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Unit 3: Introduction to Thermochemistry
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Unit 3 goes over basic thermochemistry, enthalpy, Hess' Law, and Rates of Reaction.
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Changes in Matter• Thermochemistry
◦Study of the energy changes accompanying changes in matter
• Nuclear Change
◦Change in arrangement of nuclei
‣ Formation of new atom; potential energy change is huge
‣ Fusion of hydrogen (sun): H + H -> He
‣ Fission of Uranium-235 into Krypton-92 and Barium-141 (and neutrons)
• Physical Changes
◦Change in state
‣ Solids (s) —> Liquid (l) —> Gas (g) / or aqueous (aq)
‣ Change in arrangement of molecules; potential energy change is small
‣ Boiling hydrogen: H2 (l) —> H2 (g)
• Chemical Change
◦Formation of a new substance
‣ Change in electronic structure; potential energy is large
‣ EVIDENCE: dramatic change in color; change in smell; formation of precipitate (Solid) or
bubbles (gas)
‣ Burning Hydrogen: t 02cg
21cg 2420cg
Changes in Energy
• Law of Conservation of Energy
◦Energy can’t be created or destroyed; it’s only transferred
‣ Chemical System
• Reactants / products undergoing change
‣ Surroundings
• Environment around the chemical system
• Open System
◦Both matter and energy can move in or out
• Closed System
◦Energy can move in or out; matter cannot
• Isolated System
◦Ideal system; no movement of matter or energy
Open closed Isolated
energy
energy
a
system system System
erymatter
Surroundings
Surroundings Surroundings
,Types of Energy
• Kinetic Energy
◦Energy of particles in motion
‣ Moving Electrons within an atom
‣ Vibration of Atoms within a compound / bonds (solids)
‣ Rotation an translation of molecules within a substance (liquids and gasses)
• Potential Energy
◦Energy based on an objects position with another object
‣ Nuclear potential of protons and neutrons in the atom
‣ Electronic potential of atoms connected by chemical bonds
◦Di cult to measure
‣ Study energy changes
Energy Terms
• Energy (E)
◦The ability to do work or produce heats units: J (Joules)
• Work (w)
◦The energy transferred to an object by a force causing the object to move; mechanical;
unites: J (Joules)
• Heat (q)
◦Amount of thermal energy transferred between substances
• Thermal Energy
◦Form of kinetic energy (motion of particles)
‣ “An object possesses thermal energy. But cannot possess heat.”
• Temperature
◦Measure of the average speed/motion (kinetic energy) of atoms/molecules
• Thermal Equilibrium
◦System and Surroundings have the same temperature; heat transfer stops
• Exothermic Reaction
◦Chemical energy is converted to thermal energy; system realizes energy
‣ Isolated
• Temperature increases in the system
‣ Non-Isolated
• Heat is given o to the surroundings; q < o
• Endothermic
◦Thermal energy is converted to chemical energy; system gains energy
‣ Isolated
• Temperature decreases in the system
‣ Non-Isolated
• Heat is absorbed by the system; q > o
Exothermic Endothermic
TempDrop
TempRise products
Reactants
a DH O
I Heatgivenout Heattakes
I
d I
gaco 8D
9 In
5 products Reactants
e I
Rxn progress Rxn progress
, g
Calculating Heat (Pg 301)
• Calculation of heat: q = m c ∆T
• Where: heat (q —> joules) equals mass (m —> grams) times temperature (∆T —> Degrees
Celsius0 times the speci c heat of your substance [ c —> (g X ˚C)
• Speci c Heat Capacity
◦Is the quantity of heat required to raise the temperature of a unit of mass of a substance 1˚C
or 1K (kelvin); pg301
• q is negative when heat is leaving the system; q is positive when heat is entering the system
Problem 1
When 600g of water in an electric kettle is heated from 20˚C to 85˚C to make a cup of tea, how
much heat ows into the water?
g MCAT
600 4 1851g E GSK
M 600g 8
163020J
C 4.1851g0C
200000J
AT_Tz Tz
850 200
65 C
Problem 2
Temp be if 250.0J of heatwere to beXfered
into
whatwould thefinal
at 20.0 C
10.0g of methanol initially
M lo og of MCAT
C1.9185190
C 8 D'T
MC
06 250.05
250.0J D1
T 20.0 C
GO0g 2.91851g.cz
BT
8.568 C AT
DT Tz T
D TTT Tz
28.6827
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