Download Chapter 6 - Chemistry

Chapter 6
(Essentials of General Chemistry, 2nd Edition)
(Ebbing and Gammon)
Thermochemistry
Thermodymanics
thermodynamics
- the science of the relationships between heat and
other forms of energy
thermochemistry
- one area of thermodynamics
- study of the quantity of heat absorbed or evolved
by chemical reactions
- heat measurements provide data needed to
determine whether a particular chemical reaction
occurs and, if so, to what extent
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Energy and Its Units
energy
- the potential or capacity to move matter
- exists in different forms that can be interconverted
Types of energy:
i.) kinetic energy
ii.) potential energy
iii.) internal energy
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Kinetic Energy
kinetic energy
- the energy associated with an object by virtue of its
motion
- an object of mass, m, and speed or velocity, v, has
kinetic energy, EK, equal to
EK = ½ mv2
with units of kg·m2/s2 or J
- a joule is an extremely small unit (often use kJ)
calorie (cal)
- the amount of energy required to raise the
temperature of one gram of water by one degree
Celsius
- a non-SI unit of energy commonly used by chemists
- defined in terms of a joule: 1 cal = 4.184 J
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Potential Energy
potential energy
- the energy an object has by virtue of its position in
a field of force
- Consider water of a given mass m at the top of a
dam is at a relatively high position h in the
gravitational field g of the earth.
- The potential energy would be given by
Ep = mgh
where Ep = potential energy of a quantity of water at the top
of the dam
m = the mass of the water
g = the constant acceleration of gravity
h = the height of the water measured from some
standard level
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Note: - the choice of this standard level is arbitrary since
only differences of potential are important in any
physical situation
- convenient to choose the standard level to be the
surface of the earth
- potential energy of water at top of dam is converted
to kinetic energy when water falls to a lower level
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Internal Energy
internal energy, U
- the sum of the kinetic and potential energies of the
particles making up a substance
total energy, Etot, of a quantity of water
- the sum of its kinetic energy and potential energies
as a whole (EK + Ep) plus its internal energy
Etot = EK + Ep + U
- when study in lab, substance is at rest in vessel,
which makes its kinetic energy as a whole equal to
zero
- potential energy as whole is constant and can be
taken to be zero
- therefore, the total energy of the substance equals
its internal energy, U
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Law of Conservation of Energy
Law of Conservation of Energy
- energy may be converted from one form to another,
but the total quantity of energy remains constant
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Heat of Reaction
thermodynamic system (or system)
- the substance or mixture of substances under study
in which a change occurs
surroundings
- everything in the vicinity of the thermodynamic
system
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Heat
Heat (q)
- the energy that flows into or out of a system because
of a difference in temperature between the
thermodynamic system and its surroundings
- as long as system and surroundings are in thermal
contact, energy flows between them to establish
temperature equality or thermal equilibrium
- heat flows from region of higher temperature to one
of lower temperature; once temperature becomes
equal, heat flow stops
Note: by convention:
q is positive (q = +) if heat is absorbed by system
q is negative (q = -) if heat is evolved by system
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Heat of Reaction
heat of reaction
- the value of q required to return a system to the
given temperature at the completion of the reaction
Classification of physical changes and chemical reactions:
i.) exothermic process
ii.) endothermic process
chemical reaction or
- chemical reaction or
physical change in
physical change in
which heat is evolved
which heat is absorbed
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Enthalpy and Enthalpy Change
- heat absorbed or evolved by reaction depends on
conditions under which reaction occurs
- reaction usually takes place in vessel open to the
atmosphere and therefore at constant pressure of
atmosphere
- in this case, write heat of reaction as qp (p indicates
process occurs at constant pressure)
enthalpy (H)
- an extensive property of a substance that can be
used to obtain the heat absorbed or evolved in a
chemical reaction
- a property of a substance that is related to heat of
reaction, qp
- is a state function
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State function
state function
a property of a system that depends only on its present
state, which is determined by variables such as
temperature and pressure and is independent of any
previous history of
the system
- that is, a change in
enthalpy does not
depend on how the
change was made, but
only on the initial state
and final state of the
system
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Enthalpy of Reaction
enthalpy of reaction
