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Endothermic/Exothermic Rxns and Delta H

Posted: Thu Jan 16, 2020 1:00 am
by Jasmine 2C
For question like Module 4 Q17:
Determine the shift in equilibrium, if any, which will occur for the following reactions when the temperature is increased
a) Photosynthesis:
6 CO2 (g) + 6 H2O (l) ⇌ C6H12O6(s) + 6 O2 (g) delta H° = +2802 kJ.mol-1
b) The hydrolysis of ATP:
ATP (aq) + H2O (l) ⇌ ADP + PO42-(aq) delta H° = -30 kJ.mol-1
From the delta H value, photosynthesis is an endothermic reaction. However, in the module video, Professor Lavelle says to form a bond between two molecules, heat is released, which means exothermic reaction. So isn't photosynthesis an exothermic reaction since CO2 and H20 are forming C6H12O6?
And the same for ATP hydrolysis. ATP is broken down into ADP, and from the video, breaking a bond requires energy so it's an endothermic reaction. But the delta H says otherwise.
I'm confused.

Re: Endothermic/Exothermic Rxns and Delta H

Posted: Thu Jan 16, 2020 3:09 am
by peteryim
As for the first part of the question, photosynthesis is not an exothermic reaction in the forward direction, because exothermic implies that the reaction is spontaneous, and the products have a lower free energy than the reactants. However, CO2 and H2O have less free energy than a large molecule like C6H12O6. Thus, energy must be inputed into the system in order for the reaction to take place, in order to combine the carbons and hydrogens into large carbohydrates. This energy comes from sunlight. Nonetheless, the reaction is endothermic.

Re: Endothermic/Exothermic Rxns and Delta H

Posted: Thu Jan 16, 2020 9:59 am
by Sean Cheah 1E
The net chemical equation for photosynthesis is deceptively simple and excludes many of the complex intermediate steps that must take place when converting the reactants to products. For this class, those details are not important. Just know that when you sum all the energy expended to break bonds and subtract all the energy released when forming bonds, the net total is a positive value, meaning that more energy had to be expended than released. This makes logical sense when you consider the fact that plants need energy from the sun to perform photosynthesis.

In the case of ATP hydrolysis, the amount of energy required to break bonds is relatively small in comparison to the energy released by the formation of new bonds, resulting in a negative delta H. The resulting free orthophosphate group is further stabilized by resonance, increasing that energy difference.

Re: Endothermic/Exothermic Rxns and Delta H

Posted: Thu Jan 16, 2020 10:23 am
by Madelyn Romberg 1H
The equation is extremely simplified and ignores all of the moments in which, yes, forming bonds did release energy. However, when thinking about it with common sense, it is an endothermic reaction. First, light (energy) is required for photosynthesis to happen. Secondly, our bodies use glucose as an energy source, releasing large sums of energy when glucose bonds are broken down. This results in molecules with less energy than glucose. Hence, cellular respiration--the opposite of photosynthesis-- is exothermic, following that photosynthesis is endothermic.