6.1 Energy Changes and Chemical Reactions

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6.1 Energy Changes and Chemical Reactions

• All reactions release or absorb energy.
• Common forms of energy are heat, light,
  electricity, sound
• Energy is stored in the bonds of compounds.
• Breaking a bond requires energy.
• Bonding releases energy.

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• The difference between the amount of energy
  absorbed in breaking the bonds in reactants and
  the amount of energy released in forming bonds
  determines whether the reaction is exothermic or
  endothermic.
• Reactions which need more energy to break bonds
  than they make when bonds are formed are
  endothermic (require heat).
• Reactions that make more energy as bonds form
  than is needed to break the bonds in the
  reactants are exothermic (give off heat).

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Exothermic Endothermic
Heat released Heat added
Even though energy comes out, a push is often needed. Need a continuous input of energy to react
Reactants ? Products energy energy Reactants ? Products
Example burning a fuel, an explosion Vaporizing water, cold pack
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Consider the following

• It is easier (requires less energy) to build
  small stable compounds.
• It is harder (requires more energy) to build
  large unstable compounds.
• Explosives are typically large, unstable
  compounds that form small stable compounds when
  they react.
• The leftover energy drives the explosion.

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TNT (Trinitrotoluene)

• C6H2(CH3)(NO3)2(s) O2(g) ? CO2(g) H2O(g)
  N2(g)
• Very complex, unstable compound simple,
  stable compounds
• Lots of E required to break the reactant bond
• Little energy required to make product bonds
• Leftover E drives the explosion

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Law of Conservation of Energy

• Energy cannot be created or destroyed. It can
  only be changed from one form to another.
• Example Burning Wood
• Reactants Products
• Chemical energy in wood heat energy
• chemical energy
• radiant energy
• sound energy

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• Magic Wand Demonstration
• Read pages 180 - 182
• Assignment
• p 185 1 5d BLM do sheet 6-1, read 6-2 and do
  6-4
• 6-3 may be done as a bonus assignment

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6.2 Synthesis and Decomposition Reactions

• Synthesis Reaction synthesis means to make or
  blend these chemical reactions have several
  small reactants combining to form larger
  products.
• General Form If A is an element, and X is an
  element we would have
• A X AX

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Example

• When pop is made
• CO2(g) H2O(l) H2CO3(aq)

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Decomposition Reaction

• Decompose means to break down.
• These chemical reactions begin with larger
  reactant(s) which break down into simpler
  products.

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• General From
• AX A X
• Example
• When you open up the pop can
• H2CO3(aq) CO2(g) H2O(l)
• The CO2 coming out of solution produces the fizz.

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• NOTICE the brackets give the state of the
  substance.
• (s) is solid
• (l) is liquid
• (g) is gas
• (aq) is aqueous which means dissolved.

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• Recall
• A solid takes up little volume,
• The same mass of liquid occupies more volume.
• A same mass of a gas occupies much more volume.
• Recall
• Materials expand as they get hot.
• Explosions are biggest when the reaction can
  change a material of small volume to a material
  with large volume VERY QUICKLY.
• Rockets, firearms, explosives, etc.

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Assignment

• Read p 186 188
• Assignment p 189 1 5
• BLM sheet 6-5 6-6

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6-3 Single and Double Displacement Reactions

• a chemical reaction in which one element takes
  the place of another element in a compound.
• Example Let A and B be cations. Let X be an
  anion.
• A BX ? AX B

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• Think A B are guys. X is a girl. B and X are
  dance partners and have bonded. BUT along
  comes A. Looks like A and X bond and B is left
  alone.
• How could we predict if this would happen or not?
  Well A would have to be more reactive with X
  than B was.
• In chemistry the reactivity of elements has been
  tested and the results are available in
  reactivity charts called The Activity Series of
  Metals.

