chemistry coursework 2014

[ellabarker99]

for our chemistry controlled assessment, we had to choose a salt and perform an experiment that would effect the temperature change of a salt mixed with water. we chose to use potassium chloride. the amount of water we used was 25ml, which remained constant for each test we did. we performed the test over 7 different values of salt, from 0.5 - 6g and we repeated each test 3 times to ensure accuracy. Here is a table of our results. All of the temperatures were measured in degrees Celsius.

 

amount of salt. (g) starting temperature. end temperature. temperature difference

0.5. 25. 24.5. -0.5

0.5. 24. 23. -1

0.5. 25. 24. -1

 

1. 24. 23. -1

1. 24. 23. -1

1. 23.5. 22. -1.5

 

2. 23. 20. -3

2. 23. 20. -3

2 22. 19.5. -2.5

 

3. 22. 18. -4

3. 24.5. 20.5. -4

3 23. 18. -5

 

4. 23.5. 18. -5.5

4. 23. 16.5. -6.5

4. 24.5. 18. -6.5

 

5. 24.5. 16. -8.5

5. 23.5. 15.5. -8

5. 25. 17. -8

 

6. 23.5. 15. -8.5

6. 22. 13. -9

6. 23.5. 15. -8.5

[AndresKiani]

Thermodynamics of salt dissolution in water. Yes.. what is your question though?

[mintyface:>]

We were doing a similar experiment, however, our group chose ammonium nitrate as the salt to be tested. We also used 25 ml of salt, but we tested between 2 and 6 grams of salt. We repeated each test 2 times, unless the 2 results were more than 0.5 degrees celsius apart, in which case, we did 3 tests. Here are our results:

2g:

1: 26-22-4

2: 26-22-4

 

3g:

1: 23-17-6

2: 22.5-16-6.5

 

4g:

1: 22.5-14-8.5

2: 21.5-14-7.5

3: 21-14-7

 

5g:

1: 21-12-9

2: 21-13-8

3: 21-12-9

 

6g:

1: 20-10-10

2: 20.5-10-10.5

[AndresKiani]

Why are you guys using grams?????

 

Tips... I can give you guys...

 

1) You guys should define concentrations of each salt used, so you can compare salt dissolution processes better. You would also want to choose molal concentrations for each solution. Molarity, will not give you an efficient measurement, because since your measuring enthalpy change of each salts' dissolution process, temp. change will effect the density of the water liquid solution, and since Molarity is mol/L, Liters water is a volumetric measurement, this will give you a less accurate result. However if you choose Molal concentration measurement for each solution, you mol/Kg. Kg is a mass reading, and will not change throughout your dissolution process.

 

2) Thermodynamics of each dissolution process is based on the.. state function.. were solute solute and solvent solvent interactions are broken, and solute solvent interactions are configured. Therefore if the amount energy put into your system to break those interactions is more energy J/mol than the energy released when the conformation is made between the salt and water particles, that salts' dissolution process will be endothermic. Flipside, if less energy is imputed and more energy is outputted than your Enthalpy change will be exothermic (neg.)

 

3) Since the thermodynamic process of salt dissolution is a state function process. You must be looking for the plateau curve, were the temp. plateaus off at. Not the min and max. You must first measure the temp. of the water, than measure the temp. were your temp. is constant after dissolution.

 

4) You will be than be using the Q = m x Cs x DeltaT(change.. Tfinal - Tinitial), to measure the amount of heat exchanged between the system.

 

5) You will also want to use a Styrofoam cup or something, to accurately measure this process' thermodynamics. Otherwise, you will have error in your data due to thermal energy dissipation.

You also have to take in consideration the hydrations for each salt, most salts are hydrated because that's their most stable form... Use that to get the accurate MW. for each salt, in order to give your correct moles of each salt used.

[studiot]

Both experiment posters please note that when reporting experimental measurements you should report to a consistent accuracy (broadly number of decimal places)

 

So if you are measuring temperature to 0.5 degrees then your measurement should be either 3.5, 4.0 or 4.5 for instance.

You should not report 4 .0 as 4.

Edited September 21, 2014 by studiot

[ellabarker99]

Thank you, I'm at GCSE level though, so I think that using grams is fine:) thank you for you're help.

 

the question

'how does that amount of salt placed in 25ml of water effect the temperature change?"

[studiot]

 

'how does that amount of salt placed in 25ml of water effect the temperature change?"

 

 

This is the second post about this GCSE experiment we have seen in the last few days.

Yours is better explained but still not fully so.

I wonder if the question was not made clear to the students/

 

Your statement above means (to me) that you are looking to draw a graph of temperature change with amount of salt added, after averaging your results in each of the sets of three.

Then you are probably expected to describe the shape of that graph to see if it is a straight line or some other shape.

