1 / 30

CH 6: Thermochemistry

CH 6: Thermochemistry. 6.1 Nature of Energy. Thermochemistry – study of energy changes during chemical reactions Aspects of thermochemistry are studied in both physics and chemistry. 6.1. Energy – the capacity to do work or produce heat

pete
Télécharger la présentation

CH 6: Thermochemistry

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. CH 6: Thermochemistry

  2. 6.1 Nature of Energy • Thermochemistry – study of energy changes during chemical reactions • Aspects of thermochemistry are studied in both physics and chemistry

  3. 6.1 • Energy – the capacity to do work or produce heat • Law of conservation of energy states that energy can be converted from one form to another, but it cannot be created or destroyed. • Energy of the universe is constant!

  4. 6.1 Two forms of energy 1. Potential energy – stored energy • Energy of position or composition • Examples 2. Kinetic energy – energy of motion • Heat, light, electricity • In this chapter our focus will (eventually) be on the heat aspects of thermochemistry.

  5. 6.1 • Consider the diagram on page 237. • Ball A has potential energy due to its position • Some of this energy is released as heat as A rolls down the hill • Ball A hits Ball B and work is done as B moves up the incline • Work = force acting over a distance

  6. 6.1 • The original potential energy of A is equal to the new potential energy of B plus the heat released as friction. • Energy can be released as: • Heat (friction) • Work (ball rolling)

  7. 6.1 • The total energy released when ball A rolls down the hill is fixed – how it’s released is not. • How much is released as heat vs. work depends upon the conditions • Total energy released does not depend on the pathway.

  8. 6.1 • Total energy change is a state function. • State function is a property of a system that depends only on its current state not its past • See last paragraph on page 237 • Energy of a system is a state function. • Heat and work are not state functions – they depend on the path taken

  9. 6.1 Chemical Energy • When studying chemical change we consider the system and the surroundings. • System includes the reactants and products of a given reaction • Surroundings are everything else in the universe! • Including the water if the reaction is done in solution

  10. Chemical Energy • Exothermic reactions – energy flows out of the system • Products are of lower potential energy than the reactants • Energy is released to the surroundings • Energy lost by the system = energy gained by the surroundings – There’s a loss of energy by the system…. DEnergy of the system is negative

  11. Chemical Energy • Endothermic reactions – energy flows into the system • Products are of higher potential energy than the reactants • The system absorbs energy from the surroundings • The energy gained by the system = the energy lost by the surroundings There’s a gain of energy by the system……. DEnergy of the system is positive

  12. Chemical Energy • Exothermic: DE < 0 • System _______ energy • Endothermic: DE > 0 • System ________ energy

  13. More on Energy of the System • The energy of the system can change as a result of 2 factors: • Heat (q) • Work (w) • Change of energy of the system = heat flowing in/out of system + work being done to/by the system DE = q + w

  14. DE of the System • Heat (q) • When heat flows into the system, q > 0 • Endothermic • When heat flows out of the system, q < 0 • Exothermic • Work (w) • When work is done to the system, w > 0 • When work is done by the system, w < 0

  15. More on Work • Most common form of work done by a system is the expanding or compressing of a gas, called pressure – volume work • Work – force applied over a distance • Moving an object a distance = work

  16. Pressure Volume Work • Consider a gas in a cylinder with a movable piston on top • Work = force x distance Force = pressure x area Distance = change in height of gas in cylinder (D h) • Work = P x A x D h D Volume • Page 241

  17. Pressure Volume Work • Work = P x D V • The sign ( +/-) on work is assigned so that: • When the gas expands, it is doing work on the surroundings (w < 0) • When the gas is compressed, work is done on the system (w > 0)

  18. Pressure Volume Work • Final version of the equation that shows both magnitude and sign on work: W = - P x D V • When the gas expands D V is positive and w < 0 (work is done by the system) • When the gas is compressed D V is negative and w > 0 (work is done on the system)

  19. 6.2 Entahalpy • Enthalpy (H) • Enthalpy is defined as: H = E + PV • E is the energy of the system • P is the pressure of the system • V is the volume of the system

  20. Enthalpy • In chemistry we consider enthalpy at constant pressure • After much math this results in the formula: • H = qp • H is called the heat of the reaction • H is a measure of the flow of heat (q) into/out of the system at constant pressure

  21. Enthalpy • When heat leaves the system D H < 0 • Exothermic process • When heat enters the system D H > 0 • Endothermic process

  22. Finally – the Applications! CH4 + 2 O2 CO2 + 2 H20 + energy • H = - 890 kJ • Is the reaction exothermic or endothermic? • How much energy will be ___________ if 6.50 grams of CH4 is burned at constant pressure? Next…..#44 on page 277

  23. Calorimetry • The heat changes associated with a chemical reaction are often measured in a calorimeter.

  24. Calorimetry • Exothermic: The heat released by the reaction is used to heat up a known quantity of water. • More heat released the hotter the water gets • Endothermic: The heat absorbed by the reaction comes from the water • More heat absorbed the colder the water gets

  25. Terms • Terms all describe the energy needed to heat or cool some amount of a given substance • Heat Capacity (C) • Specific heat capacity • Molar heat capacity

  26. Terms • Heat Capacity (C) • Amount of energy needed to raise the temperature of a substance by 10C • Units: J/0C • Substance is an entire/specific object…e.g.

  27. Terms • Specific Heat Capacity • Amount of energy needed to raise the temperature of 1 gram of a substance by 10C • Units: J/g0C • See page 245

  28. Terms • Molar Heat Capacity • Amount of energy needed to raise the temperature of 1 mole of a substance by 10C • Units: J/mol 0C Molar heat capacity = specific heat x molar mass J/mol 0C = J/g 0C x g/mol

  29. Calculating Heat Capacities Heat Capacity = heat absorbed D T Specific Heat = energy (mass) (D T) Page 277: 52, 54

  30. Determining Specific Heat of a Metal • Lab demonstration of experiment 27:E, page 347 of the lab manual • Assume the heat capacity of the ccc is 0 J/g0C

More Related