Bonding and Properties

Transcription

1 Table 2.7 Abbreviations for States One mark of a valid theory is its ability to explain observations. In this section, you will investigate the properties of ionic compounds and molecular compounds. You will use the theory you learned in section 1.3 to explain what you observe. Then, you will investigate the properties of two other important classes of substances. Bonding and Properties Chemists can infer many features of the structure and bonding of different substances. They do this by observing their physical and chemical properties. For example, physical properties such as structure, melting and boiling points, and electrical conductivity give important clues about bonding in ionic compounds and molecular compounds. It is important to know the state of a substance because substances have different properties in different states. For example, substances have different properties when they are dissolved in water. To avoid confusion, chemists use subscripts to show the state of a substance when writing its formula. Table 2.7 shows these subscripts. Properties of Ionic Compounds You can see some important properties of sodium chloride, NaCI(s), in Figure 2.4. Note that a red light indicates when a substance is conducting electricity.. In the solid state, ionic compounds have regular crystalline shapes with flat sides. This property suggests that the ions in the crystal are arranged in a regular, repeating pattern. As you know, scientists call this pattern a crystal lattice. ~ Like most ionic compounds, sodium chloride (A) has a relatively high melting point (801'C). It conducts electricity in solution (8), and does not conduct electricity in its solid state (C). 56 MHR. Unit I Energy and Matter in Chemical Change

2 . Ionic compounds tend to have fairly high melting points. This property suggests that the attraction between positively and negatively charged ions in the lattice is strong and extends throughout the crystal. A great deal of energy is required to break the strong ionic bonds.. In the solid state, ionic compounds do not conduct electricity. This property suggests that the ions in the lattice are not free to move.. When melted or dissolved in water, ionic compounds are electrolytes. They conduct electricity. This property suggests that melting and dissolving break down the crystal lattice and allow ions to move freely and carry charge. Properties of Molecular Compounds Properties of molecular compounds can also be used to build up a picture of their structure. Figure 2.5 shows glucose solid and solution being tested for electrical conductivity.. Most molecular compounds have fairly low melting points. This property suggests that the attractive forces between molecules must be weak and easy to overcome.. Once melted, molecular compounds can usually be heated further without decomposing into other compounds or elements. This property suggests that the covalent bonds within molecules must be strong.. Molecular substances tend to be non-electrolytes. They do not conduct electricity in any state as pure substances. This property suggests that molecular substances are not made of ions. They do not conduct electricity even in liquid form, because the liquid contains only neutral molecules.. When dissolved in water, most molecular substances do not conduct electricity. However, some molecular substances do conduct electricity when dissolved in water. This observation suggests that some molecular substances can form ions when they are dissolved in water. When a substance that is dissolved in water conducts electricity, you cannot be sure whether it is an ionic compound or a molecular compound. You must perform further tests. In the investigation that follows, you will design a procedure to identify substances as ionic or molecular, based on their properties. You will explain why theories of ionic and covalent bonding help to describe the different properties of substances. Like most molecular compounds, glucose does not conduct electricity either in solid form (A) or in an aqueousolution (8). Glucose is a solid at room temperature, but it has a relatively low melting point (146 C). Many molecular compounds, such as sulfur dioxide, 502(g), are gases at room temperature. Many others are liquids. Chapter 2 Names, Formulas, and Properties. MHR 57

3 Scientists often work in me same way as detectives. Like detectives searching for clues, chemists classify substances by matching observations against meories about ionic and molecular compounds. In this investigation, you will try to determine me type of bonding in several common substances by observing meir properties. Question Which substances in this investigation are composed of molecules, and which are composed of ions? Prediction I Read Procedure steps 1 to 6. Before you write your procedure, examine the substances you will be investigating. Describe their physical appearance. Predict whether your tests will determine whether each one is ionic or molecular. Safety Precautions mng~g. This may be the first time you work with chemicals in this course. If so, review the general laboratory safety procedures outlined on page xiv of this textbook. Your teacher may provide additional guidelines specific to your school.. Review with your teacher the safety hazards of the chemicals you will be using. Be sure you understand the MSDS information for these chemicals, and use recommended handling and disposal procedures.. If you have medical conditions, such as allergies or asthma, inform your teacher and find out appropriate precautions to take before beginning the investigation.. Be sure to wear gloves, safety glasses, and protective clothing while you are working in the lab. Apparatus magnifying lens 5 beakers (100 ml) scoopula stirring rod conductivity tester hot plate test tube rack 4 test tubes test tube holder Materials 9 labels distilled water solid honey (paraffin wax is an alternative) Epsom salts lauric acid washing soda Procedure 0 With your group, decide on a set of at least five observations, including a conductivity test, which will help you deteffi1ine whether each substance is ionic or molecular. You may include tests using any of the apparatus listed. Qualitative observations are fine, too. For example, you can describe the melting point as "low" or "high," depending on whether the substance melts in a beaker sitting on a hot plate at low heat. You can also describe the odour of the substances.. Write a step-by-step procedure for your tests. CAUTION: Uyou are going to use smell as one of your tests, be sure to include the correct procedure for doing so. CAUTION: Never. taste any substances in the laboratory. Design a data table to record your observations. Use a grid format, with space for observations of each substance in a separate row. e Have your procedure and data table approved by your teacher. 58 MHR. Unit 1 Energy and Matter in Chemical Change

