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Metallic Character and Periodic Trends Lab (Teacher Notes) PSI Chemistry Objective: the Goals of This Lab Are To:  Study the Reactivity of Three Metals

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Metallic Character and Periodic Trends Lab (Teacher Notes) PSI Chemistry Objective: the Goals of This Lab Are To:  Study the Reactivity of Three Metals Metallic Character and Periodic Trends Lab (Teacher Notes) PSI Chemistry Objective: The goals of this lab are to: Study the reactivity of three metals. Predict an order for the relative reactivities of three metals based on experimentation and knowledge of periodic trends. Demonstrate the “alkaline” properties of alkali and alkaline earth metals. Background: Most metallic elements exhibit shiny luster, conduct heat and electricity and are malleable and ductile. All metallic elements are solids at room temperature except mercury. The relative metallic character can be predicted using the element's reactivity, their electron configurations and knowledge of periodic trends including effective nuclear charge Zeff. As we move across any row or period of the periodic table the number of core electrons remains the same though the nuclear charge increases. The valence electrons that are being added do little to sheild one another and therefore the attraction of the nucleus steadily increases. Group 1 and Group 2 metals with Zeff values of 1 and 2 respectively are more reactive than Group 3 metals with an effective nuclear charge of 3. Group 1 and Group 2 metals lose their outershell electrons more readily, are more reactive and more metallic in character. Alkali and Alkaline earth metals produce basic solutions when mixed with water. This can be tested using an indicator (phenolphthalein) that turns a pink color when in basic solution. Materials: Calcium metal tongs Magnesium metal (ribbon form) bunsen burner and lighter Steel wool phenolphthalein indicator, litmus paper, or Aluminum foil pH paper water watch glass (3) evaporating dishes (3) goggles and aprons Preparation (if possible before class): This lab is best completed with teams of up to 3 students to ensure all students have an opportunity to participate in the lab completing and can clearly make and note observations. The phenolphthalein solution should be pre-made and in a glass bottle with a dropper top. 30 mL amber glass bottles with dropper tops can be purchased from Flinn Scientific. Timing: This is a one period lab. Approximate timing is as follows: 5 minutes for students to collect and familiarize themselves with the materials for the lab and their lab sheet. 15 minutes Part A 10 minutes Part B 10 minutes for class discussion and conclusion Safety Concerns: To instruct the students in the proper use of the bunsen burners and to reduce any fire hazards have them go to http://www.echalk.co.uk/Science/practicalSkills/BunsenBurner/DnD/BunsenWorksheet.pdf and complete the exercise. Stress to the students the importance of not looking directly at the combustion of magnesium reaction. Instructors should familiarize students with the location of laboratory safety equipment. Students should know where to find running water, an eyewash and shower, an emergency blanket and the fire extinguisher before beginning the lab. Students should also be instructed on the following safety guidelines before beginning the lab: 1.Always wear your goggles and aprons while completing this lab. 2.Also wear your goggles and aprons when you're near someone working with chemicals. 3.Familiarize yourself with the location of the eyewash and the emergency shower in the room. If you get any chemicals in your eye, use the eyewash. 4.If any of the chemicals come in contact with your skin rinse them off immediately with lots of water and let the instructor know what you spilled --- on your way to rinsing them off. 5.Never mix chemicals that you haven't been told to mix. 6.Tell the instructor of any accidents immediately. 7.Keep food and drinks out of the laboratory work area. Disposal: Neutralize waste basic solutions before sending down drain. Pre Lab Activity Answer Key 1. For each of the following elements write the noble gas electron configuration and calculate the effective nuclear charge. Metal Noble gas electron Zeff = Z - S configuration Ca [Ar] 4s2 20 - 18 = 2 Mg [Ne] 3s2 12 -10 = 2 Al [Ne] 3s23p1 13 – 10 = 3 2. How does an element with an effective nuclear charge of 1 compare with an element with an effective nuclear charge of 2 in terms of metallic character? The larger the effective nuclear charge the greater the attraction of the element's valence electrons to the nucleus the less readily these electrons are lost, the less the metallic character of the element. Anticipated Results: The reaction of calcium with water will produce hydrogen gas and calcium hydroxide which is caustic. Have students be careful with handling and disposal. Expected Results Table A Metal Reaction with Addition of water Phenolphthalein Calcium Bubbles, gas pink Magnesium none no change or slight pink Aluminum none none Table B Metal Reaction with oxygen in Addition of flame phenolphthalein Magnesium Bright white light Pink Aluminum No reaction none Expected reactivity/metallic nature of metals: Ca > Mg > Al Rationale: Calcium and magnesium have the same effective nuclear charge (Zeff) but calcium’s valence electrons are a considerable distance further from the nucleus than magnesium’s thereby causing less of a pull on them allowing them to be lost easier. Aluminum would be least reactive as it has a greater effective nuclear charge due to it’s additional proton relative to it’s shielding coefficient. * One possible answer from students might be the prediction of aluminum being more reactive than magnesium due to magnesium’s relative stability provided for by it’s full s orbital. However, by reactive we mean, the ability to form their ionic state and magnesium requires considerably less energy to become Mg2+ than aluminum does to become Al3+. Follow Ups (next day): The instructor should emphasize that although magnesium and calcium are both alkaline earth metals with the same effective nuclear charge of 2, calcium is more reactive because its valence shell electrons are further from the nucleus. Calcium's valence electrons feel less attraction from its nucleus as compared to magnesium's valence shell electrons which are closer. Calcium is more reactive and more metallic in character for this reason. The influence of effective nuclear charge on other periodic trends could be discussed. Atomic radius decreases from left to right due to increases in effective nuclear charge. Ionization energy increases from left to right as effective nuclear charge increases and atomic radius decreases. As you down a column, atomic radius increases, effective nuclear changes little and ionization energy decreases. Metallic character can also be related to ionization energy. Metals tend to have low ionization energies and form positive ions relatively easy. Students could be given the assignment of researching the alkali and alkaline earth metals families. Their research could include name origins, a broad description the family, a description of important members of the family, and the influence of periodic properties on the physical and chemicla properties of the elements. Presentation and Handout Materials Smart Notebook class presentation and lab handouts for this lab are available through teacher access to the PSI Chemistry website in the Periodic Trends unit. .
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