Chemistry and Energy

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Chemistry and Energy

• How are they related?

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Energy Encountered Daily
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What is Energy?

• Defined as the ability to do work or create heat.
• Many types of energy
• Thermal
• Light
• Gravitational
• Kinetic
• Potential

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Light Energy Review

• How is light energy produced?
• Electrons release light energy when they fall
  from a high energy level to a lower energy.
• Were now going to talk about energy released or
  used in a chemical reaction. Heat energy

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Thermochemistry

• The study of heat used or released in a chemical
  reaction.
• Lets investigate heat as it compares to
  temperature using the Heat vs. Temperature Handout

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Specific Heat Calculations

• q mC?T
• q heat (J or cal or Cal)
• 4.184 cal 1 Joule
• 1000 cal 1 Cal (dietary calorie)
• m mass (g)
• C specific heat (J/g oC or cal/g oC)
• ?T change in temperature (o C or K)
• Tf - Ti

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Specific Heat

• Specific heat of water 1 cal /g o C or
• 4.184 J / goC
• Specific heat of most metals lt 1 J / goC
• Do metals heat slowly or quickly compared to
  water?
• Do metals stay warm longer or shorter than water?

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Practice Problem

• How much energy is required to heat 120.0 g of
  water from 2.0 oC to 24.0oC?
• q mC?T
• m 120.0 g
• C 4.184 J/goC
• ?T (24.0 2.0)oC 22.0oC
• q (120.0g)(4.184 J/goC)(22.0oC)

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Practice Problem

• How much heat (in kJ) is given off when 85.0 g
  of lead cools from 200.0oC to 10.0 oC? (Specific
  heat of lead 0.129 J/g oC)
• q mC?T
• m 85.0 g
• C 0.129 J/g oC
• ?T (10.0 200.0)oC - 190.0oC
• q (85.0 g)(0.129 J/g oC)(- 190.0oC) -

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How Do Chemical Reactions Create Heat energy?

• Consider the combustion of gasoline (octane)
• 2 C8H18 25 O2 ? 16 CO2 18 H2O
• Potential Energy Stored energy
• Potential energy is stored in the bonds of the
  reactant s and the products
• When bonds are broken, the energy is available
• When produce bonds form, some energy is used in
  these bonds
• The excess energy is released as heat

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Kinetic Energy

• Directly related to temperature

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Is Heat Used or Released?

• Endothermic reactions used heat from the
  surroundings
• Sweating
• Refrigeration
• Exothermic heat releases heat to the surroundings
  
• Hot hands
• Combustion
• Exercise

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Endothermic Reactions

• Decrease in kinetic energy ? decrease in
  temperature ? heat will transfer from the
  environment to the system resulting in a cooler
  environment
• Absorbs heat from its surrounding.
• The system gains heat
• Positive value for q
• ?H q ?0
• Hproducts ? Hreactants

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Exothermic Reactions

• Increase in kinetic energy ? increase in
  temperature of system ?heat released to the
  environment resulting in a hotter environment
• Releases heat to its surroundings
• The system loses heat
• Negative value for q
• ?H q ?0
• Hproducts ? Hreactants

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Enthalpy

• Heat content for systems at constant pressure
• Symbol is H
• Terms heat and enthalpy are used interchangeably
  for this course
• ?H q m C ?T
• Heat moves from ________ to ___________.

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Law of Conservation of Energy

• Energy is not lost or gained in a chemical
  reaction
•  In a chemical reaction potential energy is
  transferred to kinetic energy

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Thermochemical Equations

• An equation that includes the heat change
• Example write the thermochemical equation for
  this reaction
• CaO(s) H2O(l) ?Ca(OH)2(s) ?H -65.2 kJ
• CaO(s) H2O(l) ?Ca(OH)2(s) 65.2 kJ

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Stoichiometry and Thermochemistry

• Tin metal can be extracted from its oxide
  according to the following reaction
• SnO2(s) 4NO2(g) 2H2O(l) 192 kJ ? Sn(s)
  4HNO3(aq)
• How much energy will be required to extract 59.5
  grams of tin?

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How to solve

• Use your stoichiometry
• Treat heat as a reactant or product
• SnO2(s) 4NO2(g) 2H2O(l) 192 kJ ? Sn(s)
  4HNO3(aq)
• 59.5 g Sn 1 mol Sn 192 kJ
• 1 g Sn 1 mol Sn

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• If an Object feels hot, it means it is undergoing
  a change with a ?H that is
• a. positive
• b. negative
• c. whether the object feels hot or not is
  unrelated to its ?H
• d. I dont know
•  

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• If the object feels hot, it means it is
  undergoing
• a. an exothermic reaction
• b. an endothermic reaction
• c. whether it feels hot or not is unrelated to
  whether it is undergoing an exothermic or an
  endothermic change

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Heat During a Change of State

• How does ice melt?

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Molar Heat of Fusion

• Heat absorbed by one mole of a substance during
  melting
• Constant temperature
• ?Hfus
• H2O(s) ? H2O(l) ?H 6.01 kJ/mol

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Molar Heat of Solidification

• Heat lost when 1 mole of a liquid solidifies
• Temperature is constant
• ?Hsolid
• ?Hfus -?Hsolid
• H2O(l) ? H2O(s) ?H -6.01 kJ/mol

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Molar Heat of Vaporization

• Heat needed to vaporize 1 mole of a liquid
• ?Hvap
• H2O(l) ? H2O(g) ?Hvap 40.7 kJ/mol

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Molar Heat of Condensation

• Heat released when 1 mole of vapor condenses
• ?Hcond
• H2O(g) ? H2O(s) ?Hcond -40.7 kJ/mol
• ?Hvap -?Hcond

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Phase Change Diagram for Water
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Phase Change Diagram
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The House that Heats Itself

• http//www.sciencefriday.com/videos/watch/10007

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CaLORIMETRY
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Calorimetry

• Method used to determine the heat involved in a
  physical or chemical change.
• Relies on the law of conservation of energy

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Calorimeter
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Simple Calorimeter
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Calorimetry Math

• Heat gained by the water q
• Heat lost by the system -q
• mC?T q
• ?T Tf Ti , m mass, C specific heat
• q gained by water q lost by system
• q water - q system
•  mC?T -mC?T
• (mass H2O)(spec. heat H2O)(?T H2O) - (mass
  sys)(spec. heat sys)(?T sys)

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Standard Heat of Reaction

• Heat change for the equation as it is written
• ?H ?Hf(products) - ?Hf(reactants)
• Standard Heats of Formation (?Hf)
• Change in enthalpy when 1 mole of the compound is
  formed from its elements in their standard states
  at 25oC and 101.3 kPa

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Hesss Law

•  A way to calculate the heat of a reaction that
  may be too slow or too fast to collect data from.
• Add together several reactions that will result
  in the desired reaction. Add the ?H for these
  reactions in the same way.
• ??Htotal ??Hproducts - ??Hreactants