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Energy
Energy is the capacity to do work or produce heat
Law of Conservation of Energy (First Law of Thermodynamics)
Units of energy
– Calorie (based on water)
– Joule (more general
– 1 calorie = 4.184 joules
Energy
Figure 6.3
Endothermic Process
Figure 6.2
Exothermic Process
Potential and Kinetic Energy
Potential energy –
– Stored energy
– Energy due to position
Kinetic Energy –
– energy due to motion
– vibrational, translational, rotational
– KE = ½ mv2
Temperature, Heat and Work .
Temperature –
– doesn’t measure energy – related to energy
– The measure of the average kinetic energy of the particles
Heat
– the transfer of energy between two objects due to a temperature
– Ways energy is transferred: heat (q), work (w)
– Energy changes on a system E = q + w
Work –
– force acting over a distance –
– “on” the system – work is positive
– “by” the system – work is negative
Place a rubber band loosely looped over the index fingers in contact with skin just above your
upper lip. Now quickly stretch the rubber band. What do you experience? Now let the rubber band
relax quickly. What do you feel now?
When the rubber band is stretched quickly, work is done on it, causing its internal energy to rise.
This rise reveals itself as a small increase in temperature. When the rubber band is allowed to
quickly contract, it performs work and suffers a reduction in internal energy which produces a
cooling sensation.
State Functions and Pathways
Elevation –
state function
Distance –
not a state function
State Functions and Pathways .
Pathway
– Condition for the transfer of energy which is divided between work & heat
– The ratio of heat to work depends on the pathway
– The total energy change is independent of the pathway
State Function (or property)
– A property of a system that depends on its present state – not how it got that
way
– A change in state function is independent of the pathway between two states
– Example: elevation versus distance
– Energy is a state function
– Work and heat are not state functions
STATE FUNCTIONS
Energy
Enthalpy
NOT STATE FUNCTIONS
Work
Heat
Internal Energy.
Internal Energy, E
– All the energy contained within a chemical system
– The sum of the kinetic and potential energies of all the “particles” in
the system
– An increase in the internal energy of a system can take three forms
• An increase in temperature
• A phase change
• The initiation of a chemical reaction
– A decrease in the internal energy of a system will usually result in
• A decrease in temperature
• A phase change
– The total internal energy of a system cannot be determined
– Changes in internal energy can be determined
– Can be changed by a flow of work, heat or both
E=q+w
Heat, Work and Energy.
First Law of Thermodynamics
the energy of the universe is constant
energy can be converted from one form to another
energy cannot be created or destroyed.
In a chemical system the energy exchanged between system and
surroundings can be accounted for by heat (q) and work (w)
E=q+w
Sign Conventions for Work and Heat
Process
Sign
Work done by the system on the surrounding
-
Work done on the system by the surrounding
+
Heat absorbed by the system form the surroundings (endo)
+
Heat absorbed by the surroundings from the system (exo)
-
Heat, Work and Energy.
Work refers to a force that moves an object over a distance
Only pressure/volume work (the expansion/contraction of a gas) is
significance in chemical systems.
Only significant when there is an increase or decrease in the amount of gas
present.
Example
2NH3  2N2 +3H2
There are more moles of gas in the product.
The system can be thought of as doing work on the surroundings
Pushing back the atmosphere.
So work is negative.
Internal Energy
Hot air Balloon
Is w positive or negative?
Work is done by the balloon
Expanding – work is negative
Is q positive or negative?
endothermic process
E = q + (-w)
Heat, Work and Energy.
For the expansion/contraction of a gas against constant external pressure
w = - PV
If there is no change in the total volume of gas before and after a reaction
occurs, there is no significant work done by or on the system
Then E = q because w=0
Enthalpy .
H = E + PV
consider … w = - PV …so….
= E – (- PV )
Enthalpy, H, can be considered to be energy with the work taken out.
H = E – (- PV ) substitute q + w or q + (- PV) for E
H = q + (- PV) – (- PV ) so…
H = q
at constant external pressure
For chemical reactions, H can be calculated theoretically and measured
directly.
Often is PV small –even when a gas or volume change is involved – so it is
ignored and then
H = E
Sample Questions from the AP Exam on H, E, q , w.
A gas absorbs 28.5 J of heat then then performs 15.2 J of work. The change
in internal energy of the gas is:
a.
b.
c.
d.
e.
-13.3 J
+13.3 J
-43.7 J
+43.7 J
None of these
(b) E = q + w; 28.5 J – 15.2 J = 13.3 J
Sample Questions from the AP Exam on H, E, q , w.
Which of the following statements correctly describes the signs of q and w
for the following exothermic process at 1 atmosphere pressure and 370
kelvin? H2O (g)  H2O (l)
a.
b.
c.
d.
e.
q
q
q
q
q
and w are both negative
is positive and w is negative
is negative and w is positive
and w are both positive
and w are both zero
(c) An exothermic indications q is negative and the gas is condensing to a
liquid so it is exerting less pressure on its surroundings indicating w is
positive.
