Thermochemistry
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Transcript Thermochemistry
Thermochemistry
AP Chem
Ch. 6
Thermochemistry
Thermochemistry – the study of heat
changes that accompany chemical
reactions and phase changes
Universe = System + Surroundings
Endothermic Reaction – one in which
energy (heat) is absorbed into the system
Exothermic Reaction – one in which energy
(heat) is released from the system
Energy
Energy – the ability to do work or
produce heat
the SI unit for heat is the Joule (J)
Heat CANNOT be measured
directly; it is calculated
“q” = quantity of heat
In the lab…
Calorimeter – an
insulated device
used for measuring
the temperature
change during a
chemical process
Useful Conversion Factors
1 cal = 4.184 J
1 Cal = 1000 cal
101.3 J = 1 L•atm
1 J = 1 kg•m2 / s2
Specific Heat
Specific Heat – energy required to raise
the temperature of one gram of any
substance one degree Celsius.
Each substance has its own specific heat
The higher the specific heat, the longer
it will take to raise the temp.
water has a very high specific heat
4.184 J/g°C
q = m c ΔT
q = heat absorbed or released
c = specific heat value
m = mass or moles of substance
ΔT = change in temperature
Use this when there is NO change in state
**Make sure units will cancel (usually “c” will
determine what units to use for q, m, DT)**
Heat Lost = Heat Gained
When 2 substances at different temperatures
come into contact with each other, heat is
transferred from the warmer to the cooler
substance until both substances are at the
same temperature
Enthalpy
Enthalpy (H) – measure of the amount of
energy aborbed/released by a reaction
Changes in enthalpy (ΔH rxn )can be
calculated for specific chemical reactions
Exothermic (lose heat) = -ΔHrxn
Endothermic (gains heat) = +ΔHrxn
Enthalpy (cont’d)
ΔH rxn = H products – H reactants
DH = q only at constant pressure
ΔH° = standard enthalpy change
° = occurs under standard conditions
standard conditions = 1atm, 298K (25°C)
Thermochemical Equations
A balanced chemical equation that
includes all reactants, products, and
the change in energy (ΔH)
Use energy in stoichiometric
calculations
Thermochemical Equations
Can be written 2 ways:
a. 4Fe + 3O2 2Fe2O3 + 1625kJ
or
4Fe + 3O2 2Fe2O3
ΔH = -1625kJ
b. NH4NO3 +27kJ NH4+ + NO3or
NH4NO3 NH4+ + NO3-
ΔH = 27kJ
Bond Energy
Bond energy = energy required to
BREAK a bond
Forming bonds = energy is released
Breaking bonds = energy is absorbed
DHo = bond energies broken – bond energies formed
Heat in Changes of State
Molar
Heat Values (table 11.5 on
handout)
ΔH vap = - ΔH cond
ΔH fus = - ΔH solid
Heat in Changes of State
q = mDH
m = mole or mass
DH = molar heat value
**units for m depend on units of DH**
Heating/Cooling Curve
Changes in Temp & State
Use Heating/Cooling Curve
Use q=mcDt and q=mDH to calculate
total heat
Heat of Formation
Heat of formation-change in enthalpy that takes
place when a compound is formed from it’s
elements
ΔH°f = standard heat of formation = the change in
enthalpy that accompanies the formation of 1 mole
of compound with all substances in their standard
states
Table 11.6 in handout
all in standard states: 1atm, 298K (25°C)
Heat of Formation
ΔH°rxn = Σ ΔH°f (products) - Σ ΔH°f (reactants)
**ΔH°f of a pure element in it’s standard
state is 0.0 kJ**
Hess’s Law
Add two or more thermochemical equations
together to obtain a desired thermochemical
equation
Multiply rxn by a
coefficient
Divide rxn by a
coefficient
Reverse rxn
Multiply DH by same
coefficient
Divide DH by same
coefficient
Change sign of DH