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As I have read that every reaction(endothermic or exothermic)requires activation energy.

So is it correct to say that even exothermic reactions absorb heat?

I am asking this because it is always taught that exothermic reactions release heat .

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Every exothermic reaction releases net heat - by definition.

Every reaction absorbs energy, e.g. thermal energy from heat, for its activation, but the amount of released energy is for exothermic reactions bigger than absorbed energy.

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  • $\begingroup$ Note that in the gas phase also barrierless reactions do occur (typically involving ions and neutrals). Although these reactions still have to overcome the centrifugal barrier, the $s$-wave collision is truly barrierless. $\endgroup$
    – Paul
    Jan 20 at 13:42
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    $\begingroup$ @Paul I guess any short enough and simple enough answer has its exceptions. $\endgroup$
    – Poutnik
    Jan 20 at 14:01
  • $\begingroup$ You are right of course, it was just a small addition to an excellent answer ;) $\endgroup$
    – Paul
    Jan 20 at 15:11
  • $\begingroup$ @Poutnik. Are you sure that all reactions absorb energy for its activation ? Do $\ce{AgCl}$ or $\ce{BaSO4}$ precipitations have activation energies ? Do neutralization reactions (like $\ce{H^+ + OH- -> H2O}$) have activation energies ? $\endgroup$
    – Maurice
    Jan 20 at 20:06
  • $\begingroup$ @Maurice See my last note. The answer was intended for the introduction level. For your specific cases, I imagine some energy is needed to break ion hydration layers of ordinary ions, or for charge hopping for H+ and OH-. $\endgroup$
    – Poutnik
    Jan 20 at 20:12
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Exothermic/endothermic (or exergonic/endergonic, if you equally account for changes in entropy) only compares initial state prior, and final state after the reaction.

So the height to overcome that this reaction happens (the activation energy) is irrelevant. It equally is irrelevant if this energy was invested by heat (e.g., Bunsen burner/fire, as in roasting metals), light (laser, UV; as in photocylizations), addition/removal of electrons (electrochemistry, redox-reactions), etc.

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[OP] As I have read that every reaction(endothermic or exothermic)requires activation energy.

Yes, and this activation energy is for a single set of reactants reacting to form product.

[OP] So is it correct to say that even exothermic reactions absorb heat?

No. Heat is a bulk transfer of thermal energy from one object to the next.

If you search for activation energy on the internet, you might find strange examples like lighting a candle with a match. This does not really explain activation energy. Activation energy is necessary for every single reaction, not just to "start the reaction". In the example of a candle, the combustion reaction still needs an activation energy after the candle is lit. The temperature of the system is high enough, though, that it is available.

What happens in general is that the reaction mixture has sufficient energy already (no need to provide it from the outside). You just have to wait long enough that molecules with particularly high kinetic energy collide. That means that typically the vast majority of collisions does not lead to a reaction.

In general, the picture of a chain reaction that just needs to be started is incorrect as well. Two reactants colliding don't "know" whether products have been formed elsewhere. The only thing they "know" is whether there is sufficient energy available to reconfigure the nuclei and electrons from reactants to products, i.e. cross the activation barrier.

[OP] I am asking this because it is always taught that exothermic reactions release heat.

That is also not quite correct. Exothermic reactions release heat or do work (or some combination). If you make sure that they don't do work (i.e. no electrochemical reaction, no mechanical work), then it is correct to predict a release of heat (or warming of the reaction mixture, which is a heat transfer to the solvent if there is a solvent). If you have a gas phase reaction in an insulated vessel, there is no transfer of heat - the thermal energy of the reaction mixture will simply increase.

So if you want to know whether a reaction is exothermic or not and measure the reaction enthalpy, you have to make sure there is no (non-PV) work, that the reaction mixtures returns to the same temperature, and that you "catch" all the heat the reaction gives off.

Examples of descriptions of activation energy that don't hit the mark

This energy is called activation energy. For example, activation energy is needed to start a car engine. Turning the key causes a spark that activates the burning of gasoline in the engine. The combustion of gas won't occur without the spark of energy to begin the reaction.

The reaction requires a certain temperature of the reaction mixture, and it requires reactants to be in the gas phase. The mechanism of gasoline combustion is complex and probably involves radical formation and radical chain reactions. The chain reaction steps, while probably faster the the radical formation steps, also have an activation barrier. That means that even when the engine is running, the molecules that react still need to get their activation energy, and there might be collisions that do not result in a reaction (even one as explosive as the combustion of gasoline in the cylinder of a engine).

Reactions require an input of energy to initiate the reaction; this is called the activation energy (EA). Activation energy is the amount of energy required to reach the transition state. The source of the activation energy needed to push reactions forward is typically heat energy from the surroundings.

"Heat energy" is not a technical term. It is either thermal energy or heat transfer. The source of activation energy is the thermal energy of reactants coming together. The activation energy is not needed to push reactions forward (the direction of the reaction is not governed but the height of the activation barrier).

In chemistry and physics, activation energy is the minimum amount of energy needed to start a chemical reaction. Reactants often get activation energy from heat, but sometimes energy comes from light or energy released by other chemical reactions. For spontaneous reactions, the ambient temperature supplies enough energy to achieve the activation energy.

Again, there is a misconception about the start of the reaction. Every single molecule that reacts needs activation energy, not only the first couple.

The activation energy of a chemical reaction is kind of like that “hump” you have to get over to get yourself out of bed. Even energy-releasing (exergonic) reactions require some amount of energy input to get going, before they can proceed with their energy-releasing steps. This initial energy input, which is later paid back as the reaction proceeds, is called the activation energy and is abbreviated $E_A$.

This is almost not incorrect. When the reaction has passed the transition state, it is almost done. So this is different than getting our of bed compared to the rest of the day. Also, getting out of bed usually is downhill, sitting up in bed is what requires the center of mass to be lifted.

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    $\begingroup$ Interestingly, IUPAC has disconnected their definitions of endothermic and exothermic from the direction of heat flow: chemistry.stackexchange.com/questions/122102/… $\endgroup$
    – theorist
    Jan 21 at 7:35
  • $\begingroup$ @Karsten Theis "What happens in general is that the reaction mixture has sufficient energy already (no need to provide it from the outside). You just have to wait long enough that molecules with particularly high kinetic energy collide. That means that typically the vast majority of collisions does not lead to a reaction" this paragraph of your answer says that exothermic reactions already have enough energy but it takes time to get activated . So I concluded that exothermic reactions do not absorb energy . Am I correct? $\endgroup$
    – Xyz
    Jan 21 at 10:39

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