Endo & Exothermic Rxns
• I know the factors that determine
whether a reaction will occur.
• I know the difference between endo
and exothermic reactions.
• I can identify the components of a
potential energy diagram.
Reactions
• In order for a reaction to occur the
molecules must:
– Collide
– with the proper orientation
– and set energy.
Lighting a match
In order to light a match:
• The match must collide with the side
of the box that is rough.
• The head of the match must be
oriented to face the box
• The strike must have enough energy.
Activation Energy
• The set energy required to get
reaction going.
• The larger the amount of activation
energy the slower the reaction.
DE1 > DE2 therefore the
second reaction is faster than
the first.
Endothermic
• Process that absorbs heat
from the surroundings
– Feels cold because your
hand (body) is part of
the surroundings.
• Heat, q is positive
– q stands for the quantity of
heat.
• Chemical potential energy
of reactants is lower than
the products.
~4kJ
Potential
energy
diagram
~1.5kJ
~0.5kJ
~3.5kJ
The y-axis says energy now,
could also say potential
energy or enthalpy. They all
mean the same thing. How
much energy is stored in the
bonds of the molecules!
The transition state is where the reactants are
• (Ea)f is the activation energy for
transitioning into the products. This is an
activated complex because it has bonds
the forward reaction.
breaking and forming so it could do either way. • (E ) is the activation energy for
a r
the reverse reaction.
• DH is the change in enthalpy.
The difference between the
products and the reactants. DH
is also called the heat of
reaction. It is the amount of
heat/energy absorbed or
released during a reaction.
• Notice that the (Ea)f is equal to
the sum of the (Ea)r and DH.
• Say (Ea)f is equal to 100 kJ and
DH is equal to 80 kJ. What
then would be the (Ea)r?
Catalyst
• A species that speeds up the rate of
reaction without itself being
consumed.
• Lowers the activation energy
required to get the reaction going.
Notice that the level of
where the reactants start
and where the products end
do not change!
The only thing that changes
is the amount of activation
energy required to get the
reaction going.
Exothermic Reactions
• Process in which energy is
released to the surroundings.
– Feels hot because your
hand (body) is part of the
surroundings.
• Heat, q is negative
– Scientists needed a way to tell
whether the energy was
absorbed or released. It was
decided to make the energy
released negative
• Chemical potential energy of
reactants is higher than the
products.
Potential
energy
diagram
About 4 kJ
About 1.5 kJ
Heat of rxn,
DH
~3.5 kJ
~0.5 kJ
Notice that now (Ea)f + DH = (Ea)r.
So if (Ea)f = 50kJ and DH = 50kJ,
what does (Ea)r = ?
Your turn…
Label each arrow
Your turn…
Activation energy of
forward reaction because
it starts at the reactant
level and goes to the peak
of the transition state.
Change in enthalpy or
Heat of reaction because
it goes from the reactants
to products.
Enthalpy of activated complex
because it goes from 0 to the peak
of the transition state.
Enthalpy of
reactants because
it goes from 0 to
the reactants.
Your turn take 2
2 Activation
energy of
forward
reaction
because goes
from reactants
to peak of
transition state
Activation energy of
reverse reaction
because goes from
products to peak of
transition state
5 DH, or heat of reaction
because it goes from
the reactant level to
product level
Enthalpy
of
reactants
because
goes from
0 to
reactants
3 Enthalpy of
activated
complex or
transition state
because goes
from 0 to peak
Enthalpy of
products
because
goes from
0 to
products
Differences in this PE diagram?
There are actually 3 rxns occurring one
after the other. The net result the
combination of all 3 rxns.
Notice that the activation energy of the first
reaction is that greatest, that is why that
reaction is labeled slow.
worksheet
• Complete the worksheet
• Check your answers
• Complete the online worksheet.