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Chapter 6 Thermochemistry 6'1 6'2 Notes

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Potential Energy energy due to position or composition. Chemical Energy ... Thermal energy is being converted into potential energy stored in the chemical bonds ... – PowerPoint PPT presentation

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Title: Chapter 6 Thermochemistry 6'1 6'2 Notes


1
Chapter 6 Thermochemistry6.1 6.2 Notes
  • AP Chemistry

2
Goals 35 45 aah!
  • Describe energy in terms of the capacity to do
    work or produce heat.
  • Understand the law of conservation of energy.
  • Describe energy as a state function.
  • Distinguish between exothermic and endothermic
    processes.
  • Distinguish between potential and kinetic energy.
  • Distinguish between heat and temperature.
  • Describe energy flow in the universe between
    systems and surroundings.
  • Understand the first law of thermodynamics.
  • Perform energy calculations on systems with both
    heat and work energy.
  • Describe the enthalpy of a system as the flow of
    heat energy at constant pressure.
  • Use calorimetry to determine the heat of reaction
    at constant volume and pressure.

3
The Nature of Energy
  • Energy capacity to do work or release heat
  • Law of Conservation of Energy energy can be
    converted from one form to another, but cant be
    created or destroyed
  • The energy of the universe is constant

4
Kinetic vs. Potential Energy
  • Kinetic Energy - energy an object has due to its
    motion and depends upon the objects mass and
    velocity (KE(1/2)mv2)
  • Potential Energy energy due to position or
    composition

5
Chemical Energy
  • Chemical Energy potential energy stored within
    the bonds of a substance
  • System part of the universe we are focusing on
    (reactant and products in a reaction)
  • Surroundings everything else in the universe
    (room reaction is taking place in)

6
Heat vs. Temperature
  • Heat energy transferred between two objects due
    to temperature differences between them
  • Temperature measure of the average kinetic
    energy of particles in matter

7
Exothermic vs. Endothermic
  • Exothermic heat is evolved, or given off
    energy flows out of the system
  • Endothermic heat is absorbed energy flows into
    the system
  • The energy gained by the surrounding must be
    equal to the energy lost by the system

8
Exothermic Reactions
  • Potential energy stored in the chemical bonds is
    being converted to thermal energy via heat

9
Endothermic Reactions
  • Thermal energy is being converted into potential
    energy stored in the chemical bonds

10
Thermodynamics
  • Study of energy and the changes it undergoes
  • 1st law of thermodynamics the energy of the
    universe is constant

11
Internal Energy
  • Sum of kinetic and potential energy of the
    particles in the system
  • Energy heat work
  • ?E q w
  • If pressure volume work is done, then w -P?V

12
Sign of q and w
  • q is () if energy flows into the system
    (endothermic)
  • q is (-) if energy flows out of the system
    (exothermic)
  • w is (-) if system does work on surroundings
    (expanding)
  • w is () if surroundings does work on system
    (compression)

13
Internal Energy Example
  • A system releases 125 kJ of heat and does 29 kJ
    of work. Calculate ?E for this process.

14
PV Work Example
  • Calculate the work associated with the expansion
    of a gas from 36 L to 57 L at a constant external
    pressure of 1.5 atm.

15
Internal Energy, Heat, and Work Example
  • A hot air balloon is inflated by heating the air
    inside it. The volume of the balloon increases
    from 2.00x106 L to 4.00x106 L by the addition of
    5.50x108 J of heat energy. Assuming the balloon
    expands against a constant pressure of 1 atm,
    calculate ?E for this process. (Use 1 Latm
    101.3 J)

16
Enthalpy
  • ?H E PV
  • At constant pressure, ?H q, or the change in
    enthalpy of a system is equal to the energy flow
    as heat
  • When studying reactions at constant pressure, the
    terms heat of reaction and enthalpy are
    interchangeable
  • ?H ?Hproducts - ?Hreactants

17
Enthalpy Example
  • On average, each person in the USA consumes 110 g
    of sugar daily. If all of this sugar is assumed
    to be glucose, how much heat energy from glucose
    is transferred to one person per day at constant
    pressure?
  • C6H12O6 6 O2 ? 6 CO2 6 H2O 2870 kJ

18
Calorimetry
  • Science of measuring heat
  • Calorimeter device used to measure heat change
  • Heat Capacity (C)
  • C (heat absorbed/increase in temperature)
  • Specific Heat Capacity (c or s) amount of heat
    necessary to raise 1 gram of a substance by 1 oC
  • Molar Heat Capacity - amount of heat necessary to
    raise 1 mole of a substance by 1 oC

19
Constant-Pressure vs. Constant Volume Calorimetry
  • Constant-Pressure heat of reaction is equal to
    the enthalpy change
  • ?H q specific heat x mass x ?temperature
  • q mc?T
  • Constant-Volume heat of reaction is equal to
    the internal energy change
  • ?E q ?temperature x heat capacity of
    calorimeter
  • q C?T

20
Constant-Pressure Calorimetry Example
  • A 2.00 L of 0.500 M Ba(NO3)2 solution at 22.0 oC
    is mixed with 1.00 L of 1.00 M Na2SO4 in a
    calorimeter to produce a white precipitate. The
    temperature of the solution increased to 28.1 oC.
    Assume this reaction took place at constant
    pressure and the calorimeter absorbed a
    negligible amount of heat (Density of the final
    solution is equal to 1.0 g/mL and its specific
    heat capacity is 4.18 J/(goC)). Calculate the
    enthalpy change per mole of BaSO4 formed.

21
Constant-Volume Calorimetry Example
  • 12.0 g of H2 gas was burned in a bomb calorimeter
    causing a temperature increase of 17.0 oC. The
    bomb calorimeter has a heat capacity of 11.3
    kJ/oC. What is the energy of combustion of
    hydrogen per mole?
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