Nature has given us two Thus you can use Hess's Law and a number of reactions which are

9;s Law and a number of reactions which are more easily dealt
ears, two eyes, and but one with to find the information about the reaction of magnesium and oxygen.
tongue, to the end that we
should hear and see more than Consider the following reactions:
we speak. --Socrates
Mg(s) + 2 HCl(aq) → MgCl2(aq) + H2(g) (1)
The Determination of the Heat of Formation of Magnesium Oxide
MgO(s) + 2 HCl(aq) → MgCl2(aq) + H2O(ℓ) (2)
Chemical reactions proceed with the evolution or absorption of heat. This heat
flow represents differences in chemical energy associated with the arrangement of H2(g) + ½ O2(g) → H2O(ℓ) (3)
atoms into molecules or ions. It includes the electrical potential energy arising from
attractions between electrons and nuclei in atoms or ions for each other. Thus when a For the first two reactions, the heats of reaction are easily found in a simple calorimeter.
piece of magnesium ribbon is burned in air to yield magnesium oxide, a large quantity For the third reaction, the heat of formation of liquid water, the enthalpy change is well
of heat is released to the surroundings: known (-286 kJ/mol). By reversing equation (2) and adding all three we obtain:

Mg(s) + ½ O2(g) → MgO(s) + heat Mg(s) + ½ O2(g) → MgO(s) (4)

Because heat is evolved in this reaction, we know that the sum of the chemical And the enthalpy change for this reaction (∆Hof), the heat of formation for magnesium
energies of the amounts of elemental magnesium and oxygen combining is greater than oxide, is found by a similar reversal and addition of the enthalpies of the three reactions.
the chemical energy of the amount of magnesium oxide formed.
The constant-pressure calorimeter used in this experiment consists of a
When the reaction takes place at constant pressure, the heat flow, qp, is called specially treated block of expanded polystyrene foam (commonly called “styrofoam”,
the change in ENTHALPY of the system and for MOLAR quantities is given the but not the same stuff as the rough and porous material often used in crafts). The block
symbol ∆H. The absolute value of the enthalpy, H, for a substance is indeterminate, but has a hole drilled in it with a practical volume of about 15 mL and is coated to help
∆H for a particular process is directly measurable, at least in principle, and is a quantity prevent it from absorbing liquid. Although the calorimeter is fairly well insulated some
of fundamental interest in thermochemistry and thermodynamics. heat will escape or enter through hole in the lid provided for the thermometer, and some
heat will be absorbed into or from the calorimeter contents by the calorimeter itself. For
When one mole of a substance is formed from its constituent elements in their the purposes of this experiment we will ignore the former random error. But the
standard states at standard conditions, ∆H for the reaction is called the standard heat of systematic error introduced by ignoring the calorimeter itself can be reduced by using
formation and given the symbol ∆Hof . In this experiment you will try to find ∆Hof for the calorimeter constant in heat calculations. The average calorimeter constant for this
magnesium oxide, i.e., for the reaction: set-up has been determined to be 5.6 J/oC.

Mg(s) + ½ O2(g) → MgO(s)

You are faced with a practical difficulty, however. To cause magnesium and oxygen to
react they must be heated. The reaction then proceeds in an uncontrolled and highly
exothermic fashion. The only device that can deal with this reaction directly is a bomb
calorimeter [we don't own one].

However, so long as the system starts at and returns to standard conditions, the
path it follows is not important. Any series of reactions that begins with Mg and O2 and
ends with MgO will involve the same overall change in enthalpy.

Adapted from “An Introduction to Small Scale Thermochemistry”, Heat of Reaction:

Mg-HCl-H2O System, Walter Rohr, Flinn Scientific
Preparing to experiment Technique

