What is enthalpy
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What is enthalpy

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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 ...

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.

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What is enthalpy Document Transcript

  • 1. What is Enthalpy?Author: Dr. Robert D. Craig, Ph.DDay 13This lesson and Laboratroy procedure was adapted from http://chemistry.allinfoabout.com/features/calorimeter.htmby Dr. Anne HelmenstineandPhysical properties of matter by Carl Martikenhttp://www.iit.edu/~smart/martcar/lesson5/id37.htmThe problem set was developed by S.E. Van Bramer for Chemistry 145 at Widener University and the 1997 regents ExamAim: 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.htmlhttp://www.emsc.nysed.gov/ciai/mst/sciencestand/physset45.htm lStandard 1: Analysis, Inquiry and DesignMathematical Analysis
  • 2. Students will use mathematical analysis to calculate the heat involved in a phase ortemperature change for a given sample of matter (4.2iv)Standard 4: Sciencedevelop their own mental models to explain common chemical reactions and changes instatesMotivation: How to measure heat flow and enthalpy change using a Coffee CupBomb Calorimetry ?Lesson:The term enthalpy is composed of the prefix en-, meaning to "put into", plus the Greekword -thalpein, meaning "to heat",It is often calculated as a differential sum, describing the changes within exo- andendothermic reactions, which minimize at equilibrium Enthalpy change is defined by thefollowing equation:Where,ΔH is the enthalpy changeHfinal 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 CalorimeterA calorimeter is a device that is used to measure the quantity of heat flow in a chemicalreaction. Two common types of calorimeters are the coffee cup calorimeter and the bombcalorimeter the devise has an outer insulated portion which is viewed as the end of theuniverse—no heat or work can pass it. The contents of the outer insulated containerconsists 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 ofinsulate container (including the walls of the “bomb”) are the surroundings. The wall ofthe “bomb” is the boundary. The “bomb” confines the system to constant volume.
  • 3. From the law of conservation of energy, we can deduce thatThe heat transferred from the system = the heat transferred into the surrounding the leftterm is just the heat of the reaction (qv) and the right term is the sum of the heat absorbedby the water and the heat absorbed by the bomb’s stainless steel walls so we have-qv = q water + q bombWhere 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) andQ bomb = heat capacity of bomb * ∆TThe heat capacity of the bomb is determined by first doing an experiment with somechemical for which you know the heat of combustion so that you can solve the equationsfor the heat capacity of the bomb. Then the unknown is run using the previouslydetermined value for the heat capacity of the bomb.Procedure:The coffee Cup CalorimeterStudents will begin experiment by Carl Martikenhttp://www.iit.edu/~smart/martcar/lesson5/id37.htmBack 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 knownvolume of water and a thermometer is inserted through the lid of the cup so that thethermometer is inserted through the lid of cup so that the thermometer bulb is below thesurface. The water absorbs the heat of any chemical reaction taking place in the
  • 4. calorimeter. The change in the water temperature is used to calculate the amount of heatthat has been absorbed.Heat flow is calculated using the relation:Q = (specific heat) x m x ∆TWhere q is heat flow, m is mass in grams, and ∆T is the change in temperature. Thespecific heat is the amount of heat required to raise the temperature of 1 gram of asubstance 1 degree Celcius. The specific heat of (pure) water is 4.18 J/(g.oC)For example, consider a chemical reaction which occurs in 200 grams of water with aninitial temperature of 25.0 oC. The reaction is allowed to proceed in the coffee cupcalorimeter. 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.oC) x 200 g x (31.0 oC -25.0 oC)q water = +5.0 x 103 JIn other words, the products of the reaction evolved 5000 J of heat, which was lost to thewater. The enthalpy change, ∆ H, for the reaction is equal in magnitude by opposite tothe heat flow for the water∆ H reaction = - (q water)For an exothermic reaction, ∆H < 0; q water is positive. The water absorbs heat from thereaction and an increase in temperature is seen. For an endothermic reaction, ∆H > 0; qwater is negative. The water supplies heat for the reaction and a decrease in temperatureis seenA coffee cup calorimeter is great for measuring heat flow in a solution, but it can’t beused for reactions which involve gases, since they would escape from the cup. Also, acoffee cup calorimeter can’t be used for high temperature reactions, since high heatwould meld the cup. A bomb calorimeter is used to measure heat flows for gases andhigh temperature reactions.‘A bomb calorimeter works the same way as a coffee cup calorimeter, with one bigdifference. In a coffee cup calorimeter, the reaction takes place in the water. In a bombcalorimeter, the reaction takes place in a sealed metal container, which is placed in thewater in an insulated container. Heat flow from the reaction crosses the walls of thesealed container to the water. The temperature difference of the water is measured, justas it was for a coffee cup calorimeter.
  • 5. Analysis of the heat flow is a bit more complex than it was for the coffee cup calorimeterbecause 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. oC)) x mwater x ∆TThe bomb has a fixed mass and specific heat. The mass of the bomb multiplied by itsspecific heat is sometimes termed the calorimeter constant, denoted by the symbol C withunits of joules per degree Celsius. The calorimeter constant, denoted by the symbol Cwith units of joules per degree Celsius. The calorimeter constant is determinedexperimentally and will vary from one calorimeter to the next. The heat flow of thebomb is:q bomb = C x ∆TOnce the calorimeter constant is known, calculating heat flow is a simple matter. Thepressure within a bomb calorimeter often changes during a reaction, so the heat flow maynot be equal in magnitude to the enthalpy change.
  • 6. Energy and Enthalpy Homework Problem SetThis 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 changedfrom 15.5 oC to 14.5 oC 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 oC the final temperature is now 30.72 oC. 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. f. Heat from ice at -100.0 °C to gas at 200.0 °C