Reactivity (1 Viewer)

[_Anonymous]

How does the unstable valence shell determine the Boiling/Melting point (this is more based on bonds), electrical conductivity and properties for an Element?

For example, Hydrogen is highly flammable. But people say it's due to the unstable valence shell. How does the unstable valence shell cause it to be so flammable?

Also, my teacher mentioned something about intermolecular forces being the cause for the properties of elements at room temperature (eg. Mercury being liquid at room temperature). Could someone please elaborate on that and tell me more about intermolecular forces?

Thanks.

 

[Queenroot]

How does the unstable valence shell determine the Boiling/Melting point (this is more based on bonds), electrical conductivity and properties for an Element?

For example, Hydrogen is highly flammable. But people say it's due to the unstable valence shell. How does the unstable valence shell cause it to be so flammable?

Also, my teacher mentioned something about intermolecular forces being the cause for the properties of elements at room temperature (eg. Mercury being liquid at room temperature). Could someone please elaborate on that and tell me more about intermolecular forces?

Thanks.

 

[fan96]

Boiling and melting point is determined by intermolecular forces between molecules. e.g. between water molecules exist hydrogen "bonds". When substances change states from liquid to gas, all the molecules break away from each other. Stronger intermolecular forces make it harder for this to happen. This is why water's melting point and boiling point is so high compared to similarly sized molecules without hydrogen bonds, such as carbon dioxide, methane and hydrogen sulfide, and why water is a liquid at room temperature and all those others are gases.

Electrical conductivity depends on the electrons in an atom's outer shell and whether they are free to move around (delocalised). Graphite has a lattice structure in which each carbon atom is covalently bonded to 3 others; each carbon atom has a free electron that can move across the lattice so that's why graphite conducts electricity. But in diamond's lattice, each carbon atom is bonded to four others so there aren't any free electrons to move around, hence diamond doesn't conduct electricity. Metals exist as lattices in a "sea of delocalised electrons", so they are excellent conductors of electricity. Ionic substances only conduct electricity in solution because the act of dissolving breaks up the ionic lattice and allows the electrons to move around.

Combustion is typically the reacting of a fuel with oxygen, so a flammable substance tends to be reactive. Hydrogen has one valence electron (an incomplete outer shell), so the hydrogen molecules tend to give their electrons to the oxygen to create water when you provide enough energy (e.g. a flame):

2H_{2 (g)}+ O_{2 (g)} -> 2H₂O _(l)

But noble gases like helium and xenon are not flammable because they already have complete outer shells, so they don't give away their electrons to the oxygen atoms.

 

[_Anonymous]

Boiling and melting point is determined by intermolecular forces between molecules. e.g. between water molecules exist hydrogen "bonds". When substances change states from liquid to gas, all the molecules break away from each other. Stronger intermolecular forces make it harder for this to happen. This is why water's melting point and boiling point is so high compared to similarly sized molecules without hydrogen bonds, such as carbon dioxide, methane and hydrogen sulfide, and why water is a liquid at room temperature and all those others are gases.

So does that mean Mercury's intermolecular forces are quite weak since it's a liquid at room temperature?

Electrical conductivity depends on the electrons in an atom's outer shell and whether they are free to move around (delocalised). Graphite has a lattice structure in which each carbon atom is covalently bonded to 3 others; each carbon atom has a free electron that can move across the lattice so that's why graphite conducts electricity. But in diamond's lattice, each carbon atom is bonded to four others so there aren't any free electrons to move around, hence diamond doesn't conduct electricity. Metals exist as lattices in a "sea of delocalised electrons", so they are excellent conductors of electricity. Ionic substances only conduct electricity in solution because the act of dissolving breaks up the ionic lattice and allows the electrons to move around.

Thanks, I understand now.

Combustion is typically the reacting of a fuel with oxygen, so a flammable substance tends to be reactive. Hydrogen has one valence electron (an incomplete outer shell), so the hydrogen molecules tend to give their electrons to the oxygen to create water when you provide enough energy (e.g. a flame):

2H_{2 (g)}+ O_{2 (g)} -> 2H₂O _(l)

But noble gases like helium and xenon are not flammable because they already have complete outer shells, so they don't give away their electrons to the oxygen atoms.

Why does Oxygen have to be the fuel in a fire? How come it's not Nitrogen or another gas (Nitrogen is far more reactive)? Could you explain how thermal energy (heat in this case) is created when compounds are formed and especially in a Combustion reaction?

 

[fan96]

1. I'm not sure about mercury since we didn't ever talk about it in class. Mercury is a metal so it should exist as a metallic lattice, not as molecules. Perhaps the forces holding the metal cations together are weaker than they are in other metals.

3. Nitrogen's valency is 3, and oxygen's is 2, so oxygen should be more reactive than nitrogen. As for other gases the only ones I can think of that might be applicable here are the halogens. Again I'm not sure if combustion with nitrogen or halogens instead of oxygen works (that's something you should ask your teacher), but oxygen is definitely the most common reactant.

Several things are required for a chemical reaction to occur. One of these things is activation energy (E_a) - for example, you can't just burn wood by placing it in air or even in pure oxygen. You need to give it a push with some input energy. This energy is used to get the reaction started by breaking bonds in the reactants, which releases more energy (the activation energy is also used to give particles the kinetic energy they need to collide with each other and react). The energy released is then used to create new bonds that form the products. Sometimes after all the bonds are formed, you get leftover energy, which can be released as thermal, sound, light energy, etc. That is known as an exothermic reaction, where the products have less energy than the reactants. All combustion reactions are exothermic - they give off heat.

Some reactions such as the dissolving of KOH or NH₄NO₃ are endothermic, which means that the products end up with more energy than the reactants. Endothermic reactions don't release heat, rather they will take in heat (i.e. make the surroundings cooler).

You can visualise all of this by looking at an energy profile diagram.