Chemistry question

[CroppinIt]

I'm working through RS4K Chemistry level I in preparation for next year. Bear in mind that chemistry was never my strong suit....

 

In chapter 3, we get into reactions of various kinds: combination, decomposition, displacement, and exchange. I'm following that, in theory. But then we get the following example (from the teacher's guide p21):

 

"In a decomposition reaction molecules decompose or 'break down' to form other molecules. Note that two molecules of water decompose into one molecule of oxygen gas and two molecules of hydrogen gas. This is called a balanced reaction because the number of H and O atoms is the same on both sides; only the nature of the bond changes."

 

So..... is this evaporation or is it something different? The book never addresses this question, but I'm certain my kids will ask it.

 

I've been taught that water changes forms from liquid to gas in evaporation but I've always understood that it's still water vapor (H2O). In this decomposition reaction, it looks to me like we no longer have H2O but rather two H2 molecules and an O2 molecule, not water vapor.

 

Can someone explain the difference to me? Under what conditions does water decompose into H and O rather than evaporate into water vapor? Or is this really the same thing and I'm just totally off the mark?

 

Thanks!

[SebastianCat]

It's been a while since I've studied chemistry too, but from what I can remember, water is a very stable molecule and will only decompose in the presence of a catalyst like electricity. When water evaporates, it becomes water vapor, and the water molecule (H20) is still intact. Because water vapor is a gas, it just means the molecules are much farther apart than in the liquid state.

 

But if you stick electrodes into a container of water and run an electrical current through it, you can force water to decompose into hydrogen gas and oxygen gas.

 

Someone else correct me if I'm wrong, please!

[CroppinIt]

It's been a while since I've studied chemistry too, but from what I can remember, water is a very stable molecule and will only decompose in the presence of a catalyst like electricity. When water evaporates, it becomes water vapor, and the water molecule (H20) is still intact. Because water vapor is a gas, it just means the molecules are much farther apart than in the liquid state.

 

But if you stick electrodes into a container of water and run an electrical current through it, you can force water to decompose into hydrogen gas and oxygen gas.

 

Someone else correct me if I'm wrong, please!

 

This helps. Thanks!!

[TheHomeScientist]

Water evaporating is a change of state (from liquid to gas), but not a chemical reaction because the gaseous water molecules are the same as liquid water molecules.

 

A chemical reaction requires that atoms or molecules re-arrange to form new combinations. For example, one molecule of iodine is made up of two iodine atoms. If you react that molecule of iodine with two molecules of potassium, you end up with the reactants (iodine and potassium) consumed, and the product being two molecules of potassium iodine, each of which contains one iodine atom bound to one potassium atom.

 

As another poster mentioned, molecules differ in stability, depending on their bonding energy, physical conformation, and other factors. Some molecules (nitroglycerin, for example) are very unstable. It takes almost no energy to cause them to decompose. Other molecules, including water, are very stable. Causing a water molecule (two hydrogen atoms bound to one oxygen atom) to decompose into hydrogen gas and oxygen gas requires a very high energy input (normally, in the form of electrical energy).

[cmarango]

:iagree:Decomposition is NOT evaporation.

[CroppinIt]

Water evaporating is a change of state (from liquid to gas), but not a chemical reaction because the gaseous water molecules are the same as liquid water molecules.

 

A chemical reaction requires that atoms or molecules re-arrange to form new combinations. For example, one molecule of iodine is made up of two iodine atoms. If you react that molecule of iodine with two molecules of potassium, you end up with the reactants (iodine and potassium) consumed, and the product being two molecules of potassium iodine, each of which contains one iodine atom bound to one potassium atom.

 

As another poster mentioned, molecules differ in stability, depending on their bonding energy, physical conformation, and other factors. Some molecules (nitroglycerin, for example) are very unstable. It takes almost no energy to cause them to decompose. Other molecules, including water, are very stable. Causing a water molecule (two hydrogen atoms bound to one oxygen atom) to decompose into hydrogen gas and oxygen gas requires a very high energy input (normally, in the form of electrical energy).

 

:iagree:Decomposition is NOT evaporation.

 

Thank you both! I think I'm getting a handle on this.

[GingerPoppy]

Think of it this way: Decomposition starts with molecules at the beginning, which break down into their components, which is often the individual atoms (but could also be smaller, simpler molecules).

 

Change of state (such as evaporation) does not involve any change to the particles themselves... so atoms will stay atoms, or molecules will stay molecules. The main difference is that the atoms or molecules are simply further apart and more active. This happens at a particular temperature for each element (made of atoms) or compound (made of molecules).