Section 9: Thermodynamics and Energy

Transcription

1 Section 9: Thermodynamics and Energy The following maps the videos in this section to the Texas Essential Knowledge and Skills for Science TAC (c) Law of Conservation of Energy Chemistry (11)(A) Chemistry (11)(B) Chemistry (11)(D) 9.02 Calorimetry Chemistry (11)(B) Chemistry (11)(C) Chemistry (11)(E) 9.03 Enthalpy and Constant-Volume Calorimeters Chemistry (11)(B) Chemistry (11)(E) 9.04 Thermochemical Equations Chemistry (11)(C) 9.05 Heat in Changes of State Chemistry (11)(E) 9.06 Hess s Law Chemistry (11)(C) 9.07 Standard Heats of Formation Chemistry (11)(C) Note: Unless stated otherwise, any sample data is fictitious and used solely for the purpose of instruction. Safety Note: Any chemicals mentioned in these videos are potentially harmful and should be handled with the appropriate safety precautions. Copyright 2017 Licensed and Authorized for Use Only by Texas Education Agency 1

2 9.01 Law of Conservation of Energy The law of conservation of energy states that in any chemical reaction or physical process, the total energy of the chemical reaction or physical process cannot be created or destroyed, only converted from one form to another form. Energy is defined as the ability to do work or produce heat. It exists in two basic forms: Kinetic energy is the energy of motion, like running down a hill. Potential energy is the amount of energy that results from an object s position or composition (chemical potential energy). The chemical potential energy of butane, for example, results from the arrangement of the carbon and hydrogen atoms and the strength of the bonds that join them. Thermal energy is the energy that comes from heat. Heat is generated by the movement of tiny particles in an object. It is directly related to the kinetic energy of the particles within the object being studied. The SI unit of energy, and of heat, is the joule (J). Below are a few different ways to express energy and their conversion factors: o 1 calorie (cal) = J o 1 Cal (food Calorie) = 1,000 calories = 1 kcal The heat (q) of a substance or system can be either absorbed or released. When heat is released by the substance, the value for q is. When heat is absorbed by the substance, the value for q is. The equation q = m c DT is used to calculate heat. The variable m represents the mass of the sample in grams, c represents the specific heat of the substance, and DT is the change in temperature in degrees Celsius, calculated as DT = T final T initial. The specific heat of any substance is the amount of heat required to raise the temperature of one gram of that substance by one degree Celsius. Copyright 2017 Licensed and Authorized for Use Only by Texas Education Agency 2

3 1. A can of soda contains J of energy, while a slice of pizza contains J. If you drink a six-pack of soda and eat five slices of pizza during a San Antonio Spurs basketball game, by how many Calories did you exceed the required daily intake of 2,000 Calories per day? A. 462 Calories B. 280 Calories C. 690 Calories D. 375 Calories 2. Calculate the heat released, in joules, when 54.8 grams of iron is cooled from 45.9 C to 21.0 C. The specific heat of iron is J/g C. Copyright 2017 Licensed and Authorized for Use Only by Texas Education Agency 3

4 9.02 Calorimetry When determining energy changes, identifying the system and the surroundings is very important. The system is the object or reaction being studied. The surroundings are everything around the system. In thermochemical calculations, the direction of heat flow is given from the point of view of the system. In an endothermic process, the system gains heat as the surroundings lose heat. In an exothermic process, the system releases heat to its surroundings. Calorimetry is the measurement of the heat transfer into or out of a system for chemical and physical processes. In a calorimetry experiment, the amount of heat lost by a system equals the amount of heat gained by the surroundings, and vice versa. When the temperature of the system becomes equal to the temperature of the surroundings, we say the system and the surroundings have reached thermal equilibrium, which means no heat is being transferred between them. q system = -q surroundings A calorimeter is an insulated device used to measure the absorption or release of heat in chemical or physical processes. There are two types of calorimeters: Constant-pressure calorimeters An example of this type is a foam cup, since it does not let much heat in or out. Constant-volume calorimeters An example of this type is a bomb calorimeter (not related to a military weapon). In a bomb calorimeter, a sample of a compound is burned in a constant-volume chamber in the presence of oxygen, at high pressure. Copyright 2017 Licensed and Authorized for Use Only by Texas Education Agency 4

