Chemical Energetics 1:

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Chemical Energetics 1

• All chemical reactions involve an energy change.

The transfer of energy, usually heat, into or out
of the reaction mixture. For example When
petrol burns heat is given out. When ammonium
nitrate dissolves in water heat is taken in.
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Chemical Energetics 2

• As chemists are lazy people instead of writing
  energy change all the time they use the
  symbols

?H
Pronounced delta aitch. ?greek letter delta
meaning change. Hheat. So, ?H means heat
change.
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Temperature Changes 1

• Some reactions cause the temperature of the
  reaction mixture to increase.
• This type of reaction is called exothermic.
• Heat energy is given out by the reaction mixture
  hence the surroundings increase in temperature.

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Temperature Changes 2

• Some reactions cause the temperature of the
  reaction mixture to decrease.
• This type of reaction is called endothermic.
• Heat energy is taken in by the reaction mixture
  hence the surroundings decrease in temperature.

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Representing Energy Changes

• The energy changes in a chemical reaction can be
  conveniently represented using
• energy level diagrams
• Energy level diagrams make it easier to decide
  whether a reaction is exothermic (gives out heat
  and gets hotter) or endothermic (takes in heat
  and gets cooler).

See next slide for some examples.
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Energy Level Diagrams
This energy level diagram shows a reaction where
?H is positive (an endothermic reaction).

• This energy level diagram shows a reaction where
  ?H is negative (an exothermic reaction).

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Making and Breaking Bonds

• Making chemical bonds is always exothermic.

Breaking chemical bonds is always endothermic.
All chemical reactions involve bond
rearrangements. Bonds are broken and new ones
are formed.

• Chemical reactions are in two stages
• Breaking bonds, an endothermic process.
• Making new bonds, an exothermic process.

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Reaction Profiles

• These are a more detailed version of the energy
  level diagrams previously looked at.
• They show the endothermic bond breaking and the
  exothermic bond making stages of a reaction.

Reaction profiles can be used to do numerical
calculations on the energy changes that happen
in chemical reactions.
The next slide shows the energy profile for the
reaction between hydrogen and chlorine to form
hydrogen chloride.
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Reaction Profile for

• H2 Cl2 ? 2HCl

Energy taken in to break bonds.
H, H, Cl, Cl (Atoms)
Energy given out when bonds are made.
H-H, Cl-Cl
energy
Reactants
Overall energy change, ?H
H-Cl, H-Cl
Products
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Calculating Energy Changes

• The energy change for a reaction can be
  calculated using bond energies.

Bond energy is the amount of energy required to
break a bond. This value is always endothermic,
ie. has a positive value.
When a bond is formed the energy given out is
exactly equal to, but of opposite sign, to the
energy required to break the bond. Bond making is
always an exothermic process.
.
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Reaction Profile for

• H2 Cl2 ? 2HCl

Energy taken in to break bonds.
H, H, Cl, Cl (Atoms)
Energy given out when bonds are made.
H-H, Cl-Cl
energy
Reactants
Overall energy change, ?H
H-Cl, H-Cl
Products
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An Example Calculation

• Consider the reaction between hydrogen and
  chlorine

to see the reaction profile.
H2 Cl2 ? 2HCl
In this reaction one H-H bond and one Cl-Cl bond
is broken and two H-Cl bonds are formed.
The H-H bond energy is 436kJ/mol The Cl-Cl bond
energy is 242kJ/mol So, the energy needed to
break these bonds is 436 242 678kJ
The H-Cl bond energy is 431kJ So the energy given
out when these bonds are formed is 2 x -431
-862kJ
Overall change 678 862 -184kJ, an
exothermic reaction. Next slide shows the
reaction profile with these values added.
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Reaction Profile for

• H2 Cl2 ? 2HCl

Energy in 678kJ
H, H, Cl, Cl (Atoms)
Energy out -862kJ
H-H, Cl-Cl
energy
Reactants
Overall energy change, ?H -184kJ
H-Cl, H-Cl
Products
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Chemical reactions take place in two stages
Summary

• Endothermic reactions get cold, they take in
  heat.
• Exothermic reactions get hot, they give out heat.

Bond breaking is endothermic, bond making is
exothermic.

1. An endothermic bond breaking stage.
2. An exothermic bond making stage.

The energy changes in a chemical reaction can be
represented using energy level diagrams and
reaction profile diagrams.