Thermochemistry pt 2

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Thermochemistry pt 2
2
Calorimetry

• ?H can be found experimentally or calculated from
  known enthalpy changes
• Measure heat flow with a calorimeter
• Heat capacity (J/K) amount of heat needed to
  raise the temperature by one degree
• Specific Heat capacity (J/gK) amount of heat
  needed to raise the temperature of 1g of a
  substance by one degree

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Specific Heat

• Specific heat can be found by measuring the
  temperature change (?T) a known mass (m)
  undergoes when it loses or gains a specific
  amount of heat (q)
• Specific heat (c) q / (m x ?T)
• What is the specific heat of water if 209 J of
  heat is needed to increase the temperature of
  50.0g of water by 1.00K?
• Why does it not matter whether we work in Celsius
  or Kelvin?

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Concept Practice

• How much heat is needed to warm 250g of water
  from 22C to near its boiling point, 98C? The
  specific heat of water is 4.18J/gK. What is the
  molar heat capacity of water?
• Answer 7.9x104J, 75.2J/molK

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Concept Practice

• Large beds of rocks are used in some solar-heated
  homes to store heat. Assuming the specific heat
  of the rocks is 0.82 J/gK, how much heat has
  been absorbed if the temperature of 50.0kg of
  rocks increases by 12.0C. What temperature
  change occurs if the rocks emit 450kJ of heat?
• Answer 4.9x105J, 11K11C

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Constant Pressure Calorimetry

• Assume that calorimeter prevents the loss or gain
  of heat
• qr x n completely transferred to the solution
• -qr x nqs o l n
• qs o l n mc?T -qr x n

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Concept Practice

• When a student mixes 50mL of 1.0M HCl and 50mL of
  1.0M NaOH in a coffee-cup calorimeter the
  temperature of the resultant solution increases
  from 21.0C to 27.5C. Calculate the enthalpy
  change for the reaction assuming the total volume
  of the solution is 100mL, that its density is
  1.0g/mL and the its specific heat capacity is
  4.18J/gC.
• -54kJ/mol

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Concept Practice

• When 50.0mL of 0.100M AgNO3 and 50.0mL of 0.100M
  HCl are mixed in a constant pressure calorimeter,
  the temperature of the mixture increases from
  22.30C to 23.11C. The temperature increase is
  caused by the following reaction
• AgNO3(aq) HCl(aq) ? AgCl(s) HNO3(aq)
• Calculate the heat of the reaction assuming the
  combined solution has a mass of 100.0g and a
  specific heat of 4.184.18J/gC.
• Answer -68kJ/mol

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Constant Volume Calorimetry

• Combustion reactions are studied using a bomb
  calorimeter
• To measure heat of combustion we need the heat
  capacity of the calorimeter (Cc a l)
• qr x n -Cc a l x ?T
• Remember at constant volume q ?E

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Concept Practice

• Methylhydrazine (CH6N2) is commonly used as a
  liquid rocket fuel. The combustion of
  methylhydrazine with oxygen produces nitrogen,
  carbon dioxide and liquid water. When 4.00g of
  the fuel is combusted in a bomb calorimeter, the
  temperature of the calorimeter increases from
  25.00C to 39.50C. In a separate experiment, the
  heat capacity of the calorimeter is found to be
  7.794kJ/C. What is the heat of reaction for the
  combustion of a mole of the the rocket fuel in
  this calorimeter?
• Answer -1.30x103 kJ/mol

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Concept Practice

• A 0.5865 sample of lactic acid (HC3H5O3) is
  burned in a calorimeter whose heat capacity is
  4.812kJ/C. The temperature increases from
  23.10C to 24.95C. Calculate the heat of
  combustion of a) lactic acid per gram and b) per
  mole.

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Hess's Law

• Hess's Law If a reaction is carried out in a
  series of steps, ?H for the reaction will equal
  the sum of the enthalpy changes for the
  individual steps.
• Combustion of methane
• CH4(g) 2O2(g) ? CO2(g) 2H2O(g) ?H -802kJ
• 2H2O(g) ? 2H2O(l)
  ?H -88kJ
• CH4(g) 2O2(g) 2H2O(g) ? CO2(g) 2H2O(g)
  2H2O(l)
• ?H -890kJ

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Hess's Law
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Hess's Law

• The number of steps does not matter
• ?H for any process can be measured as long as we
  know the ?H for each of the individual steps
• This allows us to find ?H for reactions that are
  difficult to measure.
• The combustion of carbon to form carbon monoxide
• The conversion of graphite to diamond

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Concept Practice

• C(s) O2(g) ? CO2(g) ?H -393.5kJ
• CO(g) ½ O2(g) ? CO2(g) ?H -283.0kJ
• Use the above reactions calculate the enthalpy of
  combustion of C to CO.
• C(s) ½ O2(g) ? CO(g)
• Answer -110.5kJ

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Concept Practice

• Carbon occurs in two forms, graphite and diamond.
  The enthalpy of combustion of graphite is
  -395.kJ/mol and that of diamond is -395.4kJ/mol.
  Calculate the ?H for the conversion of graphite
  to diamond.
• Answer 1.9kJ

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Concept Practice

• Calculate ?H for the reaction
• 2C(s) H2(g) ? C2H2(g)
• using the following pieces of data
• C2H2(g) 5/2 O2(g) ? 2CO2(g) H2O(l) ?H
  -1299.6kJ
• C(s) O2(g) ? CO2(g) ?H -393.5kJ
• H2(g) ½ O2(g) ? H2O(l) ?H -285.8kJ
• Answer 226.8kJ

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Enthalpy

• Various ?H measured according to the type of
  process.
• ?Hv a p vaporization
• ?Hf u s fusion
• ?Hf formation
• Enthalpy changes depend on temperature, pressure
  and state of reactants and products
• To compare enthalpies we define a standard state

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Standard States

• A substance is at standard state when it is in
  its pure form at one atmosphere and the
  temperature of interest.
• Standard enthalpy (?H) is when all reactants and
  products are in their standard states.
• Standard enthalpy of formation (?Hf) is the
  change in enthalpy that forms one mol of the
  compound from its elements.
• 2C(graphite) 3H2(g) ½O2(g) ? C2H5OH(l) ?Hf
  -277.7kJ
• ?Hf of elements in their standard state is zero.

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Concept Practice

• For which of the following reactions at 25C
  would the enthalpy change represent a standard
  enthalpy of formation? For those that are not,
  what changes should be made in the reaction
  conditions?
• 2Na(s) ½O2(g) ? Na2O(s)
• 2K(l) Cl2(g) ? 2KCl(s)
• C6H12O6(s) ? 6C(diamond) 6H2(g) 3O2(g)

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Calculating ?Hrxn from ?Hf

• ?Hrxn S n?Hf (products) - S n?Hf (reactants)
• Calculate the standard enthalpy change for the
  combustion of 1 mol of benzene to carbon dioxide
  and liquid water.
• Answer -3267kJ

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Standard Enthalpies of Formation
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Concept Practice

• Calculate the enthalpy change for the combustion
  of 1 mol of ethanol (C2H5OH)
• The standard enthalpy change for the reaction
• CaCO3(s) ? CaO(s) CO2(g) is 178.1kJ. Calculate
  the standard enthalpy of formation of calcium
  carbonate.
• Given the following standard enthalpy of
  reaction, calculate the the standard enthalpy of
  formation of CuO(s)
• CuO(s) H2(g) ? Cu(s) H2O(l) ?H -129.7kJ

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Homework

• 44, 47, 49, 52, 54, 57, 59, 61, 68, 70, 75, 108,
  111,