chemistry_chapter6notes

chemistry_chapter6notes - Chapter 6 Thermochemistry 6.1...

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Chapter 6: Thermochemistry 6.1 Light the Furnace: The Nature of Energy and Its Transformations Thermochemistry – the study of the relationships between chemistry and energy ex. The combustion of natural gas gives off heat The Nature of Energy: Key Definitions Energy – the capacity to do work o Types of energy: Kinetic energy – the energy possessed by an object due to its motion Thermal energy – the energy associated with the temperature of an object Potential energy – energy associated with the position or composition of an object Chemical energy – energy associated with the relative positions of electrons and nuclei in atoms and molecules, is also a form of potential energy o Ways that objects exchange energy: Work – the result of a force acting through a distance Heat – the flow of energy caused by a temperature difference o Law of conservation of energy – energy can be neither created nor destroyed. However, energy can be transferred from one object to another, and it can assume different forms. In an energy exchange, energy is transferred between the system and the surroundings. If the system loses energy, the surroundings gain energy, and vice versa. Units of Energy Definition of kinetic energy : KE = ½ mv 2 the SI unit of energy is kg m 2 /s 2 , defined as the joule (J) calorie (cal) – the amount of energy required to raise the temperature of 1 g of water by 1 °C 1 cal = 4.184 J (exact) Page | 1
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6.2 The First Law of Thermodynamics: There Is No Free Lunch Thermodynamics – the study of energy and its interconversions First law of thermodynamics (law of energy conversion) – the total energy of the universe is constant o because energy is neither created nor destroyed, and the universe does not exchange energy with anything else, its energy content does not change o ex. Although some new hybrid cars (electric and gasoline powered) can capture energy from braking and use that energy to recharge their batteries, they could never run indefinitely without adding fuel Internal Energy internal energy ( E ) – the sum of the kinetic and potential energies of all of the particles that compose the system state function – its value depends only on the state of the system, not how the system arrived at that state o The state of a chemical system is specified by parameters such as temperature, pressure, concentration, and phase (solid, liquid, or gas). o The value of a change in a state function is always the difference between its final and initial values Work and heat, however, are not state functions ex. Altitude is a state function. The change in altitude during a climb depends only on the difference between the final and initial altitudes Δ E = E final - E initial In a chemical system, the reactants constitute the initial state and the products constitute the final state Δ E system = -Δ E surroundings If the reactants have a higher internal energy than the products, Δ E system is negative and energy flows out of the system into the surroundings.
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