Chemical Reactions PDF

Okay, I will convert the images into a structured markdown format. ### Signs of Chemical Reactions How can you tell when a chemical reaction is taking place? **Figure 2** shows some signs that tell you that a reaction may be taking place. In some chemical reactions, gas bubbles form. Other reactio...

Okay, I will convert the images into a structured markdown format. ### Signs of Chemical Reactions How can you tell when a chemical reaction is taking place? **Figure 2** shows some signs that tell you that a reaction may be taking place. In some chemical reactions, gas bubbles form. Other reactions form solid precipitates. A **precipitate** is a solid substance that is formed in a solution. During other chemical reactions, energy is given off. This energy may be in the form of light, thermal energy, or electricity. Reactions often have more than one of these signs. And the more of these signs that you see, the more likely that a chemical reaction is taking place. **Precipitate**: A solid that is produced as a result of a chemical reaction in solution. **Self-Check**: What is a precipitate? ### Figure 2 Some Signs of Chemical Reactions * **Gas Formation**: The chemical reaction in the beaker has formed a brown gas, nitrogen dioxide. This gas is formed when a strip of copper is placed into nitric acid. * **Solid Formation**: Here you see potassium chromate solution being added to a silver nitrate solution. The dark red solid is a precipitate of silver chromate. * **Color Change**: Don't spill chlorine bleach on your jeans! The bleach reacts with the blue dye on the fabric and causes the color of the material to change. * **Energy Change**: Energy is released during some chemical reactions. The fire in this photo gives off light energy and thermal energy. During some other chemical reactions, energy is taken in. ### A Change of Properties Even though the signs we look for to see if a reaction is taking place are good signals of chemical reactions, they do not guarantee that a reaction is happening. For example, gas can be given off when a liquid boils. But this example is a physical change, not a chemical reaction. So, how can you be sure that a chemical reaction is occurring? The most important sign is the formation of new substances that have different properties. Look at **Figure 3**. The starting materials in this reaction are sugar and sulfuric acid. Several things tell you that a chemical reaction is taking place. Bubbles form, a gas is given off, and the beaker becomes very hot. But most important, new substances form. And the properties of these substances are very different from those of the starting substances. Figure 3 shows two photos. * The top photo shows the starting substances: table sugar and sulfuric acid, a clear liquid. * The substances formed in this chemical reaction are very different from the starting substances. **Self-Check**: What happens to the bonds of substances during a chemical reaction? ### Bonds: Holding Molecules Together A **chemical bond** is a force that holds two atoms together in a molecule. For a chemical reaction to take place, the original bonds must break and new bonds must form. ### Breaking and Making Bonds How do new substances form in a chemical reaction? First, chemical bonds in the starting substances must break. Molecules are always moving. If the molecules bump into each other with enough energy, the chemical bonds in the molecules break. The atoms then rearrange, and new bonds form to make the new substance. **Figure 4** shows how bonds break and form in the reaction between hydrogen and chlorine. **Figure 4 Reaction of Hydrogen and Chlorine** Hydrogen + Chlorine -> Hydrogen Chloride * **Breaking Bonds**: Hydrogen and chlorine are diatomic. Diatomic molecules are two atoms bonded together. The bonds joining these atoms must first break before the atoms can react with each other. * **Making Bonds**: A new substance, hydrogen chloride, forms as new bonds are made between hydrogen atoms and chlorine atoms. ### New Bonds, New Substances What happens when hydrogen and chlorine are combined? A chlorine gas molecule is a diatomic molecule. That is, a chlorine molecule is made of two atoms of chlorine. Chlorine gas has a greenish yellow color. Hydrogen gas is also a diatomic molecule. Hydrogen gas is flammable, colorless gas. When chlorine gas and hydrogen gas react, the bond between the hydrogen atoms breaks. And the bond between the chlorine atoms also breaks. A new bond forms between each hydrogen and chlorine atom and a new substance, hydrogen chloride, is formed. Hydrogen chloride is a nonflammable, colorless gas. Its properties differ from the properties of both of the starting substances. Let's look at another example. Sodium is a metal that reacts violently in water. Chlorine gas is poisonous. When chlorine gas and sodium react, the result is a familiar compound-table salt. Sodium chloride, or table salt, is a harmless substance that almost everyone uses. The salt's properties are very different from sodium's or chlorine's. Salt is a new substance. ### Quick Lab Reaction Ready 1. Place a piece of chalk in a plastic cup. 2. Add 5 mL of vinegar to the cup. Record your observations. 