Transcript Energy, rate, and equilibrium

Chapter 7
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Study of energy, work and heat
Three basic laws of thermodynamics from the
ideas of kinetic molecular theory
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Molecules and atoms in a reaction mixture are in
constant, random motion
These molecules and atoms frequently collide
with each other
Only some collisions, those with sufficient energy,
will break bonds in molecules
When reactant bonds are broken, new bonds
maybe formed and products result
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We cannot measure an absolute value for
energy stored in a chemical system
We can only measure the change in
energy when a chemical reaction occurs
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Necessary to
establish a boundary
between a system:
the process under
study and the
surroundings: the
rest of the universe
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First Law of Thermodynamics states that
the energy of the universe is constant.
(Law of Conservation of Energy)
An exothermic reaction releases energy to
the surroundings (they become warmer)
An endothermic reaction absorbs energy
from the surroundings (they become
cooler)
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A thermochemical equation shows the
energy released or absorbed
An endothermic reaction gives the energy
on the reactants side of the equation.
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22kcal + 2NH3(g) -> N2(g) + 3H2(g)
An exothermic reaction gives the energy
on the products side of the equation:
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CH4(g) + 2O2(g) -> CO2(g) + 2H2O(g) + 211 kcal
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Exothermic:
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The reaction releases
___ kcal of heat.
Endothermic:
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The reaction requires
_____ kcal of heat.
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Term used to
represent heat
Symbolized as ΔH
Combustion of
Methane =
ΔH
= -211 kcal
Decomposition of
Ammonia =
ΔH = +22 kcal
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Second Law of
Thermodynamics
states that the
universe
spontaneously tends
toward increasing
disorder or
randomness
Measure of this
randomness is
entropy.
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Exothermic or endothermic?
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Fuel oil burned in a furnace
When solid NaOH is dissoved in water, the
solution gets hotter.
S(s) + O2(g) -> SO2(g), ΔH = -71 kcal
N2(g) + 2O2(g) + 16.2 kcal -> 2NO2(g)
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Exothermic reactions
should be
spontaneous, but not
all are.
So entropy explains
why not all
exothermic reactions
are spontaneous.
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Endothermic
reactions should be
nonspontaneous, but
not all are.
Entropy also explains
why not all
endothermic reactions
are spontaneous.
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ΔS is the measurement of the difference
between entropies of products and
reactants
Reactions that are exothermic and high in
entropy occur spontaneously
Lower entropy reactions will require
energy input to occur
Which substance has the greatest entropy,
H2O(l) or H2O(g)?
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ΔG represents free energy, the combined
contribution of the enthalpy and entropy
values for a chemical reaction.
ΔG = ΔH – TΔS, where T is the Kelvin
temperature of the reaction
- ΔG will always be spontaneous and +ΔG
will always be nonspontaneous
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If the entropy is negative and enthalpy is
positive, the free energy will always be
positive.
If the entropy is positive and enthalpy is
negative, the free energy will always be
negative.
If both are negative or both are positive, it
depends on the temperature.
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Calorimetry: measurement of heat energy
in a chemical reaction
Calorimeter: device used to measure heat
changes in calories
Specific Heat: the number of calories of
heat needed to raise 1 g of a substance 1
degree Celsius
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The heat is calculated by using the
following equation:
Q = ms x ΔTs x SHs
Where ms = the mass of solution
ΔTs = the change in temperature, and
SHs = the specific heat of the solution
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With the units of calories, grams and oC
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If 1.00 x 102 g of a liquid with specific heat
capacity of 0.800 cal/g-oC was placed in a
calorimeter and 6.5x102 cal of heat was
released, what was the temperature
change observed?
Q = ms x ΔTs x SHs
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The oxidization of food substances to release
energy
Measured in fuel value, or the amount of
energy per gram of food
Reported in nutritional Calories or
kilocalories: 1C = 1kcal = 1000 cal
Practice: If the fuel value of 1.00g of a
certain carbohydrate is 3.00 nutritional
Calories, how many grams of water must be
present in the calorimeter to record a 5.00oC
temperature change?
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Chemical kinetics is the study of the rate
of chemical reactions
Kinetics also tells us the mechanism of the
reaction, a step-by-step description of how
reactants become products
Represented as the disappearance of
reactants or the appearance of a product
over time
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Structure of reactants
Concentration of reactants
Temperature of reactants
Physical state of reactants
Presence of a catalyst
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A catalyst is a substance that increases the
reaction rate
Takes an alternative path with lower activation
energy
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An equation of the form:
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Has a rate expression:
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A + B -> products
Rate = k[A]n[B]n
In a single step reaction, the coefficient in
the balanced equation and the exponent n
are numerically the same
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Write the rate expression for the singlestep reaction:
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2NO2(g) -> 2NO(g) + O2(g)
Write the general form of the rate equation
for:
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CH4(g) +2O2(g) -> CO2(g) + 2H2O(g)
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Many important chemical reactions do not
go to completion
In these equilibrium reactions, no further
change is taking place, yet both reactants
and products remain.
A physical equilibrium like sugar dissolving
in water is a reversible reaction; notated
with a double arrow ↔
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A situation in which the rate of the forward
process in a reversible reaction is exactly
balanced with the rate of the reverse
process
In a saturated solution, dissolved sugar
particles and solid sugar particles are
constantly switching places
sugar(s) ↔ sugar (aq)
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In N2(g) + 3H2(g) ↔ 2NH3(g)
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Some of the molecules collide with sufficent
energy to break N-N and H-H bonds
Rearrangement produces NH
At equilibrium, the rate of depletion of
hydrogen and nitrogen is equal to the rate of
depletion of ammonia
The rates of forward and reverse reactions
are equal
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Summarizes the relationship between the
concentration of reactants and products in an
equilibrium reaction.
When the system reaches equilibrium, the Keq
does not change unless temperature changes
aA + bB ↔ cC + dD
Keq = [C]c[D]d
[A]a[B]b
In N2(g) + 3H2(g) ↔ 2NH3(g)
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Keq = [NH3]2
[N2][H2]3
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Coefficients in balanced equation become
the exponents in the expression
Equilibrium constants are reported at 25oC
[ ] mean molarity with units of mol/L
Concentrations of solids and liquids are
not shown because they cannot change
Write the equilibrium-constant expression
for 2HI(g) ↔ H2(g) + I2(g)
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Keq = greater than 100: at equilibrium
mostly product
Keq = less than .01: at equilibrium mostly
reactant
Keq = between 100 and .01: significant
concentrations of both
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If stress is added to a system at
equilibrium, the system will respond by
altering the equilibrium composition to
minimize the stress
If product is introduced, equilibrium will
shift left
If reactant is introduced, equilibrium will
shift right
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Treat heat as a product or reactant
In an exothermic reaction, adding heat is
similar to adding product
In an endothermic reaction, adding heat is
similar to adding reactant
Only gases are affected significantly by
changes in pressure
Increase in pressure = shift to the right
Decrease in pressure = shift to the left