Chemical Formulas and Equations

Transcription

1 Part I: The Big Picture Chemical Formulas and Equations Reminder: H is the element symbol for Hydrogen. H 2 is the chemical formula for hydrogen. The subscript 2 after the H means that two atoms of hydrogen make up a single molecule of hydrogen gas. Subscripts for a particular kind of molecule never change because these numbers describe how many of an atom type are present in a particular molecule. Reminder: O is the element symbol for Oxygen. O 2 is the chemical formula for oxygen. The subscript 2 after the O means that two atoms of oxygen make up a single molecule of oxygen gas. Remember that a subscript describes how many of an atom type are in the molecule and does not change. H 2 O is the chemical formula for water. The subscript 2 after the H means two hydrogen atoms and one oxygen atom make up a single molecule of water. Remember that a subscript describes how many of an atom type are in the molecule and does not change. If an atom within a molecule is not followed by a subscript, then only one of that atom type is present in the molecule. For example: A molecule of Is represented by the chemical formula And is the same as Water H 2 O H 2 O 1 Carbon dioxide CO 2 C 1 O 2 Continue with Part I on the next page. 1

2 Part I: The Big Picture, continued Subscripts in a chemical formula do not change because they identify the substance. For example, water is always H 2 O, never H 3 O. It doesn t matter if water is a reactant or a product; water is always identified as H 2 O to represent 2 atoms of hydrogen and 1 atom of oxygen in every molecule of water. However, the total number of water molecules present in a reaction can change. For example 2H 2 O means that two molecules of water are present, 3H 2 O means that three molecules of water are present, 4H 2 O means that four molecules of water are present and so on. In these examples, the numbers of 2, 3 and 4 before H 2 O is called a coefficient and means how many molecules are present. For example: A molecule of Is represented by the chemical formula And when a coefficient appears before the chemical formula Then, that is the number of molecules present. If a coefficient does not appear before the chemical formula, that means just one molecule is present. Water H 2 O 4H 2 O 4 water molecules Carbon dioxide CO 2 3CO 2 3 carbon dioxide molecules Carbon dioxide CO 2 CO 2 Means the same thing as 1CO 2 1 carbon dioxide molecule Continue with Part I on the next page. 2

3 Part I: The Big Picture, continued Now we know that the coefficient before the H 2 and the H 2 O means there are 2 molecules of each substance. Let us break this chemical reaction down further and look at what happens. 2H 2 + O 2 2H 2 O In this chemical reaction, the black dots represent hydrogen atoms and the white dots represent oxygen atoms. The gray lines represent the atoms that are held together within the molecule. In a chemical reaction, energy (represented by red Xs) pulls the atoms apart in the reactant molecules. The atoms of the reactant molecules separate allowing them to re-combine to form the product molecules. Answer the questions for Part I in your Student Journal. Part II: Chemical Formulas Now that you know some of the basics, let us look at each part of the chemical reaction in further detail. In a chemical reaction, substances react with each other to make new substances with different properties. Each molecule is represented by a specific chemical formula. A chemical formula tells you how many atoms of each element are found within each substance. The numbers of atoms within a molecule are shown by the subscripts, which are found after each elemental symbol. If there is no subscript after the atom, then there is only one atom of that element in the substance. Continue with Part II on the next page. 3

4 Part II: Chemical Formulas, continued Glucose C 6 H 12 O 6 Number of Carbon Atoms = 6 Number of Hydrogen Atoms = 12 Number of Oxygen Atoms = 6 You will work with your group to identify the type and number of atoms in a chemical formula and make models to represent chemical formulas. Use different cube colors to represent each element in the chemical formula as follows: Procedure: Red = Hydrogen Yellow = Chlorine Orange = Copper Dark Brown = Zinc Light Brown = Oxygen Light Blue = Carbon Dark Blue = Sodium White = Potassium Green = Iodine Black = Magnesium 1. Identify the elements in the chemical formula. 2. Determine the number of atoms of each element in the chemical formula. 3. Use the color cubes to make a model of the chemical formula. 4. Draw colored boxes to represent the atoms in your chemical formula. Complete the activity for Part II in your Student Journal. 4

5 Part III: Chemical Equations A chemical equation uses chemical formulas and symbols to represent a chemical reaction. Instead of saying: six molecules of carbon dioxide plus six molecules of water react to form one molecule of glucose plus six molecules of oxygen, scientist use equations to represent the reaction. Mathematicians use equations, too. For example, two plus two equals four is the same as writing 2+2=4. The following chemical equation represents the chemical reaction known as photosynthesis. coefficient 6CO 2 + 6H 2 O! C 6 H 12 O 6 + 6O 2 reactants coefficient products coefficient The chemical formulas in a chemical equation represent the reactants and products. Reactants are the substances that take part in and undergo change during a chemical reaction. The products are the substances produced during a chemical reaction. The coefficient is a number placed in front of a chemical symbol or chemical formula to represent the number of molecules of each substance. Work with your group to identify the number and types of atoms in the reactant and products of several different chemical reactions. You will use the color cubes to model the chemical reaction represented by each chemical equation. Procedure 1. List the elements present on the reactant and product sides of the equation. 2. Determine the number of atoms of each element on the reactant and product sides of the equation. 3. Use the color cubes to make a model of the substances on the reactant side of the equation. Then draw a picture of your model in the chart in your Student Journal. 4. Place your reactant models in the plastic bag. Use your hand to break apart the models. This action represents the energy required to break the bonds between the atoms in a substance. 5. Use the color cubes in the bag to build models for the products. Then draw a picture of your model in the chart in your Student Journal. Complete the activity in Part III of your Student Journal. 5

6 Part IV: The Balancing Act Before we begin this section, turn to Part IV in your Student Journal and complete the table following the same procedures you used in Part III for the following equation. Zn + HCl! ZnCl 2 + H 2 How is this chemical equation different from the equations in Part III? Did you notice that there were not enough cubes to build the reactants? All of the equations that you used in Part III were balanced equations. The coefficients were included so that both sides of the reaction contained an equivalent number of atoms of each element. The equation above is an unbalanced equation, because it does not contain an equal number of atoms of each element on both sides of the equation. The Law of Conservation of Mass states that matter is neither created nor destroyed, but can be rearranged. Thus, the mass of all the reactants must equal the mass of all the products. In a chemical reaction, the number and types of atoms present in the reactants will be equal to the number and types of atoms in the products. A balanced chemical equation follows the Law of Conservation of Mass. Complete Part IV in your Student Journal and answer the Reflection and Conclusion questions. 6