Calorimetry

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Calorimetry

• AP Chemistry

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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.

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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).

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• 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)
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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 1 mole H2O 6.0 kJ
  18.0 g 1 mole
• 18.7 kJ will be absorbed
• q 18.7kJ

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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 1 mole 40.7 kJ
18gram 1 mole
452 kJ released
So, q - 452kJ
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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.
• Waters Specific Heat (as a liquid)
• 4.184 Joules/gram oC
• or unit cans be J/gK

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• 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.

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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
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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

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Changes In State
Solid
Freezing
Sublimation
Deposition
Melting
Vaporization
Liquid
Gas
Condensation
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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) ( 4.184 J/goC) (55oC 10oC)
q 15062 joules divide by 1000 to get
kilojoules 15062 J 1 kJ
1000J
15.06 kJ absorbed
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• 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 6.0 kJ
50 kJ
18grams 1 mole
Step 2 - Calculate heat required to heat liquid
water from 0oC to 50oC
q mC T (150g)(4.184
J/goC)(50oC)
31380 J ? convert to kJ 31.38kJ
Add both heat values together for your final
answer 50 kJ 31.38kJ 81.38 kJ heat
absorbed.
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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
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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.

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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.

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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.

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• 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
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• 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 4.013 kJ
• ?H - 4.013 kilojoules
• Since temperature of water increased, it is
  exothermic, which means a - ?H