Intro to Heat

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Intro to Heat

• Mrs. CoyleH. Chemistry

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Heat

• The energy transferred as a result of temperature
  difference.
• Symbol q
• Unit Joule (J)
• Unit Calorie (cal)
• 1 cal 4.184 J

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Food Calories

• With capital C.
• 1Cal 1kcal

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Exo- and Endothermic Reactions

• Exothermic reaction reaction gives off energy.
• reactants products
  energy
• Endothermic reaction reaction takes in energy.
• energy reactants
  products

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Bonds and Energy

• When bonds are broken energy is absorbed.
• (analogy stretching a rubber band)
• When bonds are formed energy is released.
• (analogy releasing a stretched rubber band)

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Ex Exothermic

• H2 Cl2 ? 2 HCl 183 kJ/mol

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Ex Endothermic

• 2HgO 181.7 kJ ? 2Hg O2

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Activation Energy

• Minimum amount of kinetic energy the reactants
  must have in order to react.
• The particles form an activated complex
  (transition state) that is an unstable
  arrangement of atoms that lasts a few moments.
  Then the products are formed.

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Collision Theory of Reactions
http//staff.um.edu.mt/jgri1/teaching/che2372/note
s/10/10_19.gif
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Calorimeter
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Heat qmcDT

• m mass
• DT Tf-Ti
• c specific heat , the heat needed to raise the
  temp. of 1 g of a substance by 1 degree C.
• The specific heat of water 4.184J/goC

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System

• The part of the universe that is being studied.

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Sign Convention

• When the system gains heat qgt0
• When the system loses heat qlt0

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Problem 1

• How much heat is gained by 0.050kg of aluminum
  when it is heated from 50oC to 75oC? (cAl
  0.903J/goC)
• Answer1.1x103 J

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Problem 2

• A 30.g piece of iron at 90.oC is placed in an
  insulated cup of 70.g water and 40.oC. What is
  the final temp. of the cup? (cFe0.449J/goC)
• Hint Abs(Heat Lost)Abs(Heat Gained)
• Answer 42oC