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Q & A: Predicting Exothermic/Endothermic Reactions

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Most recent answer: 10/22/2017
Q:
How do you tell whether a chemical reaction will be exothermic or endothermic?
- Brian (age 34)
Miami
A:

For our audience, let's define those terms.

An endothermic reaction soaks up heat. An exothermic reaction releases heat. Endothermicity and exothermicity depend on whether products or reactants have more energy (for reactions at constant volume) or more of something called enthalpy (for reactions at fixed pressure). The energies and enthalpies of many common substances can be looked up in tables, although often you have to make some corrections if the reactions are taking place at different temperatures or pressures than the ones used for the tables.


So if the sum of the enthalpies of the reactants is greater than the products, the reaction will be exothermic. If the products side has a larger enthalpy, the reaction is endothermic.


You may wonder why endothermic reactions, which soak up energy or enthalpy from the environment, even happen. Most spontaneous events (like water flowing downhill) release energy to the environment, heating it up. However, the principle governing which way reactions (and other events) go is that the total amount of something called entropy goes up. Entropy is a measure of how many different microscopic states things might be in. One way to increase the entropy of the environment is to release heat into it, as in exothermic reactions. However, sometimes substances can increase their own entropy a lot by soaking up heat, and then endothermic reactions can occur, even though the environment loses entropy.

A quantity called the Gibbs’ Free Energy is used to keep track of both the entropy change of the substances and the heat released to the environment (and thus the environment’s entropy change). For reactions at constant pressure, the Gibbs free energy goes down. Gibbs’ free energies are also tabulated for many substances under standard conditions.

Jason and Mike W.


(published on 10/22/2007)

Follow-Up #1: chemical equations and heat

Q:
FROM A CHEMICAL EQUATION HOW CAN WE KNOW WHETHER THE REACTION IS ENDOTHERMIC OR EXOTHERMIC?
- WALIFA (age 15)
PAKISTAN
A:
You can't in general tell whether a reaction is exothermic or endothermic just by the form of the equation. You have to either measure the heating or cooling or know enough about similar types of reactions. For example, I can bet that H2 -> 2H is endothermic, because I know that the H2 has lower energy than the 2 H's. In more complicated cases, the answer isn't so obvious.

Mike W.

(published on 05/16/2013)

Follow-Up #2: bond enthalpies

Q:
I'm actually reporting your website...on this page, you say that if bond enthalpies of products are is greater than the reactants, then the rxn is endothermic. And vise versa for exothermic. I think this is wrong. Bond enthalpy/energy is the amount required to break the bond. So if your reactants have a total enthalpy of 500 and your products have a total enthalpy of 800...then 500 would be required to break all the bonds and 800 would be released when making all the bonds in the products...so 300 extra is released, therefore this is exothermic. Going from low to high is exo, going from high to low is endo - based on the definition of bond enthalpy. Also, for the 2nd question...if you draw the structures of the molecules in the reaction, look up the enthalpies of those that are broken and made, you can do the calculation to determine if its exo or endo. I know chemistry and physics think about energy differently...so that may play into your answer. But I'm guessing that most people asking about this are doing so from a chemistry perspective - esp. when discussing reactions.
- Julie (age 34)
Whitmore Lake, Michigan
A:

Aha, this sounds like an important semantic issue. On re-reading, our answers were entirely correct. We referred to the enthalpies of the reactants, while you use the phrase "bond enthalpy". The problem is that that phrase, which we didn't use, is defined as the amount that the enthalpy is reduced  by formation of the bond. So in calculating the enthalpy of the molecules, you have to subtract bond enthalpies from the enthalpies of the un-bonded parts. 

It's important that people realize this sign convention if they are to use tables of bond enthalpies to calculate the substance enthalpies which we discussed.

Mike W.


(published on 10/22/2017)

Follow-up on this answer.