Sample Questions from the AP Exam on H, E, q , w.
A system does 130. J of work on its surroundings while 70. J of heat are
added to the system. What is the internal energy change for the system?
 E = q + w = -70. J + 130. J = -60. J
Sample Questions from the AP Exam on H, E, q , w.
What will be the volume change if 125 J of work is done on a system
containing an ideal gas? The surroundings exert a constant pressure of
5.2 atm.
-w = -PV; 125J/-5.2atm 101.3 J/L •atm = V; the volume will decrease by
0.24 liters.
Sample Questions from the AP Exam on H, E, q , w.
For the following reactions performed at constant external pressure, is work
done on the system by surroundings, by the surroundings on the system,
or is the amount of work negligible?
a.
b.
c.
d.
Sn(s) + 2F2 (g)  SnF4 (s)
AgNO3 (aq) + NaCl(aq)  AgCl(s) + NaNO3(aq)
C(s) + O2 (g)  CO2 (g)
SiI4(g) + heat  Si (s) + 2I2(g)
a. w > 0; work is done on the system as 2 moles of gas reactant decrease
to 0 moles of gas product
b. w = negligible; no change in volume
c. w = negligible; 1 mole of gas reactant to 1 mole of gas product indicates
a volume change close to zero
d. w < 0; 1 mole of gas reactant expands to moles of gas product therby
doing work on the surroundings.
Sample Questions from the AP Exam on H, E, q , w.
With the addition of heat and manganese dioxide as a catalyst, potassium
chlorate at constant pressure will decompose according to the following
equation
2KClO3 (s) + heat  2KCl(s) + 3O2 (g)
Determine whether each of the values of H, q, w and E will be positive
negative or unable to be determined.
H is positive
q is positive
w is negative
E cannot be determined from the information given. The sign of E depends
upon the magnitudes of q and w.
Sample Exercise 6.1 Internal Energy p. 233
Calculate the E for a system undergoing an endothermic process in
which 15.6 kJ of heat flows and where 1.4 kJ of work is done on
the system
“work is done on the system” Is w positive or negative?
“endothermic process”
E = q + w
Is q positive or negative?
The Nature of Energy: Vocabulary
Figure 6.4
The Piston, Moving a Distance Against a Pressure P, Does Work On the
Surroundings
P=F/A
F=PA
W = F • D
W = P A • D
W = P V or W = - P V
Expanding –
work is done by the system
work flows out of the system
work is negative
Contracting –
work is done to the system
work is going into the system
work is positive
Sample Exercise 6.2 Internal Energy p. 234
Calculate the work associated with the expansion of a gas from 46 L
to 64 L at a constant external pressure of 15 atm.
Look at the units – easy math – What does it mean?
w = - P V
For an ideal gas, work can occur only when its volume changes. So…
if a gas is heated at a constant volume, the pressure increases but
no work occurs
Sample Exercise 6.3 Internal Energy, Heat & Work p. 234
A balloon is being inflated to its full extent by heating the air inside it.
In the final stages of this process, the volume of the balloon
changes from 4.00 x 106 L to 4.50 x 106 L by the addition of 1.3 x
108 J of energy as heat. Assuming that the balloon expands
against a constant pressure of 1.0 atm, calculate E for the process.
Look at the units – easy math – What does it mean?
101.3 J = 1 L • atm
Notice: Which contributes more to the internal energy? Heat or work
Enthalpy, H .
H = E + PV
E is the internal energy, P is the pressure of the system, V is the volume of the system
Enthalpy is a state function
At A Constant Pressure
E = qp + w
E = qp - P V
qp = E + P V
H =  E + PV
H =  E + PV
where w = -P V
rearrange
PV = PV
qp = H
At constant pressure (where only PV work is allowed), the change in
enthalpy, H of the system is equal to the energy flow as heat.
For reactions at constant pressure
H = Hproducts - Hreactants
Sample Exercise 6.4 Enthalpy p. 236
When 1 mole of methane (CH4) is burned at a constant pressure, 890
kJ of energy is released as heat. Calculate H for a process in
which a 5.8 g sample of methane is burned at a constant pressure.
Calorimetry
Coffee Cup Calorimeter
Calorimeter – a devise to experimentally
determine the heat associated with a
chemical reaction.
Calorimetry – the science of measuring heat
based on observing the temperature
change when a body absorbs or
discharges energy as heat.
Heat Capacity, C
C = heat absorbed /increase in temperature
Specific Heat Capacity
J /C • g
or
J/K • g
Molar Heat Capacity
J /C • mol
or
J/K • mol
Specific Heat Capacities .
p. 237
Constant-Pressure Calorimetry
Atmospheric pressure remains constant during the process.
For reactions in solution
Assumptions
Colorimeter does not absorb or leak any heat
Solution treated like pure water with a density of 1.0 g/mL
Energy released determined from…
temperature increase,
mass of solution,
specific heat capacity of solution.
q = m C T
The Nature of Energy: Vocabulary
Figure 6.6 A Bomb Calorimeter
Specific Heat