We take it for granted that energy is neede to make something happen, in a nutshell. But, let’s look at one common event in science, dissolving. Does it take energy for a solvent to dissolve a solute? Think of NaCl. Ionic bonds are broken. But, where does the energy come from to break the molecule into ions? It is taught that the high polarity of the H20 molecule causes the disassociation of NaCl to readily occur, but surely it takes energy, right? And, if so, does this energy have a name and/or a law, like Hess’ Law or Gibbs Free Energy, which goes with it? Or, is it so incidental, no one bothered to question this? (Also, is energy needed/released when the NaCl reforms, if I evaporate the water?)
Last, is it only the polarity of a molecule that makes something a solvent? What about isopropyl alcohol? That’s a good solvent. Is it a highly polar molecule, too, or what else is at play here? - Jinx
there are two types of “energy” changes in chemical reactions, enthalpy (heat) and entropy (chaos, randomness). When many salts dissolves, it becomes more random (good), but also a little heat is released (good). Wins on both counts. The heat energy comes because more bonds (ion-dipole to be precise) are formed than broken.
Some salts dissolve and the mixture cools down. This is because enthalpy is bad (less bond energy formed). However whats drives the reaction on is the entropy, e.g. the salt spreads out and is more randomly arranged.
Actually, what drives any reaction is that the entropy of the universe must increase. The change in the entropy of the universe can be measured by (i) change in entropy in the reaction and (ii) the random heat released to the universe.
If you find entropy confusing, think of what happens in a bedroom. By itself the reaction is towards chaos and untidyness, you must work hard to keep it tidy.
Whether a solvent dissolves anything depend largely on its polarity. Non-polar solvents (isopropyl alcohol, kerosine etc) will dissolve only non polar things (grease, paint etc), whereas polar solvents such as water will only dissolve polar things (sugars, salts). Like prefers like.
Yep. Just yesterday, I used some acetone to clean out a vacuum distillation appartus that had various ketones going through it, then some hexanes to get the vacuum grease (something silicon-based) off. The acetone could handle the ketones but not the grease and the hexanes could handle the grease but wouldn’t have done anything to the ketones.
Just think of oil and water. Water is very polar. Oil is very non-polar. Polarity is a function of the relative electronegatives of the elements involved.
How, exactly, does one evaporate water without adding energy?
Aside from what scm1001 already mentioned…
Every chemical compound is at least a little stable, and some are more stable than others. For a reaction to happen, you need to overcome what stability the molecule already has. Take H-2 and O-2. They can exist together as seperate gasses, but add a tiny spark (add energy) and they react to a more stable form, H-2-O, releasing energy.
With some substances, it is only necessary to put them in close proximity and they will react on their own. In this case, you add the energy it takes to physically move the compounds together.
To answer your question- Does Everything Take Energy?
The answer is that everything requires energy. Some reactions require energy to be put in (if you like) and some reactions require energy to be taken away (if you like)
This all boils down ( ) to the First Law of Thermodynamics. Roughly speaking, energy can be neither created nor destroyed, merely changed from one form to another.
It’s a big field, so you might have to do a bit of Googling.
Vacuum grease is very annoying to clean off, especially off desiccators. I had an instructor who was obsessed with greasing everything (and always, always asked which type of grease, silicon-based or carbon-based, would be appropriate for each task). This is particularly interesting once you have the fear of frozen joints blasted into you and try asking someone who doesn’t believe in grease if they have any.
The question in the OP relates to lattice energy or energy of crystallization. This is the difference in energy between the crystalline solid and the ions in gas phase:
The lattice energy is a measure of how much the atoms in the crystal are stabilized by being in a crystal, rather than being free ions. You can read more about lattice energy here.
The degree to which these ions are stabilized in various solvents is also a factor. Basically, the difference between the amount of enthalpy in the crystal and in the dissolved ions determines the result of dissolving the crystal. When the crystal is formed from a solution, there is a decrease in its enthalpy (i.e. the crystal is more stable than the ions in solution), and heat energy is absorbed from the environment (an endothermic reaction).
When the crystal is dissolved, it loses the stability that it had because it was in crystalline form. This releases energy into the environment (an exothermic reaction). The energy comes from ‘bonds’ (electrostatic attractions, in this case) between the ions in the crystal that are broken. If the new bonds that are formed with the solvent molecules are weaker than the old bonds between the ions, the net reaction is exothermic and heat energy is released. When the ion-solvent bonds are stronger, the net reaction is endothermic and the environment becomes colder. This isn’t terribly common but it does happen, depending on the crystal and the solvent used. Generally, if the lattice energy is low (the crystal isn’t much more stable than the ions), there is a greater chance that dissolving the crystal might be endothermic.
BTW if you want a name for a law, look up ‘Born-Haber cycle’. Hess’ law also applies, and the Gibbs-Helmholtz Equation, ΔG = ΔH - TΔS (also a mathematical statement of the Second Law of Thermodynamics).
) to the First Law of Thermodynamics. Roughly speaking, energy can be neither created nor destroyed, merely changed from one form to another.