CHM101 Lab - Energy Grading Rubric

Transcription

1 Name Team Name CHM101 Lab - Energy Grading Rubric To participate in this lab you must have splash- proof goggles, proper shoes and attire. Criteria Points possible Points earned Lab Performance Printed lab handout and rubric was brought to lab 3 Safety and proper waste disposal procedures observed 2 Followed procedure correctly without depending too much on instructor or lab partner 3 Work space and glassware was cleaned up 1 Lab Report Part A (data recorded correctly, graph labeled according to directions, melting and boiling point correctly identified) Part B (data recorded with correct sig figs and units, calculations shown clearly, all questions answered) Part C (data recorded with correct sig figs and units, calculations shown clearly, all questions answered) Total 20 Subject to additional penalties at the discretion of the instructor.

2 Energy Goals 1. To measure and graph phase changes in water. 2. To measure the specific heat of a metal. Background Energy is the ability to do work or produce heat. Energy is usually expressed in units of joules (J) or calories (cal) where 1 cal = 4.18 J. In this lab you will explore energy as it relates to phase changes, specific heat and nutrition. Temperature Curves When energy in the form of heat is added to a substance, that energy can either raise the temperature or induce a phase change. Temperature curves plot temperature versus heat added (known as a heating curve) or removed (known as a cooling curve). An example of a heating curve for a substance is shown below. At the lowest temperature, before heat is added, the substance is in the solid phase. As heat is added, the temperature of the solid increases (A) until it reaches the melting point. At the melting point, all heat added is used to convert the solid to liquid, therefore the temperature is not changing and the graph appears flat (B). Once all solid has been converted to liquid, the temperature rises again and the graph again is sloped (C). When the substance reaches its boiling point, there is another flat region as the energy gets used to convert the liquid to a gas (D). Once it is completely converted to gas, the temperature rises again (E). On heating curves, sloped lines indicate that one phase is present. Flat, plateaued regions indicate that one phase is being converted to another. The temperature of a substance remains constant as it is converted from one phase to another. In this lab you will construct a partial heating curve for water, starting with a solid/liquid mixture and ending with a liquid/gas mixture. Specific Heat Capacity The specific heat (SH) of a substance is defined as the amount of energy it takes to raise the temperature of 1 gram of the substance 1 o C. Specific heat is usually expressed in units of J/g o C or cal/g o C. The higher the specific heat of a substance, the more energy is required to raise its temperature. A table of specific heats for some substances is given below. Substance Specific Heat (J/g o C) Gold Lead Brass Copper Zinc Iron Sand Aluminum Water 4.18 Water has an unusually high specific heat it takes a lot of energy to raise the temperature of water. At the beach on a hot day, the sand may burn your feet, but the water can be ice cold. This is due to sand having a much lower specific heat than water its temperature rises much more rapidly with heat from the sun. In this lab you will use the specific heat of water to determine the specific heat of a metal. You will take a hot piece of metal and place it in a cup with a known amount of water. The metal will cool down and the water will heat up until they both come to the same temperature.

3 To calculate the specific heat by this method, you will first determine the heat absorbed by the water. Do this using the specific heat formula below where q is heat, m is mass, ΔT is change in temperature, and SH is specific heat. (1) heat water = m water x ΔT water x SH water or heat water = m water x (T final T initial ) x SH water The table gives the specific heat of water, you can calculate the mass of water through its density (1.00 g/ml), and you will measure the final and initial temperatures of the water. Therefore you have all values needed to calculate the heat absorbed by the water (q water ). The heat lost by the metal as is cools is assumed to be equal to the heat absorbed by the water as given in equation (2). The sign changes because we think of heat absorbed as being positive and heat lost as being negative. (2) heat water = heat metal Finally, you will use the specific heat formula again to solve for the specific heat of the metal. (3) heat metal = m metal x ΔT metal x SH metal or heat metal = m metal x (T final T initial ) x SH metal You can compare your calculated specific heat to literature value in the table. Energy and Nutrition Energy stored in food is usually given in units of kilocalories (kcal). In the US, kcal are listed on packaging as Cal, the capital C indicating kilocalories. In this lab we will use kcal, not Cal, to reduce confusion cal = 1 kcal = 1 Cal (US notation) To determine the energy content of a food, scientists burn the food in a calorimeter. The energy absorbed by the calorimeter is equal to the energy released by the food. Energy in food comes from three major macronutrients: fats, carbohydrates and proteins. The energy stored in each of these nutrients is listed in the table below. Food Type Energy (kcal/g) Fats 9 Carbohydrates 4 Proteins 4 Note that fats are much more energy dense than carbohydrates or proteins. We will use these values to calculate the energy per serving of a food and compare to the energy listed on the nutrition panel item. Laboratory Activity Materials: Beaker 2 styrofoam cups ice tongs Hot Plate metal sample thermometer food packaging Procedure A. Heating Curve of Water 1. Cover the bottom of a 400 ml beaker with ice and add enough water to cover. The total volume should be about 150 ml. Place the beaker on in the center of a hot plate. Use a ring stand and clamp to suspend a thermometer in the water about 2 cm from the bottom.

