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Marazm

Lab Report
Design problem with variables

Hey guys!

during the upcoming week I will conduct my Design experiment. We start on tomorrow on Monday. I have chosen topic connected with energetics and enthalpy change. i am investigating one factor that affects the experimentally determined enthalpy - in this case it is concentration of one of reactants. I've sent the Design recipe to my teacher a week ago, today she gave me a feedback in which she suggested slightly changing the title and some sentences. In general, she said I know what to do and how to conduct an experiment.

But my biggest problem is with the equation and identifying the variables. as we know the equation for enthalpy change is ΔH=Q/n. I have converted it into ΔH=(cmΔT)/(CV). But what next? I know that my variables are:

-independent - molar concentration and number of moles

-dependent - enthalpy, temperature change and heat evolved

-controlled - specific heat capacity and volume.

My question is what would be the gradient? Is mass a dependent or controlled variable?

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I'm assuming you're using solutions. The problem here is that if you change the concentration you won't be keeping the same volume and mass. This is because the density of the solution will change if you change it's concentration, and if you choose to keep volume constant, the mass will differ. This is a direct consequence of how you've decided to set up your experiment as I've seen so far. There will be heat evolved from the water as well; things will go wrong if you do different trials when the mass of water is different.

To answer your question, mass would be a dependent variable, and that's not such a good thing because you'd only want to find how enthalpy is affected when you change concentration. I'm not sure if this is a doable experiment...

PS: Temperature change should be a controlled variable.

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Oh I have forgotten to add that I will prepare samples with different molar concentrations, but all of them will have the volume of 50 cm^3. Could you elaborate on why things will go wrong if mass of water changes?

Comment on temperature change: it should not be a controlled variable as it will change as a result of using sample with different molar concentration.

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For labs, you'd want to change one variable and how it affect the second variable. If you change concentration, the enthalpy will change but the mass will also change, and mass is directly proportional to enthalpy, and this messes up your results.

I guess what you could do is take the total enthalpy (do you have a tool to measure this?) and subtract heat evolved from the water from the total enthalpy. If you know what concentration you're using you'd be able to know how much water you're using, so all you get after data processing is the heat evolved of NaOH. This seems like a bit of a stretch to me though...

Changing the concentration of NaOH is redundant because it basically means you're using more or less moles(translating into mass) of NaOH if you are keeping the volume constant.

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