9.4 Thermodynamic and Kinetic Control
4 min read•january 9, 2023
Jillian Holbrook
Jillian Holbrook
This section examines the relationship between and , specifically the shortcomings of in predicting reaction behavior. Despite being spontaneous, many reactions do not occur at an observable rate because they are under kinetic control. In this guide, we will explore what kinetic control means and why it occurs.
Brief Review of Kinetics
Before getting into the thermodynamics of kinetic control, we need to briefly review topics from Unit 5. If you feel confident with , feel free to move past this section.
Recall that is the study of the rate of a reaction—essentially, how fast a reaction occurs. We measure the rate of a reaction by measuring the change in the over time: R = Δ[A]/Δt or R = -d[A]/dt (for those familiar with calculus). Higher R values indicate a quicker loss of reactants and formation of products.
Rates can also be described using , where initial reactant concentration is directly tied to the rate of a reaction as follows:
R = k[A]^n[B]^m
where [A]^n, [B]^m, etc., are various reactants raised to their (how much of an impact their concentration has on the rate).
Take a look at the following rate law as an example:
rate = k[A]
These laws help us describe how quickly a reaction will occur, with a higher rate implying a faster reaction overall.
Most important to our study of kinetic control is understanding . Due to the , chemical reactions occur when molecules hit each other at the right angle and speed/energy. This is the energy required for a chemical reaction to actually occur. The higher the , the harder it is for the reaction to occur at an observable rate.
The following diagram visually shows the concept of :
Image From Mike Sugiyama Jones
A Shortcoming of Gibbs Free Energy
A common misconception when looking at is that a thermodynamically favorable reaction occurs quickly. Many reactions that are spontaneous occur incredibly slowly. A good example of this concept that can be applied to the real world is the conversion from diamonds into graphite (represented as Cdiamond(s) → Cgraphite(s)).
For this reaction, ΔG° = -3 kJ. This tells us that the reaction for graphite formation occurs spontaneously and does not require the input of any external energy to occur. However, take a look at the nearest diamond (because people have those around the house…right?). Is it suddenly morphing into graphite? No! (I hope not. Fiveable is not responsible for diamond graphitification…).
The conversion of diamond into graphite is incredibly slow, as in thousands of years slow. We say that this reaction is in kinetic control because it is driven by the slowness of the reaction. These types of reactions are often slow because they have a high .
A spontaneous process may take either the thermodynamically controlled or the kinetic controlled pathway. A kinetically controlled path like the one above is driven by a high . A is driven by the difference in free energy between the products and reactants, the type of reaction we saw in the last section.
Reasons For Kinetic Control
As we mentioned before, the primary reason for a reaction to be under kinetic control is because of a high . Because of this, even if the reaction is thermodynamically favorable, it may not continue at a measurable rate. There is a way around this, however, and that is through the use of a ! Catalysts change the mechanism behind a reaction in order to decrease the and make reactions quicker. By reducing the , the reaction can proceed at a measurable rate.
For example, the usually occurs at an unmeasurable rate. However, when iodide ions are used as a , it creates the famous "Elephant’s Toothpaste" reaction, which you may be familiar with.
This example shows us how catalysts can transform a reaction from one at an immeasurable rate to one at a measurable rate. The concept of catalysis can be applied to kinetic control by noticing that catalysts can help get a reaction out of kinetic control by lowering the . For example, if there was a for the reaction: Cdiamond(s) → Cgraphite(s), the reaction would be able to proceed at a measurable rate. However, without one, it cannot because the rate of the reaction is incredibly slow.
Key Terms to Review (15)
Activation Energy
: Activation Energy is defined as the minimum amount of energy required to initiate or start up a chemical reaction.Catalyst
: A catalyst is a substance that speeds up a chemical reaction but isn't consumed in the process.Concentration of Reactants
: The concentration of reactants refers to the amount of reactant substances present in a chemical reaction at any given time. It is usually measured in moles per liter (M).Decomposition of Hydrogen Peroxide
: The decomposition of hydrogen peroxide refers to its breakdown into water and oxygen gas when exposed to light, heat, or the presence of a catalyst.Elephant’s Toothpaste Reaction
: This is a popular science experiment that demonstrates an exothermic reaction, specifically the rapid decomposition of hydrogen peroxide when mixed with dish soap and a catalyst.Gibbs Free Energy
: Gibbs Free Energy (G) is a thermodynamic potential that measures the maximum reversible work that a system can perform at constant temperature and pressure.Kinetic Molecular Theory
: The Kinetic Molecular Theory is a model that explains the behavior of gases. It states that gas particles are in constant, random motion and that they collide with each other and the walls of their container without losing energy.Kinetically Controlled Reaction
: This is a type of chemical reaction where rate rather than product stability determines outcome; these reactions stop at intermediate stages if those stages are slow to react further.Kinetics
: Kinetics is branch of chemistry or biochemistry concerned with measuring and studying rates of reactions.Rate Laws
: Rate laws are mathematical expressions that describe how the rate of a chemical reaction depends on the concentration of each reactant.Reaction Order
: Reaction order refers to how much changing the concentration of one reactant affects the overall rate at which products are formed.Reaction Rate
: The reaction rate is a measure of how fast a chemical reaction occurs.Spontaneous Reaction
: A spontaneous reaction is one that can occur without any external input once it has started. These reactions are thermodynamically favorable and often release energy in some form.Thermodynamic Favorability
: Thermodynamic favorability refers to the likelihood of a reaction occurring based on its change in Gibbs free energy. If the change is negative, the reaction is thermodynamically favorable and will occur spontaneously.Thermodynamically Controlled Reaction
: This is a type of chemical reaction where the products are determined by which ones have the lowest energy and are therefore most stable.9.4 Thermodynamic and Kinetic Control
4 min read•january 9, 2023
Jillian Holbrook
Jillian Holbrook
This section examines the relationship between and , specifically the shortcomings of in predicting reaction behavior. Despite being spontaneous, many reactions do not occur at an observable rate because they are under kinetic control. In this guide, we will explore what kinetic control means and why it occurs.
