Topic Students devise an experiment to determine the types of bonds in three compounds.

Transcription

1 Types of Bonds Topic Students devise an experiment to determine the types of bonds in three compounds. Introduction Molecules are made of atoms that are held together by either ionic or covalent bonds. Atoms are stable and unreactive when the electrons in their outermost orbits are arranged in an octet, a set of eight. If atoms have more or less than eight electrons in their outer orbits, they are reactive and will either share or transfer electrons until they reach a stable configuration. An ionic bond forms when a reactive atom transfers one of its outer-orbit electrons to another reactive atom. After the transfer, the two atoms are stable, but they are not electrically neutral. Each has taken on an electrical charge and is therefore referred to as an ion. An atom that receives an electron becomes a negatively charged ion, and one that gives up an electron forms a positively charged ion. Since unlike charges attract, the two ions are pulled toward one another and form a strong ionic bond, shown in Figure 1. Molecules that are made up of ions held together by ionic bonds have a rigid crystalline structure that is not easily broken. In order to split the bonds that hold ionic compounds together, a lot of energy must be used. sodium atom chlorine atom sodium ion chlorine ion Figure 1

2 TYPES OF BONDS 2 The strength of ionic bonds gives ionic compounds several distinctive characteristics: a. Ionic compounds have very high melting and boiling points. b. In the solid state, ionic compounds do not conduct electricity because their ions are rigidly held in position. When dissolved in water, the ions are free to move about and act as good electrical conductors c. Ionic compounds dissolve easily in water because water is a polar compound, one that has a slightly positive charge on one end and a slightly negative charge on the other. The charged ends of water molecules surround the ions that make up the ionic compound and separate them. Covalent compounds form when reactive or unstable atoms share some of the electrons in their outer shells, as in Figure 2. By sharing, each of the unstable atoms can reach a configuration in which they have 8 outermost electrons. Because covalent bonds are not as strong as ionic bonds, these compounds have relatively low melting and boiling points. Most do not dissolve in water, and in solution they cannot conduct electricity. electron H H H H Figure 2 Time Required 60 to 75 minutes Materials sugar (about 1 teaspoon [tsp]) salt (about 1 tsp)

3 TYPES OF BONDS 3 wax (the size of a marble) hot plate stirring rod water (about 200 milliliters [ml]) conductivity tester 100-ml beaker small square of aluminum foil (about 2 centimeters [cm] 2 cm) hot pads Safety Note Please review and follow the safety guidelines. Take care when working with the hot plate. Procedure 1. Devise an experiment to determine whether three compounds sodium chloride, magnesium sulfate, and wax are ionic or covalent. On the chart below, list the steps (the procedure) of your proposed experiment. Your experimental procedure must be one that can be carried out using the equipment available to you. You do not have to use all of the equipment. 2. On the second row of the chart, make a list of the materials or equipment you will need in your experiment. 3. After completing your proposed procedure and list of materials, ask your teacher to look over your ideas and approve them by signing or initialing your chart. 4. Create a data table to record information that you gather. 5. Carry out your experiment and record you data.

4 TYPES OF BONDS 4 Chart of Procedure, Materials, and Teacher Approval List of steps you plan to take (the procedure) List of materials you will need Teacher approval Analysis 1. What kind of bonds hold molecules of magnesium sulfate together? How do you know? 2. What kind of bonds hold molecules of sodium chloride together? How do you know? 3. What kind of bond holds molecules of wax together? How do you know? 4. Compound A has a melting point of 393 C, and compound B has a melting point of 1,320 C. Which of the two compounds is most likely a covalent compound? What is the relationship between type of bond and the melting point of a substance? What s Going On? Sodium chloride and magnesium sulfate are ionic compounds. For this reason, they dissolve in water and conduct electricity. In addition, both have very high melting and boiling points. Wax is a covalent compound that has relatively low melting and boiling points, and it will neither dissolve in water nor conduct electricity.

5 TYPES OF BONDS 5 The ions in ionic compounds have positive and negative ends. Only a few covalent compounds are polar, most notably water. Polar and nonpolar compounds will not dissolve in one another. That is why oil (a nonpolar compound) and water (a polar compound) do not mix. Want to Know More? See Our Findings.

