Heat in Chemical Reactions

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Heat in Chemical Reactions

• Chapter 12

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12-1 Chemical Reactions That Involve HeatTWO
Trends in Nature

• Order ? Disorder
• ? ?
• High energy ? Low energy
• ?

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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 lt Hreactants
Hproducts gt Hreactants
DH lt 0
DH gt 0
6.4
4
12-2 Heat and Enthalpy Changes
Is DH negative or positive?
System absorbs heat
Endothermic
DH gt 0
6.01 kJ are absorbed for every 1 mole of ice that
melts at 00C and 1 atm.
6.4
5
Exothermic Reactions
Is DH negative or positive?
System gives off heat
Exothermic
DH lt 0
890.4 kJ are released for every 1 mole of methane
that is combusted at 250C and 1 atm.
6.4
6
Thermochemical Equations

• The stoichiometric coefficients always refer to
  the number of moles of a substance

• If you reverse a reaction, the sign of DH changes

• If you multiply both sides of the equation by a
  factor n, then DH must change by the same factor
  n.

6.4
7
Thermochemical Equations

• The physical states of all reactants and products
  must be specified in thermochemical equations.

266 g P4
-6470 kJ
6.4
8
Exothermic process is any process that gives off
heat transfers thermal energy from the system
to the surroundings. Heat is a product!
Endothermic process is any process in which heat
has to be supplied to the system from the
surroundings. Heat is a reactant!
6.2
9
Exothermic
Endothermic
6.2
10
Spontaneous Physical and Chemical Processes

• A waterfall runs downhill
• A lump of sugar dissolves in a cup of coffee
• At 1 atm, water freezes below 0 0C and ice melts
  above 0 0C
• Heat flows from a hotter object to a colder
  object
• A gas expands in an evacuated bulb
• Iron exposed to oxygen and water forms rust

18.2
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spontaneous
nonspontaneous
18.2
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Does a decrease in enthalpy mean a reaction
proceeds spontaneously?
Spontaneous reactions
18.2
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Entropy (S) is a measure of the randomness or
disorder of a system.
DS Sf - Si
If the change from initial to final results in an
increase in randomness
Sf gt Si
DS gt 0
For any substance, the solid state is more
ordered than the liquid state and the liquid
state is more ordered than gas state
Ssolid lt Sliquid ltlt Sgas
DS gt 0
18.3
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Thermodynamics
State functions are properties that are
determined by the state of the system, regardless
of how that condition was achieved.
energy
, pressure, volume, temperature
DE Efinal - Einitial
Potential energy of hiker 1 and hiker 2 is the
same even though they took different paths.
6.3
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Entropy Changes in the Surroundings (DSsurr)
Exothermic Process DSsurr gt 0
Endothermic Process DSsurr lt 0
18.4
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12-4 Calorimetry
DEsystem DEsurroundings 0
or
DEsystem -DEsurroundings
Exothermic chemical reaction!
6.3
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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.
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.
C J/gºC
Heat (q) absorbed or released
q mCDt
Dt tfinal - tinitial
6.5
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C of Fe 0.444 J/g 0C
Dt tfinal tinitial 50C 940C -890C
q mCDt
869 g x 0.444 J/g 0C x 890C
-34,000 J
6.5
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Constant-Volume Calorimetry
qsur -qrxn
qrxn - qwater
qwater mCDt
Reaction at Constant V
DH qrxn
No heat enters or leaves!
6.5
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Constant-Pressure Calorimetry
qsys qwater qcal qrxn
qsys 0
qrxn - (qwater qcal)
qwater mCDt
qcal CcalDt
Reaction at Constant P
DH qrxn
No heat enters or leaves!
6.5
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6.5
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Chemistry in Action
Fuel Values of Foods and Other Substances
1 cal 4.184 J
1 Cal 1000 cal 4184 J
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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?
The standard enthalpy of formation of any element
in its most stable form is zero.
6.6
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6.6
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Hesss 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 doesnt matter
how you get there, only where you start and end.)
6.6
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Calculate the standard enthalpy of formation of
CS2 (l) given that
1. Write the enthalpy of formation reaction for
CS2
2. Add the given rxns so that the result is the
desired rxn.
6.6
27
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.
6.6
28
The enthalpy of solution (DHsoln) is the heat
generated or absorbed when a certain amount of
solute dissolves in a certain amount of solvent.
DHsoln Hsoln - Hcomponents
Which substance(s) could be used for melting ice?
Which substance(s) could be used for a cold pack?
6.7
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The Solution Process for NaCl
DHsoln Step 1 Step 2 788 784 4 kJ/mol
6.7
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12-5 What is Heat?

• Caloric Theory
• Heat is a fluid that transfers between objects.
• This was widely accepted through the 1700s
• Count Rumford showed that heat produced by
  friction could not be a fluid.
• James Joule demonstrated the mechanical
  equivalent of heat in 1850.
• This proved heat is a form of energy and
  destroyed the caloric theory
• Kinetic Theory suggested that heat results from
  the motion of particles in matter.

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• Kinetic Molecular Theory
• The volume of individual particles is so small it
  can be assumed to be zero
• Collision of particles with walls of the
  container are the cause of pressure
• Particles exert no force on each other, neither
  attract nor repel
• Average kinetic energy of a collection of gas
  particles is assumed to be directly proportional
  to Kelvin Temp. of the gas.
• Heat is the transfer of kinetic energy from a hot
  object to a cooler one, or the total kinetic and
  potential energy of the particles in a substance.
• Temperature is the average kinetic energy of the
  particles (how fast they move on average).