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Unit 2 Calorimetry II

Unit 2 Calorimetry II. Chapter 10. Temp Equalization. When we add a spoon (at 20°C) to water (at 95°C) The spoon’s temperature will rise since the spoon is cooler than the water. The water’s temperature will fall since the water is hotter than the spoon.

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Unit 2 Calorimetry II

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  1. Unit 2Calorimetry II Chapter 10

  2. Temp Equalization • When we add a spoon (at 20°C) to water (at 95°C) • The spoon’s temperature will rise since the spoon is cooler than the water. • The water’s temperature will fall since the water is hotter than the spoon. • The final temperature for both the spoon and water will be the same! • Tf(spoon) = Tf(water)

  3. Heat Lost = Heat Gained • When combining two samples at different temperatures, the amount of heat gained by the cooler substance equals the amount of heat given off by the hotter substance. • The absolute values of q are equal. • When you add a spoon to hot water: |qspoon| = |qwater|

  4. Spoon Water Combining Substances • It is difficult to measure q, but we can measure how it affects other things. • Rearrange cp equation to solve for q q = cpmT • Then, set the two equations equal: |cpmT| = q = |cpmT|

  5. Spoon Water Get Rid of Absolutes • To get rid of absolutes, add negative to ONE side (usually Exothermic side). • Since the water is hotter, that is the exothermic side (its temp will fall). cpmT = -cpmT

  6. Coffee Cups

  7. And bombs

  8. Combining Substances • The combined formula is: cp1m1T1 = -cp2m2T2 • If we want to solve for something, we just divide by variables the we know: • cp1 = -cp2m2T2 / m1T1 • m1 = -cp2m2T2 / cp1T1 • T1 = -cp2m2T2 / cp1 m1

  9. Co-mingling • This technique will work as long as we do not cross phase boundaries! • As long as a solid remains a solid, • As long as a liquid remains a liquid, • And as long as a gas remains a gas, • We can use the previous formulas; • However…

  10. Phase Diagram Ionization & Plasma Boiling Point Melting Point Temperature Gas Phase Solid Phase Liquid Phase Heat

  11. Phase Changes Boiling Pt Gas Temperature Melting Pt Liquid Notice: No T!!! Solid Heat

  12. Phase Changes • Notice on the phase change diagram, the temperature remains constant while the phase is changing. • We cannot use cp= q / mT for phase changes • Because there is no T!

  13. Two More Heat Capacities • Heat of Fusion (cfus) • the amount of heat needed to convert 1g of a substance from solid to liquid. • Heat of Vaporization (cvap) • the amount of heat needed to convert 1g of a substance from liquid to gas. • One general formula: cx = q / m

  14. Heats and Phase  • When heat problems cross phase boundaries, we need to break them apart: • 50 g Ice at -10°C going to 150°C • All three phases: • Solid: -10°C to 0°C • Liquid: 0°C to 100°C • Gas: 100°C to 150°C

  15. Steps in a Phase Change

  16. Solution • Plug in values for each step. • Add up the values of heat (q) • For phases • q = cpm∆T • For melting/boiling points • q = cxm

  17. Summary • Heat Capacity  cp = q / mT • Heat Lost = Heat Gained qendo = -qexo cpmT = -cpmT • Phase Changes • Break into steps • Heats of Fusion and Vaporization • cx = q / m

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