BOND BREAKAGE - energy released?

[matt123]

hey guys
i always thought that when bonds BREAK .. energy is RELEASED

but according to neap 2008

" Energy is released when bonds form and absorbed when bonds break"

correct? or no?

[jasoN-]

Think about an exothermic reaction.
You have reactants.
To form products you first must break the bonds in the reactants (the activation energy part)
When the bonds are reforming (to form the product), energy is released

[Potter]

Is the question asking about an exothermic reaction or endothermic? If it's the latter the first part of their solution doesn't make sense.

[will74]

Consider an energy profile diagram...

The enthalpy of reactants increases till it gets to the activation energy, then the bonds in reactant molecules break.

Thus it TAKES energy to break bonds.

Then, When new bonds are formed on the profile, the enthalpy decreases as energy is lost to the surrounding environment.

There Energy is released when new bonds form. And used/absorbed when they are broken.

Hope this helps

[year12]

Yep! That's what Kilbaha 09 exams says too - "When chemical bonds are broken, energy is absorbed. When chemical bonds form, energy is released". Think about the energy profile diagrams - whether a reaction is endo or exo, there's always activation energy barrier. For a reaction to occur - ie. 'fruitful collisions' - the particles must overcome this. And really, if you want to see a 'reaction', the bonds of your reactant particles must be broken (so that you can get new bonds!). Hence, the absorption of energy when they do so (they need energy provided to them to do this). Conversely, when bonds form, they no longer need the extra energy which they release. In the case of exothermic reactions, the energy absorbed in breaking the bonds is less than the energy released when the new bonds form.

Hope I haven't confused you more.  

Have a look at Section 1 MC Q. 6 on Kilbaha exam as well!

[matt123]

thanks people
got it in the bag now
was just little confused.

[Mao]

Energy and 'stability' go hand in hand. (In fact, force is always in the direction to reduce energy, so by definition lower energy = greater stability)

Two atoms (not bonded) have low stability, high energy, as they do not have full outer valence shells.

Two atoms (bonded) have high stability, low energy, as they have full outer shells.

Thus bonded --> non-bonded will naturally require an energy input, breaking bonds is almost always endothermic. Conversely, forming bonds is almost always exothermic.

[kyzoo]

I think of it like this.

Ice --> Liquid Water is breaking bonds
You need to heat ice to break lattice bonds, thus you need to provide energy to break bonds (molecules need to absorb energy)

Thus breaking bonds is endothermic.