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5 DidYouKnow?, Nicotine-like chemicals are being investigated for their potentially useful medicinal effects. They may be useful in the treatment of Alzheimer's disease, Parkinson's disease, some mental illnesses, and adult ADD. Some chemicals that are related to nicotine appear to improve memory and learning in rats, and may protect rat brain cells from some toxins. Other chemical relatives of nicotine are effective painkillers. Nicotine itself is a "dirty drug," whose negative effects often counteract its possible benefits. Tobacco products, such as cigarettes, with their mix of potent chemicals, have no known medical use. ~ Caffeine is present in varying amounts in all these items. Assessing the Risk: Toxic Properties of Substances Industrial workers who handle the molecular substance ethanol, C2HsOH(t'), are warned that the chemical can be harmful. The MSDS for pure ethanol states that it can cause mutations, damage unborn children, irritate the skin and lungs, cause unconsciousness, and damage the liver and nervous system. Despite these hazards, people expose themselves to ethanol solutions daily. They do so every time they drink an alcoholic beverage. Life in our modern society can involve the use of many chemicals with properties that produce pronounced physical and psychological effects. Society's attitude to these substances varies. Some of them are widely accepted. Caffeine, for example, is a powerful stimulant that is found in coffee, tea, cocoa, and chocolate (Figure 2.6). It is a diuretic, meaning that it causes more frequent urination. It also increases blood pressure by constricting blood vessels. Caffeine is not classified as a hazardous substance. Its use is not regulated. Controlling your caffeine intake is a personal decision. On the other hand, governments and other bodies strictly regulate the use of some substances. Many performance-enhancing drugs, for example, are strictly banned from athletic competitions. The use and sale of many psychoactive drugs, such as cocaine, heroin, and ecstasy (MDMA), are illegal. People under a certain age cannot legally purchase alcoholic beverages and cigarettes. In the last decade, many towns and cities have decided to ban smoking in public places. Knowing about chemistry can help people make informed personal decisions about whether to include potentially harmful chemicals as part of their lifestyle. Table 2.8 outlines some information about the chemicals in alcoholic beverages and tob:\cco products and their effects. In the next investigation, you will have a chance to find similar information about another chemical that you wish to research. Table 2.8 Ethanol and Nicotine Products 60 MHR. Unit I Energy and Matter in Chemical Change

6 Think About It Deciding to use chemicals always involves balancing risks and benefits based on what you know about their properties. In this investigation, you will create an action plan for regulating the use of a particular chemical so that people can enjoy its benefits (if any) while minimizing its risk to human health and to the environment. Try to create an action plan that is imaginative, feasible, and likely to be effective. " Creatively present your findings and action plan. For example, you may present your work as a poster, a slide show, or a web page. What to Do 0 As a group, choose a widely used chemical to investigate. Some possibilities are listed below:. chlorine or fluoride chemicals added to drinking water. formaldehyde-emitting glues used in plywood and chipboard. a medicine. "road salt" used to melt ice. addictive chemicals discussed in this section. MSG (monosodium glutamate) or another food additive. Gather information on the benefits of using the chemical and the specific hazards it presents to human health and to the environment. Such risks may arise while in producing, distributing, using, recycling, or disposing of the chemical. Use Internet and print resources. The Merck Index is a useful starting point for information about chemicals and their properties.. Find out whether use of the chemical is now regulated in Canada. If its use is regulated, find out how. 0 Brainstorm ways to reduce the hazards associated -I with the chemical. You could, for example, decide! '- r \ I' i, to ban it completely, control how it is distributed, require a permit to purchase or use it, or restrict the quantity that is allowed in products. Use your ideas to create an action plan for using the chemical. Chapter 2 Names, Formulas, and Properties. MHR 61

7 Section 2.2 Summary In this section, you learned about some of the properties of molecular compounds and ionic compounds. You used your knowledge of bonding to explain some of these properties. You have also explored some of the risb associated with chemicals and their properties. In section 2.3, you will learn about the properties of acids and bases. These substances have some dangerous properties, but they are also very useful. Check Your Understanding 1. Compare and contrast each term in the pairs below. (a) crystal lattice - molecule (b) electrolyte - non-electrolyte 2. Explain why ionic compounds tend to have very high melting points, while molecular compounds tend to have relatively low melting points. 3. Apply Examine the table on the left. The table shows melting points for sodium fluoride, sodium chloride, and sodium bromide. By examining these melting points, what can you infer about the bonding in each compound? 4. Apply You have been given two solid substances. One of them is an ionic compound, and one is a molecular compound. (a) Describe four safe tests you could perform to try to identify the substances as ionic compounds or molecular compounds. (b) For each test, explain what you would expect to see for an ionic compound, and what you would expect to see for a molecular compound. 5. Thinking Critically Examine the observations in the table below. (a) Use the observations to classify each substance as an ionic compound or a molecular compound. Give reasons to justify your choice. (b) Based on the information in the table, which of the five tests (odour, hardness, ability to dissolve in water, conductivity, or melting point) is the least useful when classifying compounds as ionic or molecular? Explain your answer. 62 MHR. Unit 1 Energy and Matter in Chemical Change