Heat and Thermochemistry Review

1. Heat and Thermochemistry Review

2. Heat and temperature • Heatis the transfer of energy between a system and its environment because of a temperature difference between them • The symbol Q = represents the amount of energy transferred by heat between a system and its environment

3. Heat and Temperature Heat Flow Below Temp OC Time in minutes

4. Methods of Heat Transfer • Need to know the rate at which energy is transferred • Need to know the mechanisms responsible for the transfer • Methods include • Conduction • Convection • Radiation

5. Conduction • The transfer can be viewed on an atomic scale • It is an exchange of energy between microscopic particles by collisions • Less energetic particles gain energy during collisions with more energetic particles • Rate of conduction depends upon the characteristics of the substance

6. Conduction example • The molecules vibrate about their equilibrium positions • Particles near the stove coil vibrate with larger amplitudes • These collide with adjacent molecules and transfer some energy • Eventually, the energy travels entirely through the pan and its handle

7. Conduction, cont. • In general, metals are good conductors • They contain large numbers of electrons that are relatively free to move through the metal • They can transport energy from one region to another • Conduction can occur only if there is a difference in temperature between two parts of the conducting medium

8. Convection • Energy transferred by the movement of a substance • When the movement results from differences in density, it is called natural conduction • When the movement is forced by a fan or a pump, it is called forced convection

9. Convection example • Air directly above the flame is warmed and expands • The density of the air decreases, and it rises • The mass of air warms the hand as it moves by

10. Convection Current Example • The radiator warms the air in the lower region of the room • The warm air is less dense, so it rises to the ceiling • The denser, cooler air sinks • A continuous air current pattern is set up as shown

11. Radiation • Radiation does not require physical contact • All objects radiate energy continuously in the form of electromagnetic waves due to thermal vibrations of the molecules

12. Radiation example • The electromagnetic waves carry the energy from the fire to the hands • No physical contact is necessary • Cannot be accounted for by conduction or convection

13. Applications of Radiation • Clothing • Black fabric acts as a good absorber • White fabric is a better reflector • Thermography • The amount of energy radiated by an object can be measured with a thermograph • Body temperature • Radiation thermometer measures the intensity of the infrared radiation from the eardrum

14. Resisting Energy Transfer • Dewar flask/thermos bottle • Designed to minimize energy transfer to surroundings • Space between walls is evacuated to minimize conduction and convection • Silvered surface minimizes radiation • Neck size is reduced

15. Global Warming • Greenhouse example • Visible light is absorbed and re-emitted as infrared radiation • Convection currents are inhibited by the glass • Earth’s atmosphere is also a good transmitter of visible light and a good absorber of infrared radiation

16. Comparison of temperature scales • Relative Scales • Fahrenheit (°F) • Celsius (°C) • Absolute Scales • Kelvin(K)

17. Absolute Zero and the Kelvin Scale Pressure remains the same, volume changes by 1/273 for each Celsius degree change.

18. Freezing and Boiling point graph 1. solid (ice) 2. (solid <-> liquid) melting <-> freezing (0 Celsius - water and ice 3. liquid (water) 4. (liquid <-> gas) (vaporization <-> condensation) boiling (100 Celsius - water and gas) 5. gas  (water vapor) BP MP or FP

19. Boil, Boil, Toil and Trouble Phase changes Step One:solid ice rises in temperature the ice has not melted yet. Step Two: solid ice melts temperature DOES NOT CHANGE (Heat of fusion) Step Three:liquid water rises in temperature The liquid has not boiled yet. Step Four:liquid water boils temperature DOES NOT CHANGE (Heat of Vaporization) Step Five:steam rises in temperature 2 4

20. A phase change occurs when the physical characteristics of the substance change from one form to another Common phases changes are Solid to liquid – melting Liquid to gas – boiling Phases changes involve a change in the internal energy, but no change in temperature Phase Changes

21. Sublimation • Some substances will go directly from solid to gaseous phase • Without passing through the liquid phase • This process is called sublimation Dry Ice CO2

22. Deposition • Some substances will go directly from gaseous phase to solid • Without passing through the liquid phase • This process is called deposition Dry Ice CO2 Frost

23. Dynamic Equilibrium:2 opposing forces taking place at the same rate No Equilibrium Equilibrium lid EVAPORIZATION Open system VAPORIZATION Closed system VAPORIZATION CONDENSATION

24. Phase Point Diagram E C D Line AB – Equal between solid ↔ gases, sublimation ↔ deposition Line BC – Equal between solid ↔ liquid, melting ↔ freezing Line BD – Equal between liquid ↔ gas, boiling ↔ vaporization Point B – Triple Point Point C – normal mp or fp standard Point D – normal bp Point E – Critical Point Triple Point – point where all three phases of matter are in equilibrium Critical Temperature – highest temp. in which pressure can be applied to reliquefy a gas Critical Pressure – the pressure required to reliquefy a gas at the critical temp. Critical Point – boiling point line comes to the end

25. Phase Diagram

26. Phase Diagrams Negative slope for BC * Solid is less dense than liquid (only water has solid that is less dense…. So it floats) Positive slope for BC (solid) * Solid is more dense than liquid

27. Boiling Point Atmospheric pressure • Vapor pressure of the gas must equal atmospheric pressure • Atmospheric pressure goes up, BP goes up • Atmospheric pressure goes down, BP goes down Vapor Pressure

28. Boiling and Freezing in a Vacuum Reduced pressure allows room temperaturewaterto boil, taking thermalenergyout of the water

29. Altitude and pressure • Boiling on a Mountain • At higher altitudes the atmospheric pressure is less, so water moleculesneed less kinetic energy • Atm Pressure temp (Cook food longer) • Boils faster at a lower temp)

30. Vapor Pressure & Boiling Point • Vapor pressure increases as temperature increases for different substances • A liquid will boil when its vapor pressure equals atmospheric pressure • Water has the greatest intermolecular forces, followed by ethyl alcohol and diethyl ether.