5 1. Hot water is dropped into an insulated cup that contains g of water at room temperature (21.00 C). The final mass of the mixture is g and the final temperature of the mixture is C. The specific heat of water is J/ g C. What was the initial temperature of the hot water? A C B C C C D C Copyright 2017 Licensed and Authorized for Use Only by Texas Education Agency 5

6 9.03 Enthalpy and Constant-Volume Calorimeters The enthalpy (H) of a system is the heat content of the system at constant pressure. The heat absorbed or released by a reaction at constant pressure is the same as the change in enthalpy, represented as DH. The numerical DH value of a reaction can be determined by measuring the heat transfer of the reaction at constant pressure. For our purposes, heat and enthalpy are interchangeable because the reactions we are dealing with occur at constant pressure. In other words, q = DH. In reactions, q sys = DH = -q surr = - m c DT. 1. When 45 ml of water containing mol HBr at 25 C is added to 45 ml of water containing mol KOH at 25 C in a foam cup calorimeter, a reaction occurs. Calculate the enthalpy change (in kilojoules) during this reaction, if the highest temperature observed is 40.0 C. Assume the densities of the solutions are 1 g/ml and that the volume of the final solution is equal to the sum of the volumes of the reacting solutions. Copyright 2017 Licensed and Authorized for Use Only by Texas Education Agency 6

7 9.04 Thermochemical Equations A thermochemical equation is a chemical equation that includes the enthalpy change. The enthalpy change in a chemical equation can be written as either a reactant or a product. For exothermic reactions, the enthalpy change is a product. For endothermic reactions, the enthalpy change is a reactant. The heat of reaction is the enthalpy change for the chemical equation exactly as it is written. It is expressed as the heat of reaction based on the quantity that caused the heat (in kj/mol). The heat of reaction can be used to convert energy to moles, or moles back to energy. If a reaction is exothermic, the chemical potential energy of the reactants is higher than the chemical potential energy of the products. The opposite is true in endothermic reactions. Examples of heats of reaction: o Heat of combustion The heat of reaction for the complete burning of one mole of a substance o Heat of solution The heat of reaction for the dissolution of one mole of a substance Enthalpy can be thought of as a formula and a conversion factor. 1. The oxidation of aluminum is harmful to the structure of buildings. Aluminum reacts with iron(iii) oxide to yield iron and aluminum oxide. 2 Al(s) + Fe 2 O 3 (s) 2 Fe(s) + Al 2 O 3 (s) The reaction of 4.72 g Al(s) evolves 73.7 kj of heat. Calculate the enthalpy change per mole of aluminum combusted. Copyright 2017 Licensed and Authorized for Use Only by Texas Education Agency 7

8 2. Elemental sulfur (S 8 ) reacts in the presence of oxygen to form sulfur dioxide gas. If the exothermic reaction releases 850 kj of heat upon the combustion of one mole of elemental sulfur, what quantity of heat is released when 127 g of oxygen is reacted? A. 127 kj B. 1,080 kj C. 850 kj D. 422 kj Copyright 2017 Licensed and Authorized for Use Only by Texas Education Agency 8

9 9.05 Heat in Changes of State The molar heat of fusion (DH fus ) is the heat absorbed by one mole of a solid substance as it melts to a liquid at constant temperature. The molar heat of solidification (DH solid ) is the heat lost by one mole of a liquid substance as it solidifies at constant temperature. The quantity of heat absorbed by melting a solid is the same as the quantity of heat released when the liquid solidifies. In other words, DH solid = -DH fus. The molar heat of vaporization (DH vap ) is the heat absorbed by one mole of a liquid substance as it vaporizes at constant temperature. The molar heat of condensation (DH cond ) is the heat lost by one mole of a vapor as it condenses at constant temperature. The quantity of heat absorbed by vaporizing a liquid is the same as the quantity of heat released when the vapor condenses. In other words, DH cond = -DH vap. When changing state from solid to liquid to gas, heat is absorbed or gained. When changing state from gas to liquid to solid, heat is liberated or released. Copyright 2017 Licensed and Authorized for Use Only by Texas Education Agency 9

