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Thermochemistry Heating Curves: Understanding Phase Changes

Learn about heating curves and how to calculate the total energy required for phase changes in water. This lesson covers temperature changes, the molar heat of fusion, and the molar heat of vaporization. Practice problems included.

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Thermochemistry Heating Curves: Understanding Phase Changes

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  1. Do NowDay 5 How many cal of heat are required to turn 567g of water into steam? K: 567g of H2O Hv=2260 J/g U: ? cal of heat q= mHv q= (567 g)(2260 J) g q=1,281,420 J 1 cal 4.184J q=306,267 cal

  2. Day 5 - Notes Unit: Thermochemistry Heating Curves

  3. After today you will be able to… • Explain what occurs on each point of a heating curve • Calculate the total energy required for water to go through phase changes

  4. Heating Curves A heating curve is a graph of temperature versus time. • It describes the enthalpy changes that take place during phase changes.

  5. Heating Curves When a solid substance is heated, its temperature will increase until it reaches its melting point (m.p.). • Temperature will then stay constantduring the melting process.

  6. Heating Curves When a substance is completely melted, its temperature will again increase until it has reached its boiling point (b.p.). • Temperature stays constant during boiling. • Once completely vaporized, the temperature will again increase.

  7. Heating curve problems involve TWO types of calculations: • q=mcΔT, use for temperature changes • q=mHf / q=mHv, use for when the phase of the substance is being changed

  8. The type of phase change will determine the ΔH to be used: • Molar heat of fusion (ΔHfus): the amount of energy required to melt/freeze a substance (solid liquid) • Molar heat of vaporization (ΔHvap): the amount of energy required to vaporize/condense (boil) a substance (liquid  gas)

  9. Example: What is the total amount of heat in Joules (J) that must be added to 165g of ice (solid H2O) at -5.00°C to change it into steam at 103°C? GAS q=mcΔT 103˚C BOILING (ΔHvap) 100˚C Temp (˚C) LIQUID q=mcΔT ΔHfus= 334 J/g ΔHvap= 2260 J/g cice= 2.01J/g°C cwater= 4.18 J/g C csteam= 2.02 J/g°C MELTING (ΔHfus) 0˚C ICE q=mcΔT The problem would give you the specific heats of water at solid/ gas state. You don’t have to memorize these #’s -5.00˚C Time

  10. ΔHfus= 334 J/g ΔHvap= 2260 J/g cice= 2.01J/g°C cwater= 4.18 J/g°C csteam= 2.02 J/g°C 103˚C (5) Example: What is the total amount of heat in Joules (J) that must be added to 165g of ice (solid H2O) at -5.00°C to change it into steam at 103°C? (4) (3) 100˚C (2) (1) Temp (˚C) • 1,658 • 55,110 • 68,970 • 372,900 • 999.9 • Ice, q=mcΔT q=(165g)(2.01J/g˚C)(5.00˚C) (4) Boiling, q=mHv q=1,658J q=(165g)(2260 J) g q=372,900J 0˚C + (2) Melting, q=mHf q=(165g)(334 J) g q=55,110 J TOTAL ENERGY (J): +499,638 J (3) Liquid, q=mcΔT (5) Gas, q=mcΔT -5.0˚C q=(165g)(4.18J/g˚C)(100˚C) q=(165g)(2.02J/g˚C)(3˚C) q=68,970J q=999.9J Time

  11. Questions? Complete WS 5

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