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This 5 page Class Notes was uploaded by Kyle A. Headen on Tuesday October 11, 2016. The Class Notes belongs to CHEM 1307 at Texas Tech University taught by Tamara Hanna in Fall. Since its upload, it has received 4 views. For similar materials see /class/226511/chem-1307-texas-tech-university in Chemistry at Texas Tech University.
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Date Created: 10/11/16
Chapter6 Energy: The scientific description of changes that take place in the physical world is centered around the concept of energy. The concept of energy is deeply entangled with the idea of objects in motion. Objects in motion are said to have kinetic energy. Stored energy that is capable of causing an object to move in the future is called potential energy. One of the fundamental postulates of science is the Law of Conservation of Energy. It states that energy is neither created nor destroyed in any physical process – it is just converted from one form to another or moved from place to place. The total energy of the universe remains constant. The Law of Conservation of Energy is also known as the First Law of Thermodynamics. The various forms of energy are classified in many ways, depending upon the identity of the objects and the forces acting upon them, the direction of the motion, and whether or not the energy is kinetic or potential. Forms of Energy Thermal energy Translational energy Nuclear energy Rotational energy Electrical energy Vibrational energy Mechanical energy Chemical energy Virtually all forms of energy can be classified as kinetic energy or potential energy. Kinetic energy is a well-defined quantity that is a function of the mass (m) of the moving object and its speed (v) relative to a fixed point. Kinetic Energy = ½ m v2 The basic S.I. unit for all forms of energy is the joule. 1 joule (J) = 1 kg m2 s-2 Note that these units come directly from the formula for kinetic energy. The kinetic energy of an object is either zero (when the object is not moving) or a positive number. Its value can always be determined if the mass and the speedof the object are known. The potential energy of an object cannot be determined, because we cannot know all of the possible amounts and all of the possible forms of energy that could be releasedby the object. Chemical transformations involve changes in the potential energy of substances. We cannot know the value of the potential energy of the substances before the process begins, and we do not know the value of the potential energy of the substances when the process ends, but we can obtain a value for the change in energy. Thermodynamics: Thermodynamics is a field of science that describes the changes in energy that occur when substances undergo a chemical or physical change. In order to measure the changes in energy that are associated with a process we divide the universe into three parts. The system is the object of study. The system is separated from the rest of the universe by a boundary. The rest of the universe outside the boundary is called the surroundings. Internal Energy: “The internal energy (E) of a system is the sum of the kinetic and potential energies of all of the particles that compose the system.” “Internal energy is a state function, which means that its value depends only on the state of the system, not on how the system arrived at that state.” “The state of a chemical system is specified by parameters such as temperature, pressure, concentration, and physical state (solid, liquid, or gas).” State Functions: A change in the value of a state function depends only upon the values for the initial state and the final state. If an object is moved from an initial point to a final point, the change in position is equal to the distance between the two points. This would always have the same value, and would therefore be a state function. The distance traveled in getting from the first point to the final point can vary, depending the route that is taken. It would therefore not be a state function. Pressure, volume, temperature, and concentration are all state functions. Thermodynamics: According to the Law of Conservation of Energy, when energy is transferred into or out of a thermodynamic system, the energy lost by the system must be equal to the energy gained by the surroundings (or vice versa) so that the total energy of the universe remains constant. Internal Energy: The change in the internal energy of the system, DE, can be determined indirectly by measuring the energy that flows into or out of the system across the boundary. The sign of DE can be positive or negative, and is always determined from the viewpoint of the system. E is positive if energy flows into the system. E is negative if energy flows out of the system. Heat and Work Energy can cross the boundary of a system as heat (q) and/or work (w). The change in internal energy of the system is the sum of the heat and work transferred between the system and the surroundings. E = q + w Heat is the transfer of random kinetic energy (motion of particles in all directions) from an object at a higher temperature to another object at a lower temperature. Work is the application of force to an object that results in a displacement of the object in a particular direction (work = force x distance). Like E, the signs of q and w can be positive or negative, and they are always determined from the viewpoint of the system. q is positive if heat flows into the system, and negative if heat flows out of the system. w is positive if work is done on the system, and negative if work is done by the system on the surroundings. Internal energy is a state function, so a change in the internal energy, E, will depend only on the energies of the initial and final states of the system. However, E is made up of two components. E = q + w The relative contributions of q and w to E can vary, depending on how the change is carried out. Heat (q) and work (w) are not state functions. Thermodynamics: The “Zeroth Law of Thermodynamics” states that when two objects are brought into contact with each other and are allowed to transfer heat, the heat will flow from the object that is at higher temperature to the object at the lower temperature until they both reach the same intermediate temperature. At this point the two objects are said to be in thermal equilibrium. An endothermic process is one in which energy is transferred as heat into the system from the surroundings (q is positive). An exothermic process is one in which energy is transferred as heat out of the system to the surroundings (q is negative). In general, the transfer of a specific amount of heat to one substance can cause a different change in temperature than the transfer of the same amount of heat to a different substance. This ability to undergo a small or a large change in temperature upon the absorption of heat is called the specific heat capacity of the substance. q = (C) (m) (T) Where: q = the heat transferred to the object C = the specific heat capacity of the object m = the mass of the object T = the change in temperature of the object T = T - T final initial q = (C) (m) (T) q is positive when heat is transferred to the object and negative when heat leaves the object. T is positive when heat is transferred to the object (T finals larger than initialnd negative when heat leaves the object (Tfinals less than Tinitial
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