Reaction Profiles
After completing this topic you should be able to :
Potential energy diagrams for reactions can be used to:
• show the energy pathway
• show and calculate the enthalpy change
• show and calculate the activation energy
• show where the activated complex forms
• Learn how a potential energy diagram can be used to describe a reaction
pathway, and to display activation energy and reaction enthalpy.
• Be able to sketch the potential energy diagram for a reaction given
relevant data eg activation energy, enthalpy change etc.
• Enthalpy change is the energy difference between the products and the
reactants.
• The enthalpy change has a negative value for exothermic reactions a
positive value for endothermic reactions
Reaction profiles – Enthalpy
Thermochemistry is the study of heat energy taken in or given out
in chemical reactions. This heat, absorbed or released, can be
related to the internal energy of the substances involved. Such
internal energy is called ENTHALPY, symbol H.
As it is only possible to measure the change in enthalpy, the symbol
 H, is used.
 H = Hp - Hr
Enthalpy (products) – Enthalpy (reactants)
Units kJ, kilojoules
Reaction profiles -Exothermic and Endothermic Reactions
Step 1: Energy must be
SUPPLIED to break bonds:
Step 2: Energy is RELEASED
when new bonds are made:
A reaction is EXOTHERMIC if more energy is RELEASED
than SUPPLIED. If more energy is SUPPLIED than is
RELEASED then the reaction is ENDOTHERMIC
Reaction profiles -Exothermic and Endothermic Reactions
-H
Enthalpy of
reactants
Enthalpy of
products
+H
Enthalpy of
reactants
Enthalpy of
products
Exothermic reactions give out thermal energy so the enthalpy of the
products is less than that of the reactants.
Endothermic reactions take in thermal energy from their surroundings.
The enthaply of the products is greater than that of the
reactants
Reaction profiles -Exothermic and Endothermic Reactions
Exothermic reactions give out heat,
causing a rise in the temperature
Endothermic reactions take in heat
The energy change in a reaction can be
shown in a potential energy diagram or
reaction profile
-  H
Path of reaction
Exothermic reactions
give out thermal energy
 H = -ve
PE kJmol-1
PE kJmol-1
Reaction profiles -Exothermic and Endothermic Reactions
+H
Path of reaction
Endothermic reactions take in
thermal energy from their
surroundings.
 H = +ve
Reaction profiles -Exothermic and Endothermic Reactions
A. Combustion of methane
CH4
H
(g)
+ 2O2
CH4
CO2
(g)
B. Cracking of ethane
C2H4
(g)
products
H
kJmol-1
+ 2 H20
products
C2H6
(g)
reactants
+ 2O2
(g)
→ CO2
(g)
+ 2 H20
(l)
(g)
 H negative, exothermic reaction
reactants
kJmol-1
(g)
C2H6
(l)
(g)
→ C2H4
(g)
+ H2(g)
+ H2(g)
 H positive, endothermic reaction
Use of the thermite reaction
This reaction is used to weld railway lines
Together
https://www.youtube.com/watch?v=iAEqu3gVN
ZM
http://www.youtube.com/watch?v=vCqG3rWtN
bc&feature=related
An endothermic reaction
Higher Chemistry Eric Alan and John Harris
https://www.youtube.com/watch?v=tTzNkr0
6Ap8
Enthalpy changes and industrial processes
For industrial processes it is essential that
chemists can predict the quantity of heat
taken in or given out.
Exothermic reactions lower the temperature,
slowing the reaction rate
Heat must be supplied to maintain the
rate of reaction – this is an expense
Enthalpy changes and industrial processes
Exothermic processes produce heat
Heat may need to be removed to prevent the
reactions proceeding beyond the capacity of
the plant