1. Calorimetry
AP Chemistry

2. Calorimetry Calorimetry is the measurement of heat flow.
It allows us to calculate the amount of energy required to heat up a substance or to make a substance change states.

3. Calorimetry Terms
Molar Heat of Fusion — The heat absorbed by one mole of a substance when changing from a solid to a liquid.
For water, it = 6.0 kiloJoules/mole
Heat of solidification is opposite of heat of fusion (heat is released).

4. For water, it = 40.7 kiloJoules/mole.
Molar Heat of Vaporization — The heat absorbed by one mole of a substance when changing from a liquid to a gas.
For water, it = 40.7 kiloJoules/mole.
Heat of condensation is the opposite of heat of vaporization (heat is released)
Heat Required For a Phase Change – Or Latent Heat Process
Heat Absorbed or Released = q
q = (moles) x (Molar Heat Fusion/Vaporization)

5. Calculating Heat Required To Change State
Example #1: How much heat is needed to melt 56.0 grams of ice into liquid (the molar heat of fusion for ice is 6.0 kJ/mol)?
56.0 g mole H2O kJ = g mole
= kJ will be absorbed
q = kJ

6. Example #2
How much heat energy will be released when 200grams steam condenses back to a liquid water?
Molar Heat condensation = 40.7kJ/mol
q = (moles) x (Molar Heat condensation)
200gram mole kJ
18gram mole
= 452 kJ released
So, q = - 452kJ

7. Heating a Substance with No Phase Change
Also known as a sensible heat process.
Depends on Specific Heat Capacity
Specific Heat Capacity--The amount of energy required to raise one gram of a substance one degree Celcius.
Every substance and every phase of that substance has its own unique specific heat capacity.
Water’s Specific Heat (as a liquid)
= Joules/gram oC
or unit cans be J/gK

8. Specific heat (J/goC) = heat capacity (J/oC)
You will see both terms used.
You can also use Molar Heat Capacity
This is the heat energy required to raise 1 mole of a substance 1oC or 1K.
Molar heat capacity can be determined from the specific heat by multiplying it by the molar mass of the substance.

9. Energy to Change Temperature
q = (mass) ( C ) ( T )
Change in Temperature
Tfinal – Tinitial
In OCelcius or kelvin
Heat
Measured in Joules
Specific Heat
Capacity
Mass
In grams

10. Exothermic vs. Endothermic
Exothermic Change – Heat Energy is released to the surroundings. q and ∆H are (-)
Molecules slow down, extra energy is transferred to surrounding.
Cooling phase changes are exothermic
Endothermic Change – Heat energy is absorbed by the system. q and ∆H are (+)
-Molecules move faster as they absorb energy
-Phase changes that require energy (or heat) are endothermic

11. Changes In State Solid Liquid Gas Freezing Sublimation Deposition
Melting
Vaporization
Liquid
Gas
Condensation

12. Example #3
How much energy is needed to heat 80grams of water from 10oC to 55oC at constant pressure?
q = m C T = m C (Tfinal – Tinitial )
q = (80grams) ( J/goC) (55oC – 10oC)
q = joules
divide by 1000 to get kilojoules
15062 J 1 kJ =
1000J
15.06 kJ absorbed

13. Example #4 -How much energy is needed to change 150grams of ice from 0oC to 50oC?
This problem requires two steps. Since water is solid ice at 0oC, we need to melt the ice and then heat it up to 50oC.
Step 1 – Calculate heat required to melt 150grams ice
150g 1 mole kJ = kJ
18grams mole
Step 2 - Calculate heat required to heat liquid water from 0oC to 50oC
q = mC T = (150g)(4.184 J/goC)(50oC)
= J  convert to kJ = 31.38kJ
*Add both heat values together for your final answer
50 kJ kJ = kJ heat absorbed.

14. Multiple Step Calorimetry Problems
Add each individual energies together for total
Use
q = Moles x Molar Heat vap/fus e
Gas
Heats
Vaporization d c
Liquid
Heats b
Solid
Heats
melting a
Use q = mC T
qtotal = a + b + c + d + e

15. Calorimetry Coffee Cup Calorimetry
Experimental way of measuring heat generation/consumption by essentially catching all the heat energy in a water bath or water bath + metal apparatus.
Coffee Cup Calorimetry
Styrofoam cup insulates the contents so any heat generated or consumed in the water can be measured by the temperature change
q = -(Hrxn) = mCH2O∆T
Reactions must take place in water, then you measure the change in temperature.

16. Bomb-Calorimeter Used to measure heats of reaction.
Usually do combustion reactions in it.
Metal innards absorb heat. You have to keep track of heat absorbed by metal and water.
q = mCH2O∆T + Cp ∆T
qwater qapparatus
Cp is the heat capacity of the apparatus – determined experimentally.

17. Conservation of Energy
Under conditions of constant pressure
Heat Lost = Heat gained
qp-lost = qp-gained
For a Reaction heat gained by water = heat released by reaction.

18. Example- When 100mls of 0. 1M HCl is mixed with 100mls of 0
Example- When 100mls of 0.1M HCl is mixed with 100mls of 0.1M NaOH in a coffee cup calorimeter, the temperature increases from 25oC to 29.8oC. Assume that the coffee cup is a perfect insulator, pressure is constant and the density of the solution is 1g/ml and specific heat capacity for the mixture is that of water, 4.18 J/gK.
What is the enthalpy change for the reaction?
∆H = q = mC∆T

19. Since density is 1g/mL , all volumes are gram values.
Total volume must be used = 200mLs = 200grams.
∆T = Tfinal – Tinitial = 29.8oC – 25oC = 4.8oC
q = mC ∆T = (200g)(4.18J/gK)(4.8K)
q = 4013 J = kJ
∆H = kilojoules
Since temperature of water increased, it is exothermic, which means a - ∆H