there are six hydrogen atoms on the product side, putting a three in front of the H 2 on the reactant side will give you a total of six hydrogen atoms. 3H 2 + N 2 à 2NH 3 4

5 Let s look at the balanced equation: 3 H 2 + N 2 à 2 NH 3 Number of Hydrogen 6 6 Number of Nitrogen 2 2 Mass of Hydrogen 6 * = * = Mass of Nitrogen 2 * = * = Total Mass = = *Once you balance the equation, both matter and mass are conserved. Let s look at another example. CH 4 + O 2 à CO 2 + H 2 O Number of Carbon 1 1 Number of Hydrogen 4 2 Number of Oxygen = 3 Mass of Carbon 1* = * = Mass of Hydrogen 4 * = * = Mass of Oxygen 2 * = * = Total Mass = =

6 In the previous unbalanced equation, the hydrogen atoms can be balanced by adding a coefficient of two to H 2 O. Then, the oxygen atoms can be balanced by adding a coefficient of two to O 2. The following shows the balanced equation for this reaction. CH O 2 à CO H 2 O Number of Carbon 1 1 Number of Hydrogen 4 4 Number of Oxygen = 4 Mass of Carbon 1* = * = Mass of Hydrogen 4 * = * = Mass of Oxygen 4 * = * = Total Mass = = At this point, there are equal numbers of carbon, hydrogen, and oxygen atoms on both the reactants and products sides of the equation. There are four atoms of hydrogen, one atom of carbon, and four atoms of oxygen on both sides of the equation. To balance a reaction, you must confirm that all atoms are balanced. In this way, the equation is written in a way that obeys the Law of Conservation of Mass. Getting Technical: Chemical Equations of Automobiles One way that you are protected in an automobile crash is from the inflating of air bags. Upon impact, the air bag inflates very quickly and provides a cushion to the passenger to prevent injuries. The following chemical equation represents the primary reaction that causes the air bag to inflate: 2NaN 3 2Na + 3N 2 6

7 NaN 3 is called sodium azide. It is a solid substance that is a reactant in the reaction. It is stored in a very small space such as the steering wheel of the car. When a crash occurs, the reactions begin. N 2 is known as nitrogen gas. The production of this gas causes the air bag to inflate. The nitrogen gas takes up a large volume that inflates the air bag. It is important that scientists understand the chemical equation of this reaction. They must know how much sodium azide is needed to produce a quantity of nitrogen gas that will inflate the air bag properly. Air bags inflate from chemical reactions that can be described by chemical equations. Why do scientists need to understand these equations? What Do You Know? Chemical equations can be written to describe chemical reactions. Look at the chemical equations in this table. For each equation, count the number of each type of atom in the reactants and the products. Then, write Balanced or Unbalanced beside each equation. Chemical equation H 2 + I 2 2HI (hydrogen reacts with iodine) 2Al + 3O 2 2Al 2 O 3 (aluminum reacts with oxygen) 2Zn + 2HCl ZnCl 2 + H 2 (zinc reacts with hydrogen chloride) Balanced or unbalanced? Balance the following equations (if no coefficient is needed, leave blank). Verify your answer by making sure the mass of the reactants equals the mass of the products: 1. K + F 2 à KF mass of reactants/ products= 2. H 2 O 2 à O 2 + H 2 O mass of reactants/ products= 3. Ca + CuF 2 à CaF 2 + Cu mass of reactants/ products= 7

8 Experiment with Chemical Reactions To help your child learn more about chemical formulas and equations, work with your child to explain how equations are similar to a recipe that is used in cooking. Interestingly, there are many different ways that chemical reactions and chemical equations are used in cooking. For example, when you bake a cake, one of the chemical reactions that occurs is the baking soda reacting with water to produce carbon dioxide gas. This gas produces the holes in the cake that give the cake its light, fluffy texture. A similar type of reaction occurs when baking soda is mixed with vinegar, which will produce CO 2 gas as a product. This reaction is generally used when making school fair volcano projects. The experiment you will perform with your child is making Gak, or silly putty. Materials 8 oz bottle of Elmer s Glue Mixing bowl Food coloring of your choice Plastic cup One teaspoon Measuring cup Borax (laundry detergent) Spoon for stirring Water Small food scale Procedure: 1. Weigh the mixing bowl and document its weight in the chart 2. Pour the 8 oz bottle of Elmer s Glue into the mixing bowl. 3. Fill the empty glue bottle halfway with warm water to rinse out any excess glue and pour into the mixing bowl. 4. Stir this mixture while adding your desired food coloring. 5. Weigh this colored glue mixture and document the weight in the chart. 6. Weigh the empty plastic cup and document the weight in the chart. 7. In a plastic cup, add ½ cup of water and 1 teaspoon of Borax. 8. Stir this mixture in the plastic cup until some of the Borax has dissolved (dissolving will work better with warm water.) 9. Weigh the Borax and water mixture and document the weight in the chart 10. Slowly pour and stir the Borax mixture into the glue and water mixture in the mixing bowl. 11. The mixture should start to solidify to form a silly putty consistency. 12. Now weigh the Gak in the mixing bowl and document its weight in the chart *Note: the more Borax and water added to the glue, the more it will solidify. *Note: do not allow kids to put this in their mouth because Borax is a detergent and should not be ingested. Use the chart on the following page and discuss the questions with your child at the end of the investigation. 8

9 Weight Weight Empty Mixing Bowl Mixing bowl with colored glue and water mixture Weight of the glue and water mixture (Subtract the mass of the empty mixing bowl from the mass of the mixing bowl with colored glue and water mixture) Mixing bowl with Gak Initial empty mixing bowl Final weight of the Product (Subtract the mass of the empty mixing bowl from the mixing bowl with Gak) Empty plastic cup Plastic cup with Borax solution Weight of the Borax solution (Subtract the mass of the empty plastic cup from the plastic cup with Borax solution) Final weight of the reactants (add the weight of the glue and water mixture to the weight of the Borax solution, the green coded sections on this chart) After performing the reaction, discuss the following questions with your child: What was the initial weight of the reactants? What was the final weight of the products? Was mass conserved? Compare and contrast the final weight of the reactants on the chart to the final weight of the products on the chart. If they are not similar, discuss what could have happened for the difference in weight. How do you know a chemical reaction occurred? Identify some properties of the reactants and properties of the products. 9