Thermochemistry

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Chapter 5

• Thermochemistry

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Energy of objects

• Objects can possess 2 kinds of energy.
• KE energy of motion
• Ek ½ mv2
• PE stored energy (energy of position)

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Units of heat

• Joule 1 kg-m2/s2
• 1 cal 4.184 Joule

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The universe

• is divided into two halves.
• the system and the surroundings.
• The system is the part you are concerned with.
• The surroundings are the rest.
• Exothermic reactions release energy to the
  surroundings.
• Endo thermic reactions absorb energy from the
  surroundings.

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Energy is...

• The ability to do work or transfer heat.
• Energy can be transferred by doing work
• Force- any kind of push or pull exerted on an
  object
• Work any energy used to move an object against a
  force
• Wf x d

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• Energy can be transferred as heat.
• Always in the direction from hotter object to a
  colder one. When you apply an ice pack your body
  part feels cooler because your body is
  transferring heat to the ice pack.

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Energy is conserved!

• First law of thermodynamics- any energy that is
  lost by a system must be gained by its
  surroundings, and vice versa.
• The change in energy that occurs during a
  reaction can be calculated

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Heat
Potential energy
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Heat
Potential energy
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Energy changes

• Every energy measurement has three parts.
• A unit
• A number (indicating magnitude)
• and a sign to tell direction.
• negative exothermic (system loses energy)
• positive- endothermic (system gains energy)

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Surroundings
System
Energy
DE lt0
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Surroundings
System
Energy
DE gt0
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• LOOK AT FIGURE 5.5 on page 150.

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Calculating energy changes

• System can change energy by work or heat
• Heat given off is negative.
• Heat absorbed is positive.
• Work done by system on surroundings is positive.

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• Work done on system by surroundings is negative.
• Thermodynamics- The study of energy and the
  changes it undergoes.

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First Law of Thermodynamics

• Law of conservation of energy.
• q heat
• w work
• DE q w
• Take the systems point of view to decide signs.
• Look at Table 5.1

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• The internal energy of a system is a state
  function, it is independent of it past history.
• ?E state function (q and w are not though)

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Practice

• Hydrogen and oxygen gases are in a cylinder. As
  a reaction occurs between them, the system loses
  1150 joules of heat to the surroundings. The
  reaction also causes a piston to rise as the hot
  gas expands. The expanding gas does 480 J of
  work on the surroundings. What is the change in
  internal energy of the system?

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Enthalpy

• Most reactions occur at constant pressure and so
  energy is transferred mainly in the form of heat.
• Enthalpy is the heat absorbed or released by a
  system under constant pressure
• abbreviated H

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More about enthalpy

• State function
• Cannot be measured, but the change can.
• ?H equals the heat (qp) gained or lost by the
  system when the process occurs under constant
  pressure.
• ?H Hfinal Hinitial qp

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• Since H is a state function it depends only on
  the starting and ending states of the system,
  not how they got there.

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Enthalpies of reactions

• ?Hrxn H(products)- H(reactants)
• Thermochemical equations
• 2H2 O2 2H2O ?H -483.6 kj
• Enthalpy is an extensive property (dependent on
  amount)

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Calorimetry

• Measuring heat.
• Use a calorimeter.
• Two kinds
• Constant pressure calorimeter (called a coffee
  cup calorimeter)
• heat capacity for a material, C is calculated
• C heat absorbed/ DT DH/ DT
• specific heat capacity C/mass

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Calorimetry

• molar heat capacity C/moles
• heat specific heat x m x DT
• heat molar heat x moles x DT
• Make the units work and youve done the problem
  right.
• A coffee cup calorimeter measures DH.
• An insulated cup, full of water.
• The specific heat of water is 1 cal/gºC
• Heat of reaction DH sh x mass x DT

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Examples

• How much heat is released when 4.5 g of methane
  is burned in a constant pressure system.
• CH4 202 ? CO2 H2O ?H-890 kJ

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Calorimetry

• Constant volume calorimeter is called a bomb
  calorimeter.
• Material is put in a container with pure oxygen.
  Wires are used to start the combustion. The
  container is put into a container of water.
• The heat capacity of the calorimeter is known and
  tested.
• Since DV 0, PDV 0, DE q

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Bomb Calorimeter

• thermometer
• stirrer
• full of water
• ignition wire
• Steel bomb
• sample

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Properties

• intensive properties not related to the amount of
  substance.
• density, specific heat, temperature.
• Extensive property - does depend on the amount of
  stuff.
• Heat capacity, mass, heat from a reaction.

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Hesss Law

• Enthalpy is a state function.
• It is independent of the path.
• We can add equations to to come up with the
  desired final product, and add the DH
• Two rules
• If the reaction is reversed the sign of DH is
  changed
• If the reaction is multiplied, so is DH

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O2
NO2
-112 kJ
180 kJ
H (kJ)
NO2
68 kJ
N2
2O2
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Standard Enthalpy

• The enthalpy change for a reaction at standard
  conditions (25ºC, 1 atm , 1 M solutions)
• Symbol DHº
• When using Hesss Law, work by adding the
  equations up to make it look like the answer.
• The other parts will cancel out.

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• Use the following data to calculate the enthalpy
  of combustion of C? CO
• ( C ½ O2 ? CO )
• 1. C O2 ? CO2 ?H -393.5 kJ/mol
• 2. CO ½ O2 ? CO2 ?H-283.0 kJ/mol

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Standard Enthalpies of Formation

• Made a table of standard heats of formation. The
  amount of heat needed to for 1 mole of a compound
  from its elements in their standard states.
• Standard states are 1 atm, 1M and 25ºC

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• ?Hrxn Sn?Hf (products) Sm?Hf (reactants)
• Use appendix C to calculate the standard
  enthalpy change for the combustion of 1 mol of
  benzene (C6H6).

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Since we can manipulate the equations

• We can use heats of formation to figure out the
  heat of reaction.
• Lets do it with this equation.
• C2H5OH 3O2(g) 2CO2 3H2O
• which leads us to this rule.

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Since we can manipulate the equations

• We can use heats of formation to figure out the
  heat of reaction.
• Lets do it with this equation.
• C2H5OH 3O2(g) 2CO2 3H2O
• which leads us to this rule.

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• You are responsible for all of chapter this
  includes the part we did not talk about on foods
  and fuels. Make sure you study it. It will be
  on the test!