[student]

if two pounds of sodium metal are thrown into 100L of water what is the final temp of the water as well as what is the final pH of the water?

[jdurg]

Well, show me how far you've gotten with the problem and where exactly you're getting stuck.  We're here to help, but not do your homework for you.  (Please read the top of the page).

[dexangeles]

don't you have the initial temps of the materials, and other information?

[Mitch]

good luck keeping the water around.

[jdurg]

This type of problem was described in my alkali metals part 1 article.  However, I was very vague and didn't go into too much detail.  In reality, this problem can be quite tricky since as the reaction proceeds, water slowly is decomposed thus lowering the amount of water that's there.  I think this is a good question to give to general chemistry students as well as advanced chemistry students depending on how accurate you want to get.  (Since to be really accurate you'd have to know the rate of reaction to see how much of the reactants are consumed each second.  Then you'd need to recalculate the temperatures every single second until one of the reactants is done.  You'd also have to factor in the heat absorbed by the sodium metal itself and the heat it absorbs while it melts.  Then you have to figure out how much heat is created as the sodium hydroxide forms and so on and so on.  So as you can see, this would be a great question to ask an advanced group).

[gsomd1980]

I'm working on a similar problem.  qw=-qobj right?  so i can't figure out how to solve for the final temp using SHmdT=-SHmdT.  Am I on the right track or am I way off, thanks for your help.

[Mr Amino]

I dont get the question.  The only way to get a solution is to assume totally unrealistic circumstances.  I mean, what form is the Na in? a block? pellets? the exposed surface area will determine the rate of reaction.  In real life, a reaction that proceeds quickly will surly have a higher temp than one that proceeds slowly.  Are we assuming that no heat is dissapated from the system?  Are we assuming that no water is evaporated when the sodium hits the H2O?

Im not trying to be nit picky.  I understand that you can ask this question totally theoretically but it is just a terrible question unless there is something you left out.  

Jdurg, why would the heat absorbed by the Na matter as it will be released back in the system anyways...unless it speeds up the reaction in which case that would be impossible to quantify.

lol, maybe im just looking into this too deeply.  if so, someone please correct me!  

[jdurg]

The heat absorbed my the sodium matters because you can't overlook the energy needed to actually melt a substance.  This is called the heat of fusion.  It defines how much energy is needed for one mole of a substance at its melting point to completely melt.  As it's melting, it's temperature is not changing since it's absorbing that energy in order to melt.  Therefore, if you have a large amount of a substance, the amount of energy its absorbing as it melts can have a substantial effect on the overall temperature.  (This is why in a completely isolated system you can have both liquid water and ice exist at 0 degrees Celcius in complete equllibrium.  At 0 degrees Celcius, ice will only melt/form if there is enough energy present.  This energy is quantified in water's Heat of Fusion).