# Thermal Equilibrium Temperature Question Sodium and Water

1. Apr 3, 2016

### RJLiberator

1. The problem statement, all variables and given/known data

Expose the error: When sodium is placed in water, a violent reaction always occurs; therefore sodium and water can never be at the same temperature.

2. Relevant equations
Thermal equilibrium and temperature. Zeroth law. Isotherms.

3. The attempt at a solution

The error is that sodium and water CAN be at the same temperature.
The system will reach a thermal equilibrium.

2. Apr 3, 2016

### Merlin3189

Does the sodium have to be in contact with the water?
The question says it has to be in thermal equilibrium, not in contact.
You do mention the zeroth law.

3. Apr 3, 2016

### RJLiberator

"When the sodium is placed in water" So I assume yes to this question.

I don't think the actual question says anything about thermal equilibrium. Or am I wrong?

Those are just general topics of what I am learning right now. IT may or may not have to do with the Zeroth law.

The question:
Expose the error: When sodium is placed in water, a violent reaction always occurs; therefore sodium and water can never be at the same temperature.

4. Apr 3, 2016

### Merlin3189

Yes, I'm sorry, I inserted the thermal equilibrium in my own head!
I agree that, when sodium is placed in water, a reaction occurs.
I don't see that any conclusion follows about sodium at all times. (Edit: Never implies all times, before during and after reaction.)

What about just before it is put in the water? What temperature limitations are there on it then? And which piece of sodium are we talking about? As I read the second part of the sentence, it refers to any piece of sodium anywhere at any time.
If you do choose one specific piece of sodium, which is kept (in its oil) in a jar next to the trough of water, where they are both at the same temperature, right up to the moment it contacts the water (or water vapour) it can be at the same temperature.

If I asked you to measure the heat generated by the reaction, by doing the experiment in an insulated sealed container (very dangerous of course!) and measuring the temperature after the reaction, then you would need to know the temperature of both water and sodium at the start of the reaction. It would probably be convenient if they were both at the same temperature, but there is no reason why the sodium should not be hotter or colder: other than convenience there is no constraint on the temperature of the sodium.

In your interpretation of the question, which seems to be that the sodium cannot be at the same temperature as the water while the reaction is taking place, your conclusion might be correct. But that would IMO be a strange question. What IS the temperature of a lump of sodium which started at room temperature and whose surface is now heating up very rapidly? What is the temperature of a trough of water which is being strongly heated in one small area? Neither has a clear, well defined temperature at any instant during the reaction.
At the surface of the sodium I would say that the water and sodium could be at exactly the same temperature just where they are in contact.

Chemically, I would suspect the sodium cools down as energy is used to remove sodium atoms and ionise them. Similarly the water should cool as its molecules are removed and torn asunder. Only when the ions come together is great energy released, which presumably spreads into the sodium and water at random, heating them more or less equally. Or possibly heating the water more, as the ions proceed to dissolve in it.

When I think that this is a physics question rather than a chemistry one, I think it makes more sense to see it as, maybe a bit of a catch question, but testing understanding of the zeroth law - that two objects do not have to be in contact to be at the same temperature.

But see what the real physicists say when they get here.