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• Real Life Example
• Mg is more reactive than hydrogen so
• Mg(s) HCl(aq) ? MgCl2(aq) H2(g)
• Balance the above reaction.
• 1Mg(s) 2HCl(aq) ? 1MgCl2(aq) 1H2(g)

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• Example 2
• Na is very reactive so
• Na(s) H2O(l) ? NaOH(aq) H2(g)
• Think of water as being ionic Hydrogen Hydroxide
• Na(s) H(OH)(l) ? Na(OH)(aq) H2(g)
• Balance the above reaction.
• 2Na(s) 2H(OH)(l) ? 2Na(OH)(aq) 1H2(g)

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• Double Displacement Reaction a reaction in
  which the cations from 2 different compounds
  exchange forming 2 new compounds.
• AX BY ? BX AY
• Think of 2 couples and the guys switched places.
• AX BY

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• Example
• 2KI 1Pb(II)(NO3)2 1Pb(II)I2 2K(NO3)
• Notice an interesting event takes place in the
  reaction. Two liquids are mixed and one of the
  materials produced is a solid. The solid forms
  as particles that slowly rain down to the
  bottom of the liquid. The solid is called a
  precipitate.

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Neutralization

• Neutralization a double displacement reaction
  that occurs when an acid and a base are combined.
  The products are always the same
• Acid Base ? Water Salt
• A salt is an ionic compound.

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• Ex)
• HNO3 NaOH ? H2O NaNO3
• (NOTE it often helps to think of H2O as HOH-)
• Read P 190 202 and do P 202 1-4, BLM 6-7, 6-8
  (read), 6-9 6-12

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6.4 Organic Chemistry

• Organic molecules contain carbon (carbon is found
  in all living things).
• Carbon Bonding
• Carbon has 4 valence electrons
• .
• . C .
• .

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• Carbon does not form ions, but bonds through
  covalent bonds (shares e-)
• Carbon forms chains with other carbon atoms
• Examples Methane - CH4

H lX H lX C lX H lX H
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• Ethane C2H6
• Draw the Lewis dot diagram for this
• We will use carbon chain drawings
• See board
• Carbon chains are most often found with hydrogen

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• Atoms complete all open ends to the chains
• Examples butane (C4H10) and propane (C3H8)
• Draw these
• Hydrocarbons (CH) are carbon chains that contain
  hydrogen
• Hydrocarbons are very useful fuels

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Combustion

• Combustion means burning
• Hydrocarbons are excellent fuels
• When fuels burn they combine with oxygen in
  exothermic reactions
• HC O2 CO2 H2O Thermal E
• We use the thermal energy
• for heating
• to expand gas quickly and we use the force
  produced by the expanding gas (internal
  combustion engine)

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Complete Combustion

• In a perfect world every hydrocarbon molecule
  would combine with oxygen to produce the maximum
  amount of heat, and only CO2 and H2O

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Incomplete Combustion

• Reactions are often limited by a limited amount
  of one of the reactants (O2 or fuel).
• Incomplete combustion
• reduces thermal energy
• wastes fuel (fuel in vehicle exhaust)
• produces Carbon monoxide (CO) which is deadly
  odorlous, colourless, binds with rec blood cells
  better than O2 but is not useable by the body)
• produces carbon (soot) and causes smog and
  respiratory diseases including cancer

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• HC O2 CO2 H2O CO C (soot) Reduced
  Thermal E

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Assignment

• CYU P 206 1 4 BLM 6-14, 6-15, 6-16
• Ch 6 Review
• BLM 6-21
• Ch Review P 209 1-18
• Pop Can Canon Demonstration

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Ch 6 - Lab

• Copper (II) Chloride CuCl2
• Sodium Hydroxide NaOH
• Sodium Chloride NaCl
• Potassium Nitrate KNO3
• Sodium Carbonate Na2CO3
• Calcium Chloride CaCl2
• Potassium Iodide KI
• Lead (II) Nitrate Pb(NO3)2

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• H2 and O2 Gas Test
• Wooden splint glows/ignites (2)
• Lighter explodes/pops (2)
• Mg and O2
• Exothermic, flame and light, heat (2)
• 2Mg O2 2MgO thermal E (4)
• Ionic, metal/non-metal (2)
• 3Mg N2 Mg3N2 thermal E (4)

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• Mg and HCl
• Bubbles and heat (2)
• Single displacement, Mg and H switch places (2)
• Mg and HCl (1)
• MgCl2 and H2 (1)
• Mg 2HCl MgCl2 H2 thermal E (4)

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• Dble. Displacement
• Blue precipitate (gelatin) (2)
• CuCl2 2NaOH Cu(OH)2 2NaCl (4)
• None (2)
• NaCl KNO3 NR (4)
• Turned foggy/milky, white precipitate (2)
• Na2CO3 CaCl2 CaCO3 NaCl (4)
• Yellow precipitate (2)
• 2KI Pb(NO3)2 PbI2 2KNO3 (4)

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