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Thermochemistry

Thermochemistry. Energy Changes in Chemical Reactions Chemistry 12AP. Thermodynamics. The study of energy and its interconversions. Types of Energy. Energy – the capacity to do work Work : energy used to cause an object with mass to move against a force. Types: Radiant energy

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Thermochemistry

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  1. Thermochemistry Energy Changes in Chemical Reactions Chemistry 12AP

  2. Thermodynamics The study of energy and its interconversions.

  3. Types of Energy • Energy – the capacity to do work • Work: energy used to cause an object with mass to move against a force. Types: • Radiant energy • Thermal energy • Chemical energy • Potential energy Law of Conservation of Energy Energy can be converted from one form to another.

  4. Energy Changes in Chemical Reactions • Almost all chemical reactions absorb or produce (release) energy. • Heat is the transfer of thermal energy between two bodies that are at different temperatures. (sometimes referred to as heat flow) • Temperature is the average kinetic energy of the molecules.

  5. Electrostatic Potential Energy • k – constant of proportionality, 8.99 x 109Jm/C2 • When dealing with molecular-level objects, the electrical charges on Q1 and Q2 are typically on the order of magnitude of the charge of the electron. When Q1 and Q2 have the same sign (i.e. both positive or both negative) the two charges repel one another, pushing them apart; E would be positive. If they have opposite charges, then they would attract one another, making the E negative. • The lower the energy (E) of a system, the more stable it is. Thus, the more strongly opposite charges interact, the more stable the system.

  6. First Law of Thermodynamics • The system is the specific part of the universe that we are “studying”. The surroundings are the rest of the universe outside the system. system + surroundings = universe ∆E = Ef – Ei ∆Esys + ∆Esurr = 0 Or ∆Esys = -∆Esurr

  7. Examples of Systems Open Closed Isolated Water vapor heat heat

  8. More on Energy Total change in energy (∆E) aka. First Law of Thermodynamics ∆E = q + w (heat + work) q = heat absorbed by/lost from the system w = work done on/by a system on its surroundings

  9. Work and Heat

  10. Example Problem Example 1. There is a flow of 31.2kJ of heat to a system. At the same time, the system does 2.5kJ of work on the surroundings. Calculate the change in energy.

  11. A reaction is exothermic if it gives off heat energy flows from the system to the surroundings (q is negative) • A reaction is endothermic if it absorbs/takes in heat energy flows from the surroundings into the system (q is positive)

  12. Calorimetry • Is the laboratory technique used to measure the heat released or absorbed during a chemical or physical change

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