Energy changes

1. Chemical Energetics 1:Chemical Energetics 1:
AllAll chemical reactions involve an chemical reactions involve an
energy change.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.

2. Chemical Energetics 2:Chemical Energetics 2:
Instead of writing ‘energy change’ all the Instead of writing ‘energy change’ all the
time chemists use the symbols:time chemists use the symbols:
DDHH
Pronounced ‘delta aitch’.
D=greek letter ‘delta’ meaning change.
H=heat.
So, DH means ‘heat change’.

3. Temperature Changes Temperature Changes
11
Some reactions cause the Some reactions cause the
temperature of the reaction mixture temperature of the reaction mixture
to to increase.increase.
This type of reaction is called This type of reaction is called
exothermic.exothermic.
Heat energy is given out by the Heat energy is given out by the
reaction mixture hence the reaction mixture hence the
surroundings increase in surroundings increase in
temperature.temperature.

4. Temperature Changes Temperature Changes
22
Some reactions cause the Some reactions cause the
temperature of the reaction mixture to temperature of the reaction mixture to
decrease.decrease.
This type of reaction is called This type of reaction is called
endothermic.endothermic.
Heat energy is taken in by the Heat energy is taken in by the
reaction mixture hence the reaction mixture hence the
surroundings decrease in surroundings decrease in
temperature.temperature.

5. Representing Energy Changes:Representing Energy Changes:
The energy changes in a chemical reaction The energy changes in a chemical reaction
can be conveniently represented usingcan be conveniently represented using
energy level diagramsenergy level diagrams
Energy level diagrams make it easier to Energy level diagrams make it easier to
decide whether a reaction is exothermic decide whether a reaction is exothermic
(gives out heat and gets hotter) or (gives out heat and gets hotter) or
endothermic (takes in heat and gets cooler).endothermic (takes in heat and gets cooler).See next slide for some examples.

6. Energy Level DiagramsEnergy Level Diagrams
This energy level diagram This energy level diagram
shows a reaction where shows a reaction where DDH is H is
negativenegative (an exothermic (an exothermic
reaction).reaction).
This energy level diagram shows a
reaction where DH is positive (an
endothermic reaction).
Reactants
Products
DH negative,
exothermic.
energy
Reactants
Products
DH positive,
endothermic.
energy

7. Making and Breaking Bonds:Making and Breaking Bonds:
Making chemical bonds is Making chemical bonds is alwaysalways
exothermic.exothermic.
Breaking chemical bonds is alwaysalways endothermic.
All chemical reactions involve bond ‘rearrangements’.
Bonds are broken and new ones are formed.
Chemical reactions are in two stages:
3.Breaking bonds, an endothermic process.
4.Making new bonds, an exothermic process.

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

9. Reaction Profile for:Reaction Profile for:
HH
22

+ Cl+ Cl
2 2 ®® 2HCl 2HCl
energy
H-H, Cl-Cl
Reactants
H, H, Cl, Cl
(Atoms)
H-Cl, H-Cl
Products
Energy taken in
to break bonds.
Energy given out when
bonds are made.
Overall energy
change, DH

10. Calculating Energy Changes: Calculating Energy Changes:
The energy change for a reaction can be The energy change for a reaction can be
calculated using ‘bond energies’.calculated using ‘bond energies’.
‘Bond energy’ is the amount of energy required to
break a bond. This value is always endothermicalways 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 always an
exothermicexothermic process.
.

11. Reaction Profile for:Reaction Profile for:
HH
22

+ Cl+ Cl
2 2 ®® 2HCl 2HCl
energy
H-H, Cl-Cl
Reactants
H, H, Cl, Cl
(Atoms)
H-Cl, H-Cl
Products
Energy taken in
to break bonds.
Energy given out when
bonds are made.
Overall energy
change, DH

12. An Example An Example
Calculation:Calculation:
Consider the reaction between hydrogen and chlorine:Consider the reaction between hydrogen and chlorine:
H
2
+ Cl
2
® 2HCl
to see the
reaction profile.
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.

13. Reaction Profile for:Reaction Profile for:
HH
22

+ Cl+ Cl
2 2 ®® 2HCl 2HCl
energy
H-H, Cl-Cl
Reactants
H, H, Cl, Cl
(Atoms)
H-Cl, H-Cl
Products
Energy out = -862kJ
Overall energy
change,
DH = -184kJ
Energy in
= +678kJ

14. Chemical reactions take place in two stages:Chemical reactions take place in two stages:
EndothermicEndothermic reactions get cold, they take in reactions get cold, they take in
heat.heat.
ExothermicExothermic reactions get hot, they give out reactions get hot, they give out
heat.heat.
Summary
Bond breaking is endothermic,
bond making is exothermic.
The energy changes in a chemical reaction can
be represented using energy level diagrams and
reaction profile diagrams.
1.An endothermic bond breaking stage.
2.An exothermic bond making stage.