POGIL: Chemical Bonding Students will be able to: • Understand the differences between ionic, covalent and metallic bonding • Link ionic, covalent, and metallic bonding with the physical properties of matter Why? Have you ever accidentally used salt instead of sugar? Salt and sugar may look the same, but they obviously taste very different and also very different chemically. They are held together by different chemical bonds. Chemical bonds hold atoms together in a compound. We will look at the differences between the chemical bonds that hold salt and sugar together. Model 1: Determining Type of Bonds (Type of Atoms) Directions: Use the chart below and your periodic table to answer the questions that follow. Group One and Group Two both show different compounds made by bonding different elements together. There are two types of bonding in food science, as shown in these two groupings. Let’s take a closer look at the elements in each compound to see what makes a compound IONIC or COVALENT. Group 1 IONIC BONDS

Group 2 COVALENT BONDS

NaCl (Sodium Chloride) KI (Potassium Iodide)

H2O (Dihydrogen Monoxide) C6H12O6 (Dextrose)

MgO (Magnesium Oxide)

C6H4Cl2 (Paradichlorobenzene)

Li2O (Lithium Oxide)

CO2 (Carbon Dioxide)

KF (Potassium Fluoride)

HF (Hydrofluoric Acid)

FeBr3 (Iron (III) Bromide)

NH3 (Ammonia)

CaCl2 (Calcium Chloride)

NO2 (Nitrogen Dioxide)

NiI2 (Nickel(II)Iodide)

CH4 (Methane)

BaS (Barium Sulfide)

CF4 (Carbon Tetraflouride)

Critical Thinking Questions: 1. What type of element are all of the FIRST elements in the GROUP ONE compounds? Where are they found on the periodic table?

2. What type of element are all of the SECOND elements that make up each compound in GROUP ONE? What side of the periodic table do you find these SECOND elements on?

BIG IDEA #1: When a _____________________ and a _____________________ come together, they make an IONIC BOND.

3. What do you notice about ALL of the elements in GROUP TWO? 4.

What side of the periodic table do you find ALL of these elements on?

BIG IDEA #2: When a _____________________________ and another _____________________ come together, they make a COVALENT BOND. Model 2: Properties of Ionic and Covalent Compounds BACKGROUND- read and annotate and answer the pre-lab questions Chemical compounds are combinations of atoms held together by chemical bonds. These chemical bonds are of two basic types—ionic and covalent. Ionic bonds result when one or more electrons from one atom or group of atoms is transferred to another atom. Positive and negative ions are created through the transfer. In covalent compounds no electrons are transferred; instead electrons are shared by the bonded atoms. The physical properties of a substance, such as melting point, solubility, and conductivity, can be used to predict the type of bond that binds the atoms of the compound. In this experiment, you will test six compounds to determine these properties. Your compiled data will enable you to classify the substances as either ionic or covalent compounds. The different forces within these substances that you will test account for the many physical properties of ionic and covalent compounds such as solubility, melting point, and ability to conduct an electric current. Melting -- Many substances remain in a solid state at room temperature. In order to melt an ionic compound, it is necessary to break ionic bonds. Therefore, ionic compounds usually have high melting points. To melt a covalent compound, it isn’t necessary to break bonds. It is only necessary to overcome the much weaker intermolecular forces that hold the particles together. Solubility – Solubility is a complex phenomenon. For the purposes of this lab it is necessary to understand the golden rule of solubility “like dissolves like.” What this means is that a solvent (the dissolving medium in a solution) will dissolve a solute (a substance that is dissolved into a liquid) that is similar in structure. More specifically a polar solvent will dissolve a polar solute and a non polar solvent will dissolve a nonpolar solute. Ionic compounds are compounds with extreme polarity. Ionic compounds tend to be soluble (or dissolve in) water because water is a polar compound that can exert enough force to overcome the ionic bond and cause the ions to go into solution. In general covalent compounds are less soluble in water. The tendency of compounds to dissociate or ionize in water tells a great deal about the way in which bonds hold the compound itself together. Conductivity – A substance that conducts electricity when it is dissolved in water is referred to as an electrolyte. If a compound is ionic, then when it dissolves in water it will form ions which will allow electric current to flow through the solution. One way to assess the dissociation tendency of a compound in water is to test for the solutions ability to conduct electricity. If an aqueous solution of the compound does not conduct, it is called a non-electrolyte. If there is conduction in an aqueous solution, the compound is called an electrolyte. Charged particles must be present and free to move in order for an electric current to flow. The amount of conduction by the solution gives an indication of

the compound’s ionic character. Indeed, conduction or non-conduction by the solution gives an indication of the bond type that exists in the compound. The physical properties of a substance can be used to predict the type of bond that binds the atoms of the compound. In this experiment, you will test six compounds to determine these properties. Your compiled data will enable you to classify the substances as either ionic or covalent compounds. Pre-Lab Questions 1. How are ionic bonds formed? What types of elements make up ionic compounds?

