Activation Energy

What everyday analogy is used to introduce activation energy?

Needing extra energy to get out of bed in the morning even when an exciting day lies ahead, representing the “hump” that must be overcome before proceeding smoothly.

What is activation energy?

The initial energy input required to start a chemical reaction, later paid back as the reaction proceeds; abbreviated
𝐸
𝐴
E
A
​

.

Why do even energy-releasing (exergonic) reactions require activation energy?

Because reactant molecules must have their bonds broken and be contorted into an unstable, high-energy transition state before new product bonds can form.

What must happen to bonds in reactants for a reaction to occur?

Some or all of the chemical bonds must be broken so that new product bonds can form.

What is the transition state?

An unstable, high-energy, contorted (deformed or bent) state that reactant molecules must reach for bonds to break and reactions to proceed.

Why is the transition state considered unstable?

Because molecules do not remain there long and quickly proceed to the next step of the reaction once that state is reached.

Why is activation energy always positive?

Because the transition state is always at a higher energy level than the reactants or products, so energy must always be added to reach it, regardless of whether the reaction overall is exergonic or endergonic.

In an exergonic forward reaction, what does the activation energy arrow represent?

The amount of free energy that must be added to go from the energy level of the reactants to the energy level of the transition state.

How would the activation energy differ for the reverse reaction of the same system?

It would be larger, because the products (being lower in energy) would need more energy added to reach the same transition state.

What does the reaction coordinate diagram show about exergonic reactions?

Even though products are at a lower energy level than reactants, there is still a “hump” reflecting formation of the high-energy transition state.

What is the typical source of activation energy?

Heat absorbed from the surroundings, which provides thermal energy to the reactant molecules.

How does thermal energy help molecules reach the transition state?

It increases the frequency and force of their collisions and jostles atoms and bonds within molecules, making bond-breaking more likely.

What happens once a molecule absorbs enough energy to reach the transition state?

It can proceed through the remainder of the reaction.

How is activation energy related to reaction rate?

The higher the activation energy, the slower the reaction, because fewer molecules have enough energy to reach the transition state at any moment.

Why do many reactions effectively not proceed at all without an energy input?

Their activation energies are so high that virtually no molecules can reach the transition state at ambient temperature.

What real-world example illustrates a reaction with extremely high activation energy?

The combustion of a fuel such as propane, which has an effectively zero reaction rate at room temperature despite being energy-releasing.

Why is it good that propane has such high activation energy at room temperature?

It prevents propane canisters from spontaneously combusting on a shelf.

What initiates combustion in propane?

A spark that provides enough energy for some molecules to get over the activation energy barrier.

How does combustion become self-sustaining once started?

The molecules that react release energy, and that released energy helps more fuel molecules reach the activation energy barrier, producing a chain reaction.

Why can’t cells use sparks to overcome activation energy?

A spark would damage the cell, so it is not a viable way to provide activation energy for biological reactions.

How do cells overcome high activation energies safely?

They lower the activation energy through catalysis.

What is catalysis?

The process of speeding up a reaction by reducing its activation energy.

What is a catalyst?

A factor added to a reaction that lowers the activation energy.

What are biological catalysts called?

Enzymes.

What question did a student ask about exothermic vs. exergonic reactions?

They asked whether energy-releasing reactions are called exothermic and reactions taking in energy are endothermic, and whether this matches the terms exergonic and endergonic used in the article.

What explanation was given about the difference?

Exothermic and endothermic refer specifically to heat; exergonic and endergonic refer to energy in general.

What question was asked about products returning to reactants?

A student asked whether a product could go back to being a reactant after going through the activation energy hump.

What was the response about reversing reactions?

Theoretically yes, but practically no, because it requires additional energy to go from product back to reactant.

What example was used to explain reversing reactions?

Combustion of wood: energy from a match pushes carbon over the activation energy to form CO₂, and reversing this requires energy (like photosynthesis).

What point was emphasized regarding reversing reactions?

It is possible but generally requires additional energy, so practically it does not happen spontaneously.

What question did a student ask about endothermic/exothermic vs. endergonic/exergonic?

They asked if there is a difference between the terms and noted they learned endothermic means storing energy and exothermic means releasing energy.

What explanation was provided?

Endothermic/exothermic describe enthalpy changes (heat), while endergonic/exergonic describe Gibbs free energy changes.

What question was asked about using science in daily life?

A student asked, “When will I ever use science in day-to-day life?”

What responses were given about everyday uses of science?

Understanding nutrition labels, recognizing molecular ingredients, and understanding active ingredients in medicines.

What misunderstanding about activation energy was asked by a student?

They asked: if a molecule has more activation energy, shouldn’t it complete the reaction faster because it is easier to reach the barrier?

What clarification was given?

Activation energy is the amount of energy needed to activate the transition state; it is not energy that a molecule already possesses.

What question was asked about the abbreviation of activation energy?

A student asked why activation energy is abbreviated as
𝐸
𝐴
E
A
​

instead of
𝐴
𝑒
A
e
​

.

What explanation was given for using
𝐸
𝐴
E
A
​

?

Energy is expressed with E, and subscripts specify type; reversing it would cause confusion with many other symbols and constants.

What question was asked about whether energy released after activation energy can be harnessed industrially?

A student wondered whether the energy released from bond formation could be used industrially once activation energy is supplied.

What explanation was provided about industrial use?

Exothermic reactions may release heat that can reduce heating costs, but typically the amount of energy released is not ideal, and mixtures must be heated or cooled to maintain optimal temperatures for reaction yield.

What simple definition of activation energy was given in the discussion?

“Activation energy is the amount of energy needed to initiate a reaction.”

What question was asked about ∆G?

A student asked what ∆G means, why it becomes positive or negative, and what endergonic and exergonic mean.

What explanation was provided about ∆G?

∆G is Gibbs free energy, indicating whether a reaction can occur spontaneously (negative ∆G, exergonic) or requires input energy (positive ∆G, endergonic).

How did the discussion explain the reaction diagram in relation to ∆G?

The reactants need energy to get over the hill (positive ∆G locally), but overall the reaction releases energy and ends with lower energy than it began with, giving a negative ∆G.

What question was asked about spontaneous reactions?

A student asked: “What is a spontaneous reaction?”

What definition was given?

A spontaneous reaction is one that occurs between molecules with little or no catalyst.