Energetics/Thermochemistry

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Energetics/Thermochemistry
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Outline for Energetics

• 1. endothermic and exothermic reactions
• 2. units of energy
• 3. specific heat, and molar heat
• 4. molar heats of combustion
• 5. heats or enthalpies of formation ?Hf
• 6. Hesss Law
• 7. Bond enthalpies (juniors only)

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Endothermic and exothermic reactions

• Endothermic
• Reactions which absorb energy or have energy
  added in order to occur.
• Products have more stored energy than reactants
• Products are less stable than reactants
• Writing the reactions
• kJ are written on the reactant side
• ?H is positive

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EXAMPLE OF AN ENDOTHERMIC REACTION
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Endothermic and exothermic reactions continued

• Exothermic
• Reactions which release energy
• Products have less stored energy than the
  reactants
• Products are more stable than the reactants
• Writing the reactions
• The kJ are written on the products side
• ?H is negative

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EXAMPLE OF AN EXOTHERMIC REACTION
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(No Transcript)
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Writing endothermic or exothermic reactions

• Endothermic
• Example
• kJ 2H2O?2H2 O2
• The kJ are written on the reactant side.
• 2H2O?2H2 O2 ?HkJ The ?H is positive.

• Exothermic
• Example
• 2H2 O2 ? 2H2O kJ
• The kJ are written on the product side.
• 2H2 O2 ? 2H2O
• ?H - kJ The ?H is negative.

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Enthalpy or change in heat

• Symbol for enthalpy is ?H.
• - ?H means energy is released such as with an
  exothermic reaction.
• ?H means energy is used or absorbed such as
  with an endothermic reaction.

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Units of Energy

• Joule (J) (the unit used in the math)
• 1newton.meter2/second2
• Is the SI unit for energy
• 4.18J1 calorie
• 1000J 1 kilojoule (kJ)
• calorie (c)
• energy required to raise the temperature of 1
  gram of water by 1?C
• 1000calories 1kilocalorie (kcal) or 1 food
  calorie (Calorie)
• 0.24cal 1 joule

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Bomb calorimeter
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Math with energetics
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Specific and molar heatThe heating or cooling of
a substance

• 1)Heat Capacity- the amount of energy a substance
  can absorb before its temperature is increased.
  General equation is
• C heat absorbed/increase in temp
• a) Molar heat capacity the energy required to
  raise the temp of 1 mole of a substance by 1?C.
  (units J/molC)
• b) Specific Heat the energy required to raise
  the temperature of 1 gram of a substance by 1?C
• symbol cp units J/gC
• examples 4.18J/gC for H2O, 0.45J/gC for Fe,
  0.71J/gC for carbon.

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Equation for Specific heat

• Equation using specific heat
• q cp x m x ?T where
• cp is specific heat
• mmass in grams
• ?T change in temp.
• q energy in joules

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Problems with specific heat

• Example 1 Find the energy needed to raise the
  temp of 5.00x102ml of water from 20. C to 100. C.
  Assume no energy is lost to the surroundings.
• Qcpx m x ?T Q?, m 5.0x102g (1g1ml for water),
  ?T 100-20 80 C, cp4.18J/gC
• Substitute into the equation
• Q 4.18J/g C x 5.0x102g x 80. C
• Q 1.7x105J or 170kJ

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Problems with specific heat

• Example 2 A 20.0 g metal sample is heated to 200
  C and then dropped into 100.ml of water. Both the
  metal sample and the water ended up with a final
  temp of 20 C. Find the metals specific heat if
  the water was 15 C before the metal was placed
  into it.

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Example 2 continued

• In order to find the cp of the metal, there is
• m 20.0g. ?T 200-20 180 C. but Q?, and cp?
• There are too many variables. However, the metal
  released its heat into the water so we can find
  the Q by finding the energy that went to heat the
  water.
• Step 1 Find the energy to heat the water
• Q ?, m 100g H2O, cp 4.18J/gC, ?T 20-15C 5C
• Q 4.18J/gC x100g x 5C Q 2090 J or 2.1 x 103J
• Step 2 Find the specifc heat of the metal
• Q 2090J, m 20.0g, cp ? ?T 200-20180C
• 2090J cp x 20.0g x 180C, cp 0.58J/gC

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Heats or Enthalpies of Formation ?Hf?

• What is a ?Hf ??
• Used to calculate the energy involved in a
  reaction with out experimenting.
• It is the energy content for one mole of a
  compound.
• It is the energy involved in making (forming)
• one mole of a compound
• From its simplest elements
• At 25 C and 1 atm.
• These values are used to determine the ?H for a
  reaction.
• Writing the equation for a ?Hf
• examples H2O(g) H2 ½ O2? H2O
• ?Hf - 242kj/mole This value is from an
  appendix
• ?

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Bond Enthalpies

• Bond Enthalpies
• Energy is required to break bonds, energy is
  released when making bonds.
• Exothermic more energy was released when making
  bonds in the products than what energy was
  absorbed to break bonds in the reactants.
• Endothermic more energy was required to break
  the bonds in the reactants than what energy was
  released when making bonds in the products.

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Bond Enthalpies(continued)

• 2. Bond Enthalpies
• The average energy required to break a covalent
  bonds.
• -_at_ 25 degrees Celsius, 1 ATM, always work w/
  gases
• -compound is turned into single gaseous atoms,
  not its simplest, stable form in nature
• Ex C-H(g)?CH 413 KJ/mol
• O2(g)?OO 495 KJ/mol
• Single bonds require less energy to break than
  double bonds, lt triple bonds.
• EXAMPLES WRITE ON BOARD

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Molar Heats of Combustion

• 1.Combustion reactions
• a) requirements for combustion
• i) fuel
• ii) ignition
• iii) O2
• b) products of complete combustion
• i) energy light
• ii) stable compounds, each w/ oxygen in it
• C?CO2, H?H2O, S? SO2

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Piston in the internal combustion engine
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continued

• c) Writing Combustion Reactions
• CH 4(g)2O2 ? CO2(g)3H2O(g)KJ
• 2. Incomplete Combustion
• a) Why this happens i) not enough O2
• ii) not enough time
• iii) not enough surface area
• b) products made from incomplete combustion
• CO, ash, soot as well as CO2 and H2O