Energy Changes in Reactions

1. Energy Changes in Reactions Chapter 3

2. Heat vs. Temperature • Heat is a form of energy, it is transferred from one system to another • Temperature is an indication of the intensity of heat, it measures the agitation of particles

3. Transfer of Energy • the transfer of kinetic energy can occur by work or by heat • when work is done, particle motion is orderly (inflating a balloon, surface expands in the same direction) • when heat is responsible for the transfer of energy, particle motion is random (feeling the warmth of a cup of hot coffee)

4. when your hands feel warm they are receiving the energy of the hot coffee, but there is no visible movement • when your hands feel cold, you are feeling the loss of energy to a colder substance

5. the natural tendency in nature is to move toward higher levels of disorder (entropy), this is why heat ALWAYS flows from hot to cold (think of a lava flow)

7. T1 < T2 T1 < Tf < T2

8. Conservation of Energy • Energy can be transferred or transformed, but NEVERcreated or destroyed • When studying energy changes, the type of "system" must be considered • A system is a location being observed (beaker, appliance, humans, solar system, etc)

9. systems can be in contact with and influenced by surroundings which are part of their environment Fireworks are an example of an energy change

10. Types of Systems • OPEN: In contact with surroundings, exchanges matter and energy with it (beaker, can be heated or cooled or added to)

11. 2. CLOSED: No exchanges of matter but allows energy to be exchanged with surroundings (a tied balloon, no matter can be added but can be heated or cooled)

12. 3. ISOLATED: No exchanges of matter or energy with surroundings (a thermos is temporarily an isolated system)

13. Calorimetry • used to determine quantities of heat involved in a reaction • calorimeter is a closed system (internal chamber) that allows the transfer of energy to water surrounding the core

14. Calculating Heat Energy Q = mcΔt • Q = heat energy (J) • m = mass (g) • c = specific heat capacity (J/goC • Δt = change in temperature (Tf - Ti)

15. Specific heat capacity refers to the amount of energy required to raise the temperature of one gram of a substance by one degree Celsius • Amount of heat energy required to create a given change in temperature depends both on mass of substance and specific heat capacity

16. Why are coastal regions more temperate?

17. Why does the desert experience such drastic temperature changes?

18. Example 1. Calculate the quantity of thermal energy absorbed by a 5.00 kg block of concrete (use table p.134) when its temperature increases from 17.1°C to 35.5°C.

19. 2. A 1.35 g piece of aluminum is heated to 205 oC and then removed from the heat. After a few seconds, the aluminum has released 176 J of heat. What is its final temperature? • Q = • m = • c = • ΔT =

20. Calculating energy transfer in mixtures • When mixing two substances, heat will flow between them until the final temperature of both is the same • The heat energy lost by one substance is gained by the other • In other words, -Q = +Q

21. -mcΔT = +mcΔT HOTCOLD • The hotter substance ALWAYS loses heat • The colder substance ALWAYS gains heat • The final temperature of BOTH substances is ALWAYS the same

22. Examples: • 1. Calculate the mass of water at 10.0°C needed to cool a 10.0 g piece of glass from 95.0°Cto 30.0°C.

23. 2. A 13.7 g piece of copper is heated in an oven to a temperature of 125.0°C. It is then removed and dropped into a beaker containing 240.0 ml of water at 22.0°C. What is the final temperature of the mixture?

24. Really cool chemistry! • How do people walk on hot coals? http://videos.howstuffworks.com/science-channel/14375-skeptical-inquirer-the-ritual-of-firewalking-video.htm The coals of many kinds of wood are poor conductors of heat, so if you walk fast enough you won't get burned!