IB Chemistry  Energetics
Investigation: Hess’s Law
Name:__________________
Date:___________________
The combustion of magnesium is very rapid and exothermic, and is represented by the equation:
Mg(s) +
1
O2(g)  MgO(s) + heat
2
It is possible to observe and measure a series of reactions that enable us, with the use of Hess’s law, to
determine the enthalpy change for this reaction.
(1) Magnesium reacts in acid to form hydrogen gas and a salt:
Mg(s) + 2HCl(aq)  H2(g) + MgCl2(aq)
∆H1 = ? kJ
(2) Magnesium oxide reacts in acid to form water and a salt:
MgO(s) + 2HCl(aq)  H2O(l) + MgCl2(aq)
∆H2 = ? kJ
The values ∆H1 and ∆H2 can be determined empirically using a simple calorimeter. The enthalpy of
reaction for a third reaction can be found in reference table.
(3) Hydrogen and oxygen gases react to form water:
H2(g) +
1
O2(g)  H2O(l)
2
∆H3 = -285.8 kJ
______________________________________________________________________________
PURPOSE:
The purpose of this investigation is to use Hess’s law to determine the molar enthalpy of
combustion, ∆Hcomb, of magnesium, using calorimetry.
QUESTION:
What is the molar enthalpy of combustion, ∆Hcomb, of magnesium?
EXPERIMENTAL DESIGN:
Measured masses of magnesium and magnesium oxide will be added to measured volumes of
hydrochloric acid solution of known concentration. The temperature changes will be determined.
Calculated enthalpies of reaction will be combined, using Hess’s law, to determine the enthalpy
of combustion of magnesium.
*ALWAYS include: formulas and correct units, where applicable.
*Answer in complete sentences, where applicable.
PREDICTION:
(a) Show how the three known equations and their enthalpies of reaction may be combined, using
Hess’s law, to yield the target equation and its enthalpy of combustion.
(I /3)
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MATERIALS:
eye protection
steel wool
electronic balance
thermometer
polystyrene calorimeter
- 100 mL graduated cylinder
- scoopula
- 10 to 15 cm strip of magnesium ribbon
- magnesium oxide powder
- 1.00 mol/L hydrochloric acid
PROCEDURE:
1. Measure 100.0 mL of 1.00mol/l hydrochloric acid into a polystyrene cup. Measure the initial
temperature of the acid solution to the nearest 0.1°C.
2. Polish a length of magnesium ribbon with steel wool. Determine the mass, to the nearest 0.01
g, of approximately 0.3 g of magnesium metal. Add the solid to the solution and stir it, and
record the maximum temperature that the solution attains.
3. Dispose of the products in the waste beaker located in the fume hood.
4. Repeat the first three steps, using approximately 1 g of magnesium oxide powder measured to
the nearest 0.01 g.
ANALYSIS:
(b) Were the changes exothermic or endothermic? Explain.
(C /2)
______________________________________________________________________________
Reaction (1):
(c) For the first reaction, calculate the enthalpy change per mole of magnesium, ∆H1.
(I /4)
∆H1 = __________
(d) Write the thermochemical equation for the reaction of magnesium in acid, including your
experimental value.
(C /2)
______________________________________________________________________________
Reaction (2):
(e) For the second reaction, calculate the enthalpy change per mole of magnesium oxide, ∆H2.
(I /4)
∆H2 = __________
(f) Write the thermochemical equation for the reaction of magnesium oxide in acid, including
your experimental value.
(C /2)
2
(g) Using Hess’s law, the values you have found experimentally, and the given value for the
enthalpy change for the formation of water from its elements, determine the molar enthalpy of
combustion of magnesium.
(I / 4)
EVALUATION:
(h) Explain why (and how) your calculated enthalpies of reaction would be inaccurate if some
heat were transferred to the air or the Styrofoam cup.
(C / 3)
(i) The accepted value for the molar enthalpy of combustion of magnesium is
-601.6 kJ/mol. Calculate a percentage difference by comparing your experimental value
with the accepted value.
(I / 2)
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DATA:
Reaction (1)
mMg = _______
Molar mass (Mg) = _______
nMg = _______
Ti = ______ ;
Tf = ______ ;
∆T = ______
_______________________________________________________________________
Reaction (2)
mMgO = _______
Molar mass (MgO) = _______
Ti = ______
Tf = ______
;
;
nMgO = _______
∆T = ______
________________________________________________________________________
Recall:
∆Hrxn =
mcT
n
or
c = _______________
%diff. =
∆Hrxn =
;
q
n
m =______________
exp erimental  accepted
accepted
x 100%
*This page will not be marked. Include all formulas and show all calculations in the
spaces provided on pages 2 and 3.
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