1 / 21

Phase Changes and Heat Calculations

Phase Changes and Heat Calculations. Obj. 1…Vapor Pressure. Vapor pressure (VP) is the P exerted at the surface of a. liquid by particles trying to escape the liquid. Obj. 2…VP and Temperature. As T , KE will. (direct relationship).

Download Presentation

Phase Changes and Heat Calculations

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. Phase Changes and Heat Calculations

  2. Obj. 1…Vapor Pressure • Vapor pressure (VP) is the P exerted at the surface of a liquid by particles trying to escape the liquid.

  3. Obj. 2…VP and Temperature • As T , KE will . (direct relationship) • If liquid molecules gain enough KE, they will overcome the intermolecular bonds that hold them together. • become a gas

  4. Obj. 3…Boiling/Melting Points • Boiling point (BP) = the temp. at which the VP of a liquid is equal to the external pressure. • KE increases pressure enough to break intermolecular bonds. • BP is directly related to atmospheric pressure. • a pot of water in Denver (mountains…low pressure) will boil at a lower temp. than a pot of water in Houston (sea level). • normal BP is always measured at sea level. • Melting Point (MP) = the temp. at which a solid turns into a liquid. • as KE of solid increases, molecules begin to vibrate • if vibrations are strong enough, molecules will break away from liquid their fixed positions

  5. Obj. 3…Boiling/Melting Points

  6. Obj. 4… Freezing/Melting and Boiling/Condensation Points • The freezing point (FP) and melting point (MP) of a substance occur at the same temp. • FP (liquid solid) is used as a substance loses KE (heat) • molecules get slower and lock into place. • MP (solid liquid)is used as a substance gains KE (heat) • molecules break away from solid bonds. • The boiling point (BP) and condensation point (CP) of a substance occur at the same temp. • BP (liquid gas) is used as a substance gains KE (heat) • molecules break away from liquid bonds. • CP (gas liquid)is used as a substance loses KE (heat) • molecules get slower and more attracted to each other.

  7. Obj. 5…Sublimation • sublimation = a solid changing directly into a vapor (gas) w/out passing through the liquid stage. • only occurs in certain solids with high VP. • Ex…naphthalene (moth balls), CO2 (dry ice) etc…

  8. Obj. 6…Boiling vs. Evaporation • for a liquid to boil, the VP of the liquid MUST = the atmospheric pressure. • to accomplish this, we can… • increase temp. of liquid ( KE = VP) • reduce atmospheric pressure ** entire pot of water boils at the same time!!! • evaporation occurs w/out changing temp. or pressure. • surface molecules exposed to more KE (sun/atmosphere) than particles below surface. • this is a cooling process (high KE molecules leave, low KE molecules stay). ** only occurs at the SURFACE of a liquid!!!

  9. Obj. 7…Volatile vs. Non-Volatile • volatile substances evaporate very easily and boil at low temps. • vapors are typically very strong and distinct. • Ex…ammonia, gasoline, rubbing alcohol, acetone • non-volatile substances contain stronger bonds and do not evaporate easily. • Ex…molasses, glue, paint

  10. Obj. 8…KE and Intermolecular Bonds • As KE , the strength of intermolecular bonds will . (inverse relationship) • heat causes KE to • enough movement eventually breaks intermolecular bonds. • heat causes KE to • molecules get slower, move less. • eventually lock into place. • bond strength increases.

  11. Obj. 9…Heating/Cooling Curves Heating Curve: • KE is (melting and boiling) • Plateaus = phase changes! • temp. remains constant until EVERY molecule changes phase. BP Gas (vapor) Boiling MP Temperature (KE) Liquid Melting Solid Time

  12. Obj. 9 cont… Cooling Curve: • KE is (condensation and freezing) Gas (vapor) Condensation Temperature (KE) CP Liquid Freezing FP Solid Time

  13. Obj. 10…Vocabulary Obj. 11…Heat Calculations • as a substance changes phases, temp. remains constant until all molecules have completed the change! • plateaus on heating/cooling curves. • to calculate heat gained/lost during a phase change… total heat (q) = mass x H(f or v) heat of fusion… use when melting! ** Both Hf and Hv will be given to you! heat of vaporization… use when boiling!

  14. Obj. 11 cont… • Ex… How many kilojoules (kJ) of heat are required to melt a 10.0 gram ice cube at 0°C and 101.3 kPa? (Hf° = 0.334 kJ/g) total heat (q) = mass x Hf total heat (q) = 10.0 x 0.334 kJ/g = 3.34 kJ • This can be used for any phase change, as long as temp. remains constant (plateaus).

  15. Obj. 12 and 14…Temp. Changes • To calculate a temp. change (slope)… heat (q) = m x Cp x Δ T mass change in temp…(Tf – Ti) specific heat capacity **given…changes w/ phases!** • Ex… The temp. of a 64.0g sample of H2O is raised from 20.0°C to 40.0°C. How much heat is required? (Cp water = 4.184 J/g°C) heat (q) = m x Cp x ΔT 40-20 = 20 4.184 x q = 64 x 20° = 5360 joules

  16. add together = Obj. 12 and 14 cont… • We can combine the phase change eq. and the Δ temp. eq. to find the total heat absorbed on a heating curve. • Ex… How much heat is needed to change 32.0 grams of H2O at -30.0°C to 45.0°C? (Hf water = 333.9 J/g, Cp ice = 2.1 J/g°C, Cp water = 4.184 J/g°C) ** must do 3 separate equations… (m x Cp x ΔT) 1) Δ temp. from -30°C to 0°C (m x Hf) 2) Phase change (melting) (m x Cp x ΔT) 3) Δ temp. from 0°C to 45°C 45°C 3 temp. 1) 32.0 x 2.1 x 30 = 2016 J 2 1 2) 32.0 x 333.9 = 10684.8 J -30°C 6024.96 J 3) 32.0 x 4.184 x 45 = 18700 J time

  17. Obj. 12 and 14 cont… • +q (heat) = endothermic reaction (heat absorbed) (heat released) • -q (heat) = exothermic reaction • graphically… *R havemore heat than P = *R haveless heat than P = lost heat = gained heat = exothermic! endothermic! R P Enthalpy (ΔH) Enthalpy (ΔH) R P Time Time

  18. Obj. 12 and 14 cont… • to calculate heat of reaction (Hr) from a graph… Hr = ΔHproducts – ΔHreactants (ΔH) (ΔH) R P 561 kJ 113.5 kJ P R 154 kJ 45.2 kJ Time Time Hr = 113.5 – 45.2 = 68.3 kJ Hr = 154 – 561 = -407kJ -q = exothermic! +q = endothermic!

  19. Obj. 13 and 16…Phase Diagrams • a phase diagram represents the P-T relationships b/n the different phases of the same substance. • each point on the curves shows the T and P at which two phases are in equilibrium. (conditions for phase changes to occur!)

  20. Obj. 13 and 16 cont… • the point at which all 3 curves intersect = triple point. • represents T and P at which all 3 phases exist simultaneously! • triple point for water is 0.016°C and 0.61 kPa • every substance has its own triple point.

  21. Obj. 15…Liquefying Gases • two ways to liquefy (condense) a gas… • atmospheric pressure. • VP of gas would be lower than atmospheric pressure • temperature of gas. • KE of gas decreases causing bond strength to increase

More Related