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I did an experiment to determine enthalpy change when Zinc is added to Copper(II) sulphate solution.

I recorded values and plotted a temperature-time graph to obtain values of the temperature change to calculate heat transfer .

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The trend line that I obtained is when temperature of the mixture started to drop.

I never knew why I needed to extend that trend line and draw a vertical line at the point when Zinc is added and thus never understood why the difference between the interception point and the point when Zinc is added is $\Delta T$

To summarise, I don’t understand why I need to extend the trend line and the interception point gives the highest ‘T’ value...

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In an ideal world you would not have to do that. You'd simply wait until the reaction completes and then make a single measurement of temperature.

Too bad our world isn't ideal. The moment your reaction vessel becomes slightly hotter than the environment, it starts losing heat. If you wait for too long, it will cool all the way down to room temperature, so there will be nothing to measure in the first place. If you make your measurement in the first seconds so as not to let any heat escape, that's too early; the reaction has barely started. If you wait just until completion (which is a few minutes into the process, judging by your graph), that's too late, because by then some amount of heat is already lost.

So what do we do? We account for the loss. We assume that the temperature drop is linear (which of course is just an approximation, but still, it is much better than nothing), make a dozen or so measurements, build that trendline, and extend it to the origin. That's where your temperature would stop if there were no drop.

So it goes.

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  • $\begingroup$ Thanks alot! One more question from me, I did another exercise where I just recorded the initial and highest temperature it reached to calculate enthalpy change. As compared to the style I went for above (when I recorded the temperature in intervals), why would I get a more accurate $\Delta H$ value for the style I went for above rather than the exercise when I record the initial and highest temperature ? $\endgroup$ – user175089 Jan 12 '18 at 16:29
  • $\begingroup$ Is it because of the same explanation ? When we assume heat lost to the surrounding for this but not for the one when I just used highest and initial temperature . $\endgroup$ – user175089 Jan 12 '18 at 16:31
  • $\begingroup$ That's right, the highest temperature is reached when some heat is already lost. $\endgroup$ – Ivan Neretin Jan 13 '18 at 9:17

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