Ploest
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So throughout this first semester of my AS Level course, I’ve gathered up a few (silly) doubts that Google didnt clear.

If any of those doubts are wrong, please do correct me. The list goes something like this:

1- Why is it
- A. Not possible to measure the enthalpy change of hydration/dehydration of crystals (can’t you just use/remove excess of water?”
- B. Not possible to measure the enthalpy change during a decomposition reaction
- C. Possible to measure the enthalpy change of formation of CO2

2- Why electron density maps are circularly shaped when orbitals are not.

3- When to know if a bond is ionic or covalent. (Without an electronegativity table) Sometimes they just mention a compound in a multiple choice question and ask whether it is ionic or covalent.

4- Why the reactions in dilute cold KMNO4 are dramatically different than those in hot concentrated KMNO4.

Those questions do sound really dumb, probably because they are, but they have bothered me for way too long. Thanks in advance!
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redonks
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1. a. I think it's because in an experiment you'd dissolve something solid so if you measure the enthalpy change it'll include the dissociation enthalpy and not just the hydration enthalpy
1. b. Decomposition is endothermic so you have to heat it and you don't know exactly how much heat was absorbed and how much was lost to the surroundings
1. c. why wouldn't you be able to? the reaction is exothermic so you don't need to heat it and carbon and oxygen combine easily to make carbon dioxide so it would work practically

2. you're looking at lots of atoms so it's not really close up enough to see the individual orbital shapes i think? (that's my guess after googling what electron density maps actually are because i haven't learnt about them)

3. metal and non metal combined is ionic, two non metals combined are covalent

4. don't really know but generally if something is hot and concentrated it's easier for reactions to happen so maybe it's just that some reactions have a high activation energy so they won't happen in cold dilute solutions
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Ploest
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(Original post by redonks)
1. a. I think it's because in an experiment you'd dissolve something solid so if you measure the enthalpy change it'll include the dissociation enthalpy and not just the hydration enthalpy
1. b. Decomposition is endothermic so you have to heat it and you don't know exactly how much heat was absorbed and how much was lost to the surroundings
1. c. why wouldn't you be able to? the reaction is exothermic so you don't need to heat it and carbon and oxygen combine easily to make carbon dioxide so it would work practically

2. you're looking at lots of atoms so it's not really close up enough to see the individual orbital shapes i think? (that's my guess after googling what electron density maps actually are because i haven't learnt about them)

3. metal and non metal combined is ionic, two non metals combined are covalent

4. don't really know but generally if something is hot and concentrated it's easier for reactions to happen so maybe it's just that some reactions have a high activation energy so they won't happen in cold dilute solutions
Can’t even enough words to describe how GRATEFUL I am for your contribution!!! 😭🙏 Thanks a ton for clearing them up; I can finally revise chemistry in peace! 🤣
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MexicanKeith
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(Original post by Ploest)
So throughout this first semester of my AS Level course, I’ve gathered up a few (silly) doubts that Google didnt clear.

If any of those doubts are wrong, please do correct me. The list goes something like this:

1- Why is it
- A. Not possible to measure the enthalpy change of hydration/dehydration of crystals (can’t you just use/remove excess of water?”
- B. Not possible to measure the enthalpy change during a decomposition reaction
- C. Possible to measure the enthalpy change of formation of CO2

2- Why electron density maps are circularly shaped when orbitals are not.

3- When to know if a bond is ionic or covalent. (Without an electronegativity table) Sometimes they just mention a compound in a multiple choice question and ask whether it is ionic or covalent.

4- Why the reactions in dilute cold KMNO4 are dramatically different than those in hot concentrated KMNO4.

Those questions do sound really dumb, probably because they are, but they have bothered me for way too long. Thanks in advance!
Just thought I'd add to whats already been said

electron density maps add up the density from each occupied orbital (not sure what you mean by 'circularly shaped' because molecular electron density maps arent in general) so the individual orbital shapes aren't what you see.

KMnO4 is used as an oxidising agent. It's oxidising power depends on how easily it is reduced (this can be measured by reduction potentials). You may or may not know that reduction potentials can be influenced by the concentrations of the species involved (this is described by the Nernst Equation). The redox reactions that occur in solutions of KMnO4 involve protons (H+), so the reduction potential has a dependence on the pH.

An easy way to represent this is in a frost diagram. There are various things you can learn from a frost diagram, but the most useful thing to know for this situation is that the lower down a species appears, the more stable it is. So the steeper the line between two species, the stronger the driving force for reaction from the higher species to form the lower species. You can clearly see on the diagram, the driving force for reduction of KMnO4 in acid to Mn2+ is roughly 4 times the size of that for basic conditions (neutral will be somewhere in between). This additional instability in acid is what leads to KMnO4's reactivity under these conditions.

Frost Diagram: https://upload.wikimedia.org/wikiped..._manganese.png
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Ploest
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(Original post by MexicanKeith)
not sure what you mean by 'circularly shaped' because molecular electron density maps arent in general
Thanks a lot for your brilliant reply! I’m extremely sorry for my poor wording. What I essentially meant is that if orbitals are regions where there is the highest probability of finding electrons, and if electron density maps are the measure of the probability of an electron being present at a specific location, then why is the electron density map of N2 drawn like picture 1 (attached) and not picture 2 (attached)?

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