Experiment 1: Determine the Enthalpy Change of a Chemical Reaction Take a calori

Question

Experiment 1: Determine the Enthalpy Change of a Chemical Reaction

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

Take a balance from the Instruments shelf and place it on the workbench.

Place the calorimeter directly on the balance to weigh it. Record the mass of the empty calorimeter in your Lab Notes. Remember to press Save Notes.

Take a thermometer from the Instruments shelf and attach it to the calorimeter. Record the temperature of the calorimeter in your Lab Notes.

Take 1.0 M hydrochloric acid from the Materials shelf and add 50 mL to the calorimeter.

Take magnesium from the Materials shelf and add 0.150 g to the calorimeter.

Watch the thermometer and record the maximum temperature displayed in your Lab Notes.

Measure and record the mass of the calorimeter and its contents in your Lab Notes.

Discard the calorimeter by dragging it to the recycling bin beneath the workbench.

Repeat the experiment with a new calorimeter, using 50 mL of hydrochloric acid and 0.250 g of magnesium.

Repeat the experiment with a new calorimeter, using 50 mL of hydrochloric acid and 0.350 g of magnesium.   

The Enthalpy Change of a Chemical Reaction

Figure 1. To Go Coffee Cups

What Is the Best Coffee Cup?

You may have noticed that coffee and tea shops sell their drinks to go in a variety of cups and mugs. But, what is the best container for your drink? Some people prefer to have a cup that gets warm in the winter to warm up their hands, whereas others prefer cups that can keep their drink warm longer. The cups that keep your drink warm the longest are made from materials that are good insulators such as Styrofoam. These materials do not lose a lot of heat and therefore they keep the drinks warm longer. Similarly, an insulated container can be used in the lab to measure the heat released or absorbed during a physical or chemical transformation.

Enthalpy of Reaction

The change in energy associated with a chemical reaction is called enthalpy of reaction (Hrxn), and is measured in kJ/mol. Its value depends on the difference in energy between the intramolecular and intermolecular bonds broken and formed, respectively. If the energy of the bonds broken is greater than the energy of the bonds formed, the process is called endothermic and occurs with absorption of energy from the surroundings (H > 0). If however, the energy of the bonds broken is smaller than the energy of the bonds formed, the process is called exothermic and occurs with release of energy (H < 0). The change in enthalpy can be difficult to measure. Instead, the temperature of the surroundings can be monitored and used to determine the heat flow between the sample and its surroundings.

Calorimetry

Calorimetry is a technique used to measure the change in heat associated with a chemical reaction or physical change. This is usually performed using an insulated container called a calorimeter. One of the most used calorimeters in the laboratory is the constant pressure calorimeter that is open to the atmosphere and therefore experiences the constant pressure. It is often used to measure the heat flow in aqueous reactions.

Heat of Reaction

The heat released or absorbed when using a constant pressure calorimeter is termed q and is expressed in Joules (J). Assuming that the calorimeter does not lose any heat to the surroundings, the net heat flow into or out of the system has to be zero, as shown in the equation below.

qrxn + qsoln + qcal = 0

where qrxn, qsoln, and qcal are the heat gained or lost by the reaction, solution, and calorimeter, respectively. Given this formula, the heat of reaction can be computed as follows.

qrxn = – (qsoln + qcal)

The change in temperature (T) during the reaction can be used to calculate the heat of the reaction (qrxn) using the equation below.

where m is the mass of the solution in grams, c is the specific heat of the solution in J/g°C, T is the difference between the final and initial temperatures in degrees Celsius (°C), and Ccal is the calorimeter constant in J/°C. The specific heat of a substance is the amount of heat required to increase the temperature of 1 g of it by 1 °C. For dilute aqueous solutions, the specific heat of water, 4.18 J/g°C, can be used for c. Each calorimeter has a unique calorimeter constant. The calorimeter constant for the calorimeter you will use in this virtual lab is 9.30 J/°C.