3. What evidence of a chemical reaction do you see? 4. What type of new substance was formed? ### Summary * A chemical reaction is a process by which substances change to produce new substances with new chemical and physical properties. * Signs that indicate a chemical reaction has taken place are a color change, formation of a gas or a solid, and release of energy. * During a reaction, bonds are broken, atoms are rearranged, and new bonds are formed. ### Using Key Terms 1. In your own words, write a sentence showing the relationship between chemical reaction and precipitate. ### Understanding Key Ideas 2. Most Chemical reactions * a. Have starting substances that collide with eachother. * b. Do not break bonds. * c. Do not rearrange atoms. * d. Cannot be seen. 3. If the chemical properties of a substance have not changed, has a chemical reaction occurred? ### Critical Thinking 4. **Analyzing Processes**: Steam is escaping from a teapot. Is this a chemical reaction? Explain. 5. **Applying Concepts**: Explain why charcoal burning in the grill is a chemical change. ### Interpreting Graphics Use the photo below to answer the questions that follow. The photo shows a substance in transparent container emitting light 6. What evidence of a chemical reaction is shown in the photo? 7. What is happening to the bonds of the starting substances? ## SECTION 2 Chemical Formulas and Equations ### Objectives * Interpret and write simple chemical formulas. * Write and balance simple chemical equations. * Explain how a balanced equation shows the law of conservation of mass. ### Vocabulary * chemical formula * chemical equation * reactant * product * law of conservation of mass ### Reading Strategy Discussion Read this section silently. Write down questions that you have about this section. Discuss your questions in a small group. **Chemical Formula**: A combination of chemical symbols and numbers to represent a substance. ### Chemical Formulas and Equations How many words can you make using the 26 letters of the alphabet? Many thousands? Now, think of how many sentences you can make with all of those words. Letters are used to form words. In the same way, chemical symbols are put together to make chemical formulas that describe substances. Chemical formulas can be placed together to describe a chemical reaction, just like words can be put together to make a sentence. ### Chemical Formulas All substances are formed from about 100 elements. Each element has its own chemical symbol. A **chemical formula** is a shorthand way to use chemical symbols and numbers to represent a substance. A chemical formula shows how many atoms of each kind are present in a molecule. As shown in **Figure 1**, the chemical formula for water is $H_2O$. This formula tells you that one water molecule is made of two atoms of hydrogen and one atom of oxygen. The small 2 in the formula is a subscript. A subscript is a number written below and to the right of a chemical symbol in a formula. Sometimes, a symbol, such as O for oxygen in water's formula, has no subscript. If there is no subscript, only one atom of that element is present. Look at **Figure 1** for more examples of chemical formulas. **Figure 1 Chemical Formulas of Different Substances** | Water | Oxygen | Glucose | | -------------- | --------------- | -------------- | | $H_2O$ | $O_2$ | $C_6H_{12}O_6$ | | Water molecules are made up of 3 atoms - 2 atoms of hydrogen bonded to 1 atom of oxygen | Oxygen is a diatomic molecule. Each molecule has 2 atoms of oxygen bonded together | Glucose molecules have 6 atoms of carbon, 12 atoms of hydrogen, and 6 atoms of oxygen| | Carbon dioxide | Dinitrogen monoxide | | -------------- | ------------------- | | $CO_2$ | $N_2O$ | | The absence of a prefix indicates one carbon atom. The prefix di-indicates two oxygen atoms. | The prefix di-indicates two nitrogen atoms. The prefix mono-indicates one oxygen atom | **Figure 2** The formulas of these covalent compounds can be written by using the prefixes in the names of the compounds. | Number | Prefix | | ------ | -------- | | 1 | mono- | | 2 | di- | | 3 | tri- | | 4 | tetra- | | 5 | penta- | | 6 | hexa- | | 7 | hepta- | | 8 | octa- | | 9 | nona- | | 10 | deca- | ### Writing Formulas for Covalent Compounds If you know the name of the covalent compound, you can often write the chemical formula for that compound. Covalent compounds are usually composed of two nonmetals. The names of many covalent compounds use prefixes. Each prefix represents a number, as shown is **Table 1**. The prefixes tell you how many atoms of each element are in a formula. **Figure 2** shows you how to write a chemical formula from the name of a covalent compound. ### Writing Formulas for Ionic Compounds If the name of a compound contains the name of a metal and the name of a nonmetal, the compound is ionic. To write the formula for an ionic compound, make sure the compound's charge is zero. In other words, the formula must have subscripts that cause the charges of the ions to cancel out. **Figure 3** shows you how to write a chemical formula from the name of an ionic compound. | Sodium chloride | Magnesium chloride | | --------------- | ------------------ | | NaCl | $MgCl_2$ | | A sodium ion has a 1+ charge. A chloride ion has a 1- charge. One sodium ion and one chloride ion have an overall charge of (1+) + (1-) = 0 | A magnesium ion has a 2+ charge. A chloride ion has a 1- charge. One magnesium ion and two chloride ions have an overall charge of (2+) + 2(1-) = 0 | **Self-Check** What kinds of elements make up an ionic compound? **Figure 3** The formula of an ion compound is written by using the least number of each ion so that the overall charge is 0.