You will be provided with the following materials: 1. Transfer errors

1. 3.0 cm of Mg ribbon In calculating the heat released by the reactions it is important to know the
[the mass in grams per metre will be given in the lab--be sure to record it!] mass of the reaction mixture. Because we are using small amounts in this experiment
2. 2.0 M HCl (use about 15 mL each time) [see Technique section] we need to try and minimize the error that would normally not be very significant when,
3. a calorimeter, lid and stirring bar for example, liquid is added from a graduated cylinder.
4. a magnetic stirrer
5. MgO (use about 0.5 g) [see Technique section] In the Carbonate Project you used a technique of massing the amount of liquid
6. a thermometer probe added to a reaction by the difference of the graduated cylinder full of the liquid and the
cylinder after the liquid had been poured out. That would be appropriate in this
Design an experiment to determine the heat of reaction when a measured amount of Mg experiment once again.
reacts with HCl (the amounts suggested make Mg the limiting reagent).
The MgO is a very light powder and tends to stick to glass to some extent so it
Design an experiment to determine the heat of reaction when a measured amount of too is a problem if you plan on measuring some into a beaker and dumping it into the
MgO reacts with HCl (the amounts suggested make MgO the limiting reagent). calorimeter. You could mass by difference but there is the additional problem of adding
the powder to a calorimeter full of HCl. The reaction begins immediately at the surface
BE SURE TO BRING YOUR TI-83/84 CALCULATOR and heat is lost (spray too) before the lid is replaced. One way to reduce this error is to
measure the MgO directly into the calorimeter and then add the HCl quickly.
TO CLASS FOR THIS EXPERIMENT. YOU WILL
ALSO NEED A COPY OF THE HCHEM.83G FILES IN For this to work it is important that the calorimeter is dry.
YOUR CALCULATOR MEMORY. 2. Drying the calorimeter
Pre-lab take-home quiz These blocks are fairly soft and you can damage the surface coating if you are
careless handling them. Drying the inside with our normal paper toweling requires a
Answer these questions on a separate sheet of paper to be turned in on the day you do gentle touch. A final drying with a softer Kimwipe is a good idea.
this experiment.
3. Dealing with the thermometer probe
1. Most Mg ribbon is uniform enough that the mass of a small measured length is
proportional to the mass of a larger measured length. Assume that the mass of 1.00 You need to suspend the probe so that the tip is in the solution but will not hit
metre of Mg ribbon is 0.4368 g. Show that using 3.0 cm of this ribbon and 15 mL of 2.0 the stirring bar. One way to do this is to use your ring and ring stand. The probe cable
M HCl makes Mg the limiting reagent in the reaction between the two substances. can be draped over the ring and the probe will dangle over the calorimeter. A little
adjustment to line things up, and it’s done!
2. Show how the three reactions given in the introduction actually add up to give the _____________________________________________________
formation reaction for MgO (i.e., set up the three equations and show how they add up,
canceling like terms, etc.) The chemicals

Magnesium oxide occurs in nature as the mineral periclase. It is a white, very fine,
odorless powder. It gradually picks up CO2 and water from the air, combining with the
water to form magnesium hydroxide (milk of magnesia).

Magnesium oxide is used the manufacture of refractory crucibles and bricks as well as
in medical applications as a laxative and antacid.
Analysis

These questions should be answered in your laboratory notebook following your data
and observations.

1. Use your data to determine the heat of reaction per mole of Mg for the reaction
between Mg and HCl. You did not use one mole of Mg, but enthalpy changes are
proportional to moles. Don't forget to include the calorimeter constant in the
calculations [the specific heat of the reaction mixture is 3.75 J/goC]

2. Use your data to determine the heat of reaction per mole of MgO for the reaction
between MgO and HCl. You did not use one mole of MgO, but enthalpy changes are
proportional to moles. Don't forget to include the calorimeter constant in the
calculations [the specific heat of the reaction mixture is 3.75 J/goC]

3. Use the molar enthalpies from (1) and (2) above, and the enthalpy value given earlier
for the formation of water to determine the heat of formation for MgO (∆Hof ) as you
demonstrated in your answer to question #2 in the pre-lab quiz.

4. With the final result the question arises (as it does in any experiment) as to its
accuracy. Using values obtained from handbooks, the following heats of reaction have
been calculated:
Reaction (1) -462 kJ/mol Mg

Reaction (2) -146 kJ/mol MgO

Reaction (4) -602 kJ/mol MgO

For each, determine the % error* in your experimental results and briefly discuss any
significant deviations with specific reference to things you did and would not do again,
etc.

* experimental - actual
actual x 100