This is how I understand the link between bonds and enthalpy.
Bonds contain energy. So to break bonds requires an energy input, and to make bonds, releases energy. ATP is energy currency and when it breaks down to form ADP, bonds are broken but why is energy released rather than needing to be inputted?
However, in a thermal decomposition reaction, a substance, such as calcium carbonate requires energy to break down, it decomposes only when heated, because energy is needed to break the bonds. This is why things don't spontaneously decompose to their simplest parts.
From another point of view, it's because atoms 'want' to have a full outer shell. Surely that would mean compounds with bonds contain less energy, because nature favours things with lower energy, but then bonds contain energy, so this doesn't quite fit. What have I missed here? I know we're going to do this in more detail in Year 13 when we do the Kreb's cycle, but is it the making or breaking of ATP that releases energy?
Is the enthalpy of something with bonds greater or smaller than the enthalpy of something without bonds?
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Well done for making the link between what we're doing in Chemistry at the moment with your AS Biology, Vicky! As ever, you're clearly thinking out loud and trying to apply your understanding to much more complex situations, which is fantastic!
It's been a while since I looked at ATP and ADP so will need to do some reading up on it myself, but in terms of deciding whether a reaction is exothermic or endothermic, it is worth remembering that we do not look just at the bonds being broken or just at the bonds being made. If the enthalpy needed to break the bonds in the reactants is LOWER THAN the enthalpy given out to form the new bonds in the products, the reaction will be EXOTHERMIC - and vice versa. So, it's the difference between these enthalpy values, rather than considering them individually, that informs us whether the reaction is exothermic or endothermic.
Which bonds are broken/made in the formation of ADP from ATP?
What are the bond enthalpy values for each of these bonds?
Can you calculate a value for the enthalpy change for this reaction?
I hope this helps! I'm not sure that I've got to the core of what you're trying to ask or even if you want an answer, as I think you're just trying to work things out for yourself in your head but thank you for making me think on a Sunday evening! I always enjoy reading what you have to say!
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