Enthalpy (5.3)
• Objectives
– Today I will be able to:
• Define state function
• Calculate the enthalpy of a system
• Informal assessment – monitoring student
interactions and questions as they complete
the practice problems
• Formal assessment – analyzing student
responses to the practice problems
Lesson Sequence
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•
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Evaluate: Warm Up
Explain: Enthalpy
Elaborate: Enthalpy Calculations
Evaluate: Closure
Warm Up
Calculate the change in internal energy for a
process in which a system absorbs 30 J of heat
from the surroundings and does 44 J of work
on the surroundings.
Answer
• E=q+w
• q = 30 J because heat was absorbed
• w = -44 J because work was done by the
system
• E = 30 J + (-44 J)
• E = - 14 J
Objectives
• Today I will be able to:
– Define state function
– Calculate the enthalpy of a system
Homework
• Organic Functional Groups Quiz
– Thursday, October 2
• Bring textbook to exchange
• Finish practice problems
Agenda
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Warm Up
Enthalpy Notes
Practice Problems
Exit Ticket
Enthalpy (5.3)
What two components make up
the total energy of a system?
Work
• Mechanical work is the focus for chemical and
physical changes
• Associated with a change in volume
• Constant pressure is maintained
Zn(s) + 2H+(aq)  Zn2+(aq) + H2(g)
Work cont.
• Work involved in the expansion or
compression of gases is called pressure –
volume work
w = - PΔV
• Units: L-atm
• Conversion factor
1 L-atm = 101.3 J
Practice Problem
• A fuel is burned in a cylinder equipped with a
piston. The initial volume of the cylinder is
0.250 L, and the final volume is 0.980 L. If the
piston expands against a constant pressure of
1.35 atm, how much work (in J) is done?
Answer
• w = -PΔV
• W = - (1.35 atm)(0.730 L) = -0.9855 L-atm
• -0.9855 L-atm (101.3J / 1 L-atm) = -99.8 J
• W = -99.8 J
Practice Problem 2
Answer
Enthalpy (H)
• Internal energy plus the product of the
pressure and volume of a system
H = E + PV
• The equation is used to account for the
absorption/release of heat and work during a
chemical or physical change
• Relates mainly to heat flow
Enthalpy is a state function
• State function
– A property of a system that is determined by
specifying the systems condition or state
– Value of a state function depends only on the
present state of the system, not on the path the
system took to reach the state
Example
Potential energy of hiker 1 and hiker 2 is the same
eventhough they took different paths.
6.7
Which of the following variables are
examples of state functions?
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ΔE
q
w
H
PV
Enthalpy Change (ΔH)
• Change in heat exchange between a system
and its surroundings at constant external
pressure
ΔH = ΔE + PΔV
Keep in mind…
• ΔH = ΔE + PΔV
• ΔH = (qp + w) – w
• ΔH = qp
• For most reactions the difference between ΔH
and Δ E is small because there is not a lot of
work
• If PΔV is small it can be ignored from
calculations
Closure
• Complete practice problems:
– 5.31, 5.32, 5.37,