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Thermochemistry: An Intro

Thermochemistry: An Intro. Chem 12. What is thermochemistry?. The study of the energy involved in chemical reactions. Law of Conservation of Energy:. The total energy in the universe is constant ∆E universe = 0 But energy can be transfered from one substance to another.

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Thermochemistry: An Intro

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  1. Thermochemistry: An Intro Chem 12

  2. What is thermochemistry? • The study of the energy involved in chemical reactions

  3. Law of Conservation of Energy: • The total energy in the universe is constant ∆Euniverse = 0 • But energy can be transfered from one substance to another

  4. Kinetic Molecular Theory and Energy: • Kinetic molecular theory: substances are composed of particles that are continually moving and colliding with other particles. • Kinetic energy: energy of motion • Potential energy: energy that is stored • Ex: Think about a rock…. • Heat is a form of energy • SI Units: joule (J)

  5. Cont’d… • When these collisions occur, a chemical reaction may also occur resulting in the breaking and forming of chemical bonds • This requires energy! • The average kinetic energy of all the particles is proportional to the temperature • The higher the temperature the faster the particles move, the harder the collisions between particles. • But what is temperature???

  6. Temperature (T ) • A measure of the average kinetic energy of the particles that make up a substance or system • A temperature change is explained as a change in kinetic energy • Temperature depends on the quantity of heat (q) flowing out or in of the substance. • Measured with a thermometer

  7. Heat (q): • Refers to the transfer of kinetic energy between objects with diff temps • Units = joules (J) • 1st law thermodynamics can be expressed as: qsystem = - qsurroundings

  8. First Law of Thermodynamics Any change in the energy of a system is equivalent by an opposite change in the energy of the surroundings. ΔEsystem = -ΔEsurroundings According to this law, any energy released by a system is gained by the surroundings, and vice versa So for heat, qsystem = -qsurroundings A negative sign denotes heat lost (exothermic), while a positive sign denotes heat gained (endothermic)

  9. Specific Heat Capacity: • Amount of heat that can be transfered depends on the type of substance • SHC = the energy required to change 1 gram of the substance by 1°C. • Units: J/g ∙ °C

  10. Specific heat capacity formula: q = m ∙ c ∙ ∆T • Remember: ∆T = Tf - Ti

  11. Example: • 15 g of ice was added to 60g of water. The initial temp of water was 26.5 oC, the final temp of the mixture was 9.7 oC. How much heat was lost by the water? q=mc ∆T q=(60g) (4.18 J/g oC) (9.7-26.5 oC) q= - 4213.44 J

  12. Heat Capacity (C): • Water in bathtub has higher heat capacity then water in tea cup due to greater mass C = m x c • C  heat capacity KJ/°C • m  mass (kg) • c  SHC KJ/Kg ∙ °C

  13. Heat Capacity cont… • Can use heat capacity to determine heat associated with temp change q = C∆T

  14. Try it : • Read pages 626-635 • Practice Problems: (pg 634)1-4,(pg 636) 8, 11-14 (pg 637)

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