2. How are covalent bonds formed? What types of elements make up covalent compounds?

3. What does melting point mean? How can you test for a melting point?

4. What does solubility mean? How can you test a substance’s solubility?

5. What does electrical conductivity mean (hint: think about what the words mean)? How would you test for conductivity?

LAB: We will do this in class next period, so skip it for now. Safety precautions: Always wear safety goggles to protect your eyes.

•

Do not touch any chemicals.

• •

Do not heat glassware that is broken, chipped, or cracked. When using a flame, confine long hair and loose clothing.

Materials: • • • • • •

Well-Plate Bunsen burner/Sparker Conductivity tester Ring Stand Safety Goggles Mortar and Pestle

Procedure 1. Put on safety goggles.

• • • • • •

Aluminum foil square Distilled Water Ethanol (CH3CH2OH) CaCl2 (calcium chloride) KI (potassium iodide) NaCl (sodium chloride)

• • •

C13H18O2 (ibuprofen) C8H9NO2 (acetaminophen) C12H22O11 (sucrose)

Test #1: Melting point test: 2. Before you begin, write a brief description of each of the six substances in Table 1. 3. Label your piece of aluminum foil with each of the substances to help you keep track of each metal. For example: CaCl2

NaCl

C8H9NO2 (acetaminophen)

KI

C13H18O2 (ibuprofen)

C12H22O11(sucrose)

4. Place a folded square of aluminum foil on a hot plate. 5. Place a few crystals of sucrose, sodium chloride, acetaminophen, calcium chloride, ibuprofen, and potassium iodide in separate locations on the square of aluminum foil. Do not allow the samples of crystals to touch. 5. For this experiment, it is not necessary to have exact values for the melting point. The foil will continue to get hotter as it is heated, so the order of melting will give relative melting points. Note the substance that melts first by writing a 1 in Table 1. Record the order of melting for the other substances. 6. After 2 min, record an n in Table 1 for each substance that did not melt. Allow the foil to cool completely before removing. Test #2: Solubility test MUST BE DONE PRIOR TO THE CONDUCTIVITY TEST 7. Put a few crystals (2-3) of each of the white solids in separate test tubes. Fill the test tube 2/3 full of water (tap or distilled). Cover the top with your thumb and shake the test tube. Record the solubility of each substance in Table 1 as soluble or non-soluble in water. Rinse your thumb. Wash the test tubes with water and a test tube brush. 8. Do this part in a fume hood. Repeat step 7 but fill with ethanol instead of water. Record the solubility of each substance in Table 1 as soluble or non-soluble in ethanol. Rinse your thumb. Test #3 Conductivity test (Teacher Demo): 9. Test the conductivity of each water solution made in the solubility test (top row) by dipping both electrodes into each well of the well plate. Be sure to rinse the electrodes and dry them with a paper towel after each test. If the bulb of the conductivity apparatus lights up, the solution conducts electric current. Record your results in Table 1. Clean UP!! 10. Clean the test tubes by rinsing it with water into the sink. If any wells are difficult to clean, use a cotton swab. Wash your hands thoroughly before you leave the lab and after all work is finished. If the aluminum foil is cool, throw it in the garbage.

TABLE 1: CHARACTERISTICS OF COMPOUNDS

Compound

Description

*Melting point

Solubility in Solubility in H2 O ethanol

**Conductivity

Calcium chloride

Ibuprofen

Acetaminophen

Potassium iodide Sodium chloride

Sucrose

*Rank your Melting Point 1-6…1 being the first substance that melted and 6 being the last. Some may not even melt. **Consider putting a symbol such as +, ++, +++ to represent the brightness of the bulb if it conducts. If the substance does not conduct electricity, then write “does not conduct.”