Brief Review of Kinetics
Before getting into the thermodynamics of kinetic control, we need to briefly review topics from Unit 5. If you feel confident with , feel free to move past this section.
Recall that is the study of the rate of a reaction—essentially, how fast a reaction occurs. We measure the rate of a reaction by measuring the change in the over time: R = Δ[A]/Δt or R = -d[A]/dt (for those familiar with calculus). Higher R values indicate a quicker loss of reactants and formation of products.
Rates can also be described using , where initial reactant concentration is directly tied to the rate of a reaction as follows:
R = k[A]^n[B]^m
where [A]^n, [B]^m, etc., are various reactants raised to their (how much of an impact their concentration has on the rate).
Take a look at the following rate law as an example:
rate = k[A]
These laws help us describe how quickly a reaction will occur, with a higher rate implying a faster reaction overall.
Most important to our study of kinetic control is understanding . Due to the , chemical reactions occur when molecules hit each other at the right angle and speed/energy. This is the energy required for a chemical reaction to actually occur. The higher the , the harder it is for the reaction to occur at an observable rate.
The following diagram visually shows the concept of :
Image From Mike Sugiyama Jones
A Shortcoming of Gibbs Free Energy
A common misconception when looking at is that a thermodynamically favorable reaction occurs quickly. Many reactions that are spontaneous occur incredibly slowly. A good example of this concept that can be applied to the real world is the conversion from diamonds into graphite (represented as Cdiamond(s) → Cgraphite(s)).
For this reaction, ΔG° = -3 kJ. This tells us that the reaction for graphite formation occurs spontaneously and does not require the input of any external energy to occur. However, take a look at the nearest diamond (because people have those around the house…right?). Is it suddenly morphing into graphite? No! (I hope not. Fiveable is not responsible for diamond graphitification…).
The conversion of diamond into graphite is incredibly slow, as in thousands of years slow. We say that this reaction is in kinetic control because it is driven by the slowness of the reaction. These types of reactions are often slow because they have a high .
A spontaneous process may take either the thermodynamically controlled or the kinetic controlled pathway. A kinetically controlled path like the one above is driven by a high . A is driven by the difference in free energy between the products and reactants, the type of reaction we saw in the last section.
Reasons For Kinetic Control
As we mentioned before, the primary reason for a reaction to be under kinetic control is because of a high . Because of this, even if the reaction is thermodynamically favorable, it may not continue at a measurable rate. There is a way around this, however, and that is through the use of a ! Catalysts change the mechanism behind a reaction in order to decrease the and make reactions quicker. By reducing the , the reaction can proceed at a measurable rate.
For example, the usually occurs at an unmeasurable rate. However, when iodide ions are used as a , it creates the famous "Elephant’s Toothpaste" reaction, which you may be familiar with.
This example shows us how catalysts can transform a reaction from one at an immeasurable rate to one at a measurable rate. The concept of catalysis can be applied to kinetic control by noticing that catalysts can help get a reaction out of kinetic control by lowering the . For example, if there was a for the reaction: Cdiamond(s) → Cgraphite(s), the reaction would be able to proceed at a measurable rate. However, without one, it cannot because the rate of the reaction is incredibly slow.
Key Terms to Review (15)
Activation Energy
: Activation Energy is defined as the minimum amount of energy required to initiate or start up a chemical reaction.Catalyst
: A catalyst is a substance that speeds up a chemical reaction but isn't consumed in the process.Concentration of Reactants
: The concentration of reactants refers to the amount of reactant substances present in a chemical reaction at any given time. It is usually measured in moles per liter (M).Decomposition of Hydrogen Peroxide
: The decomposition of hydrogen peroxide refers to its breakdown into water and oxygen gas when exposed to light, heat, or the presence of a catalyst.Elephant’s Toothpaste Reaction
: This is a popular science experiment that demonstrates an exothermic reaction, specifically the rapid decomposition of hydrogen peroxide when mixed with dish soap and a catalyst.Gibbs Free Energy
: Gibbs Free Energy (G) is a thermodynamic potential that measures the maximum reversible work that a system can perform at constant temperature and pressure.Kinetic Molecular Theory
: The Kinetic Molecular Theory is a model that explains the behavior of gases. It states that gas particles are in constant, random motion and that they collide with each other and the walls of their container without losing energy.Kinetically Controlled Reaction
: This is a type of chemical reaction where rate rather than product stability determines outcome; these reactions stop at intermediate stages if those stages are slow to react further.Kinetics
: Kinetics is branch of chemistry or biochemistry concerned with measuring and studying rates of reactions.Rate Laws
: Rate laws are mathematical expressions that describe how the rate of a chemical reaction depends on the concentration of each reactant.Reaction Order
: Reaction order refers to how much changing the concentration of one reactant affects the overall rate at which products are formed.Reaction Rate
: The reaction rate is a measure of how fast a chemical reaction occurs.Spontaneous Reaction
: A spontaneous reaction is one that can occur without any external input once it has started. These reactions are thermodynamically favorable and often release energy in some form.Thermodynamic Favorability
: Thermodynamic favorability refers to the likelihood of a reaction occurring based on its change in Gibbs free energy. If the change is negative, the reaction is thermodynamically favorable and will occur spontaneously.Thermodynamically Controlled Reaction
: This is a type of chemical reaction where the products are determined by which ones have the lowest energy and are therefore most stable.
Resources
© 2024 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.