Transcript Entropy - AP Bio

Entropy
Section 16-2
Enthalpy
• What is enthalpy?
• What does a +ΔH mean in terms of the energy of
reactants and energy of products?
• What does a +ΔH mean in terms of energy production or
consumption?
• What is an exothermic reaction in terms of ΔH?
Spontaneous Rxn?
• Spontaneous – proceeds without inputting
energy
• How is this similar to Exothermic (-ΔH)?
• How is this different than Exothermic (-ΔH)?
Enthalpy & Reaction Tendency
• Most reactions in nature are exothermic
• Tendency in nature for a reaction to proceed
in a direction that has less energy
• Lower energy = More stability
– Energy is inversely proportional to Stability
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• Melting is one example of a naturally occurring
endothermic process.
• An ice cube melts spontaneously at room temperature as energy is
transferred from the warm air to the ice.
• The well-ordered arrangement of water molecules
in the ice crystal is lost, and the less-ordered liquid
phase of higher energy content is formed.
• A system that can go from one state to another
without an enthalpy change does so with an
increase in entropy.
Question
•The decomposition of ammonium nitrate:
• 2NH4NO3(s)  2N2(g) + 4H2O(l) + O2(g)
Which side of the reaction has less order (simpler)?
***The arrangement of particles on the right-hand side of the
equation is more random than the arrangement on the left
side and hence is less ordered.
Entropy
• There is a tendency in nature to proceed in a direction
that increases the randomness of a system.
• A random system is one that lacks a regular arrangement of
its parts.
• This tendency toward randomness is called entropy.
• Entropy is a measure of chaos or disorder
• Entropy, S, can be defined in a simple qualitative way as
a measure of the degree of randomness of
the particles, such as molecules, in a system.
Entropy Changes for some rxns
States of Matter
• To understand the concept of entropy, consider
the comparison between particles in solids,
liquids, and gases.
• In a solid, the particles are in fixed positions, and we
can easily determine the locations of the particles.
• In a liquid, the particles are very close together, but
they can move around. Locating an individual particle
is more difficult. The system is more random, and the
entropy is higher.
Higher entropy?
• So which has higher entropy, solids or liquids?
• In a gas, the particles are moving rapidly and are far
apart. Locating an individual particle is much more
difficult, and the system is much more random. The
entropy is even higher.
• Rank the states of matter in terms of entropy.
Entropy Change
• Absolute entropy, or standard molar entropy, of
substances are recorded in tables and reported in
units of kJ/(mol•K).
• Entropy change, which can also be measured, is
defined as the difference between the entropy of
the products and the reactants.
• An increase in entropy is represented by a positive
value for ∆S, and a decrease in entropy is
represented by a negative value for ∆S.
ΔG or Free Energy
• Processes in nature are driven in two directions:
toward least enthalpy and toward largest entropy.
• As a way to predict which factor will dominate for
a given system, a function has been defined to
relate the enthalpy and entropy factors at a given
temperature and pressure.
• This combined enthalpy-entropy function is called
the free energy, G, of the system; it is also called
Gibbs free energy.
Spontaneous
• Spontaneity is determined by the sign of ΔG
• If ΔG is (-), then spontaneous rxn
• Will proceed w/o energy input
• If ΔG is (+), then NONspontaneous rxn
• Will only proceed with energy input
Free Energy Equation
• Only the change in free energy can be measured.
It can be defined in terms of enthalpy and
entropy.
• At a constant pressure and temperature, the freeenergy change, ∆G, of a system is defined as the
difference between the change in enthalpy, ∆H,
and the product of the Kelvin temperature and the
entropy change, which is defined as T∆S:
• ∆G0 = ∆H0 – T∆S0
∆G0 = ∆H0 – T∆S0
• Use various combinations of signs of ΔH & ΔS,
to determine:
– Which combination guarantees a spont. Rxn?
– Which combination guarantees a NONspont. Rxn?
– Which combination depends on temperature to
determine spontaneity?
Sample Problem
•For the reaction NH4Cl(s)  NH3(g) + HCl(g),
at 298.15 K, ∆H0 = 176 kJ/mol and
∆S0 = 0.285 kJ/(mol•K). Calculate ∆G0, and tell
whether this reaction is spontaneous in the
forward direction at 298.15 K.
Sample Problem Solution
• Given: ∆H0 = 176 kJ/mol at 298.15 K
∆S0 = 0.285 kJ/(mol•K) at 298.15 K
• Unknown: ∆G0 at 298.15 K
• Solution: The value of ∆G0 can be calculated
according to the following equation:
• ∆G0 = ∆H0 – T∆S0
• ∆G0 = 176 kJ/mol – 298 K [0.285 kJ/(mol•K)]
• ∆G0 = 176 kJ/mol – 84.9 kJ/mol
• ∆G0 = 91 kJ/mol
Which of the following two conditions
will favor a spontaneous reaction?
A. an increase in entropy and a decrease in enthalpy
B. an increase in entropy and an increase in enthalpy
C. a decrease in entropy and a decrease in enthalpy
D. a decrease in entropy and an increase in enthalpy
Which of the following two conditions
will favor a spontaneous reaction?
A. an increase in entropy and a decrease in enthalpy
B. an increase in entropy and an increase in enthalpy
C. a decrease in entropy and a decrease in enthalpy
D. a decrease in entropy and an increase in enthalpy
Which of the following processes has a
negative ∆S?
A. evaporating 1 mol of a liquid
B. raising the temperature of 1 L of water from
295 K to 350 K
C. freezing of 1 mol of a liquid
D. None of the above.
Which of the following processes has a
negative ∆S?
A. evaporating 1 mol of a liquid
B. raising the temperature of 1 L of water from
295 K to 350 K
C. freezing of 1 mol of a liquid
D. None of the above.
• At a constant pressure, the following reaction is
exothermic: 2NO2(g)  N2O4(g). Which of the
following statements is true about the reaction (as
written)?
A. The reaction is always spontaneous.
B. The reaction is spontaneous at low temperatures, but not at
high temperatures.
C. The reaction is spontaneous at high temperatures, but not at
low temperatures.
D. The reaction is never spontaneous.
• At a constant pressure, the following reaction is
exothermic: 2NO2(g)  N2O4(g). Which of the
following statements is true about the reaction (as
written)?
A. The reaction is always spontaneous.
B. The reaction is spontaneous at low temperatures, but not at
high temperatures.
C. The reaction is spontaneous at high temperatures, but not at
low temperatures.
D. The reaction is never spontaneous.
• For certain molecules, enthalpies of formation can be
determined from combustion data. Using the diagram
below, calculate the enthalpy of formation of methane
gas, CH4(g), and the enthalpies for two of the combustion
reactions shown on the diagram below.
C(s) + O2(g)  CO2(g)
∆H = –393.5 kJ
H2(g) + O2(g)  H2O(l)
∆H = ?
CH4(g) + 2O2(g)  CO2(g) + 2H2O(l) ∆H = ?
Answer
•
The formation reaction for CH4(g) is C(s) + 2H2(g)  CH4(g).
•
From the graph, ∆H for H2(g) + O2(g)  H2O(l) is
[965.1 kJ  (393.5 kJ)]/2 = 285.8 kJ.
•
The ∆H for CH4(g) + 2O2(g)  CO2(g) + 2H2O(l) can be read
directly from the graph and is 890.2 kJ.
•
The enthalpy of formation for CH4(g) is 965.1 kJ + 890.2 kJ =
74.9 kJ.