 

Is this how you see it?

[AndresKiani]

I explained all of that in my previous post "Primate" your just reiterating what I've said.., they should not be looking for max temp and min temp. Instead your looking for a plateau. Take the difference between the initial temp. of the water and the final temp being the temp. at which it plateaus. Enthalpy change is state function.

 

Also, its not ok to use grams... you cannot accurately compare the thermodynamics of salt dissolution in water with grams. Grams will not give you accurate data, you need to use moles.

[Essay]

The question

'how does that amount of salt placed in 25ml of water effect the temperature change?" -ellabarker99

===

 

How does it "effect" or cause the temperature change, is a qualitative question.

 

How does it "affect" or change the temperature change, is a quantitative question.

 

Are we sure which question is being asked?

~

Edited September 21, 2014 by Essay

[AndresKiani]

Lol yes.. but

 

The better question is. Is what is the enthalpy change of the system based on the thermodynamic properties of each salt's dissolution process in an aqueous solvent.

 

This can be qualitative and quantitative. Good science is both.

[studiot]

 

I explained all of that in my previous post "Primate" your just reiterating what I've said.., they should not be looking for max temp and min temp. Instead your looking for a plateau. Take the difference between the initial temp. of the water and the final temp being the temp. at which it plateaus. Enthalpy change is state function.

 

Please be aware that GCSE is at a considerably more junior level.

 

This is the specification for what the posters are doing

 

 

 

carrying out some reactions and measuring the energy produced, assuming that it is only the water in the

solution that is being heated and that 4.2 joules will raise the temperature of 1cm3 of water by 1°C.

 

The specification for the "controlled assessment experiment" is in section 3.6 here

http://filestore.aqa.org.uk/subjects/AQA-4402-W-SP-14.PDF

Edited September 21, 2014 by studiot

[studiot]

ellabarker

 

I note you have returned to this thread several times so I guess you have a continued interest, and you have obviously put some good effort into the experiment.

 

But no one here knows what you are expecting us to do. (you haven't said)

 

That is why there is a shortage of replies.

 

However lots of good chemists will be happy to discuss with you so fire away and ask.

 

I said in post 7 that I expect you should draw a graph like the one below (which is what I made very quickly of your figures).

Yes grams would be just fine, don't worry about moles.

 

 

 

Over to you

 

[mintyface:>]

Following on from my previous post on this forum, I did mean to write 25 ml of WATER, not salt.

[fiveworlds]

 

May I ask why the change in starting temperatures? Why is there a larger temperature difference in the second 25 than the first? Was the salt and the water at the same temperature or not? Was the room temperature stable? Was the sample of salt pure? ie were any isotopes present?

The second table is a bit better.

 

 

Edited October 4, 2014 by fiveworlds

[hypervalent_iodine]

The starting temperatures really don't matter in this case. All that really matters for that section of the results is the temperature change. Unless the OP needs to calculate delta H, I'm not sure they need to worry about impurities so long as each experiment used salt from the same source. Isotopes wouldn't matter regardless.

[studiot]

 

the question

'how does that amount of salt placed in 25ml of water effect the temperature change?"

 

 

I'm sure also that one of the things you are meant of gain from this exercise is experience in the presentation and evaluation of data.

 

So do what do you think of the difference between my presentation and that of fiveworlds?

Is the bar chart / histogram an improvement?

 

Also you should be able to make comment on the two variables you have measured, viz temperature difference and weight (mass) of salt.

Is there any impact on your data from the quality of the measurement?

[sophieowen212]

Following on from recent posts in this thread:

 

For my chemistry controlled assessment, I chose to investigate the effects of changing the mass of salt added to a constant volume of water. I was used ammonium nitrate salt . I used 50ml of water (dependant variable), and changed the amount of salt being added from 0.5g - 5g. I measured at every 0.5g interval. to enhance reliability and accuracy I repeated each test 3 times. Note all temperatures were measured in degrees Celsius. See attached table of data

 

SALT AMOUNT(G) START TEMPERATURE(°) END TEMPERATURE(°) TEMPERATURE CHANGE(°)

0.5 23 25 24 22 25 24 1 0 0

 

1.0 23 24 24 23 24 23 0 0 1

 

1.5 24 24 24 22 23 23 2 1 1

 

2.0 24 24 23 22 23 22 2 1 1

 

2.5 24 25 24 21 22 22 3 2 2

 

3.0 24 25 25 21 22 22 3 3 3

 

3.5 23 25 21* 20 21 20 3 4 1

 

4.0 24 25 25 21 20 20 3 4 5

 

4.5 23 25 24 18 20 19 5 4 5

 

5.0 23 25 24 18 19 19 5 6 5

 

 

* OUTLIER

[hypervalent_iodine]

And?