Energy and Chemical Change UNIT XI Ch' 16

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Energy and Chemical ChangeUNIT XI(Ch. 16)
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Energy

• Energy is the ability to do work or produce heat.
• There are two forms of energy
• Potential Energy - the energy due to the
  composition or position of an object
• Kinetic Energy - the energy of motion

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The First Law of Thermodynamics

• In any chemical reaction or physical process,
  energy can be converted from one form to another,
  but it can neither be created nor destroyed.

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Second Law of Thermodynamics

• When energy changes form, it is never 100
  efficient. Some energy is always lost, usually in
  the form of low quality heat.

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Chemical Potential Energy

• Chemical Potential Energy is the energy stored in
  a substance because of its composition.
• e.g., octane (C8H18) in gasoline.

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Measuring Heat

• calorie - the amount of heat required to raise
  the temperature of one gram of pure water by one
  degree Celsius (1 oC).
• Calorie 1000 calories 1 Calorie 1 kcal 1
  food Calorie
• The metric unit of heat and energy is the joule
  (J).
• 1 joule 0.2390 calories
• 1 calorie - 4.184 joules

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Specific Heat

• The specific heat of any substance is the amount
  of heat required to raise the temperature of 1
  gram of that substance by 1 degree Celsius. Each
  substance has its own specific heat.

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Calculating Heat Absorbed or Heat Evolved

• Amount of heat q
• q is proportional to the amount of mass of the
  object (m)
• q is proportional to the temperature change (T2 -
  T1), ?T
• Therefore, q m c (T2 - T1) mc ?T, where q
  heat, m mass, c specific heat, T2 is the
  final temperature and T1 is the initial
  temperature.

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Calorimeter

• A calorimeter is an insulated device used for
  measuring the amount of heat absorbed or released
  during a physical or chemical process.
• Heat lost Heat gained
• m c ?T m c ?T

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Chemical Energy and the Universe

• Thermochemistry is the study of heat changes that
  accompany chemical reactions and phase changes.
• System - that part of the universe that contains
  the process you want to study.
• Surroundings - everything else in the universe
  except the System under study
• Thus, Universe System Surroundings

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Enthalpy and Enthalpy Changes

• Scientists are interested in changes in energy
  (heat) rather than the absolute amount of heat
  (energy) a system possesses.
• Enthalpy (H) is the heat content of a system at
  constant pressure.
• Enthalpy of reaction (Heat, ?Hrxn) is the change
  in enthalpy for a reaction.
• ?Hrxn Hfinal - Hinitial
• ?Hrxn Hproducts - Hreactants

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• Exothermic DH lt 0
• Endothermic DH gt 0

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Exothermic Reaction

• 4 Fe 3 O2 -----gt 2 Fe2O3 1625 kJ
• The reactants have more energy than the product.
  Therefore, Hproducts lt Hreactants. Thus, ?Hrxn is
  a negative number and the reaction is exothermic.

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Endothermic Reaction

• 27 kJ NH4NO3 -----gt NH4 NO3-
• In this reaction the products have more energy
  than the reactants. Therefore, Hreactants lt
  Hproducts. Thus, ?Hrxn is a positive number and
  the reaction is endothermic.

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Thermochemical Equations

• A thermochemical equation is a balanced chemical
  equation that includes the physical states of all
  the reactants and products and the energy change,
  usually expressed as the change in enthalpy, ?H.
• C6H12O6(s) 6 O2(g) 6 CO2(g) 6 H2O(l)
• ?Hcomb -2808 kJ, exothermic

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Changes of State

• ?Hvap the molar heat of vaporization, the heat
  required to vaporize one mole of a liquid
  substance.
• ?Hfus the molar heat of fusion, the heat
  required to melt one mole of a solid substance.
• ?Hvap and ?Hfus are positive numbers therefore,
  endothermic.

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Reaction Spontaneity

• A spontaneous process is a physical or chemical
  change that occurs with no outside intervention.
• Entropy (S) is a measure of the disorder or
  randomness of the particles that make up a
  system.
• Law of Disorder - states that spontaneous
  processes always proceed in such a way that the
  entropy of the universe increases.

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Entropy, the Universe and Free Energy

• ?Gsystem ?Hsystem - T?Ssystem

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?Gsystem and reaction spontaneity
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• End of Presentation

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• Additional slides
• Supplemental info.

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Reaction Energy and Reaction Kinetics

• Thermochemistry - the study of the changes in
  heat energy that accompany chemical reactions and
  physical changes.
• Heat and Temperature
• Calorimeter - a device designed to measure the
  amount of heat absorbed or released during a
  physical or chemical change.
• Temperature is the measure of the amount of
  kinetic energy of particles in matter
• Heat is the total of all the kinetic energy of
  the particles in matter
• Heat Capacity and Specific Heat
• Specific Heat is the amount of heat energy
  required to raise the temperature of one gram of
  substance by one degree Celsius (or one degree
  Kelvin).
• cp q / m DT
• or, q m cp DT

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Heat of Reaction

• Heat of Reaction is the quantity of heat released
  or absorbed during a chemical reaction. It is
  written as a thermochemical equation
• 2 H2(g) O2(g) ---gt 2 H2O(g) 483.6 kJ
• The heat released (483.6 kJ) is given the symbol
  DH. In this reaction 483.6 kJ is released when 2
  moles H2 reacts with 1 mole O2 to form 2 moles
  H2O.
• Exothermic DH lt 0
• Endothermic DH gt 0
• H, enthalpy, is the heat content of a system at
  constant pressure.
• If the chemical reaction describes the formation
  of one mole of a compound from its elements, then
  DH is given the subscript f and is referred to
  as the Heat of Formation, DHf.

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Stability and Heat of Formation

• DHf for a pure element is 0 and is given the
  symbol DHf0. Thus, DHf0 for water is -241.8 kJ.
• Compounds with high negative heats of formation
  are very stable. Those with positive or only
  slightly negative heats of formation are unstable
  and will decompose spontaneously.

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Heat of Combustion

• The heat of combustion, DHc0,is the amount of
  heat released when one mole of a substance reacts
  with oxygen.
• C3H8(g) 5 O2(g) ---gt 3 CO2(g) 4 H2O(g)
  2219.2 kJ
• Hesss Law The overall enthalpy change in a
  reaction is equal to the sum of enthalpy changes
  for the individual steps in the process.
• C(s) 2 H2(g) ---gt CH4(g) DHf0 ?
• C(s) O2(g) ---gt CO2(g) DHc0 -393.5
  kJ/mol
• H2(s) 1/2 O2(g) ---gt H2O(l) DHc0
  -285.8 kJ/mol
• CH4(g) 2 O2(g) ---gt CO2(g) 2 H2O(g) DHc0
  -890.8kJ/mol
• Combine the reactions as indicated in test
  (p.520) and you get DHf0 -74.3 kJ/mol

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Driving Forces of Chemical Reactions

• Entropy is a measure of the degree of randomness
  of the particles in a system (chaos).
• Free Energy
• DGo DHo - TDSo