- the change in enthalpy for a reaction at a given
temperature and pressure
- obtained by subtracting the enthalpy of the
reactants from the enthalpy of the products
H = Hfinal Hinitial
- since you start from reactants and end with
products, enthalpy of reaction is
H = H(products) H(reactants)
- is a state function
- key relation: enthalpy and heat of reaction
H = qp
- the enthalpy of reaction equals the heat of reaction
at constant pressure
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Enthalpy Diagram
2 Na (s) + 2 H2O (l)
2 NaOH (aq) + H2 (g)
Figure 6.6:
Enthalpy Diagram
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Enthalpy and Internal Energy
- enthalpy, H, defined precisely as the internal energy, U,
plus pressure, P, times volume, V
H = U + PV
- if we label the initial quantities (those for reactants)
with subscript i and final quantities (those for products)
with subscript f, then
H = Hf Hi = (Uf + PVf) - (Ui + PVi)
- collecting internal-energy terms and the pressurevolume terms, you can rewrite
H = (Uf - Ui ) + P(Vf - Vi) = U + P V
- rearranging, gives
U= H-P V
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Thermochemical Equations
thermochemical equation
- the chemical equation for a reaction (including
phase labels) in which the equation is given a molar
interpretation, and the enthalpy of reaction for
these molar amounts is written directly after the
equation
- phase labels are very important (must be included)
eg. Reaction of sodium and water
2 Na (s) + 2 H2O (l)
2 NaOH (aq) + H2 (g)
H = -368.6 kJ
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Two important rules for manipulating thermochemical
equations:
1.) When a thermochemical equation is multiplied by
any factor, the value of H for the new equation is
obtained by multiplying the value of H in the
original equation by that same factor.
2.) When a chemical equation is reversed, the value of
H is reversed in sign.
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Stoichiometry and Heats of Reaction
Consider the reaction of methane, CH4, burning in
oxygen at constant pressure.
CH4 (g) + 2 O2 (g)
CO2 (g) + 2 H2O (l)
H = -890.3 kJ
How much heat could you obtain from 10.0 g of methane?
grams of CH4
moles of CH4
kilojoules of heat
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Measuring Heats of Reaction
Heat capacity © of a sample of substance
- the quantity of heat needed to raise the temperature
of the sample of substance one degree Celsius (or
one Kelvin)
- changing the temperature of the sample from an
initial temperature ti to a final temperature tf
requires heat equal to
q=C t
where t = tf
ti
- heat capacity is directly proportional to amount of
substance
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molar heat capacity of a substance
- the heat capacity for one mole of substance
- that is, the quantity of heat needed to raise the
temperature of one mole of a substance one degree
Celsius
specific heat capacity (or specific heat)
- the quantity of heat required to raise the
temperature of one gram of a substance by one
degree Celsius (or one kelvin) at constant pressure
- to find heat required to raise the temperature of a
sample:
q =s x m x t
where s = specific heat of substance
m = mass in grams
t = temperature change
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Table 6.1
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Measurement of Heat of Reaction
Figure 6.10
A Simple Coffee-cup
Calorimeter
Figure 6.11
Bomb Calorimeter
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Using Heats of Reaction
Hess s Law
- enthalpy change for chemical reaction is
independent of path by which products are obtained
Hess s Law of Heat Summation
- states that for a chemical equation that can be
written as the sum of two or more steps, the
enthalpy change for the overall equation equals the
sum of the enthalpy changes for the individual steps
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Enthalpy Diagram Illustrating Hess s Law
Figure 6.12
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Standard Enthalpies of Formation
standard state
- the standard thermodynamic conditions chosen for
substances when listing or comparing
thermodynamic data:
1 atm pressure
the specified temperature (usually 25oC)
- standard conditions are indicated by a superscript
degree sign (o)
eg. The enthalpy change for a reaction in which reactants
in their standard states yield products in their standard
states is denoted Ho
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allotrope
- one of two or more distinct forms of an element in
the same physical state
reference form
- the stablest form (physical state and allotrope) of
the element under standard conditions
Figure 6.13
Allotropes of
Sulfur
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Standard enthalpy of formation, Hof
(standard heat of formation)
- the enthalpy change for the formation of one mole
of the substance in its standard state from its
elements in their reference form and in their
standard states
- values of standard enthalpies of formation are listed
(Table 6.2)
- standard enthalpy of formation of an element is
dependent on the form of the element
Using standard enthalpies of formation to find the standard
enthalpy of a reaction:
Ho = n Hof (products)
m Hof (reactants)
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