6 OUR FINDINGS TYPES OF BONDS Analysis 1. Ionic. The material dissolves easily in water and conducts electricity. 2. Ionic. The material dissolves easily in water and conducts electricity. 3. Covalent. The material does not dissolve in water. 4. A. Covalent compounds have lower melting points than ionic compounds

7 SAFETY PRECAUTIONS Review Before Starting Any Experiment Each experiment includes special safety precautions that are relevant to that particular project. These do not include all the basic safety precautions that are necessary whenever you are working on a scientific experiment. For this reason, it is necessary that you read and remain mindful of the General Safety Precautions that follow. Experimental science can be dangerous, and good laboratory procedure always includes carefully following basic safety rules. Things can happen very quickly while you are performing an experiment. Materials can spill, break, or even catch fire. There will be no time after the fact to protect yourself. Always prepare for unexpected dangers by following the basic safety guidelines during the entire experiment, whether or not something seems dangerous to you at a given moment. We have been quite sparing in prescribing safety precautions for the individual experiments. For one reason, we want you to take very seriously every safety precaution that is printed in this book. If you see it written here, you can be sure that it is here because it is absolutely critical. Read the safety precautions here and at the beginning of each experiment before performing each activity. It is difficult to remember a long set of general rules. By rereading these general precautions every time you set up an experiment, you will be reminding yourself that lab safety is critically important. In addition, use your good judgment and pay close attention when performing potentially dangerous procedures. Just because the text does not say be careful with hot liquids or don t cut yourself with a knife does not mean that you can be careless when boiling water or punching holes in plastic bottles. Notes in the text are special precautions to which you must pay special attention. GENERAL SAFETY PRECAUTIONS Accidents caused by carelessness, haste, insufficient knowledge, or taking an unnecessary risk can be avoided by practicing safety procedures and being alert while conducting experiments. Be sure to check the individual experiments in this book for additional safety regulations and adult supervision requirements. If you will be working in a lab, do not work alone. When you are working off site, keep in

8 SAFETY PRECAUTIONS 2 groups with a minimum of three students per group, and follow school rules and state legal requirements for the number of supervisors required. Ask an adult supervisor with basic training in first aid to carry a small first-aid kit. Make sure everyone knows where this person will be during the experiment. PREPARING Clear all surfaces before beginning experiments. Read the instructions before you start. Know the hazards of the experiments and anticipate dangers. PROTECTING YOURSELF Do not horseplay. Tie back long hair. Follow the directions step-by-step. Do only one experiment at a time. Locate exits, fire blanket and extinguisher, master gas and electricity shut-offs, eyewash, and first-aid kit. Make sure there is adequate ventilation. Keep floor and workspace neat, clean, and dry. Clean up spills immediately. If glassware breaks, do not clean it up; ask for teacher assistance. Never eat, drink, or smoke in the laboratory or workspace. Do not eat or drink any substances tested unless expressly permitted to do so by a knowledgeable adult. USING EQUIPMENT WITH CARE Set up apparatus far from the edge of the desk. Use knives or other sharp-pointed instruments with care. Pull plugs, not cords, when removing electrical plugs. Clean glassware before and after use. Check glassware for scratches, cracks, and sharp edges.

9 SAFETY PRECAUTIONS 3 Clean up broken glassware immediately. Do not touch metal conductors. Do not use reflected sunlight to illuminate your microscope. Use alcohol-filled thermometers, not mercury-filled thermometers. USING CHEMICALS Never taste or inhale chemicals Label all bottles and apparatus containing chemicals Read labels carefully. Avoid chemical contact with skin and eyes (wear safety glasses, lab apron, and gloves). Do not touch chemical solutions. Wash hands before and after using solutions. Wipe up spills thoroughly. HEATING SUBSTANCES Wear safety glasses, apron, and gloves when boiling water. Keep your face away from test tubes and beakers. Use test tubes, beakers, and other glassware made of Pyrex glass. Never leave apparatus unattended. Use safety tongs and heat-resistant gloves. If your laboratory does not have heat-proof workbenches, put your Bunsen burner on a heat-proof mat before lighting it. Take care when lighting your Bunsen burner; light it with the airhole closed, and use a Bunsen burner lighter in preference to wooden matches. Turn off hot plates, Bunsen burners, and gas when you are done. Have a fire extinguisher on hand. Keep flammable substances away from flames and other sources of heat.

10 SAFETY PRECAUTIONS 4 FINISHING UP Thoroughly clean your work area and any glassware used. Wash your hands. Be careful not to return chemicals or contaminated reagents to the wrong containers. Do not dispose of materials in the sink unless instructed to do so. Clean up all residues and put them in proper containers for disposal. Dispose of all chemicals according to all local, state, and federal laws. BE SAFETY CONSCIOUS AT ALL TIMES!