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Thermochemistry
Unit 7
Energy is the capacity to do work
•
Thermal energy is the energy associated with
the random motion of atoms and molecules
•
Chemical energy is the energy stored within the
bonds of chemical substances
•
Nuclear energy is the energy stored within the
collection of neutrons and protons in the atom
•
Electrical energy is the energy associated with
the flow of electrons
•
Potential energy is the energy available by virtue
of an object’s position
6.1
Energy Changes in Chemical Reactions
Heat is the transfer of thermal energy between two bodies that
are at different temperatures.
Temperature is a measure of the thermal energy.
Temperature = Thermal Energy
900C
A Video – Eureka!
400C
greater thermal energy
Thermochemistry is the study of heat change in chemical
reactions and changes of state (physical changes)
The system is the specific part of the universe that is of
interest in the study.
SURROUNDINGS
SYSTEM
open
Exchange: mass & energy
closed
isolated
energy
nothing
6.2
Exothermic process is any process that gives off heat –
transfers thermal energy from the system to the surroundings.
2H2 (g) + O2 (g)
H2O (l)
2H2O (l) + energy
H2O (s) + energy
Endothermic process is any process in which heat has to be
supplied to the system from the surroundings.
energy + 2HgO (s)
energy + H2O (s)
2Hg (l) + O2 (g)
H2O (l)
Exothermic and Endothermic Videos
Mythbusters Exothermic Reaction
Death of a Gummy Bear
The Most Common Endothermic Reaction
Enthalpy (H) is used to quantify the heat flow into or out of a
system in a process that occurs at constant pressure.
DH = H (products) – H (reactants)
DH = heat given off or absorbed during a reaction at constant pressure
Hproducts < Hreactants
DH < 0
Hproducts > Hreactants
DH > 0
6.3
Thermochemical Equations
Is DH negative or positive?
System absorbs heat
Endothermic
DH > 0
6.01 kJ are absorbed for every 1 mole of ice that
melts at 00C and 1 atm.
H2O (s)
H2O (l)
DH = 6.01 kJ
Thermochemical Equations
Is DH negative or positive?
System gives off heat
Exothermic
DH < 0
890.4 kJ are released for every 1 mole of methane
that is combusted at 250C and 1 atm.
CH4 (g) + 2O2 (g)
CO2 (g) + 2H2O (l) DH = -890.4 kJ
Drawing Enthalpy Diagrams
Draw the energy diagram & thermochemical equation for the
following reactions:
4 H2 +
2 O2

4 H2O
+
2H2O(g) + 116 kcal → 2H2(g) + O2(g)
967.2kJ
The MATH of Thermochemical Equations
How much heat is evolved when 266 g of white
phosphorus (P4) burn in air?
P4 (s) + 5O2 (g)
P4O10 (s)
DH = -3013 kJ
6470kJ
The MATH of Thermochemical Equations
How much heat is absorbed when 32.5g of O2(g) is reacted
with N2 (g)?
N2(g) + 2 O2(g) ----> 2 NO2(g)
ΔH = +67.6 kJ
Introduction to
Specific Heat and
Heat Capacity
What is specific heat?
How is heat capacity
different?
How do we measure the
specific heat of substances?
How do we calculate specific
heat and heat capacity?
The specific heat (s) of a substance is the amount of heat (q)
required to raise the temperature of one gram of the
substance by one degree Celsius. (specific amount)
The heat capacity (C) of a substance is the amount of heat
(q) required to raise the temperature of a given quantity (m)
of the substance by one degree Celsius. (general amount)
Heat (q) absorbed or released:
q = msDt
Dt = tfinal - tinitial
6.4
A quick Video
A simulation
How much heat is given off when an 869 g iron bar cools
from 940C to 50C?
s of Fe = 0.444 J/g • 0C
= -34,000 J
Multiple Methods to Calculate ΔH
Heats of Formation
Calorimetry
Hess’s Law
Because there is no way to measure the absolute value of
the enthalpy of a substance, must I measure the enthalpy
change for every reaction of interest?