4 2. Record the initial temperature to the nearest 0.1 o C. Turn the hot plate to 100 o C, and to 300 o C once the ice has melted. Continue to record the temperature every minute. Stir the beaker right before each measurement. Stop recording when the water has been at a full boil for 5 minutes. 3. You will need boiling water for Part B, so leave your beaker on the hot plate. 4. Graph your data on the grid provided in the data sheet. Label time on the x- axis and temperature on the y- axis such that the data take up most of the graph. Label the phases present at each part of the graph and label where phase changes are taking place. (see example graph in Background) Give the graph a descriptive title. B. Specific Heat of Metals 1. Obtain and record the mass of a piece of piece of metal. 2. Place the metal in beaker of boiling water using tongs. Boil the metal for 10 minutes. (Graph while waiting) 3. In a graduated cylinder, measure then pour 80.0 ml of cool water into a two nested Styrofoam cups. Measure the temperature of the water (This is T initial water ). Calculate the mass of the water using density (1.00 g/ml) don t put liquids on scales.) 4. After 10 minutes, measure the temperature of the boiling water bath (Assume the metal is at the same temperature as the boiling water, this is T initial metal ). 5. Remove the metal from the beaker. Working quickly, blot excess water on a paper towel and place the metal into the water in the Styrofoam cups. Record the maximum temperature that the water in the cup reaches. (This is T final for both the water and the metal) 6. Calculate the heat absorbed by the water (q water ) using equation (1). Use 4.18 J/g o C for the specific heat of water. 7. Determine the heat lost by the metal using equation (2). 8. Calculate the specific heat of the metal using equation (3) and compare it to the value given in the table. C. Energy and Nutrition 1. Choose a food product. Record values for grams of fat, carbohydrates, and protein per serving. 2. Calculate the energy in kilocalories for each nutrient. Add the values to calculate the total kilocalories per serving. Use this value to find the percent of total kilocalories for each nutrient. 3. Compare the caluclated total kilocalories to the Cal (kcal) per serving listed on the package. Waste Disposal deionized water sink

5 Energy: Data Sheet Name A. Heating Curve of Water Time (min) Temperature ( o C) Time (min) cont Temperature ( o C) cont Plot your data below. Follow the guidelines in the instructions. Melting point of water according to graph: Boiling point of water according to graph: Report Page 1 of 3

6 B. Specific Heat of Metals Metal used Volume of water in cup Mass of water in cup (m water ) T initial water T initial metal T final water = T final metal ΔT water (T final T initial ) ΔT metal (T final T initial ) watch the sign! Calculate the the heat absorbed by the water (heat water = m water x ΔT water x SH water ) heat water = heat metal = Calculate the the specific heat of the metal (heat metal = m metal x ΔT metal x SH metal ) Look up the specific heat of your metal in the table. How does this compare to your calculated specific heat? Report Page 2 of 3

7 Energy: Data Sheet C. Energy and nutrition Fats Name Grams per serving kcal per serving Percent of total kcal Carbohydrates Protein Total % 1. How many kcal ( Cal ) per serving are listed on the product label? Compare this to your calculated value. (Note food energy on packaging is rounded to the nearest 10 kcal ( Cal ) 2. a) Starting with your total calculated kcal, how many cal per serving are in this food product? b) How many calories of energy would be needed to heat g (1 L) of water from 25 o C to 100.C o? SH water = 1.00 cal/g o C c) Does one serving of your food contain enough energy to heat the water in b)? Report Page 3 of 3