10 A change-of-state diagram (also known as a heating curve) summarizes the enthalpy changes that occur as a solid is heated to a liquid and then to a vapor. Below is a generic example of such a diagram. The diagram above can be used to calculate the heat released or required by a physical process. For a segment with a temperature change, use the formula q = m c DT. For a segment with a phase change, use the formula q = m DH. o For fusion, the formula is q = m DH fus. o For freezing, the formula is q = m -DH fus. o For vaporization, the formula is q = m DH vap. o For condensation, the formula is q = m -DH vap. Copyright 2017 Licensed and Authorized for Use Only by Texas Education Agency 10

11 1. How much heat (in kilojoules) is released when 45.9 g of H 2 O(g) at C and kpa is converted to H 2 O(l) at C? DH vap for water is 40.7 kj/mol and boiling point of water is C 2. You place a bottle containing 2.0 L of water at 32 C into a refrigerator to cool to 5.0 C. How many kilojoules of heat does the water lose? Assume the density of water is 1 g/ml and the specific heat of water is J/g C. Copyright 2017 Licensed and Authorized for Use Only by Texas Education Agency 11

12 9.06 Hess s Law Hess s law of heat summation states that if you add two or more reactions to give a final equation, then you can also add the heats of reaction to give the final heat of reaction. This law allows you to determine the heat of reaction indirectly by using the known heats of reaction of two or more reactions. The process for using Hess s law to determine the heat of reaction for a final equation is outlined below: Match the reactants and the products with the desired equation. Make sure the reactants and the products have the same stoichiometric coefficients as the desired equation. If the same reactant or product ends on different sides of the yield arrow, then cancel them out. If an equation is reversed, then the heat of reaction should be reversed as well. If an equation is multiplied by any factor, make sure to multiply the heat of reaction by that same factor. Think of this as a puzzle. You need to manipulate equations to get the result. Copyright 2017 Licensed and Authorized for Use Only by Texas Education Agency 12

13 1. What is the enthalpy change (DH) for the reaction below? P 2 O 3 (l) + H 2 O 2 (l) 2 PO 2 (l) + H 2 O(g) You will need the following information from reactions: 4 PO 2 (l) + 2 C(s) 2 P 2 O 3 (l) + 2 CO(g) DH = kj CO(g) C(s) + ½ O 2 (g) DH = kj H 2 O(g) + ½ O 2 (g) H 2 O 2 (l) DH = kj A kj B. -1,096 kj C kj D kj Copyright 2017 Licensed and Authorized for Use Only by Texas Education Agency 13

14 9.07 Standard Heats of Formation The standard heat of formation (DH f ) of a compound is the change in enthalpy that accompanies the formation of one mole of a compound from its elements, with all substances in their standard states. Na(s) + ½ Cl 2 (g) NaCl(s) DH f = "# $%& The standard state of an element is defined as the most stable form of the substance in the physical state that exists at a pressure of 1 atm (101.3 kpa) and 25 C. Examples of elements in standard states include diatomic elements, lone metals, C graphite, S 8 (s), and P 4 (s). The DH f of a free element in its standard state is zero. If a reaction occurs under standard conditions, an alternative to Hess s law is to use the standard heats of formation to find the heat of reaction. The standard heat of reaction is the difference between the standard heats of formation of all reactants and products. ( =, ( -./012345, ( - /78348,45 To use this method, you must be given a table of all the DH f in the final reaction. Copyright 2017 Licensed and Authorized for Use Only by Texas Education Agency 14

15 1. The combustion of one mole of gaseous C 2 H 4 in excess oxygen to produce carbon dioxide and water releases 1,411 kj per mole of C 2 H 4 combusted. Determine the molar enthalpy of formation of gaseous ethene (C 2 H 4 ). DH f [H 2 O (l)] = kj/mol DH f [H 2 O (g)] = kj/mol DH f [CO 2 (g)] = kj/mol A kj/mol B kj/mol C kj/mol D kj/mol Copyright 2017 Licensed and Authorized for Use Only by Texas Education Agency 15

16 Copyright 2017 Licensed and Authorized for Use Only by Texas Education Agency 16