Hella confused with exo and edothermic reactions

[prsndwg]

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I thought bond breaking is endothermic and bond making is exothermic. that going from liquid to gas is endothermic and liquid to solid to be exothermic.

Does gas gets cooler when you add energy to it? or is it just the system surrounding it?

is this exo or endo? its opposite of combustion and combustion is exo, so is this endo?

6CO2 + 6H2O ----> C6H12O6 + 6O2

why is combustion exo while bonds are being broken??

help me out here please

 

[UndergradGuy7]

You need to add energy to break a bond to overcome the activation energy (endothermic). When bonds break energy is released (exothermic). So you had that right. To go from liquid to gas you would have to add energy to chase the phase change and evaporate the liquid, so endothermic.

Not sure what you mean by this: Does gas gets cooler when you add energy to it? or is it just the system surrounding it?

If you add energy, temp would increase or dissipate to the surroundings if possible.

Also check this diagram if might help you http://www.gcsescience.com/Energy-Diagram-Exothermic.gif
This diagram is exothermic. The products are lower in energy than the reactants. If we went the reverse, from products to reactants then it would be endothermic.

 

[wantVCUdental]

I thought bond breaking is endothermic and bond making is exothermic. that going from liquid to gas is endothermic and liquid to solid to be exothermic.
This is all correct. It's something they teach you in high school, but you really got to memorize this and apply to situation where appropriate. In the combustion examples and natural phenomena, bonds are broken and formed simulataneously, to determine the overall heat of reaction, you need Hess's law and the free energy equation G=H-TS.


Does gas gets cooler when you add energy to it? or is it just the system surrounding it?

is this exo or endo? its opposite of combustion and combustion is exo, so is this endo?

6CO2 + 6H2O ----> C6H12O6 + 6O2

why is combustion exo while bonds are being broken??

help me out here please

Combustions are exothermic, why? does fire feel hot or cold? fire gives off heat, anything that gives off warmth is exothermic.
And going in reverse of combustion must be endothermic 🙂 (if you draw energy diagram that will be apparent too)
Hope that helps

 

[flybry2000]

i remember from chad's videos that when a gas expands inside of a system it cools down. This question could be answered by looking "q" (heat change) in thermodynamics of a system. And when the gas expands and performs work on a system you have a loss in energy. 😱

 

[prsndwg]

can we determine the heat of the rxn w/o its delta H value? I know the entropy is possible, but what should we look for in regards to enthalpy?

 

[UCB05]

can we determine the heat of the rxn w/o its delta H value? I know the entropy is possible, but what should we look for in regards to enthalpy?

Combustion of carbohydrates is exothermic because the bonds of the products are more stable than the bonds of the reactants. If you look at the chemical breaking of bonds, yes that requires energy, yes it's endothermic. In the same reaction, bonds are formed, which releases energy and is exothermic. The important thing to realize is that the bonds formed are more stable than the bonds that were broken, which means the reaction releases more energy than was put into it, which makes the overall combustion reaction exothermic. Conversely for the reverse reaction, you would have to put in more energy breaking bonds than you get back forming the new bonds, so that reaction is endothermic.


you can't be absolutely sure, but it's usually the case that chemical changes (bond breaking/forming) trump physical changes (entropy) in terms of enthalpy change. Unless you already know the kinetics of the particular reaction or memorized the table of bond energies, it's not really something you can just look at and know whether it's endothermic or exothermic. A carbohydrate combustion is something you could assume to be exothermic. It involves the breaking of more reduced, higher energy C-O-H bonds to fully oxidized, stabler O=C=O and O=O bonds

 

[joonkimdds]

I thought bond breaking is endothermic and bond making is exothermic. that going from liquid to gas is endothermic and liquid to solid to be exothermic.

are you sure? I thought bond breaking is exothermic and bond forming is endothermic. Would you double check this statement for me?

 

[UCB05]

are you sure? I thought bond breaking is exothermic and bond forming is endothermic. Would you double check this statement for me?

The bond dissociation values are for homolytic cleavage reactions whereby the products have incomplete octets. these are higher in energy and less stable than the bonded starting molecule. This implies that bond dissociation requires energy input and is endothermic, whereas bond formation releases energy and is exothermic