4 replies to this topic

[krykr0]

penicillin G potassium is mixed with water in a tank. I think the following reaction occurs:

 

C₁₆H₁₇KN₂O₄S(s) + H₂O(l) -------> C₁₆H₁₈N₂O₄S(aq) + KOH(aq)

 

how can I determine if this is exothermic or endothermic. Heat of formation/solution data isn't available for penicillin compounds.

Any ideas?

 

many thanks

[PingPong]

In general one can calculate heat of reaction (reaction enthalpy) from the bond energies.

 

Determine which existing bonds are broken (which costs energy) and which new bonds are formed (yielding energy).

[krykr0]

yes, thank you for this. I understand that I can use bond energies for calculating the energy involved for breaking/forming covalent bonds (H2O). can this also apply for ionic bonding? The potassium (+) breaks from oxygen (-)  and new ionic bonds are formed (KOH)

[PingPong]

I had not really looked at the equation before.

 

Note that there is no KOH fromed, it is simply K⁺ ions and OH^- ions swimming in the water without any interaction between the two.

 

If I understand the reaction correctly, it is just a potassium salt of penicillin that is being dissolved in water?

 

And you assume that the H⁺ from the water takes all places of the K⁺ in the penicillin salt?

 

I think it is more complicated, because probably the new compound is a weak acid with a K_a, so part of the penicillin is present as R⁺ and part as R-H so the reaction is really:

 

R-K + y H2O --> y R-H + (1-y) R^- + K⁺ + y OH^-

 

while y depends on the K_a of the R-H

 

At least, that is what I think.

Edited by PingPong, 15 July 2014 - 09:32 AM.

[Ajay S. Satpute]

How about dipping the thermometer in the water and noting the temperature change?

 

Regards.

 

Ajay S. Satpute