Whatisenthalpy 121129113000-phpapp02


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Whatisenthalpy 121129113000-phpapp02

  1. 1. What is Enthalpy? Author: Dr. Robert D. Craig, Ph.D Day 13 This lesson and Laboratroy procedure was adapted from http://chemistry.allinfoabout.com/features/calorimeter.htm by Dr. Anne Helmenstine and Physical properties of matter by Carl Martiken http://www.iit.edu/~smart/martcar/lesson5/id37.htm The problem set was developed by S.E. Van Bramer for Chemistry 145 at Widener University and the 1997 regents Exam Aim: What is Enthalpy? IO/SWBAT : 1. Understand that Energy is exchanged or transformed in all chemical reactions and physical changes of matter. As a basis for understanding this concept: (a) Students know how to describe temperature and heat flow in terms of the motion of molecules (or atoms) and (b) Students know chemical processes can either release (exothermic) or absorb (endothermic) thermal energy. CONTENT STANDARD: (THE PHYSICAL SETTING) www.nysed.gov 1. Chemical and physical changes can be exothermic or endothermic (4.1b) 2. Distinguish between endothermic and exothermic reactions, using energy terms in a reaction equation, ∆H, potential energy diagrams or experimental data (4.1i) PROCESS STANDARDS: http://www.emsc.nysed.gov/ciai/mst/sciencestand/physset13.html http://www.emsc.nysed.gov/ciai/mst/sciencestand/physset45.html Standard 1: Analysis, Inquiry and Design Mathematical Analysis
  2. 2. Students will use mathematical analysis to calculate the heat involved in a phase or temperature change for a given sample of matter (4.2iv) Standard 4: Science develop their own mental models to explain common chemical reactions and changes in states Motivation: How to measure heat flow and enthalpy change using a Coffee Cup Bomb Calorimetry ? Lesson: The term enthalpy is composed of the prefix en-, meaning to "put into", plus the Greek word -thalpein, meaning "to heat", It is often calculated as a differential sum, describing the changes within exo- and endothermic reactions, which minimize at equilibrium Enthalpy change is defined by the following equation: Where, ΔH is the enthalpy change Hfinal is the final enthalpy of the system, measured in joules. In a chemical reaction, Hfinal is the enthalpy of the products. Hinitial is the initial enthalpy of the system, measured in joules. In a chemical reaction, Hinitial is the enthalpy of the reactants. The Bomb Calorimeter A calorimeter is a device that is used to measure the quantity of heat flow in a chemical reaction. Two common types of calorimeters are the coffee cup calorimeter and the bomb calorimeter the devise has an outer insulated portion which is viewed as the end of the universe—no heat or work can pass it. The contents of the outer insulated container consists of the stell “bomb”, a sample dish within the “bomb”, ignition wires into the “bomb” and touching the chemical sample dish, water surrounding the “bomb”, a stirrer, and a thermometer. The contents of the “bomb” are the system and the other contents of insulate container (including the walls of the “bomb”) are the surroundings. The wall of the “bomb” is the boundary. The “bomb” confines the system to constant volume.
