化学原理Chemistry课件-post+2+thermochemistry

H2O (s) H2O (l)
H2O (l) H2O (s)
DH = 6.01 kJ
DH = -6.01 kJ
• If both sides are mutiplied by n, DH must change by n.
2H2O (s)
2H2O (l) DH = 2 x 6.01 = 12.0 kJ
DH0f (C, graphite) = 0
DH0f (O3,g) = 142 kJ/mol DH0f (C, diamond) = 1.90 kJ/mol
DHorxn = (H2 + H3) – H1
H1
CaCO3(s)
1atm
H2
CaO(s) +
H3
CO2(g)
DHoa = H4 – H1 = -DHof (CaCO3,s)
2H2 (g) + O2 (g)
2H2O (l) + energy
H2O (g)
H2O (l) + energy
Endothermic process, gets the heat from the surroundings.
energy + 2HgO (s)
2Hg (l) + O2 (g)
A sample of nitrogen gas expands in volume from 1.6 L to 5.4 L at constant temperature. What is the work done in joules if the gas expands (a) against a vacuum and (b) against a constant pressure of 3.7 atm?
Attention:
E is a state function, but q and w are not a state function, since the latter two are only related to a process.
Exothermic process, gives off the heat to the surroundings.
No one knows how much the exact energy is!
The thermodynamics deals only with the energy changes that accompany chemical reactions.
System and Surroundings
Nothing exchanged
What we can measure is only the energy change!
(1) Heat is the energy transfer between a system and its surroundings that are at different temperatures.
A Comparison of DH and DE
2Na (s) + 2H2O (l) 2NaOH (aq) + H2 (g) DH = -367.5 kJ/mol DE = DH - PDV
At 25 0C and 1 atm, 1 mole H2 = 24.5 L
PDV = 1 atm x 24.5 L = 2.5 kJ DE = -367.5 kJ/mol – 2.5 kJ/mol = -370.0 kJ/mol
C (s, graphite) + O2 (g) = CO2 (g) DHrxn0 = - 393.5 kJ Then, DHf0(CO2, g) = - 393.5 kJ/mol
i.e. DHfo = 0 for any element in its most stable form.
DH0f (O2, g) = 0
H2O (s)
H2O (l) DH = 6.01 kJ
Thermochemical Equations
• The physical states must be all specified
H2O (l)
H2O (g) DH = 44.0 kJ
• If the equation is reverse, the sign of DH must change
qp = H2 – H1 = DH
at constant pressure, the heat adsorbed or released is totally used to increase or decrease the system enthalpy.
H is a state function, but it is not the system energy. Instead, H is only a measure of the system energy!
Significance it makes the reasoning and calculation simple.
The First Law of Thermodynamics:
The energy of the universe can be converted from one form to another, but it can not be created or destroyed.
Hproducts > Hreactants DH > 0
Thermochemical Equations
System absorbs heat Endothermic DH > 0
It means that: 6.01 kJ are absorbed by one mole of ice that melts at 00C and 1 atm.
DEsystem + DEsurroundings = 0
The system energy change DE = Efinal - Einitial
DE = q + w
“+” gain by the system “–” lost by the system
q: the heat exchange with the surroundings. w: the work done on (or by) the system.
w = -P DV
(a) DV = 5.4 L – 1.6 L = 3.8 L P = 0 atm
W = -0 atm x 3.8 L = 0 L•atm = 0 joules
(b) DV = 5.4 L – 1.6 L = 3.8 L P = 3.7 atm
w = -3.7 atm x 3.8 L = -14 L•atm
= 6470 kJ
How to make a theoretical calculation?
the absolute value of enthalpy can not be measured. Can we establish a reference point?
DHfo Standard enthalpy of formation the heat change results when one mole of a compound is formed at 1 atm from its elements in the most stable form.
DE = Efinal - Einitial
Other changes:
DP = Pfinal - Pinitial DV = Vfinal - Vinitial DT = Tfinal - Tinitial
The potential energy of hiker 1 is the same as that of hiker 2.
6.4
H (Enthalpy): describe the heat flow in a process that occurs at constant pressure.
DH (Enthalpy change): H (products) – H (reactants)
Hproducts < Hreactants DH < 0
Chapter 6 Thermochemistry
Energy change of chemical reaction.
What is the Energy?
It is the capacity to do work, and exists in a variety of the forms.
➢ Thermal energy the random motion. ➢ Chemical energy the bonds of substances ➢ Nuclear energy: neutrons and protons in the atom ➢ Electrical energy: the flow of electrons ➢ Potential energy: virtue of an object’s position
Temperature is a measure of the thermal energy. Temperature = Thermal Energy
900C greater thermal energy ?
400C
(2) Work, like heat, is the energy transfer between a system and its surroundings.
H4
Ca(s)
DHoc = H2 – H5 DHod = H3 – H6
= DHof (CaO,s)