AS Chemistry

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AS Chemistry

• Unit 2, Module 3, Energy
• Enthalpy 4
• Standard enthalpy of Combustion

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Objectives - Enthalpy

• 2.3.1 Enthalpy Changes, Candidates should be able
  to
• (a) explain that some chemical reactions are
  accompanied by enthalpy changes, exothermic (?H,
  negative) or endothermic ?H, positive)
• (f) define and use the terms
• (i) standard conditions,
• (ii) enthalpy change of reaction,
• (iii) enthalpy change of formation,
• (iv) enthalpy change of combustion

3
Definitions

• Without referring to your notes, write a
  definition of
• Enthalpy change of combustion
• The enthalpy change that takes place when one
  mole of a substance reacts completely with oxygen
  under standard conditions, all reactants and
  products in their standard states.

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Standard Enthalpy Change of combustion

• Standard enthalpy change of combustion, ?Hc?
• Close your text books
• Write equations for the standard enthalpy of
  combustion of
• CH4 ?Hc? -890
• H2 ?Hc? -286
• Al ?Hc? -1676
• Write out the equations in full including state
  symbols, and the appropriate units.

5
Experimental determination of standard enthalpy
change of combustion

• What do you need to do to measure a ?Hc??
• Burn a known mass of fuel
• Heat a known mass of water
• Measure the temperature change of the water
• On the paper, draw apparatus to do this.

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Experimental determination of standard enthalpy
change of combustion
What is wrong with this method?
How could we improve it?
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Practice

• 2g of propan-1-ol heated 250cm3 of water by
  50C. Find the enthalpy change of combustion of
  propan-1-ol
• Specific heat capacity water 4.18 J.g-1.K-1
• Density of water 1g.cm-3
• Help
• Find heat change of experiment
• Find the amount in moles of propanol that reacted
• Work out heat loss in kJ.mol-1
• Report answer ?Hc?

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Experimental determination of standard enthalpy
change of combustion

• In these experiments we often end up with a value
  for ?Hc? that is too low.
• Why?
• There may have been incomplete combustion
• There may have been heat loss to the surroundings
  (not heating the water)
• How could we reduce these?

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Experimental determination of standard enthalpy
change of combustion
A bomb calorimeter How does this reduce errors?
10

• Practical 7.6 Nuffield
• Pages 154 - 156

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• Questions page 195
• Calculate the enthalpy changes of combustion for
  butan-1-ol and decane using the experimental
  results below. Give both answers to three
  significant figures.
• 1.51 g of butan 1 ol, CH3CH2CH2CH2OH, heated
  300 cm3 of water by 42.0 oC.
• 0.826 g of decane heated 180 cm3 of water from
  22.0 oC to 71.0 oC

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• Past exam questions