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AP®︎/College Chemistry
Course: AP®︎/College Chemistry > Unit 9
Lesson 4: Thermodynamics vs. kineticsThermodynamics vs. kinetics
AP.Chem:
ENE‑4 (EU)
, ENE‑4.D (LO)
, ENE‑4.D.1 (EK)
, ENE‑4.D.2 (EK)
Thermodynamics tells us what can occur during a process, while kinetics tell us what actually occurs. Some processes, such as the conversion of diamond to graphite, are thermodynamically favored but kinetically unfavored. In these cases, the processes do not occur to any measurable extent. Created by Jay.
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I still don't quite understand what moles of reaction are or when to use them. can someone explain it to me, please?(1 vote)
The mole of reaction, or mol rxn, refers to the coefficients of a balanced chemical equations. You usually see this with thermodynamics like with enthalpy and Gibbs free energy.
For example, the reaction 2H2(g) + O2(g)⇌2H2O(g) the ΔG°rxn is -457.1 kJ/molrxn. Which means that every time 2 moles of hydrogen gas react with 1 mole of oxygen gas to produce 2 moles of water vapor, 457.1 kJ of energy is lost from the system (the reaction).
Hope that helps.(3 votes)
Video transcript
- [Tutor] In chemistry, it's important to distinguish between thermodynamics and kinetics. For example, if we think about the conversion of carbon as a solid in the diamond form to carbon as a solid in the graphite form, thermodynamics tells us what will happen. Will this conversion happen
at a specific temperature? Whereas kinetics tells us when
the conversion could happen. Let's start with thermodynamics. We can calculate if this
conversion will happen at room temperature of 25 degrees Celsius by calculating the standard
change in free energy for this reaction at that temperature. To calculate the standard change in free energy for this reaction we need to know the standard change in free energy of
formation of the products. And from that we subtract
the standard change in free energy of
formation of the reactants. For this conversion our product is carbon
in the graphite form. And because carbon in the graphite form is the most stable
elemental form of carbon at a pressure of one atmosphere, the standard change in
free energy of formation of carbon in the graphite
form is equal to zero. Next, we think about our reactant, which is carbon in the form of diamond. The standard change in free
energy of formation of carbon in the diamond form is
2.84 kilojoules per mole. Looking at our balanced
equation for this conversion, we're converting one mole of diamond into one mole of graphite. Since there's a one as a
coefficient in front of diamond, we multiply the standard
change in free energy of formation of diamond
by one mole of diamond. Moles cancel, and we get
negative 2.84 kilojoules as the standard change in
free energy for this reaction. Instead of kilojoules, the units could've been written as kilojoules per mole of reaction. Because there's one mole of
carbon in the diamond form for how the balanced equation is written, we could write a conversion
factor of one mole of carbon in the diamond
form per mole of reaction. And because the standard
change in free energy of formation of diamond is equal to 2.84 kilojoules
per mole of diamond, moles of diamond would cancel out and give kilojoules per mole
of reaction as the units. Because the standard change in free energy for this
reaction is negative. We know that the conversion
of diamond into graphite at 25 degrees Celsius is
thermodynamically favorable. Next, let's think about the
kinetics of this reaction. If we look at the structure of diamond, each carbon atom is covalently
bonded to four other carbons. For example, this carbon right here is bonded
to four other carbon atoms and the tetrahedral geometry
around the central carbon atom. Graphite has a very different structure. In graphite, carbon atoms form covalently
bonded layers or sheets. And these layers or sheets are held together by
London dispersion forces. So in order to convert
diamond into graphite we would have to break a
lot of carbon-carbon bonds. And that would take a lot of energy. Since it takes a lot of energy to break the carbon-carbon
bonds in diamond, if we look at the energy
profile for this reaction, this reaction would have a
very high activation energy, symbolized by EA. And the higher the activation
energy for the reaction the slower the rate of the reaction. At room temperature, it's estimated this reaction
could take billions of years. Therefore we would say this reaction is kinetically unfavorable. Finally, let's summarize
what we've learned about the conversion of
diamond into graphite. Thermodynamics answers the
question, will the reaction go? And the answer to that question is yes. At room temperature, diamond will convert into graphite because delta G naught for
the reaction was negative. Kinetics answers the question,
how long will it take? And the answer to that question is it would take billions
of years for diamond to turn into graphite at room temperature. The reason why the reaction is so slow is because of the extremely
high activation energy. So even though the reaction
is thermodynamically favorable at room temperature, because
the reaction is so slow, the reaction is kinetically unfavored. And for all practical purposes,
the reaction doesn't happen.