Phase Changes Enthalpy

[Nigah Fatima]

After watching Wednesday's lecture, I was confused about how the enthalpies of the phase changes work. In the lecture, it was stated that for melting, vaporization, and sublimation, there is the energy required for these phase changes because it is breaking a bond (endothermic). So I was confused on why does breaking a bond makes it endothermic and forming a bond makes it exothermic? Like why is there energy required to break a bond (endothermic), but not extra energy required when forming a bond (exothermic)? In addition, if it can be explained with the 6 different phase changes and why the breaking bond phase changes are endothermic and the forming bond phase changes are exothermic?

[Hannah Kim 3C]

Breaking a bond requires more energy than the amount of energy in the bond. Forming a bond also requires energy, but the net energy after the molecule is formed is still positive which translates to the release of energy. I’m sure someone else will know the answer in regards to the connection of enthalpy and phase changes.

[Kevin Zhang 3K]

breaking a bond is putting energy into the substance and heating it up and forming a bond is to take away energy from the bond and cool it down

[Trisha Badjatia 2L]

Hi! I believe the root of your question is why breaking bonds takes energy and why forming bonds gives off energy. Forming bonds releases energy because bonds make each atom more stable, often because they allow atoms to complete an octet of valence electrons. Thus, breaking the bond would require energy. Realistically, if breaking bonds released energy, a lot of the bonds we see wouldn't exist and there would just be a bunch of singular atoms. Hope that helps!

[Adithi Ayyala 2G]

Hello! As others have said, when we form bonds, we tend to make atoms more stable, which tends to release energy from a given species. Therefore, when we go through these phase changes, the breaking of bonds creates heat, whereas the formation of bonds releases heat.

[Kimia Rategh 2A]

Hi! Just adding onto what others have said, breaking a bond is endothermic because you require energy to break the bond. As we have learned bonds hold things together, often with great strength and therefore to break one you would have to put in energy. An endothermic reaction is when the reactants have more energy than the products and therefore because you have to put energy to break the bonds, you would start out with more energy than you end up with making it endothermic. Forming a bond is exothermic because forming a bond results in a more stable formation of that molecule, and as a result, energy is released. Because energy is released, that makes it so that the products have more energy or heat than the products and therefore this makes it an exothermic reaction.

Hope this helps!

[amara ajon 1d]

Hi,
Breaking bonds is difficult to do and typically requires more energy than is released when forming a bond — leading to a net endothermic reaction.

[205678283]

Breaking a bond takes energy, so delta H would be a positive value. Energy must be put in, in order to separate two atoms/molecules/elements that were stable bonded together. In contrast, forming a bond releases energy, so delta H would be negative, because energy is lost. Think about the reaction H+(aq) + OH-(aq)--> H2O(l). The formation of this bond gives off energy/heat, so it is an exothermic reaction.

[Matthew Vu 3C]

When you break a bond, you need to invest energy into doing so. Putting energy in means it's endothermic. When you form a bond, it releases energy, which would be exothermic.

[Sabira Mohammed 3I]

Hi, atoms or molecules generally form bonds because it makes them have lower energy/be more stable, which means they release energy during formation. On the other hand, breaking the bond makes them less stable and require a higher energy to stay in that broken form, so you need energy to break the bond because the molecules/atoms want to stay in that bonded state