Kinetics vs Thermodynamics

Kinetics vs Thermodynamics
Table of Contents
1. Introduction
2. Kinetics Overview
3. Thermodynamics Overview
4. Thermodynamics vs. Kinetics
5. Outside links
6. References
7. Problems
8. Answers
9. Contributors
Kinetics and thermodynamics are related to each other in
ways that can be explained by using chemical reactions. A
discussion of kinetics and thermodynamics requires an
explanation of the underlying relationships between the
two, through application to chemical reactions and several
examples from natural processes.
Introduction
It is important to mention that a chemical reaction has
kinetic and thermodynamic aspects. The quantity related
to kinetics is the rate constant k; this constant is
associated with the activation energy required for the
reaction to proceed, that is, the reactivity of the reactants.
The thermodynamic quantity is the energy difference
resulting from the free energy (ΔG) given off during a
chemical reaction—the stability of the products relative to
the reactants. Although kinetics describes the rates of
reactions and how fast equilibrium is reached, it gives no
information about conditions once the reaction equilibrates.
In the same measure, thermodynamics only gives
information regarding the equilibrium conditions of products
after the reaction takes place, but does not explain the rate
of reaction.
Kinetics Overview
The rate constant, k, measures how fast a chemical
reaction reaches equilibrium assuming the reactants were
supplied with enough activation energy to enable the
reaction to proceed in the forward direction—reactants to
products. This requirement for input of energy symbolizes
the fact that the reactants are unreactive under certain
conditions The reaction must have some sort of energy
input before it can proceed; otherwise, the reactants
cannot cross the activation energy threshold and convert
to products. The reaction is activated by energy supplied
to the reactants by different energy sources. The rate of
reaction , the rate constant, and the kinetic energy required
for activation of reaction indicate how fast the reaction
reaches equilibrium. See Diagram #1.
Diagram #1: Depicted in the graph below are the main
points discussed in the previous paragraph. The transition
state represents a threshold the reactants must pass
before the reaction can proceed in the forward direction.
The activation energy is the energy required to reach the
transition state. Once this threshold is reached, the
reaction proceed in the favorable "downhill" direction. It is
important to remember that each reaction has a different
transition state threshold, with different activation
energies, and determined by the reactants and the
conditions in which the reaction is taking place. The value
of k is affected by these two factors, and can be increased
in the presence of a catalyst (such as an enzyme), which
increases reaction rate. In chemical reactions, specifically,
the catalyst can both provide more energy to the reactants
and lower the transition state energy. The provider of
activation energy can also be a spark, heat, or anything
else that gives off energy. Regardless of what provides the
activation energy, a kinetic or nonspontaneous reaction is
one in which the most stable state is that of the
reactants. The change in energy between the reactants
and products, also known as ΔG, relates to
thermodynamics and will be discussed shortly.
Diagram #1 link: http://www4.nau.edu/meteorite/
Meteorite/Images/EnergyDiagram.jpg
Example 1: Fuel
The gas in a fuel tank is not "wasted" or burnt away while
the car is sitting in the parking lot. Fuel is unreactive
under standard conditions; the spark created while turning
on the engine is what provides the activation energy to the
reactants, beginning the process of fuel-burning that
powers the car. For more information about the way fuel-
burning reactions are driven, visit 'outside link' number 1.
For a video that shows why two elements do not
spontaneously combust (as fuel would, had it not needed
activation energy), go to 'outside link' number 5.
Thermodynamics Overview
Thermodynamics can be considered in terms of the energy
stored within a reaction, a reactant, or a product. Most
often, thermodynamics is thought of as the different forms
of energy that are converted every time a reaction emits
energy or is initiated by energy. With respect to Gibbs free
energy (ΔG), thermodynamics refers to either (1) the
energy released during a reaction, in which case ΔG will be
negative and the reaction exergonic or spontaneous, or (2)
the energy consumed during a reaction, in which case ΔG
will be positive and the reaction endergonic or
nonspontaneous. A thermodynamic reaction favors the
products, resulting in a spontaneous reaction that occurs
without the need to constantly supply energy. This
indicates that the reactions' most stable state is that of
the products.
Thus, going back to Diagram #1, thermodynamics is what
describes the free energy between the reactants and the
products. Because thermodynamic values apply only after
the reactants have turned into products, they are said to
describe the equilibrium state. The relationship between
free energy (aka, Gibbs free energy) and other
thermodynamic quantities is expressed mathematically in
the following equation:
Because "U" is the variable representing the internal
energy of a system, it is closely correlated with the free
energy. Changes in internal energy change the value of the
free energy, in turn affecting chemical reactions in several
ways: the rate of reaction, whether the reaction is
spontaneous or non-spontaneous, and even whether or not
activation energy will be needed to initiate the reaction.
Example 2: Systems
The best way to understand thermodynamics is by
realizing that anything that transfers, receives, or contains
heat can be described as a system. Heat can enter or
leave a system, which affects the amount of thermal
energy it contains. Consider a kettle of water sitting on a
stove. As it is heated, thermal energy is added to the
system (the kettle with the water). As the stove is turned
off, the kettle cools down as the heat diffuses back to the
room; the kettle slowly equilibrates to room temperature.
This is an example of the system losing thermal energy.
To view an animated diagram of a thermodynamic system,
click on 'Outside Link' number 2.
Thermodynamics vs. Kinetics
As mentioned above, the most stable states of a kinetic
reaction are those of the reactants, in which an input of
energy is required to move the reaction from a state of
stability, to that of reacting and converting itself to
products. Kinetics is related to reactivity. In contrast, the
most stable state of a thermodynamically favorable
reaction is the products, because the reaction occurs
spontaneously, without the need for energy to be added.