Conclusions 1. Inferring Conclusions Which substances were ionic compounds and which were covalent compounds? i. Ionic:

ii. Covalent:

2. Relating Ideas Write a statement to summarize the properties of ionic compounds and another statement to summarize the properties of covalent compounds. i. Ionic:

ii. Covalent:

3. Summarizing Ideas: Fill in the following table comparing ionic compounds and covalent molecules in each of the four categories. Type of Solubility in Melting Point Conductivity Elements water (High or low) (yes or no) (metals and (yes or no) nonmetals) Ionic Compounds Covalent Molecules

Model 3: Determining Type of Bonds (Electronegativity) Ionic or Covalent? Bonding between atoms of different elements is rarely purely ionic or purely covalent. It usually falls somewhere between these two extremes, depending on how strongly the atoms of each element attract electrons. Recall that electronegativity is a measure of an atom’s ability to attract electrons. The degree to which bonding between atoms of two elements is ionic or covalent can be estimated by calculating the difference in the elements’ electronegativities (∆EN). For example, the electronegativity difference between fluorine, F, and cesium, Cs, is 4.0 - 0.7 = 3.3. (See Figure 1 below for a periodic table of electronegativity values.) So, according to Figure 2 below, cesium-fluorine bonding is ionic. Figure 1: Electronegativity Values

Figure 2: Electronegativity Difference and Bond Type Electronegatvity Difference(∆EN) Most Probable type of Bond 0.0 – 0.4 Nonpolar covalent 0.4 – 1.0 Moderately Polar 1.0 – 2.0 Very Polar Covalent ≥ 2.0 Ionic ∆EN is the difference in electronegativity (absolute) between the two elements Critical Thinking Questions: 5. Suppose that a free electron was placed between each of the following sets of atoms. Predict the outcome of each contest by matching each set of atoms with a description. F with F

__________

One atom attracts the electron strongly and pulls the electron towards itself.

Li with O

___________ Both atoms attract the electron strongly. The electron stays between the atom.

6. Use figure 1 to calculate the ∆EN for each pair of elements, use figure 2 to state what type of bond the compounds have, and use the periodic table to state what type of elements they are. ∆EN (use figure 1)

Type of Bond (use figure 2 and ∆EN)

Type of Elements (metal, non-metal, metalloid)

K and Cl H and Br Cl and F Br and Br

Model 4: Metallic Bonding Do metals tend to have high or low electronegativities? (Look back at Figure 1 if you need to)

What does this mean about how metals hold on to their electrons?

So, what does this mean for how metals bond together? We know that something must be holding the metal atoms together since they are (mostly) solids at room temperature. It is a type of bonding called metallic bonding. Without metallic bonding, metal atoms would remain as separate, individual atoms, like the Noble gases. Metallic bonding is defined as the force of attraction between atoms of the SAME metal due to pooling of their valence electrons to form a delocalized “sea” of electrons.

As you stated above, metals have low electronegativity, so their valence electrons are only loosely held to the nucleus, making them relatively free to move. In metallic bonding, the positive metal cations are packed closely together, something like marbles in a box. The valence electrons are not associated with a single atom, but are “delocalized” and free to move from atom to atom. That is, the positive metal ions are surrounded by a “sea” or “cloud” of delocalized electrons. This is called the “free-electron” or “electron sea” model of metallic bonding. This model explains many of the observed properties of metals. Match each explanation of what’s happening inside the metallic bond to the metallic property it makes possible. a. Malleable & ductile

b. Conductors of heat

c. Electrical conductor

d. Lustrous

______ 1. When light hits the surface of a metal, the free valence electrons on the surface absorb and then re-emit the light energy ______ 2. When an electric current (a stream of electrons) is passed through a metal, the electrons are free to move from metal ion to metal ion. ______ 3. When a piece of metal is hammered or stretched, the atoms slide or roll past one another. The metal atoms rearrange themselves within the “electron sea” to assume the new shape without breaking their metallic bonds. ______ 4. If energy is added to a metal, the metal ions and electrons move faster. As these particles vibrate and move, they collide with their neighbors, which transmits the heat through the piece of metal. It is important to note that this type of bonding is only happening between atoms of the same type of metal. An alloy is a mixture of a metal with one or more other elements. The other elements may be metals, nonmetals, or both. An alloy is a solid solution. It is formed by melting a metal and dissolving the other elements in it. The molten solution is then allowed to cool and harden. Several other examples of alloys and their uses are shown in the figure below .

Exercises: 1. Suggest a type of compound which would have the following characteristic a) conducts electricity in the solid state b) has a high melting point and is brittle c) has a low melting point d) conducts electricity in the molten state e) does not conduct electricity 2. Based on your knowledge of how each type of bond is formed, match the type of bond with the model representing a compound with that type of bond. Justify your answers.

__________

__________

_________

3. Based on what you just learned, put the following into the chart as either IONIC or COVALENT COMPOUNDS. KCl Fe2O3

Rb2S Na

Cd3Mg NaBr

IONIC COMPOUNDS

Cl2 CaF2

PH3 Mg

SCl6

COVALENT COMPOUNDS

Zn

CH3Cl

N2O5

SO2 CuF2

MgCl2 CS2

METALLIC COMPOUNDS