Spark
Loading...

Mechanisms and Activation Energy

← Builds on Integrated Rate Laws and k Units

Terms you will need

  • activation energyActivation energy is the minimum amount of energy that reactants need to have for a chemical reaction to happen. It's like a hill that molecules must climb over to turn into products.
  • reaction mechanismA reaction mechanism is the step-by-step process that shows how a chemical reaction happens. It breaks the overall reaction into smaller, simple steps that explain how reactants become products.
  • rate lawThe rate law is a mathematical expression that shows how the speed of a chemical reaction depends on the concentrations of the reactants. It helps predict how fast a reaction will happen.
  • rate constantThe rate constant is a number in the rate law that shows how fast a reaction happens under certain conditions. It depends on factors like temperature and the presence of a catalyst.
  • elementary stepAn elementary step is a single, simple step in a reaction mechanism where reactants directly form products in one collision. It's the smallest possible part of a chemical reaction.
  • intermediateAn intermediate is a short-lived chemical species that forms during a reaction but is not present in the final products. It is made in one step of the mechanism and used up in another.

How this topic runs

  1. 1Start here
  2. 2How it works
  3. 3At competition depth

Chemical reactions depend on two key factors: whether they can occur and how quickly they happen. The study of reaction mechanisms and activation energy helps explain these processes. A reaction mechanism is the detailed, step-by-step sequence of events that describes how reactants become products. Each step involves specific molecular interactions, and the overall mechanism must account for all intermediates, transition states, and energy changes. Activation energy is the minimum energy required for a reaction to proceed. It represents the energy barrier that reactant molecules must overcome to form products. By analyzing mechanisms and activation energies, scientists can optimize conditions to speed up desired reactions or slow down harmful ones.

Figures (10)

Henry Taube's experiment establishing the role of a bridging ligand in inner sphere electron transfer.
Benzoin condensation reaction mechanism. Cyanide ion (CN−) acts as a catalyst here, entering at the first step and leaving in the last step. Proton (H+) transfers occur at (i) and (ii). The arrow pushing method is used in some of the steps to show where electron pairs go.
Example of an enzyme-catalysed exothermic reaction
SN2 reaction mechanism. Note the negatively charged transition state in brackets in which the central carbon atom in question shows five bonds, an unstable condition .
The Michael addition reaction mechanism with ammonia's lone pair as a Michael donor, using curved arrows (electron or arrow pushing method)
The sparks created by striking steel against a piece of flint provide the activation energy to initiate combustion in this Bunsen burner. The blue flame sustains itself after the sparks stop because the continued combustion of the flame is now energetically favorable.
Figure 12.13 Illustrated are two collisions that might take place between carbon monoxide and oxygen molecules. The orientation of the colliding molecules partially determines whether a reaction between the two molecules will occur.
Figure 12.14 Reaction diagram for the exothermic reaction A+B⟶C+D.A+B⟶C+D.
Figure 12.15 Molecular energy distributions showing numbers of molecules with energies exceeding (a) two different activation energies at a given temperature, and (b) a given activation energy at two different temperatures.
Figure 12.16 This graph shows the linear relationship between ln k and 1T1T for the reaction 2HI⟶H2+I22HI⟶H2+I2 according to the Arrhenius equation.

Chemical reactions happen when molecules collide with the right energy and direction. For a reaction to occur, molecules must not only bump into each other but also have enough energy to break old bonds and form new ones. The energy they need to start the reaction is called activation energy.

Activation energy is the minimum energy needed for a reaction to start. It acts as a barrier that reactants must overcome. If the molecules don’t have enough energy, they bounce apart without reacting.

Higher temperatures give molecules more energy, so more collisions can overcome the barrier. This is why reactions often go faster when it’s hotter. Reaction mechanisms explain the exact steps a reaction takes.

They break the process into smaller, simple steps. Each step has its own energy changes and may involve short-lived molecules called intermediates. The slowest step in the mechanism usually controls the overall speed of the reaction.

By studying these steps, scientists can understand how to change reaction conditions to make reactions faster or slower.

Key Points

  • Activation energy is the minimum amount of energy that must be available to reactants for a chemical reaction to occur.
  • A reaction mechanism is the step by step sequence of elementary reactions by which an overall chemical reaction occurs.
  • The rate law for a reaction is an expression that relates the rate of the reaction to the concentrations of the reactants.
  • The rate constant is a proportionality constant in the rate law that reflects the intrinsic reactivity of the reactants under specific conditions.
  • An elementary step is a single step in a reaction mechanism that describes a specific molecular event occurring during the reaction.
  • An intermediate is a chemical species that is formed and consumed during the course of a reaction mechanism but does not appear in the overall balanced equation.

Terms

Tap a term for a plain-language explanation.

Sources & licensing(4)