Thermodynamics is related to stability .
Therefore, something that is unreactive will desire to stay
in the form of reactants, which will require an input of
energy to cause the reaction to go forward, converting
reactants into products. This is illustrated in example #3
below. A reactive species does not require an input of
energy to be converted from reactants to products,
because its most stable and preferred state is that of the
products. Instead, a thermodynamically favorable reaction
requires energy to be converted from products back to
reactants.An energy source moves the reaction forward
(kinetics corresponds to movement). The same is for
thermodynamically favorable reactions, except that the
reaction must be stimulated backward from products to
reactants.
Example 3: ATP
Adenosine triphosphate, also known as ATP, provides the
energy cells require in order to maintain metabolic
pathways, DNA synthesis and repair, and any other cellular
function necessary for survival. ATP itself is a reactive
molecule that has three phosphate groups. Molecules tend
toward stable states, converting to states of lower
energies. Thus, ATP, a high-energy molecule, tends to lose
a phosphate group and become adenosine diphosphate,
ADP. In order for this to happen, an enzyme strips one
phosphate group off of ATP, converting it to the more
stable molecule ADP. This enzyme provides the energy of
activation that enables ATP to become ADP, indicating
that ATP is kinetically stable.
Example 4: Water and Sugar
The following example involves solvents and polarity:
consider a simple situation, a spoonful of sugar is added to
a cup of water. If the two are left to react, over time the
sugar dissolves in the water, becoming the product of
sugar+water. The natural charges and polarity of water
causes the sugar molecules to react with it, eventually
dissolving within the water. There is no required input of
energy, indicating that this reaction is thermodynamically
favorable, and therefore spontaneous. Clearly, the two
reactants prefer to react and maintain stability as
products.
Note: although this is a thermodynamically favorable or
spontaneous reaction and does not require energy input,
the use of kinetic energy will force this reaction to happen
faster. If sugar is added to the cup of water and the
system is heated, the kinetic energy of the reactants is
increased by the thermal energy of the heat, which causes
the molecules to react with one another at a much faster
rate than if they been left alone at room temperature. This
is an example of how thermodynamics and kinetics are
closely related.
Outside links
1. Fuel Reactions and Kinetics: http://
www.explainthatstuff.com/fuelcells.html
2. Thermodynamic System: http://
upload.wikimedia.org/wikipedia/commons/8/8a/
Triple_expansion_engine_animation.gif
3. Nonspontaneous Reaction (compare with Diagram #
1): http://www.biology.arizona.edu/biochemistry/
problem_sets/energy_enzymes_catalysis/
graphics/16t.gif
4. Thermodynamics: http://en.wikipedia.org/wiki/
Thermodynamics#Thermodynamic_systems
5. Demonstration of two kinetically-stable elements in
a mixture, after given enough energy of activation:
http://www.metacafe.com/watch/908325/solid_
rocket_fuel_ignition/
References
1. Journal of Coordination Chemistry, Volume 62, Issue
1 Oct. 2009, pages 108-109
2. Thermodynamic stability and crystal structure of
lanthanide complexes with di-2-pyridyl ketone. S.
Dom iacute nguez a; J. Torres b; J. Gonz aacute lez-
Platas c; M. Hummert d; H. Schumann - e; C.
Kremer b
3. Role of Solvation Barriers in Protein Kinetic Stability.
RODRIGUEZ-LARREA David ; MINNING Stefan ;
BORCHERT Torben V. ; SANCHEZ-RUIZ Jose M.
Problems
1. Is it possible that graphite is thermodynamically
stable and diamond is less reactive under standard
conditions?
2. Explain how kinetics relate to thermodynamics. Use
the terms 'energy of motion', 'energy of heat', and an
example from the module in your answer.
3. Why would it be beneficial for a thermodynamically-
stable reaction to use an energy input in the form of
an enzyme or a catalyst even if it does not require
energy to proceed?
4. How come gas does not spontaneously combust
inside a fuel tank?
5. How is the rate constant k related to equilibrium?
How does the rate constant change if heat is added
to the reaction?
6. If the difference in energy between the reactants and
products is negative, is the reaction spontaneous or
nonspontaneous?
Answers
1. Yes. Their different structures will differentiate their
polarity and charge, and will cause the two
compounds to act differently. Thus, one can be
thermodynamically stable, while the other can be
less reactive.
2. The energy of motion is related to kinetics, which
determines how fast the reaction will reach
equilibrium, related to thermodynamics. The energy
of motion (kinetics) added to a reaction causes the
reaction to happen faster, using energy of heat as a
way by which to accelerate the reaction. An example
of this is the cup of water with the sugar while it is
being heated. The heat energy converts into kinetic
energy (energy of motion), accelerating the reaction
between the water molecules and the sugar crystals.
3. A catalyst or enzyme will still be beneficial in a
thermodynamically-favorable reaction because it will
simply accelerate it.
4. Fuel is unreactive to standard conditions and regular
atmosphere, which means it'll require an energy input
in order to react. The energy input is the spark
caused by the ignition of the car.
5. The rate constant k is related to equilibrium in that
it tells us about how fast the reaction reaches
equilibrium. If heat is added to a reaction, its rate
will increase due to increased kinetic energy.
6. The reaction will be spontaneous, thermodynamically
favorable. This is because the energy is given-off,
not consumed by the reaction.