# CHM135H1 Chapter Notes - Chapter 9, 17: Internal Energy, Chemical Equation, Thermodynamics

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16 Dec 2016
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Chapter 9
9.1: Energy and Its Conservation
Energy: the capacity to do work or supply heat
o Classified as either kinetic or potential
Kinetic Energy (): the energy of motion
o SI Unit: Joule (J)
=

Potential Energy (): stored energy
o Stored in an object because of its height or in a molecule because of its reactions
it can undergo
Conservation of Energy Law: energy cannot be created nor destroyed; it can only be
converted from one form into another
o Also known as the First Law of Thermodynamics
Thermal Energy: the kinetic energy of molecular motion
o Found by measuring temperature of an object
Low temperature: atoms moving slowly
High temperature: atoms moving rapidly
Heat: the energy transferred from one object to another as the result of a temperature
difference between them
Chemical Energy: potential energy in which chemical bonds act as the storage medium
9.2: Internal Energy and State Functions
System: substances in an experiment, the starting reactants and the final products
Surroundings: everything found around the system
Internal Energy (E): the sum of all the kinetic and potential energies for every
molecule/ion in the system
First Law of Thermodynamics (restated): the total internal energy E of an isolated
system constant
Not possible that truly isolate a chemical reaction from its surroundings
o Chemicals are in physical contact with flask/container
o Important to measure any energy that enters the system from the surroundings
or leaves the system and flows to the surroundings
Change in the internal energy of the system, E
∆ = � − ���
Difference in internal energy between the final state of the system after reaction and
the initial state of the system before the reaction
Energy changes are measures from the point of view of the system
o Energy that flows from a system to the surroundings has a negative sign
The system has lost it
� < ���
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o Energy that flows to a system from the surroundings has a positive sign
The system has gained it
� > ���
Internal energy of a system depends on many things:
o Chemical identity
o Sample size
o Temperature
o Pressure
o Physical State (gas, liquid, or solid)
State Function: a function/property whose value depends only on the present condition
of the system, not on the path used to arrive at that condition
o Internal energy is said to be a state function
o The overall change in any state function is zero when the system returns to its
original condition
9.3: Expansion Work
Work: the force that produces the movement of an object times the distance moved
o Work = Force * Displacement
o Most common type of work in chemical systems is the expansion work
Done as a result of a volume change in the system
If a reaction takes place inside a container outfitted with a moveable piston, the greater
volume of gas in the product will force the piston outward against the pressure of
atmosphere
o Air particles move aside and therefore, do work
= − ∗ 
= ∗  = − ∗  ∗ 
∆ =  ∗ 
= −∆
The amount of work done is equal to the pressure the gas exerts against the piston
times the volume change
o Work energy is leaving the system (negative sign)
o System has lost energy
When a reaction takes place with a contraction in volume rather than expansion, the ∆
term has a negative sign
o Work has a positive sign
o System has gained energy
When there is no volume change, ∆ =  and there is no work
9.4: Energy and Enthalpy
Change in the internal energy of a system (∆):
∆ = +  = − ∆
q has a positive sign if the system gains heat
q has a negative sign if the system loses heat
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= ∆ + ∆
Energy change in a system is due entirely to heat transfer when ∆ = 
o = ∆ @ ∆ = 
Energy change in the system is due to both heat transfer and PV work if ∆ = 
o = ∆ + ∆
Heat of Reaction: the enthalpy change for a reaction
o Also known as Enthalpy Change (∆)
∆ = ∆ + ∆
Enthalpy is a state function whose value depends on the current state of the system
o We do ot eed to ko the systes ethalpy efoe ad afte a eatio
o Only need to know the difference between final and initial states
Only the enthalpy change during a reaction is important
9.5: Thermochemical Equation and the Thermodynamic Standard State
Thermochemical Equation: gives a balanced chemical equation along with the value of
the enthalpy change, the amount of heat released or absorbed when reactants are
converted to products
o Specifies the amount of each substance
Thermodynamic Standard State: conditions under which thermodynamic measurements
are reported; 298.15 K, 1 atm, 1 M
of the superscript °
Standard Enthalpy of Reaction: enthalpy change under standard-state conditions
o Indicated by the symbol ∆°
o Refer to the reaction going in the direction written
o For a reverse reaction, the sign of ∆° must be changed
Equal in magnitude but opposite in sign for the corresponding forward
reaction
o Physical states of reactants and products must be specified as solid (s), liquid (l),
gaseous (g), or aqueous (aq)
9.6: Enthalpies of Chemical and Physical Changes
Enthalpies of Chemical Change
Endothermic: a reaction in which heat is absorbed and the temperature of the
surroundings fall
o ∆ has a positive sign
o Heat flowed into the system from the surroundings
Exothermic: a reaction in which heat is evolved and the temperature of the
surroundings rises
o ∆ has a negative sign
o Heat has flowed out of the system to the surroundings
The value of ∆° given for an equation assumes:
o Equation is balanced to represent the # of moles of reactants and products
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