Standard enthalpy of formation (DH0f) is the heat change
that results when one mole of a compound is formed from
its elements at a pressure of 1 atm.
The standard enthalpy of formation of any element in its
most stable form is zero.
DH0f (O2) = 0
6.5
0 ) is the enthalpy of
The standard enthalpy of reaction (DHrxn
a reaction carried out at 1 atm.
aA + bB
cC + dD
DH0rxn = [ cDH0f (C) + dDH0f (D) ] - [ aDH0f (A) + bDH0f (B) ]
DH0rxn = S nDH0f (products) - S mDHf0 (reactants)
Example
What is the ΔH of the following reaction using Heats of
Formation?
Zn
+
2AgNO3
→
Zn(NO3)2
+
2Ag
Benzene (C6H6) burns in air to produce carbon dioxide and
liquid water. How much heat is released per mole of
benzene combusted? The standard enthalpy of formation
of benzene is 49.04 kJ/mol.
2C6H6 (l) + 15O2 (g)
12CO2 (g) + 6H2O (l)
= - 2973 kJ/mol C6H6
Hess’s Law: When reactants are converted to products, the
change in enthalpy is the same whether the reaction takes
place in one step or in a series of steps.
(Enthalpy is a state function. It doesn’t matter how you get
there, only where you start and end.)
6.5
From the following data,
CH4(g) + 2O2(g) → CO2(g) + 2H2O(l)
ΔHo = -890 kJ/mol
H2O(l) → H2O(g)
Δ Ho = 44 kJ/mol
Calculate the enthalpy of the reaction
CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(g)
Δ Ho = ?
Calculate the standard enthalpy of formation of CS2 (l)
given that:
C(graphite) + O2 (g)
CO2 (g) DH0rxn = -393.5 kJ
S(rhombic) + O2 (g)
CS2(l) + 3O2 (g)
SO2 (g)
DH0rxn = -296.1 kJ
CO2 (g) + 2SO2 (g)
C(graphite) + 2S(rhombic)
0 = -1072 kJ
DHrxn
CS2 (l)
Spontaneous Processes
• Spontaneous: process that does occur
under a specific set of conditions
• Nonspontaneous: process that does
not occur under a specific set of
conditions
Copyright McGraw-Hill 2009
Spontaneous vs
Nonspontaneous
Why would an endothermic
reaction happen spontaneously?
Entropy
• Entropy (S): Can be thought of as a
measure of the disorder of a system
• In general, greater disorder means
greater entropy
Entropy in Action
Entropies
Trends in Entropy
• Entropy for gas phase is greater than that of
liquid or solid of same substance
– I2 (g) has greater entropy than I2 (s)
• More complex structures have greater
entropy
– C2H6 (g) has greater entropy than CH4 (g)
• Spure <
Saqueous
Slower temp
<
Shigher temp
• Sfewer moles
<
Smore moles
Entropy Changes in a System
Qualitative
Determine the sign of DS for the following
1.
2.
3.
4.
Liquid nitrogen evaporates
Salt is dissolved in water.
Liquid water is heated from 22.5 C
to 55.8 C
Water condenses on outside of bottle
Entropy Changes in the
System
Entropy can be calculated from the table
of standard values just as enthalpy
change was calculated.
DSrxn = SnDS products  SmDS reactants
Standard Entropy
2NH3(g)  N2(g) + 3H2(g)
DSrxn = 198.5 J/K · mol
Your Turn!
• Calculate the standard entropy change for the
following using the table of standard values.
(first, predict the sign for DS qualitatively)
2H2(g) + O2(g)  2H2O (g)
Gibbs Free Energy
DG = DH – T DS
• The Gibbs free energy, expressed in
terms of enthalpy and entropy, refers
only to the system, yet can be used to
predict spontaneity.
Gibbs Free Energy
• If DG < 0,negative, the forward reaction
is spontaneous.
• If DG = 0, the reaction is at equilibrium.
• If DG > 0, positive, the forward reaction
is nonspontaneous
Predicting Sign of DG