Heat Calculations Q = mH v & Q = mH f

1. Heat CalculationsQ = mHv& Q = mHf You will need a calculator and your shoulder partner

2. Objective Students will calculate the amount of heat by using the formulas Q = mHf and Q = mHvand content language (heat, mass, heat of vaporization, heat of fusion (melting)) supports include class notes, calculator, background knowledge from math class and shoulder partners.

3. Heating Curve/Phase Changes Exothermic: Down the heating curve Losing Heat Energy CONDENSATION VAPORIZATION GAS LIQUID FREEZING MELTING Endothermic: Up the heating curve Gaining Heat Energy SOLID

4. Endothermic • Gaining Energy • Going UP the heating curve • Reactants have LESS energy than products (pg. 538) • POSITIVE Hf or Hv

5. Exothermic • Losing Energy • Going DOWNthe heating curve • Reactants have MORE energy than products • NEGATIVE Hf or Hv

6. Heating Curve Heat of Fusion Q = mHf Hf = 80 cal/g Heat of Vaporization Q = mHv Hv= 540 cal/g

7. Q = mHf • Use if the question uses the word “freezes” or “melts” (1st straight line on heating curve) • Q = Heat or Heat Energy Units: Joules or Calorie • m = Mass Units : grams • Hf = Heat of Fusion • ALWAYS: 80 cal/g if “melting” - 80 cal/g if “freezing”

8. I Do: Q= mHf • Read the question: • How much energy is released when 20.0 g of water is frozen at 0oC? • Find your Q, m, Hf from the question. Q = ? m = 20.0 g Hf = -80 cal/g Q = (20.0 g) (-80 cal/g) Q = -1600.0 cal

9. We Do: Q = mHf • Read the question: • How much energy is required to melt 25.9 g of ice at its melting point? • Find your Q, m, Hffrom the question. Q = ? m = Hf=

10. Q = mHv • Use if the question uses the word “vaporizes” OR “condenses” (2nd straight line of heating curve) • Q = Heat or Heat Energy Units: Joules or Calorie • m = Mass Units : grams • Hv = Heat of Vaporization • ALWAYS: 540 cal/g if “vaporizing” • - 540 cal/g if “condensing”

11. I Do: Q= mHv • Read the question: • Determine the amount of heat (Q) needed to vaporize300 g of water at 100oC. • Find your Q, m, Hv from the question. Q = ? m = 300 g Hv = 540 cal/g Q = (300 g) (540 cal/g) Q = 162,000 cal or 162 Kcal

12. We Do: Q = mHv • Read the question: • How much energy is released when 23.5 g of steam is condensed at 100oC? • Find your Q, m, Hv from the question. Q = ? m = Hv =

13. Heating Curve Q = mHv Hv= 540 cal/g Q = mHf Hf = 80 cal/g Q=mcΔT (cliquid = 1 cal/g oC) Q=mcΔT (cice = 0.5 cal/g oC)

14. “I Do”… Calculate the TOTAL amount of heat required to completely convert 50 g of ice at -10 ºC to steam at 100 ºC. Step 1: Draw a heating curve… Q=mHv 100oC 100oC Q=mcΔT 0oC Q=mHf - 10oC Q=mcΔT

15. This questions takes 4 steps…(4 lines on the heating curve) Step 1: First Line: Heat taken up heating the ice from -10 ºC to the melting point, 0 ºC. Q=mcΔT: Q = (50g) x (0.5 cal/g oC) x(10 oC) = 250 cal Step 2: Second Line: Heat taken up for converting ice from 0 ºC to water at 0 ºC. Q=mHf: Q = (50g) x (80 cal/g) = 4,000 cal Step 3: Third Line: Heat taken up heating the water from 0 ºC to the boiling point, 100 ºC Q = mcΔT: Q = (50g) x (1 cal/g oC) x (100 oC) = 5,000 cal Step 4: Forth Line: Heat taken up vaporizing the water. Q = mHv: Q = (50g) x (540 cal/g) = 27,000 cal Step 5: Add ALL the HEAT: 250 cal + 4,000 cal + 5,000 cal + 27,000 cal = 36,250 cal

16. “YOU DO….” Calculate the amount of heat required to completely convert 25 g of ice at -5ºC to steam at 100 ºC. Step 1: Draw a heating curve… Step 2: Q = mcΔT Step 3: Q = mHf Step 4: Q = mcΔT Step 5: Q = mHv Step 6: Add all the Q’s up