[jb1524]

Hi,

I wanted to know what the controlled variables (factors held constant) were for this experiment, and why. My friends were telling me that mass of water and initial temperature didn't need to be held constant, which confused me, so know I'm not sure. I need to know 3 controlled variables.

Thanks

[Rabn]

Start by looking at the balanced chemical equation, then take a look at the thermodynamic equations involving enthalpy and consider a theoretical experimental setup.  From your investigation you're sure to find some logical options.

[jb1524]

Balanced chemical equation as in the combustion reaction? and thermodynamic equation meaning the enthalpy one (deltaH = Cp x deltaT, etc.)?

well, if you DO mean that, from the enthalpy of combustion equation, i would say the mass of water and change in temperature would need to be the same (but I'm not sure how to explain why...), initial temperature should be the same because more energy is needed to heat the water to a higher temperature?

and, not looking at the equations but the experiment in general, other controlled variables would be position of the flame (of the spirit burner), and the gap between the wick and beaker. One of my friends said that pressure would need to be kept constant, but you can't really do that in a school can you? Or should I still use it?

Thanks again. hope you can help me explain why those variables should be kept constant (if they are correct!)

[jb1524]

anyone? 

[Rabn]

I could use more background info. Tell me what the experiment was, include reagents and experimental setup and the purpose. That info should help clarify some things for me to better help.

[jb1524]

Okay, so the experiment was called the Enthalpy of Combustion of Alcohols. The purpose of the experiment was I guess just to compare the enthalpy of combustion of the alcohols.

We had to fill spirit burners with the alcohols we were testing (methanol, ethanol, propanol and butanol) and weigh them (for the initial weight). Then we filled a 250mL beaker with 100mL of water and held it in a clamp on a stand. A thermometer was put in the beaker to measure the initial temperature, and then once that was recorded, we lit the spirit burner of one of the alcohols so that the top of the wick of the burner was 1cm away from the base of the beaker. We tried to put the flame in the centre of the beaker for each alcohol.

After lighting the spirit burner, I would constantly stir the beaker with the thermometer until a temperature change of 20ºC was reached, then we would blow out the flame and re-weigh the spirit burner.

[Rabn]

Great! That experimental information really clarified a lot.  There are definitely 3 variables that were held constant.  Why not post a list of the different enthalpy equations you know? The mass of water and the change of temperature are definitely two of the three.  So great job there! Now we need to explore what the third might be. Listing the different enthalpy equations will help. 

p.s. Sorry for the delay in my response, I had a lot to do over the last couple weeks.