Transcript U1 S1 L2 PE diagrams

U1 S1 L2
Energy Diagrams
MHR:
Questions:
Upon completion of this lesson, you will be able to:
• define activation energy, activated complex, and heat of
reaction.
• draw, label, and interpret potential energy diagrams (or
energy profile curves) for exothermic and endothermic
reactions.
• calculate the activation of the forward reaction or the
activation energy of the reverse reaction or the heat of
reaction given two of these three values for a reversible
chemical system.
Temperature and Kinetic Energy
• Kinetic energy is energy of motion.
– KE depends on the mass and velocity (speed) of a
particle.
E  1 mv 2
k
2
• Temperature is an measure of the average kinetic
energy.
– For example, the air temperature in your room is probably
close to 20°C. That means that molecules in the air are
moving faster than they do at 0°C, but slower than at 40°C.
– Temperature is a measure of average motion of all the
particles in the room.
• Ie: the air in contact with the cold window is moving slower that the
air next to the heater!
Potential Energy
• Potential energy is defined as energy of position or the
energy possessed by something due to its position
relative to something else.
H2 (g) + O2(g) → H2O(l) + Energy
• An energy profile diagram (also known as a potential
energy diagram) shows the potential energy changes
that occur during a chemical reaction.
• Activated complex
– The point at which the reactants gain enough energy to form products.
– Threshold point (the highest point on the graph)
– An unstable group (complex) that instantly breaks apart to form products.
• Activation Energy (Eact)
– The energy barrier to the reaction
– The maximum amount of energy needed to start a reaction
– The energy required to reach the activated complex
Eact = Eactivated complex-Ereactants
• Heat of reaction (∆H)
– The heat released (or consumed) when a reaction takes place.
• Exothermic reaction  heat is released (-∆H)
• Endothermic reaction  heat is absorbed (+∆H)
∆H = EP - ER
Exothermic Vs: Endothermic
Question
• The heat of reaction for the formation of water is -286 kJ. Assume
that the activation energy for the formation of water from hydrogen
and oxygen gas is 112 kJ.
• Calculate the activation energy for the decomposition of water.
1.
What is defined as being proportional to the average kinetic
energy of the particles in a sample of matter?
A.
B.
C.
D.
a Joule
heat
potential energy
Temperature
2.. Which part of the graph represents the activation energy?
a)
b)
c)
d)
A
B
C
D
3.
Which symbol represents the activation energy for the reverse
reaction?
a)
b)
c)
d)
W
X
Y
Z
4.
What is the activation energy of the reverse reaction if the heat of
reaction is -60 kJ and the activation energy of the forward reaction
is 40 kJ?
a)
b)
c)
d)
20
40
60
100
Textbook Readings
page 471: Collision Theory and Temperature
page 472: Reactions and Activation Energy
pages 473-474: Transition State Theory
pages 474-476: Tracing a Reaction with a Potential Energy Diagram
Textbook Practice Items
page 476: # 1, 2, 3, and 4
page 484: # 3 and 4
page 487: # 16
pages 538-541: # 1, 11, and 14