ch.10 Energy Changes in Chemical Reactions-Study Guide - CHM2045 Energy Changes in Chemical Reactions Study Guide 10 Energy Changes in Chemical

# ch.10 Energy Changes in Chemical Reactions-Study Guide -...

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CHM2045 Energy Changes in Chemical Reactions 1 Study Guide 10 Energy Changes in Chemical Reactions Energy and Energy Changes The system is a part of the universe that is of specific interest. The surroundings constitute the rest of the universe outside the system. The system is usually defined as the substances involved in chemical and physical changes. Universe = System + Surroundings Thermochemistry is the study of heat (the transfer of thermal energy) in chemical reactions. Heat is the transfer of thermal energy. Heat is either absorbed or released during a process. An exothermic process occurs when heat is transferred from the system to the surroundings. “Feels hot!” An endothermic process occurs when heat is transferred from the surroundings to the system. “Feels cold” Introduction to Thermodynamics Thermodynamics is the study of the interconversion of heat and other kinds of energy. In thermodynamics, there are three types of systems: An open system can exchange mass and energy with the surroundings. A closed system allows the transfer of energy but not mass. An isolated system does not exchange either mass or energy with its surroundings. State functions are properties that are determined by the state of the system, regardless of how that condition was achieved. The magnitude of change depends only on the initial and final states of the system. Energy Pressure Volume Temperature The first law of thermodynamics states that energy can be converted from one form to another, but cannot be created or destroyed. Δ U sys + Δ U surr = 0 Δ U is the change in the internal energy. “sys” and “surr” denote system and surroundings, respectively. Δ U = U f U i ; the difference in the energies of the initial and final states. Δ U sys = –Δ U surr Work and Heat The overall change in the system’s internal energy is given by: Δ U = q + w
CHM2045 Energy Changes in Chemical Reactions 2 q is heat q is positive for an endothermic process (heat absorbed by the system) q is negative for an exothermic process (heat released by the system) w is work w is positive for work done on the system w is negative for work done by the system Δ U = q + w 1. Calculate the overall change in internal energy, Δ U , (in joules) for a system that absorbs 188 J of heat and does 141 J of work on its surroundings. 2. Calculate the change in total internal energy for a system that releases 1.34 x 10 4 kJ of heat and does 2.98 x 10 4 kJ of work on the surroundings. 3. Calculate the magnitude of q for a system that does 7.05 x 105 kJ of work on its surroundings and for which the change in total internal energy is -9.55 x 103 kJ. Indicate whether heat is absorbed or released by the system. Enthalpy Sodium azide detonates to give a large quantity of nitrogen gas.

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