  3. 3. From the law of conservation of energy, we can deduce that The heat transferred from the system = the heat transferred into the surrounding the left term is just the heat of the reaction (qv) and the right term is the sum of the heat absorbed by the water and the heat absorbed by the bomb’s stainless steel walls so we have -qv = q water + q bomb Where the negative sign is required because heat is lost from the system (exothermic) To determine the above we will need the individual values: Q water = mass of water*(specific heat of water)*(∆T) and Q bomb = heat capacity of bomb * ∆T The heat capacity of the bomb is determined by first doing an experiment with some chemical for which you know the heat of combustion so that you can solve the equations for the heat capacity of the bomb. Then the unknown is run using the previously determined value for the heat capacity of the bomb. Procedure: The coffee Cup Calorimeter Students will begin experiment by Carl Martiken http://www.iit.edu/~smart/martcar/lesson5/id37.htm Back ground Information: A coffee cup calorimeter is essentially a polystyrene (Styrofoam) cup with a lid. Really, any well-insulated container will work. The cup is partially filled with a known volume of water and a thermometer is inserted through the lid of the cup so that the thermometer is inserted through the lid of cup so that the thermometer bulb is below the surface. The water absorbs the heat of any chemical reaction taking place in the
  4. 4. calorimeter. The change in the water temperature is used to calculate the amount of heat that has been absorbed. Heat flow is calculated using the relation: Q = (specific heat) x m x ∆T Where q is heat flow, m is mass in grams, and ∆T is the change in temperature. The specific heat is the amount of heat required to raise the temperature of 1 gram of a substance 1 degree Celcius. The specific heat of (pure) water is 4.18 J/(g.o C) For example, consider a chemical reaction which occurs in 200 grams of water with an initial temperature of 25.0 o C. The reaction is allowed to proceed in the coffee cup calorimeter. As a result of the reaction, the temperature of the water changes to 31.0C. the heat flow is calculated: q water = 4.18 j/(g.o C) x 200 g x (31.0 o C -25.0 o C) q water = +5.0 x 103 J In other words, the products of the reaction evolved 5000 J of heat, which was lost to the water. The enthalpy change, ∆ H, for the reaction is equal in magnitude by opposite to the heat flow for the water ∆ H reaction = - (q water) For an exothermic reaction, ∆H < 0; q water is positive. The water absorbs heat from the reaction and an increase in temperature is seen. For an endothermic reaction, ∆H > 0; q water is negative. The water supplies heat for the reaction and a decrease in temperature is seen A coffee cup calorimeter is great for measuring heat flow in a solution, but it can’t be used for reactions which involve gases, since they would escape from the cup. Also, a coffee cup calorimeter can’t be used for high temperature reactions, since high heat would meld the cup. A bomb calorimeter is used to measure heat flows for gases and high temperature reactions. ‘ A bomb calorimeter works the same way as a coffee cup calorimeter, with one big difference. In a coffee cup calorimeter, the reaction takes place in the water. In a bomb calorimeter, the reaction takes place in a sealed metal container, which is placed in the water in an insulated container. Heat flow from the reaction crosses the walls of the sealed container to the water. The temperature difference of the water is measured, just as it was for a coffee cup calorimeter.
  5. 5. Analysis of the heat flow is a bit more complex than it was for the coffee cup calorimeter because the heat flow into the metal parts of the calorimeter must be taken into account: q reaction = -(q water + q bomb) Where q water = 4.18 J/ (g. o C)) x mwater x ∆T The bomb has a fixed mass and specific heat. The mass of the bomb multiplied by its specific heat is sometimes termed the calorimeter constant, denoted by the symbol C with units of joules per degree Celsius. The calorimeter constant, denoted by the symbol C with units of joules per degree Celsius. The calorimeter constant is determined experimentally and will vary from one calorimeter to the next. The heat flow of the bomb is: q bomb = C x ∆T Once the calorimeter constant is known, calculating heat flow is a simple matter. The pressure within a bomb calorimeter often changes during a reaction, so the heat flow may not be equal in magnitude to the enthalpy change.
  6. 6. Energy and Enthalpy Homework Problem Set This problem set was developed by S.E. Van Bramer for Chemistry 145 at Widener University. 1. What occurs when the temperature of 10.0 grams of water (June ’93) is changed from 15.5 o C to 14.5 o C a. The water absorbs 10.0 calories b. The Water releases 10.0 calories c. The water absorbs 155 calories d. The water releases 145 calories 2. A piece of titanium metal (mass 452.398 g) is placed in boiling water (100.00 °C). After 20 minutes it is removed from the boiling water and placed in a 1.000 liter container of water at 20.00 °C. The temperature of the water increases to 24.28 °C. What is the specific heat of titanium? 3. Next the same piece of titanium is heated in acetylene flame (like that used for welding) to an unknown temperature. When the pieced of titanium is placed in a 10.000 liter container of water at 20.00 o C the final temperature is now 30.72 o C. What is the temperature of the flame? At what temperature does titanium melt? 4. Calculate the energy required to heat a 155.4 g ice cube that starts in a freezer at -100.0 °C (VERY COLD): a. Heat from the freezer to ice at 0.0 °C. b. Heat from ice at 0.0 °C to liquid at 0.0°C. c. Heat from liquid at 0.0 °C to liquid at 100.0 °C. d. Heat from liquid at 100.0 °C to gas at 100.0 °C. e. Heat from gas at 100.0 °C to gas at 200.0 °C.
  7. 7. f. Heat from ice at -100.0 °C to gas at 200.0 °C