I have a project due tomorrow and i need Help Calorimeter Experiment

[CRich]

I have a project due tomorrow and i need Help ... Calorimeter Experiment !

Homework Statement

My groups is terrible and is basically refusing to do any work---that's the real problem
My (other) problem is this:

I have a piece of copper that I am using as a calorimeter, I'm placing it inside of a Crockpot, then I have placed water inside the copper cap, next I allow the water to boil. I am doing this to make sure everything works properly, because tomorrow I have to replace the water with an unknown liquid and determine what it is by finding its' specific heat.

However, when i do the calculations I find the specific heat of copper to be 1.991 kJ/kg*K
and if i use the specific heat of copper to find the specific heat of water I find it to be 0.824 kJ/kg*Kboth of these are wrong... someone please tell me what I am doing wrong

Homework Equations

ScMc(Tic-Tfc) = SwMw(Tiw-Tfw)

Sc -specific heat of copper, Mc - mass of copper, (Tic-Tfc) - change in temperature of copper

rearranging I get:

Sc = SwMw(Tiw-Tfw) / Mc(Tic-Tfc)

The Attempt at a Solution

Entering my values I get:

Sc = $$\frac{ (41.9 kJ/kg*K)(0.0151kg)(101.0-21.9K) }{ (0.0315kg)(100.3-21.9K) }$$
= 1.991 kJ/kg*K

So I'm off by 410%

What am I doing wrong?
Am I using the wrong equation or am I doing the experiment wrong? I'm open to any and all suggestions!

 

[MATLABdude]

Can you post the values you're using for your calculations? Plugging your values into my calculator give 20 kJ/(kg*K). Additionally, 0.015 kg of water is only 15 mL, and according to Wikipedia, the specific heat capacity of water is 4.19 J/(g*K) or kJ/(kg*K)

EDIT: Actually, what's your methodology here? Are you boiling water in the cap to get the cap up to 100 C, and then immersing the hot cap in water, and measuring the final temperature?

 

[CRich]

I used 4.19 as the specific heat and yes the mass if water is very small at 15.1g
DeltaTc=79.1
DeltaTw=78.4
mass of copper=31.5g

 

[MATLABdude]

Okay, good. So can you explain your methodology? If the water is being boiled, you have to factor in mass of water lost, and the phase transfer.

 

[CRich]

no the water isn't boiling...Ive changed my method...which yields a better result... I'm heating up an object to a known temp and submersing it into the liquid and using that to find the Sc

 

[MATLABdude]

no the water isn't boiling...Ive changed my method...which yields a better result... I'm heating up an object to a known temp and submersing it into the liquid and using that to find the Sc

That sounds about right. Good luck with the lab!

 

[vk6kro]

Also be aware of errors. Copper has a very low specific heat compared with water, so it takes a lot of copper to heat up a small amount of water.

Ideally, you need to start with a cubic inch of copper. Schools often have these for density experiments. They weigh about 146 grams, but weigh yours.

To avoid tarnishing the copper, you can put some boiling water into a weighed polystyrene foam cup (with a lid).
Measure its temperature, then drop the room temperature copper cube into the water. Allow time for the water temperature to stabilise by taking readings until the water temperature is constant. Do this with the lid on.

Then weigh the cup plus water plus copper. So, you can work out the weight of the water.

This method is good because you can handle the copper with tongs and not lose heat because it is at room temperature.

You know the final temperature of the copper and the water (they are the same) so you can work out the specific heat of water.
If you have to do this with a different liquid, beware of solvents that attack polystyrene.