Chemistry
Name ________________________
Heat of Reaction and Hess’s Law
Partner _______________________
(adapted from Flinn Scientific, Inc., Heat of Reaction and Hess’s Law, AP6441)
PURPOSE:
The heat of combustion for magnesium is too high to be measured in a high school
laboratory setting. Hess’s Law allows the calculation of this value using related reactions
that are much safer to measure.
The purpose of this experiment is to use Hess’s Law to determine the heat of reaction
for the combustion of magnesium. The heats of reaction for magnesium and
hydrochloric acid and magnesium oxide and hydrochloric acid will be measured and
combined with the heat of formation of water using Hess’s Law.
Mg(s) + 2HCl(aq)  MgCl2(aq) + H2(g)
MgO(s) + 2HCl(aq)  MgCl2(aq) + H2O(l)
H2(g) + ½O2(g)  H2O(l)
MATERIALS:
Magnesium ribbon (s)
Magnesium oxide (s)
1.0 M HCl(aq)
Aluminum foil
Centimeter ruler
Balance
25 mL graduated cylinder
Coffee cup calorimeter
Vernier LabQuest & temperature probe
SAFETY:
 HCl is toxic and corrosive. Wear goggles and lab coats at all times and wash
hands immediately and thoroughly if substances contact skin.
 Do NOT let thermometer rest on side of calorimeter--it may tip over.
PROCEDURE:
Reaction 1 – Mg(s) and HCl(aq)
1. Measure the mass of the clean, dry calorimeter (cups only).
2. Use UNEQUAL lengths of magnesium in each trial. Measure the length of the
magnesium sample to the nearest 0.01 cm and convert to mass using the
conversion factor on the board. Record mass of shorter piece as trial 1.
3. Measure about 15 mL of 1.0 M HCl. Pour into calorimeter. Record the mass of
the acid and calorimeter.
4. Measure initial T (Ti) of HCl and record to nearest 0.1ºC.
5. Carefully add first (shorter) piece of Mg to calorimeter. Swirl calorimeter gently;
stop when the temperature remains constant for 15 s. Record the highest T (Tf)
reached to nearest 0.1°C.
6. The solution is neutral and may be poured in the sink with excess water.
Thoroughly rinse and dry the cups for reuse.
7. Repeat steps 1-6 for a 2nd trial using 2nd (longer) piece of Mg.
Reaction 2 – MgO(s) and HCl(aq)
1. Measure the mass of the clean, dry calorimeter (cups only).
2. CAREFULLY measure 15 mL of 1.0 M HCl. Pour into calorimeter. Record the
mass of the acid and calorimeter.
3. Weigh out about 0.20 g of magnesium oxide. Record the mass accurately.
4. Measure initial T (Ti) of HCl and record to nearest 0.1ºC.
5. Carefully add magnesium oxide sample to calorimeter. Swirl calorimeter gently;
stop when the temperature remains constant for 15 s. Record the highest T (Tf)
reached to nearest 0.1°C.
6. The solution is neutral and may be poured in the sink with excess water.
Thoroughly rinse and dry the cups for reuse.
7. Repeat steps 1-6 for a 2nd trial using 2nd sample of magnesium oxide.
QUESTIONS:
1. Determine the heat absorbed by the solution for each reaction. (use average
values of trials for mass and ΔT).
2. Calculate the moles of solid used in each reaction (use average of masses for
each trial). Calculate the enthalpy of the reaction in kJ/mol.
(remember qsoln = -qrxn)
3. Write thermochemical equations for reactions 1 and 2.
4. Show the use of Hess’s Law to calculate heat of combustion for magnesium. Use
measured values for first two reactions and the reference value for the third
reaction listed in purpose section.
5. Calculate the % error of calculated value of heat of combustion for magnesium.
DATA TABLE:
Name: _________________
Reaction 1
Trial 1
Trial 2
mass of calorimeter (g)
mass of HCl + calorimeter (g)
mass of acid (g)
mass of solid (g)
mass of solution
Ti (ºC)
Tf (ºC)
ΔT of solution
Reaction 2
Trial 1
Trial 2