Last Updated 29 Aug 2020

# Calorimetry and Hess’s Law Purpose

Category Chemistry
Essay type Report
Words 483 (1 page)
Views 241

The purpose of this lab is to determine the enthalpy of reaction for the burning of one mole of magnesium in oxygen. Although the reaction is exothermic, the ∆HRXN will be determined by using calorimetry and then using Hess’s Law to manipulate the data collected to yield the answer needed.

## Part A

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 Volume of cold water 49. 9 mL Temperature of cold water (in a cup) 23. 50 C Volume of hot water 49. 9 mL Temperature of hot water (in a cup) 550 C

Calculations

 Mass of cold water 49. 9 g Tf from the graph by extrapolation 34. 90 C ∆THW for hot water -20. 1 ∆TCW for cold water 11. 4 qHW for hot water -4196. 5 J qcw for cold water 2380. 1 J qCal for the cup 1816. 4 J Ccup for the cup 159. 3 J

## Part B

 Description of sample Metal fizzed when dissolved in HCl Volume of HCl 100 mL Initial Temperature 220 C Mass of Mg 0. 1485 g

Calculations

 Tf from graph 50 C Mass of HCl 100 g ∆TCW for HCl -2092 J qHCl for solution -204. 4 J qCal for cup -2296. 4 J qRXN -47594 J ∆HRXN for Mg -

NET REACTION: 2HCl(aq) + Mg(s)>MgCl2(aq) + H2(g)

## Part C

 Description of sample - Volume of HCl 100 mL Temperature of HCl 220 C Mass of MgO 0. 5052 g

Calculations

 Tf from graph 240 C ∆TCW for HCl 1. 50 C qHCl for HCl 627. 6 J qCal for cup 61. 32 J qRXN 608. 92 J ∆HRXN for Mgo 55469 J/mol

Net Equation: 2HCl(aq) + MgO(s)>MgCl2(aq) + H2O(l)

## Conclusion

In this experiment, we had to find the heat capacity of the calorimeter cup using two trials of hot and cold water. When we obtained the data after 10 minutes of recording 30-second intervals of the calorimeter cup temperature, we created a line graph to show the trend line.

With the trend line, we could figure out the final temperature and delta H in the process. With the equation we could find the heat of reaction for hot water q HW = m c DTHW and using qCW = m c DTCW we could find the heat of reaction for cold water. By using |qHW| = |qCW |+ qCal, we can find the heat of reaction of the cup and multiply that by the change in temperature to find the heat capacity. After placing Mg into HCl and then the same thing with MgO, we could find the heat of solution of HCl with q CW = m c DTHCl.

When we found the values of DHRXN for B and C we can use Hess's Law, which is shown in the calculations section. All the data is in the data section of the report. The percent error of Mg was about 30%. The actual value of Mg was 601200 J/mol and the experiment we obtained was 420611 J/mol. There could have been human error in order to obtain the results that could have accounted for the 30% error, although this is not so high. Also, the heat of formation we used was MgO(s) for the actual value, although in the experiment it was a gas. This could have also accounted for some errors.

## References:

1. “ Calorimetry and Hess’s Law. ” Laboratory Manual. Publishing, 2012, pp. 168-177.

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