chem161sg_chapter6

chem161sg_chapter6 - Chapter 6 Thermochemistry...

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Chapter 6 Thermochemistry Thermochemistry is a branch of thermodynamics (later) that deals with the heat effects (absorption or evolution) during a chemical reaction. When heat is evolved (manifested by increase in temperature of the reaction vessel) or heat is absorbed (manifested by decrease in temperature of the reaction vessel) transfer of energy occurs between the system and the surrounding . Definitions System: The part, the properties of which one wants to study. This is the reaction vessel and its contents. Surrounding: The part of the universe (outside the system) with which the system interacts. Energy: The capacity to do work or to supply heat. Kinetic Energy: mv 2 m = mass (kg); v = velocity (meters/second) The energy that a moving object possesses. 1 2 Potential energy: energy due to position (high or low) or composition of the system. Recall: C 4 H 10 + 13/2 O 2 Æ 4 CO 2 (g) + 5 H 2 O + heat (exothermic reaction) Interactions between the system and the surrounding refer to the transfer of energy or matter between them. Energy transfers can occur as heat (q) or work (w). Energy transfer (as q or w) affect the total amount of energy of the system. 1
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Systems could be… - hot H 2 O energy (as heat) is transferred from the system to the surrounding but no H 2 O vapor (matter) escapes a) open b) closed c) isolated - hot H 2 O energy (as heat) and matter (H 2 O vapor) are transferred from the system to the surrounding. - hot H 2 O in thermos theoretically, no heat or matter should transfer from the system to the surrounding (but eventually the thermos cools down) Internal Energy (U): Is the sum of all forms of energy: kinetic + potential. (Kinetic energy results from translational, vibrational, and rotational motions and potential energy results from the interactions of particles with each other). Heat:(q) A form of energy that is transferred across the boundary between the system and the surrounding as a result of temperature difference between the two. We think of it as if it were a substance that “flows” [heat lost, heat gained, etc.] Warmer bodies lose kinetic energy to the colder bodies through collisions till the two bodies acquire the same temperature. Temperature is a property of a substance that measures the random motion of its molecules. The extent or the degree of an object being hot or cold is quantitatively measured by measuring its temperature. Fahrenheit and Celsius scales give us relative temperatures (with respect to boiling H 2 O and freezing H 2 O) whereas the Kelvin scale gives us the absolute temperature . 2
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surrounding heat system If the system is at a higher temperature than the surroundings… Energy, as heat, flows out of the system to the surrounding. Therefore… the
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chem161sg_chapter6 - Chapter 6 Thermochemistry...

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