Thermochemistry Chapter 6 and 17 AP Chemistry. thermodynamics: the study of energy and its...
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Transcript of Thermochemistry Chapter 6 and 17 AP Chemistry. thermodynamics: the study of energy and its...
ThermochemistryChapter 6 and 17
AP Chemistry
thermodynamics: the study of energy and its transformations
-- thermochemistry: the subdiscipline involving chemical reactions and energy changes
temperature.
Energy
kinetic energy: energy of motion; KE = ½ mv2
-- all particles have KE
-- Thermal energy is due to the KE of particles.
We measure the average KE of acollection of particles as...
potential energy: stored energyChemical potential energyis due to electrostatic forcesbetween charged particles. -- related to the specific arrangement of atoms in the substance
+ +
Units of energy arejoules (J),kilojoules (kJ),calories (cal), ornutritional calories (Cal or kcal).
-- conversions:
JamesPrescott
Joule(1818-1889)
SI unit
4184 J = 4.184 kJ = 1000 cal = 1 Cal = 1 kcal
system: the part of the universe we are studyingsurroundings: everything else
-- In chemistry, a closed system can exchange energy but not matter with its surroundings.
-- Usually, energy is transferred to...
…(1)
...or...(2)
change an object’s state of motion
cause a temperature change
Work (w) is done when a force movesthrough a distance. W = F d
Heat (q) is an amount of energy transferred from a hotter object to a colder one.
Find the kinetic energy of asingle dinitrogen monoxidemolecule moving at 650 m/s.
N2O (laughing gas)
KE = ½ mv2
m = 44 amu
= 7.31 x 10–26 kg
KE = ½ (7.31 x 10–26 kg) (650 m/s)2
= 1.5 x 10–20 J
?
First Law ofThermodynamics
Law of Conservationof Energy
=
-- Energy morphs between its various forms, but the total amount remains the same.
(pretty much)
In endothermic processes, heat is _________ bythe system.
e.g.,
absorbed
meltingboilingsublimation
e.g.,
released
freezingcondensationdeposition
In exothermic processes, heat is ________ bythe system.
• Exothermic process is any process that gives off heat – transfers thermal energy from the system to the surroundings.
2H2 (g) + O2 (g) 2H2O (l) + energy
H2O (g) H2O (l) + energy
Endothermic process is any process in which heat has to be supplied to the system from the surroundings.
energy + 2HgO (s) 2Hg (l) + O2 (g)
energy + H2O (s) H2O (l)
>
internal energy (E) of a system: the sum of all theKE and PE of the components of a system
-- The change in the internal energy of a system would be found by: ΔE = Efinal – Einitial
And for chemistry, this equation would become:
ΔE = Eproducts – Ereactants
ΔE is + if Efinal Einitial (i.e., system... )
ΔE is – if Efinal Einitial (i.e., system... ) <
gains energy
loses energy
→ ENDOTHERMIC
→ EXOTHERMIC
But we ARE able to find ΔE by measuring twotypes of “energy” quantities:
ΔE = q + w
q = heat: +/– q = system absorbs/releases heat
w = work: +/– w = work done on/by system
** KEY: Sign conventions are based on the system’s point of view.
To go further, we must introducethe concept of enthalpy (H).
-- Enthalpy (H) is defined as... H = E + PV
where E = system’s internal energy
P = pressure of the system
V = volume of the system
Heike Kamerlingh Onnes1853–1926
The Dutch physicist and Nobel laureate H.K. Onnes coined the term enthalpy, basing it on the Greek term enthalpein, which means “to warm.”
ΔH = Hfinal – Hinitial = qP
P indicates constant pressure conditions.
When ΔH is +, the system... has gained heat.
When ΔH is –, the system... has lost heat.
Enthalpy is an extensive property, meaning that…
(ENDO)
(EXO)
Enthalpy is used to quantify the heat that is either gained or lost by a system that
Is at constant pressure.
the amount of material affects its value.
In the burning of firewood at constant pressure, theenthalpy change equals the heat released. ΔH is (–)
and depends on the quantity of wood burned.
For exothermic rxns, the heat content of thereactants is larger than that of the products.
enthalpyof reaction:
ΔHrxn = Hproducts – Hreactants
(also called “heat of reaction”)
ΔH = –2390 kJ
2 H2(g) + O2(g) → 2 H2O(g) ΔH = –483.6 kJ
What is the enthalpy change when178 g of H2O are produced?
178 g H2O
The space shuttle was poweredby the reaction above.
ΔH for a reaction and its reverseare the opposites of each other.
Enthalpy change depends on thestates of reactants and products.
2 H2(g) + O2(g) 2 H2O(g) (ΔH = –483.6 kJ)
2 H2O(g) 2 H2(g) + O2(g) (ΔH = +483.6 kJ)
Enthalpy/energyis a reactant.
2 H2(g) + O2(g) 2 H2O(g) (ΔH = –483.6 kJ)
2 H2(g) + O2(g) 2 H2O(l) (ΔH = –571.6 kJ)
Calorimetry: the measurement of heat flow
-- device used is called a... calorimeter
heat capacity of an object: amount of heat needed to
raise object’s temp. 1 K = 1oC molar heat capacity: amt. of heat needed to raise
temp. of 1 mol of a substance 1 K
specific heat (capacity): amt. of heat needed to raisetemp. of 1 g of a substance 1 K
i.e.,
molar heat capacity = molar mass X specific heat
cX = heat of fusion (s/l) or heat of vaporization (l/g)
We calculate the heat a substance loses or gains using:
where q = heat
m = amount of substance
cP = substance’s heat capacity
ΔT = temperature change
q = m cP ΔT
(for within a givenstate of matter)
AND q = +/– m cX
(for between twostates of matter)
HEAT
Tem
p.
ss/l
l
l/gg
heat added (+q) →← heat re
moved (–q)
Typical Heating Curve
What is the enthalpy change when 679 g of water at27.4oC are converted into water vapor at 121.2oC?
HEAT
Tem
p.
s
s/ll
l/g
g
q = m cP ΔT
cf = 333 J/gcv = 40.61 kJ/molcP,l = 4.18 J/g-K
cP,s = 2.077 J/g-KcP,g = 36.76 J/mol-K
Heat liquid…
= 679 g (4.18 J/g-K) (100 – 27.4) = 206 kJ
q = +m cXBoil liquid… = +37.72 mol (40.61 kJ/mol) = 1532 kJ
q = m cP ΔTHeat gas…
= 37.72 mol (36.76 J/mol-K) (121.2–100) = 29.4 kJ
ΔH = 1767 kJ
+
With a coffee-cup calorimeter, a reaction iscarried out under constant pressure conditions.-- Why is the pressure constant?
calorimeter isn’t sealed,atmospheric pressure is constant
-- If we assume that no heat is exchanged between the system and the surroundings, then the solution must absorb any heat given off by the reaction.
-- For dilute aqueous solutions, it is a safe assumption that cP = 4.18 J/g-K
the specific heatof water
i.e., qabsorbed = –qreleased