HL5-2.ppt

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HL5-2.ppt

• Enthalpy
• 5.1.1
• 5.1.3
• 5.1.4
• 5.2.3
• 5.2.4

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Defining the term

• Enthalpy
• Is defined as the heat content contained by a
  substance.
• The word is derived from a Greek word meaning
  heat inside.
• In Chemistry, it is noted by H
• The problem is that you cant directly measure H,
  only changes in H.
• So we usually talk about Enthalpy as ?H.

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The units of Enthalpy
H(g) H(g) ? H2(g)
H2(g) ? H(g) H(g)
What units are used to measure Enthalpies?
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Exothermic vs. Endothermic Reactions their
Enthalpies
H(g) H(g) ? H2(g)
H2(g) ? H(g) H(g)
Which reaction was Exothermic? Why?
5.1.3 Apply the relationship between temperature change, enthalpy change and the classification of a reaction as endothermic or exothermic.
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Enthalpy and stability
H(g) H(g) ? H2(g)
H2(g) ? H(g) H(g)
Which reaction do you think has the most stable
products?
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Discuss the following graph
5.1.4 Deduce, from an enthalpy level diagram, the relative stabilities of reactants and products and the sign of the enthalpy change for the reaction.
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Defining the term

• Standard Enthalpy Change of Reaction (?H)
• Is defined as the heat gained or lost by a system
  during a specified chemical reaction when
• The temperature is 298K (25C)
• The pressure is 100 kPa (1 atm)
• All concentrations are 1 ml dm-3 (1 M)
• All substances are in their standard state (the
  state that they would normally be in at these
  conditions)
• These values can be looked up in a book (or
  online)

5.1.1 Define the terms exothermic reaction, endothermic reaction and standard enthalpy change of reaction (?H?)
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Calculation of energy changes

• Accomplished using calorimeters and is based on
  the premise that
• Heat change of reaction - heat change of water
  in the calorimeter - mH2O?TH2OcH2O
• As the water has gained the heat produced by the
  reaction, the heat change of the reaction is
  negative when the temperature of the water
  increases.
• If you measure the change at constant pressure
  (like an open contaner), ?H is also known as qp
  (so q - m?Tc as well)

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A look at the variables

• ?H - mH2O?TH2OcH2O
• ?H Enthalpy of the reaction (measured in J)
• mH2O the mass of the water in the calorimeter
  (measured in grams)
• ?TH2O the change of temperature of the water in
  the calorimeter (C)
• cH2O the specific heat capacity of water (4.18
  J/g or 1 cal/g for liquid water)

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Heat Capacity vs. Specific Heat Capacity

• Heat Capacity the amount of heat energy needed
  to change the temperature of an entire system 1 K
• The system could be a calorimeter, a room, a
  building, a planet
• Specific Heat Capacity the amount of heat
  energy needed to change the temperature of 1 g of
  a substance by 1 K
• It is more specific than Heat Capacity

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Calorimeters
Cup Calorimeter Constant P
Bomb Calorimeter Constant V
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Enthalpy Problem

• Calculate the enthalpy of combustion from the
  following data. Assume all the heat from the
  reaction is absorbed by the water. Compare your
  value with the IB Data Booklet value and suggest
  reasons for any differences.
• Mass of water in copper calorimeter/g200.00
• Temperature increase in water/C13.00
• Mass of ethanol burned/g0.45
• Q Should your answer be gt or lt zero?
• Group up and try it!

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Answer

• Step 1 Determine the ?H of the reaction.
• Mass of water in copper calorimeter/g200.00
• Temperature increase in water/C13.00
• Mass of ethanol burned/g0.45
• ?H - mH2O?TH2OcH2O
• ?H - (200.00)(13.00)(4.18)
• ?H - 10,900 J or -10.9 kJ

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Answer

• Step 2 Convert the ?H of the reaction to a
  standard ?H(?H /moles ?H)
• Mass of water in copper calorimeter/g200.00
• Temperature increase in water/C13.00
• Mass of ethanol burned/g0.45
• Moles mass/M 0.45/46.08 9.8E-3
• ?H ?H /moles -10.9/9.8E-3
• ?H -1100 kJ mol -1

5.2.3 Calculate the enthalpy change for a reaction using experimental data on temperature changes, quantities of reactants and mass of water.
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Answer

• Compare your value with the IB Data Booklet value
  and suggest reasons for any differences.
• IB Data Booklet Value -1367 kJ mol -1
• Lab value for ?H - 1100 kJ mol -1
• The booklet value suggests that more heat should
  have been gained by the calorimeter. The lab
  data suggests that not all of the heat lost by
  the reaction was gained by the calorimeter.

5.2.4 Evaluate the results of experiments to determine enthalpy changes.
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Homework (Due Tomorrow)

• Exercise 5.1
• Pages 137-138
• 1-5
• Exercise 5.2
• Page 140
• 1-6