Chemical Forums
Chemistry Forums for Students => High School Chemistry Forum => Topic started by: Nedgy on February 13, 2020, 04:11:57 PM
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I keep seeing definitions of chemical energy, like: "chemical energy is the energy stored within the chemical bonds of a molecule" and that makes sense but recently I learnt that in a chemical reaction it takes energy to break the reactants bonds and energy is released in forming new product bonds. My line of thinking was that if a reaction is exothermic then some of the chemical energy of the molecule must've been converted into thermal energy leaving us with products that contain less chemical energy. But if stronger bonds take more energy to break/release more energy when formed, doesn't that mean in an exothermic reaction our products have more chemical energy because stronger chemical bonds were formed?
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Energy released during the bond creation is that of the bond itself - so the stronger the bond, the more energy is released. To break these bonds you need to deliver exactly the same amount of energy - so again, the stronger the bond, the more energy must be delivered.
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I hear you, but what I'm thinking is, if you have an exothermic reaction, then that must mean the products have stronger bonds right? Because more energy had to be released in forming those bonds, if those bonds are stronger and chemical energy is the energy stored within the chemical bonds of molecules, then doesn't that mean the products of an exothermic reaction would have more chemical energy than, that of the reactants? Which I know is not true, but I can't get my head around why it doesn't work out.
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if you have an exothermic reaction, then that must mean the products have stronger bonds right?
Correct (in general; you have to think about other things like intermolecular interactions too, but on the whole, yes).
and chemical energy is the energy stored within the chemical bonds of molecules
No it isn't. Energy is not stored in bonds. Bonding is a low-energy situation, the stronger the bond, the lower the energy. Bond has less energy than no-bond.
doesn't that mean the products of an exothermic reaction would have more chemical energy than, that of the reactants?
No, for the above reason.
Consider for example the reaction
AB + C :rarrow: A + B + C :rarrow: A + BC
The 3 separated atoms A + B + C represent the maximum in chemical potential energy. When a bond is formed (A-B or B-C), potential energy is lost (ultimately being converted to thermal kinetic energy of the molecules). If the B-C bond is stronger than the A-B bond, the reaction AB + C :rarrow: A + BC is exothermic; the products have lower chemical energy than the reactants.
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Paraphrasing:
Making bonds releases energy, that's why they exist. As compared with lone atoms, molecules contain less energy. If the energy reference were lone atoms, all molecules would have a negative energy.
Lone atoms are very rare where we live, so reactions rearrange atoms from molecules to other molecules. Because lone atoms would be inconvenient as an energy reference, the "heat of formation" of molecules is always referred to other molecules, like O2 or solid C for instance.
In a reaction, produced heat comes from the products, like CO2 or H2O for instance. The reactants take energy to break.
So while chemical energy does relate with chemical bonds, the strength of the bonds in the reactants reduces the heat produced by a reaction. Reactions that provide much heat, like combustions, make molecules with strong bonds as products, like CO2 or H2O, from molecules with weaker bonds as reactants, like O2 or hydrocarbons.