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# Calorimetry Lab - Average molar heat of reaction

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Lab Tools :
Thermometer, Balance
Lab Glassware :
Calorimeter
Lab Chemicals :
H2O, Mg, HCl
Lab Background :

When an exothermic reaction takes place in a container, heat is released and warms up the container and its contents. The contents of the container include the reaction products and a solvent (such as water), if present. The measurement of heat transfer under controlled conditions is called "calorimetry" (from the unit of heat energy, the calorie), and a container used to measure heat transfer is called a calorimeter. The calorimeter prevents heat transfer to or from the surroundings, thereby isolating the system being measured.

When a reaction occurs in relatively large amount of water, we can relate to the water as the sole medium absorbing the heat of reaction. We can then calculate the heat of reaction by measuring the temperature change of the water, according to the following relationships:

q(water) = mass(water)*Cp(water)*deltaT(water) = q(reaction)

where Cp(water) is the specific heat of water, 4.18 J/g*C, and deltaT is the difference between the initial and final temperatures.

The absorption of heat by the calorimeter should also be taken into account and added to the heat absorbed by the water. If you have not already done so, the "Calibrating the Calorimeter" lab can be performed in order to find the calorimeter constant, Ccal. Once this constant has been obtained, the total heat of the reaction is given by:

q = Ccal * DeltaT + mass(water) * Cp(water) * Delta T

In the current experiment you will measure the heat generated by the reaction:

Mg(s) + 2HCl(aq) -> Mg+2(aq) + 2Cl-(aq) + H2(g)

The heat of the reaction is expressed in Joules per moles of reactant, and in this case you will find the heat of reaction per moles of Mg.

Assignments :
Procedure #1

1. Take a calorimeter from the Glassware shelf and place it on the workbench.

2. Take a balance from the Tools shelf and drop it on the calorimeter. Record the mass of the empty calorimeter.

3. Take a thermometer from the Tools shelf and drop it onto the calorimeter. Record the temperature of the calorimeter.

4. Add 50 mL of 1.0M hydrochloric acid (HCl) to the calorimeter.

5. Add .15g of magnesium (Mg) to the calorimeter.

6. Immediately measure and record the MAXIMUM temperature displayed by the thermometer in the calorimeter.

7. Measure and record the mass of the calorimeter and its contents.

8. Repeat the experiment with a new calorimeter, using 50 mL of HCl and .25g of magnesium. Record the MAXIMUM temperature displayed by the thermometer in the calorimeter.

9. Repeat the experiment with a new calorimeter, using 50 mL of HCl and .35g of magnesium. Record the MAXIMUM temperature displayed by the thermometer in the calorimeter.

Question #1 :

1. Record the following for each of the three experiments:

(a) Mass of the empty calorimeter (g):

(b) Initial temperature in the calorimeter (C):

(c) Maximum temperature in the calorimeter from the reaction (C):

(d) Calculate deltaT as Tmaximum - Tinitial (C):

(e) Mass of the calorimeter and its contents after the reaction (g):

(f) Calculate the mass of the contents of the calorimeter (g):

(g) Calculate the moles of Mg reacted (MW=24.305 g/mole):

2. Calculate the heat released into the solution for the 3 reactions, according to:

q(reaction) = Ccal * Delta T + mass(contents) * Cp (contents) * deltaT

(If you have not measured the calorimeter constant yet, assume that it is zero for this experiment.)

3. Find the molar heat of reaction for each experiment in units of Joules / (mole of Mg) by dividing the heat of reaction by the moles of Mg used.

4. Calculate and record the average molar heat of reaction from the three results.

1a. 11.6g for all 3
b. 21 degrees C for all 3
c. for .15g-max temp is 29.27 degrees C, for .25g-max temp is 34.78 degrees C, for .3mg, max temp is 40.29 degrees C
d..15-8.27, .25-13.78, .35g-19.29
e. .15-61.557g, .25-61.649g, .35-61.741g
f. .15-49.957g, .25-50.019g, .35-50.141g
g..15-0.006 .25-0.103, .35-0.014

I'm not sure if (g) is right. I took the answers to (f) used a gram to moles converter that I found on the internet http://www.convertunits.com/from/grams/to/moles
Is this correct?
I still need help with 2-4 if possible