Molarity model 1 lemonade mixtures

Chemistry Lab

https://youtu.be/YW8j8Gbl7uc (for lab 8)

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Heat of Formation of Magnesium Oxide
Objective
· Hess’ Law will be used to determine the heat of formation of magnesium oxide shown in the following equation:

Mg(s) + ½ O2(g) MgO(s)

· A video will be used to observe the experiment.

· Data will be provided on the Report Sheet for doing the calculations.

Introduction
The heat of reaction, (H), is defined as the heat lost or gained as a reaction proceeds from reactants to products. It is often given as part of a thermochemical equation, such as that shown for the combustion of hydrogen:

2 H2(g) + O2(g) 2 H2O(l) H = 572 kJ

The negative sign indicates that this reaction is exothermic. Thus, when two moles of hydrogen gas and one mole of oxygen gas combine to form two moles of liquid water, 572 kJ of heat energy is released. Conditions of constant atmospheric pressure (reaction vessel is an open container) and near room temperature (25 oC) are assumed in the above value, although heats of reaction do not vary appreciably with temperature.

A calorimeter is a device for measuring the heat of reaction. One carries out the reaction in the calorimeter and measures the temperature change, (T). Assuming no heat loss, the reaction heat all goes into warming the solution and the calorimeter. This heat quantity can be calculated as follows:

qrxn = qsoln + qcal

where qrxn is the heat of reaction, qsoln is the heat absorbed by the solution, and qcal is the heat absorbed by the calorimeter.

The heat absorbed by the solution is calculated as follows:

qsoln = msoln x c x T

where q is heat (J), m is mass of the solution (g), c is specific heat (J/g oC), and T is final temperature initial temperature (oC).

The heat absorbed by the calorimeter is calculated as follows:

qcal = Ccal x T

where Ccal is heat capacity of cup (J/oC), and T is final temperature initial temperature (oC).

The solution has specific heat equal to 4.184 J/g oC and density of 1.00 g/mL. The estimated calorimeter heat capacity is 2.6 x 101 J/oC. The final heat of reaction, (H) calculation requires a stoichiometric conversion from grams of limiting reactant to moles as expressed in the balanced equation. In this experiment, the Mg is the limiting reactant in the reaction.

You will measure the H value for the following reaction:

Reaction 1: Mg(s) + 2 HCl(aq) MgCl2(aq) + H2(g) H = (experimentally determined)

Using the thermochemical equation for this reaction along with the following two reactions, you will then calculate the H value (using Hess’s law) for the combustion of magnesium (which in this case, is the same as the heat of formation of MgO):

Reaction 2: MgO(s) + 2 HCl(aq) MgCl2(aq) + H2O(l) H = 157.0 kJ

Reaction 3: 2 H2(g) + O2(g) 2 H2O(l) H = 572.0 kJ

This information will then be used to calculate the H value for the combustion of magnesium (which in this case, is the same as the heat of formation of MgO):

Final Reaction: Mg(s) + ½ O2(g)  MgO(s) H = ?

The calculation is done by applying Hess’ Law as described in the chemistry textbook. Temperature measurement in this experiment is made with a digital thermometer. The initial and final temperature will be determined using the thermometer.

Pre-Lab Assignment
As preparation for this experiment, do the following:

· Open the lab folder on Blackboard for this experiment and follow the instructions for this experiment.

Calculation of the Heats of Reaction
Use the data provide on the Report Sheet to calculate qrxn for Mg and HCl.
1. Calculate T for the reaction of Mg and HCl.

2. Using the heat capacity of 2.6 x 101 J/oC for the calorimeter determine the amount of heat absorbed by the calorimeter (qcal) for the reaction of Mg and HCl.

3. Calculate the amount of heat absorbed by the solution (qsoln) for the reaction of Mg and HCl.

4. Calculate the total amount of heat (qrxn) lost by the reaction for the reaction of Mg and HCl.

5. Calculate the amount of heat lost by the reaction per gram of limiting reagent (Mg).

6. Find the heat of reaction per mole (H) for the reaction.

Heat of Combustion of Mg = Enthalpy Change for 1 mol of Mg turnings

7. Use Hess' Law to calculate H for the combustion of magnesium, as discussed in the introduction.

8. The Heat of the Reaction for MgO and HCl, the formation of water is provided.

9. Find the experimental error.

Page 2 of 2 T. Mewhinney & V. Johnson, B.Smith BAKS 04/28/20