31. Units of Heat • Calorie • A calorie is the amount of energy necessary to raise the temperature of 1 g of water from 14.5° C to 15.5° C . • A Calorie (food calorie) is 1000 cal • 1 cal = 4.186 Joules • This is called the Mechanical Equivalent of Heat

32. Specific Heat • Every substance requires a unique amount of energy per unit mass to change the temperature of that substance by 1° C • The specific heat (c) of a substance is a measure of this amount

33. Units of Specific Heat of water • SI units • 4.18 joules/g °C • Historical units • 1.0 cal/g °C

34. HEAT and Its MEASUREMENT • Heat (energy) measured in units of calories or joules. • When there is a temperature change(ΔT), heat (Q) can be calculated using this formula Q = mass x specific heat capacity x ∆T (∆T = T Final –T Initial) Q = m x C x ΔT heat = (mass in gram) (specific heat) (temperature change) Q = mass x ΔH fusion(or ΔH vaporization) or Q = moles x ΔH fusion (vaporization)

35. 1. How many joules of heat are given off when 5.0 g of water cool from 75 °C to 25 °C? (Specific heat of water = 4.18 j/g °C ) 1) mass = 5.0g 2) Change in temperature: 75 °C -25 °C = 50 °C Energy required = mass * specific heat capacity * change in temperature Q = m x ΔT x C = 5.0 g H2O * 50 °C* 4.18 j/g °C =1045 J 1.0 x x 103 J Complete problems on practice study sheet. Video: pHASE Changes at Work " A Home"

36. HEAT and ITS MEASUREMENT(Freezing & Boiling Point Graph) CHANGE IN TEMPERATURE: Q = M x cp(s) x ΔT or Q = M x cp(l) x ΔT or Q = M x cp(g) x ΔT NO CHANGE IN TEMPERATURE: Q = M x ΔHfus or Q = M x ΔHvap 100 °C O°C

37. HEAT and ITS MEASUREMENT(Freezing & Boiling Point Graph) ΔHfus = Heat of Fusion– energy required to melt 1.00 gram of substance (measured in J/g or cal/g ) ΔHvap = Heat of vaporization– energy required to vaporize (or boil) 1.00 gram of Substance (measured in J/g or cal/g ) 100 °C O°C

38. 1. How much energy in Joules is needed to raise the temp. from –7OC to 107OC ? (use 25.0g H2O ) cp(s) = 2.06 J/g O Ccp(l) = 4.18 J/g O Ccp(g) = 2.02 J/g O C F.P (H2O) = O OCB.P (H2O) = 100 OC 1. Q = 25.0g H2O x 2.06 J/g oC x 7 oC = 360.5 J • Q = 25.0g H2O x 333 J/g oC = 8,325 J 3. Q = 25.0g H2O x 4.18 J/g oC x 100 oC = 10,450 J 4. Q = 25.0g H2O x 2260 J/g = 56,500 J 5. Q = 25.0g H2O x 2.02 J/g oC x 7 oC = 353.5 J 360.5 J 8,325.0 J 10,450. J 56,500 J + 353.55 J 75,989 J 76,000 J Or 7.60 x 104 J Q = m x ΔT x C Q = M x Δhfus or Δhvap

39. Heat and Specific Heat • Q = m c ΔT • ΔT is always the final temperature minus the initial temperature • When the temperature increases, ΔT and ΔQ are considered to be positive and energy flows into the system • When the temperature decreases, ΔT and ΔQ are considered to be negative and energy flows out of the system

40. Latent Heat (hidden) • SI units of latent heat are J/kg • Latent heat of fusion, Hfus, is used for melting or freezing • Latent heat of vaporization, Hvap, is used for boiling or condensing

41. Graph of Ice to Steam

42. Warming Ice • Start with one gram of ice at –30.0º C • During A, the temperature of the ice changes from –30.0º C to 0º C • Use Q = m c ΔT • Will add 62.7 J of energy

43. Melting Ice • Once at 0º C, the phase change (melting) starts • The temperature stays the same although energy is still being added • Use Q = m Hfus • Needs 333 J of energy

44. Warming Water • Between 0º C and 100º C, the material is liquid and no phase changes take place • Energy added increases the temperature • Use Q = m c ΔT • 419 J of energy are added

45. Boiling Water • At 100º C, a phase change occurs (boiling) • Temperature does not change • Use Q = m Hvap • 2260 J of energy are needed

46. Heating Steam • After all the water is converted to steam, the steam will heat up • No phase change occurs • The added energy goes to increasing the temperature • Use Q = m c ΔT • To raise the temperature of the steam to 120°,40.2 J of energy are needed