CHM135H1 Chapter Notes - Chapter 9: Kinetic Energy, Chemical Energy, Chemical Reaction

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CHM135 CHEMISTRY TEXTBOOK CH. 9 NOTES/SUMMARY
Chapter 9 Thermochemistry: Chemical Energy
Chapter 9.1: Energy and Its Conservation
- Energy is the capacity to do work or supply heat
o E.g. water falling over a dam contains energy that can be used to turn a turbine and
generate electricity
o E.g. a tank of propane gas contains energy that, when released in the chemical process of
combustion, can heat a house or barbecue a hamburger
- Energy is classified as either kinetic of potential
- Kinetic energy, Ek, is the energy of motion:
 ; where m is the mass, and v is the velocity
- Derived SI unit for energy is (kg ∙ m2/s2) = 1 J
- Potential energy, Ep, is stored energy stored in an object due to height or in a molecule because
of reactions it can undergo
o E.g. water sitting in reservoir behind dam has potential energy because of its height above
stream at bottom of dam, and when water falls, potential energy is converted to kinetic
energy
- Conservation of energy law energy cannot be created or destroyed; it can only be converted
from one form into another
o E.g. hydroelectric dam water sitting motionless in reservoir behind dam has potential
energy due to height above stream, but has no kinetic energy since it isn’t moving; height
and potential energy decrease while velocity and kinetic energy increase and moving
water spins turbine of a generator, converting kinetic energy into electrical energy
o (Textbook, Page 313)
- Energy has many forms E.g. thermal energy is just the kinetic energy of molecular motion;
measured by finding temperature of an object
o Objects at low temperatures are perceived as cold if its atoms or molecules are moving
slowly
o Objects at high temperatures are perceived as hot if its atoms or molecules are moving
rapidly and colliding forcefully
o Heat is the amount of thermal energy transferred from one object to another as the result
of a temperature difference between the two rapidly moving molecules in hotter
objects collide with more slowly moving molecules in colder objects, transferring kinetic
energy and causing slower moving molecules to speed up
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- Chemical energy is a kind of potential energy in which chemical bonds act as storage medium
chemicals can release their potential energy in the form of heat or light when they undergo
reactions and form more stable products
- First law of thermodynamics energy cannot be created or destroyed; it can only be converted
from one form into another
Chapter 9.2: Internal Energy and State Functions
- Substances in an experiment the starting reactants and final products are collectively called
the system, while everything else the reaction flask, solvent, room and building etc. are
collectively called the surroundings
- If system could be truly isolated from surroundings so that no energy transfer could occur
between them, then total internal energy € of system, defined as sum of all kinetic and potential
energies for every molecule or ion in system would be conserved and remain constant throughout
reaction
- First law of thermodynamics (restated) total internal energy E of an isolated system is constant
- Not possible to truly isolate a chemical reaction from surroundings its important to accurately
measure any energy that enters system from surroundings or leaves system and flows to
surroundings
- Change in internal energy of system must be measured; energy change represents the difference
in internal energy between final state of system after reaction and initial state of system before
reaction:  
- Energy changes measured from point of view of system; any energy flowing from system to
surroundings has a negative sign because system has lost it , Efinal < Einitial any energy flowing
to system from surroundings has a positive sign because system has gained, Efinal > Einitial
(Textbook, Page 314)
- Internal energy of a system depends on many things:
o Chemical identity
o Sample size
o Temperature
o Pressure
o Physical state (gas, solid, or liquid)
- Internal energy doesn’t depend on system’s past history it doesn’t matter what system’s
temperature of physical state was an hour ago, and it doesn’t matter how the chemicals were
obtained; only present condition of system matters
- Internal energy is a state function state function is a function or property whose value depends
only on the present state, or condition, of the system, not on the path used to arrive at that state
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- The overall change in any state function is zero when the system returns to its original condition
for a nonstate function, the overall change is not zero when system returns to its original
condition
Chapter 9.3: Expansion Work
- Work (w) is defined as the force (F) that produces the movement of an object times the distance
moved (d): 
- E.g. when you run up stairs, leg muscles provide a force sufficient to overcome gravity and lift
you higher; when you swim, you provide a force sufficient to push water our of the way and pull
yourself forward
- Expansion work is most common type of work encountered in chemical systems (also called
pressure volume, or PV, work) done as a result of volume change in system
- If a reaction takes place inside a container outfitted with a movable piston, the greater volume of
gas in product will force piston outward against pressure of atmosphere (P), moving air molecules
aside and thereby doing work
(Textbook, Page 316)
- Force (F) is area (A) times pressure (P):
; where P is the external atmospheric pressure; if piston is pushed out a distance d,
then amount of work done is equal to force times distance, or pressure times distance
; area of piston times distance the piston moves is just the volume
change in the system:
; amount of work done is equal to pressure exerted by gas against piston times
volume change: 
- Since work is done by system to move air molecules aside as the piston rises, work energy must
be leaving the system negative sign of work in preceding equation is consistent with
convention established for ΔE; any energy flowing out of the system has a negative sign because
the system has lost it (Efinal < Eintial)
- Pressure is given as atm, and volume change is given as L, so amount of work done has units of
(L ∙ atm), where 1 atm = 101 103kg/(m ∙ s2), so 1 L ∙ atm = 101 J:

 


- When reactions take place with a contraction in volume instead of expansion, the ΔV term has a
negative sign and work has a positive sign since the system has now gained work energy
(Efinal > Einitial)
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