10.2 – Thermal Energy Transfer

1. 10.2 – Thermal Energy Transfer (pages 375 – 387 in your text)

2. A Blue Planet • Our planet, Earth, has been given the nickname “The Blue Planet”. This is because Earth’s surface is covered with 70% water.

3. The Hydrosphere • The hydrosphere is made up of water from the Earth’s oceans, rivers, streams, lakes, ground water, glaciers and water in the atmosphere. Water plays a very important role in the movement of energy through the biosphere.

4. Specific Heat Capacity • Specific heat capacity - the amount of heat required to raise the temperature of a substance by 1°C. • The specific heat capacity of water is high compared to other common fluids. This characteristic of water makes it suitable in hot-water heating systems, in the cooling systems of automobiles, and in Candu nuclear reactors. In each of these applications, water is used to carry thermal energy.

5. Water has a much higher heat capacity than air. By using the specific heat capacity (c) values in the table above and the formula “Q = mcΔT”, you can calculate the amount of heat (Q) required to raise the mass of a substance (m) by a given temperature (ΔT).

6. Quantity of Thermal Energy Formula Q = mcΔT where, Q is the amount of heat absorbed or released in joules (J) m is the mass of the substance in grams (g) c is the specific heat capacity of the substance in J/g • °C ΔT is the change in temperature in °C * if data is given in initial (T1) and final (T2) temperatures instead of the change in temperature, calculate ΔT using ΔT = T2 – T1

7. Model Problem 1 (page 376) A) A house contains 170 kg of dry air. The furnace has broken, and the temperature of the air has fallen to 2.0°C. How much energy is needed to heat the air to 20°C? (Use the GRASP method to answer the question)

8. B) Suppose that the 170 kg of air in the house is moist. How much energy is released if the temperature of the air drops from 20°C to 2.0°C? (Use the GRASP method to answer the question)

9. Homework Practice: Do Questions 1-9 on page 377 (pick any 5)

10. Conduction, Convection, and Radiation Transfer Energy • A – Radiation • Electromagnetic radiation (EMR) carries energy through the vacuum of space and the gases of the atmosphere. • EMR that reaches Earth is absorbed by molecules in the atmosphere, water, and land, which converts the EMR into thermal energy. • Radiation that is absorbed by the land and water is emitted as infrared radiation, which is absorbed by the atmosphere.

11. B – Conduction • Energetic molecules collide with other molecules and transfer their energy. • Energetic molecules on the surface of the land and water, which have absorbed radiant energy, collide with gas molecules in the atmosphere and transfer energy to them. • Gas molecules that are warmer than the surrounding molecules transfer thermal energy to these less energetic molecules. • Conduction is most effective at transferring energy in dense substances (solids and liquids) since the molecules in these substances are closer together.

12. C – Convection • As air near Earth’s surface warms due to conduction, it becomes less dense and rises. • Rising warm air is replaced by descending cool, dense air, thus generating convection currents. • Convection currents occur only in liquids and gases.

14. Answer the questions 1. Label the arrows on the diagram to describe the processes of energy transfer. Use the following terms: conduction, convection, radiation.

15. 2. Which mechanisms - convection, conduction, and/or radiation - best describe the following energy transfers? (a) a warm breeze blowing inland ___________________  (b) a damp cloth cooling your forehead ___________________  (c) a spoon warming in a coffee cup ___________________  (d) a microwave heating a bowl of soup ___________________ (e) a boy warming his hands by a fire ___________________

16. 3. Consider a snowy hill, Earth’s atmosphere, and a cornfield. Which absorbs the most solar energy directly from the Sun? Which absorbs the least solar energy? Provide two reasons to explain your answers.

17. Phase Changes • You can be badly scalded by steam from water boiling in a kettle. When steam condenses, it changes from a gaseous phase to a liquid phase. In such a phase change, steam releases a great deal of thermal energy. It is the thermal energy released as steam condenses on your skin that is so damaging.

18. When you perspire, water evaporates from your skin and leaves in the gas phase. In evaporation, a substantial amount of thermal energy is absorbed from your skin. This absorption provides you with a cooling effect. Whenever water changes phase, thermal energy is either released or absorbed.

19. The constants heat of fusion and heat of vaporization give an indication of the quantities of thermal energy involved in phase changes. • Heat of Fusion – the amount of energy required to melt one mole of a substance. solid  liquid • Heat of Vaporization – the amount of energy required to convert one mole of a substance from a liquid to a gas. liquid  gas

21. Heats of Fusion and Vaporization of Some Common Substances:

22. Ice at 0°C changes to water at 0°C; the energy is used to break the bonds between water molecules in ice. When liquid water at 0°C freezes, it releases energy. • Water and other substances can absorb or release energy with no change in temperature. Energy is required to melt ice and to cause liquid water to evaporate. The same amount of energy is released when water vapour condenses into a liquid or when liquid freezes to a solid.

23. Ice is undergoing a phase change between “B” and “C”, it is melting. Liquid water is undergoing a phase change between “D” and “E”, it is evaporating. • When these changes occur the temperature of the water does not change.

24. Formula to calculate energy required for melting(fusion): Q = nHfuswhere Q is the quantity of energy, heat, required in Joules (J) n is the number of moles of the substance (no units) Hfus is the heat of fusion in Joules per mole (J/mol)

25. Formula to calculate energy required for converting a liquid to a gas (vaporization): Q = nHvapwhere Q is the quantity of energy, heat, required in Joules (J) n is the number of moles of the substance (no units) Hvapis the heat of vaporization in Joules per mole (J/mol)

26. *** reminder! To calculate the number of moles of a substance use: n = m/M where n is the number of moles (no units) m is the mass of the substance in grams (g) M is the molar mass of the substance in grams per mole (g/mol). Use the PTOE to calculate the molar mass.

27. Find the molar mass of methanol, CH3OH The atomic molar mass of methanol is ___________ g/mol

28. Model Problem 2 (page 382) • Part A) A beaker contains exactly 360.4 g of water in the liquid state at 100°C. How much energy is required to convert he liquid water to water vapour at 100°C?

29. Part B) A container holding exactly 360.4 g of ice at 0°C melts to form liquid water at 0°C. How much energy must be added to convert the ice at 0°C to liquid water at 0°C?

30. Homework Practice: Do Questions 10-18 (pick any 5) on page 383 • Use the GRASP method to solve the problems, answers are on page 485 of the text.

31. Water’s Properties Influence Climate • In the spring, days become longer and the Sun begins to warm the land. If large amounts of snow and ice remain from the winter, then much of the Sun’s energy is used to melt them. Therefore, the air temperature does not rise as much as it would in the absence of snow and ice.

32. As winter sets in, the days get shorter and the Sun provides less heat than in summer. As liquid water freezes, however, it releases heat. Therefore, the air temperature does not drop as much as it would if there were no water to freeze.

33. The Hydrologic Cycle • The hydrologic cycle distributes water by evaporation, condensation, and precipitation. When water changes phases from a liquid to a gas or to a solid, heat is absorbed or released. • Because of these phase changes, the hydrologic cycle moves not only lots of water, but it moves vast amounts of thermal energy.