The enthalpy of reaction (Hrxn) is expressed in kJ/mol and can be obtained from qrxn as shown below.

where qrxn is the heat of reaction in kJ of reaction and n is the number of moles of reaction. The mol of reaction is a measure of how many times the reaction happens. To calculate it, the balanced equation needs to be known.

About This Lab

In this lab, you will perform a reaction inside a calorimeter and study the heat absorbed or released during the reaction. You will react solid magnesium with aqueous hydrochloric acid inside a calorimeter to produce magnesium chloride and hydrogen gas. The reaction follows the chemical equation below.

Mg(s) + 2HCl(aq) MgCl2(aq) + H2(g)

You will use a coffee cup calorimeter to measure the temperature change that occurs as the result of the chemical reaction between magnesium and hydrochloric acid. You will use the change in temperature to calculate the heat of reaction.

qrxn = – (qsoln + qcal)

Short Answer The Enthalpy Change of a Chemical Reaction Experiment 1: Determine the Enthalpy Change of a Chemical Reaction 1.Fathe table below with your resurs from of the calorimeter and its coreerts reer the reaction trial 3 350 trial 2 25o 2. Calculate the mass of the contents the calorimeter MA Lo 2.901 caleulate the number of moles of agneskum used inrial t The molar massofmagnesiumis 24.305 gmot Calculate the change in temperature in tal 1. calculate the heat of reaction in trial 1 Calculate enthalpy of reacson using your resurs fon nal 7. Calculate the heat of reaction in rial 3. 6. Calculate Pen ennalpy ofreacson using your mesults form rial 3. Conclusions endo enmie? t the reacson of magnesum and hydrochloric acid exothermic or

Explanation / Answer

2) Mass of the contents in the calorimeter in trial 1 = (mass of the calorimeter and its contents after the reaction in grams) – (mass of the empty calorimeter in grams) = (68.788 g) – (18.600 g) = 50.188 g (ans).

3) Molar mass of Mg = 24.305 g/mol.

Mass of Mg used = 0.150 g.

Therefore, moles of Mg used = (mass of Mg used)/(molar mass of Mg) = (0.150 g)/(24.305 g/mol) = 6.171*10-3 mole.

4) Change in temperature in trial 1 = (final temperature of the calorimeter in C) – (initial temperature of the calorimeter in C) = (34.5 C) – (21.5C) = 13.0C.

5) qsolution = (mass of solution)*(specific heat of solution)*(change in temperature of the calorimeter) = (50.188 g)*(4.18 J/g.C)*(13.0C) = 2727.216 J (the specific heat of a dilute solution is given as 4.18 J/g.C).

Qcal = (specific heat of calorimeter)*(change in temperature of the calorimeter) = (9.30 J/C)*(13.0C) = 120.9 J.

Qrxn = -(qsolution + qcal) = -(2727.216 J + 120.9 J) = -2848.116 J

The heat of the reaction is -2848.116 J (ans).

6) The balanced chemical equation is

Mg (s) + 2 HCl (aq) ------> MgCl2 (aq) + H2 (g)

As per question (3) above, moles of Mg reacted = 6.171*10-3 mole.

The enthalpy of the reaction is

Hrxn = qrxn/n where n = number of moles of Mg used.

Therefore,

Hrxn = (-2848.116 J)/(6.171*10-3 mole) = -461532.329 J/mol = (-461532.329 J/mol)*(1 kJ/1000 J) = -461.532 kJ/mol (ans).

7 + 8) Need the complete data for trial 3.

Conclusion:

1) Since the value of the enthalpy of the reaction is negative (as deduced for trial 1 in question 6 above), the reaction is exothermic. This means that the reaction releases energy to the surroundings. Therefore, the surroundings will be at a higher temperature after the reaction.

2) The final temperature will be lower than the one measured in the laboratory. This is because some of the heat of the reaction will be lost to the surroundings and hence the calorimeter will not get as warm as we expect it to be. Hence, the calorimeter will have a lower temperature.

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