1. Heat Calculations Q = 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 = mHv and 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 DOWN the 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 = 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, Hf from 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 vaporize 300 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 Q=mcΔT (cliquid =
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 -5 ºC 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