Module6_1112R_V3 - Thermochemistry and Calorimetry Why...

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Thermochemistry and Calorimetry Why? While we typically think of atoms (mass) as being involved in chemical reactions, there is another possible product or reactant energy. Some reactions (melting ice) require energy, and some reactions (combustion of charcoal to grill hot dogs) release energy. Thermochemistry is the area of chemistry where energy is included as a consideration in the chemical reactions. We will only consider thermal energy in this module, you will learn more about mechanical energy in your physics classes. Learning Objectives: 1. Understand how the energy of a given reaction can be measured. 2. Understand how to treat the energy of the reaction as a stoichiometric consideration in a balanced thermochemical reaction. Success Criteria 1. Successfully interpret calorimetry measurements. 2. Successfully apply the above principles to thermochemical equations.
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Model 1. Basic Thermodynamic Definitions Thermal energy is the energy exchanged when two objects with different temperatures are placed together. Thermal energy is transferred from the hot object to the cold object, or “ heat flows ” from the hot to the cold object. When the two objects are at the same temperature, the system is said to be at thermal equilibrium , and no further thermal energy or heat is exchanged. Notice that the energy is exchanged from one object to another object. We call the object we are interested in the system , and the other object the surroundings . The universe is composed of the system and the surroundings. If a block of hot copper is placed on a block of ice, energy is transferred or “heat flows” from the copper to the ice. If we choose to define the copper as the system, we would say the system lost thermal energy (“ exothermic ). However, if we choose to define the ice is the system, we would say the system gained energy (“ endothermic ”). Note that the energy gained or lost by the system is equal in magnitude, but opposite in sign from the surroundings. This is because however much the energy of the system changes, that energy must come from or go into the surroundings. The system could be in three possible states. An open system can exchange both mass and energy with the surroundings, a closed system can exchange energy but not mass with the surroundings, and an isolated system can exchange neither mass nor energy with the surroundings. The SI unit for energy is the joule, J . 4.184 J is the amount of energy required to raise the temperature of 1.00 g of pure water by 1.00 ºC. Later, in organic chemistry, you will use the calorie (1 cal = 4.184 J). We use “q” to indicate heat, it can be either positive (endothermic) or negative (exothermic). Heat capacity, C, is the proportionality constant between q and the change in temperature, q = C T (Eq. 1).
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Key Questions: 1. If a block of copper at 25 ºC is placed in contact with a block of iron at 50 ºC, will there be “heat flow?” If so, from what to what? The final temperature of the copper should be between what two values?
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