Heat Calculations Q = mHv & Q = mHf You will need a calculator and your shoulder partner

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.

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

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

Exothermic Losing Energy Going DOWN the heating curve Reactants have MORE energy than products NEGATIVE Hf or Hv

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

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”

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

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 =

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”

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

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 =

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)

“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

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

“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