Download Potential Energy Diagrams

Potential Energy
Diagrams
Chemical Kinetics
Mrs. Kay
Review of Exothermic
► Reactants
Ep is higher
than Products Ep.
► Now, we must
consider the activation
energy (the energy
needed so that the
reactants bonds will
break and reform to
make product)
Review of Endothermic
► Reactants
Ep is lower
than Products Ep.
► Need to add more
energy to the system
for the forward
reaction to take place.
► Still need to consider
activation energy
Activated Complex
► Is
the short-lived, unstable structure formed
during a successful collision between
reactant particles.
► Old bonds of the reactants are in the
process of breaking, and new products are
forming
► Ea is the minimum energy required for the
activation complex to form and for a
successful reaction to occur.
Fast and slow reactions
► The
smaller the activation energy, the faster
the reaction will occur regardless if
exothermic or endothermic.
► If there is a large activation energy needed,
that means that more energy (and
therefore, time) is being used up for the
successful collisions to take place.
Sample Problem
► The
following reaction has an activation
energy of 120kJ and a ΔH of 113kJ.
2NO2  2NO + O2
 Draw and label a potential energy (activation
energy) diagram for this forward reaction.
 Calculate the activation energy for the reverse
reaction (if the reaction went backwards)
Another Problem
► The
following hypothetical reaction has an
activation energy of 70kJ and a ΔH of 130kJ
A+BC+D
 Draw and label a potential energy diagram for
the reaction
 Calculate the activation energy for the reverse
reaction.
Watch the following Flash
► Review
of what is occuring during a
chemical reaction for both endothermic and
exothermic.
► KNOW
THIS!!
► http://mhhe.com/physsci/chemistry/essenti
alchemistry/flash/activa2.swf
Practice:
1.
The following hypothetical reaction has an
Ea of 120kJ and a ΔH of 80kJ
2a + B  2C + D




Draw and label a potential energy diagram for
this reaction.
What type of reaction is this?
Calculate the activation energy for the reverse
reaction.
Calculate the ΔH for the reverse reaction.
2.
Analyze the
activation energy
diagram below.
►
What is the Ea for the
forward reaction? For
the reverse reaction?
What is the ΔH for the
forward reaction? For
the reverse reaction?
What is the energy of
the activated complex?
►
►
Answer to #2
►
The activation energy (Ea) for the forward reaction is shown by (a):
Ea (forward) = H (activated complex) - H (reactants) = 400 - 100 =
300 kJ mol-1
The activation energy (Ea) for the reverse reaction is shown by (b):
Ea (reverse) = H (activated complex) - H (products) = 400 - 300 =
100 kJ mol-1
The enthalpy change for the reaction is shown by (c):
H = H (products) - H (reactants) = 300 - 100 = +200 kJ mol-1
for the forward and reverse reaction.