Exothermic and Endothermic Reactions

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18/02/2014
Exothermic and Endothermic Reactions

• L.O.
• I understand that reactions can be exothermic or
  endothermic and can give an example of each.
• I can show experimentally if an energy change
  has taken place by using temperature changes

237 PM
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When fuels are burned energy is released as heat.
This is combustion. e.g. methane oxygen ?
carbon dioxide water heat energy (natural
gas) ethanol oxygen ? carbon dioxide water
heat energy (methylated spirits) carbon
oxygen ? carbon dioxide heat energy (coal)
butane
oxygen ? carbon dioxide water heat
energy (camping gas)
octane oxygen ?
carbon dioxide water heat energy (in petrol)

237 PM
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Exothermic Reactions
Exothermic reactions increase in temperature.

• Examples include
• Burning reactions including the combustion of
  fuels.
• Detonation of explosives.
• Reaction of acids with metals.

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Exothermic Reactions

• Magnesium Hydrochloric acid

Heat energy given out
Gets hot
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Endothermic Reactions
Endothermic reactions cause a decrease in
temperature.

• Endothermic chemical reactions are relatively
  rare.
• A few reactions that give off gases are highly
  endothermic - get very cold.
• Dissolving salts in water is another process that
  is often endothermic.

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Endothermic Reactions
Endothermic reactions cause a decrease in
temperature.
Cools
Starts 25C
Cools to 5C
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• Whenever chemical reactions occur, energy is
• usually __________ to or from the surroundings.
• An exothermic reaction is one which gives out
• _________ to the surroundings, usually in the
• form of heat and usually shown by a ____ in
• temperature.
• In exothermic reactions heat exits the reaction.
• The best example of an exothermic reaction is
• __________ fuels, also known as ____________.
• Heat exits the reaction.

transferred
energy
rise
combustion
burning
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• An endothermic reaction is one which takes in
• energy from the ___________, usually in the form
• of heat and usually shown by a ____ in
• temperature.
• In endothermic reactions, heat enters the
• reaction. Photosynthesis is endothermic because
  it
• takes in energy from the ___. Energy enters the
• reaction.

surroundings
drop
Sun
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Remember
Exothermic Heat exits (is given out). The
temperature outside goes up.
Endothermic. Heat enters (is taken in). The
temperature outside goes down.
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• Your teacher will show you how to carry out the
  practical.

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Breaking chemical bonds

• Most chemicals will decompose (break up) if we
  heat them strongly enough.
• This indicates that breaking chemical bonds
  requires energy is an endothermic process.

Heat taken in
Energy needed to overcome the bonding between the
atoms
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Making chemical bonds

• It is reasonable to assume that bond making will
  be the opposite of bond breaking
• Energy will be given out in an exothermic process
  when bonds are formed.

Heat given out
Energy given out as bonds form between atoms
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Plenary Use traffic light cards to show whether
or
endo
exo

• Are these endothermic or exothermic?
• 1. A red glow spread throughout the mixture and
  the temperature rose.
• 2. The mixture bubbled vigorously but the
  temperature dropped 150C.
• 3. Hydrazine and hydrogen peroxide react so
  explosively and powerfully that they are used to
  power rockets into space.
• 4. The decaying grass in the compost maker was
  considerably above the outside temperature.

exo
endo
exo
exo
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• 5. Which of the following is not an exothermic
  reaction.
• The burning of petrol
• Respiration
• Photosynthesis

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• 6. What is an endothermic reaction?
• One which requires heat to start it.
• One which gives out heat.
• One which takes in energy.

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• 7. Two chemicals at room temperature (250C) were
  mixed together. The reaction was slightly
  endothermic. Which of the following could be the
  final temperature?
• 270C
• 270C
• c) 220C