expansion/compression of a gas

[capn jazz]

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Can someone explain why compression of a gas is exothermic while expansion of a gas is endothermic?

 

[G1SG2]

Can someone explain why compression of a gas is exothermic while expansion of a gas is endothermic?

I believe when you compress a gas, it gains internal energy which makes it hot. When a gas expands, it loses internal energy (which is also used to gain potential energy as the gas moves up-think about a gas expanding in a cylinder as it moves the piston up).

I also like to think of it as the compression of a gas makes the gas molecules closer together and makes them form bonds-bond formation is exothermic. Expansion breaks bonds as the gas molecules move farther away, and so its endothermic.

I'm pretty sure that's correct-if not, then someone else can correct me.

 

[capn jazz]

Wait... I thought forming bonds was endothermic and breaking bonds is exothermic...

Okay, so I'm wrong - it's the opposite. But if it's the opposite, why does breaking a phosphate linkage in ATP release energy? Am I confusing heat with energy?

 

[G1SG2]

Wait... I thought forming bonds was endothermic and breaking bonds is exothermic...

Okay, so I'm wrong - it's the opposite. But if it's the opposite, why does breaking a phosphate linkage in ATP release energy? Am I confusing heat with energy?

No no, bond formation ALWAYS releases energy, and bond breaking ALWAYS requires energy-the Bond Dissociation Energy.

ATP is a special case. You need energy to break the bonds of ATP, as you need energy to break any bond. The thing with ATP is that when bonds reform in the transition state between ADP and inorganic phosphate Pi, it releases so much energy (as bond formation is exothermic) that it exceeds the energy that you required to initially break the bond.

ATP-->ADP + Pi

Think of it this way. Suppose you needed 20 Kcal (endothermic, need energy) to hydrolze 1 molecule of ATP (this isn't the exact number-but let's just say it is, hypothetically). So you broke the bonds. Now, when ADP and Pi rearrange themselves in the transition state to finally come out as ADP and Pi, they release -27 KCal (exothermic, releasing energy).

Net energy: 20 K cal (for breaking) + (-27 k Cal by making)=-7kcal (and 1 molecule of ATP actually does yield around -7kcal).

So, for ATP, the bond formation step releases so much energy that the overall energy makes it negative. Bond breaking NEVER releases energy-it is the formation of bonds that releases energy.

 

[Charles_Carmichael]

Just a side note, when a reaction breaks ATP into ADP and Pi, it doesn't just mean the phosphate group is hydrolyzed and that this provides energy.

In actuality the phosphoryl group is transferred from ATP to the molecule that's undergoing the reaction (the phosphoryl group becomes covalently bonded to that molecule); this increases the free-energy of that molecule. Then, in the next step, this phosphoryl group is displaced and generates Pi. When this phosphoryl group is added to the molecule, the molecule has more free energy to give up during the rest of the reaction/pathway.