Lab – Determining the Enthalpy Change of a Reaction
The reaction of hydrochloric acid with magnesium metal is represented by the following
equation:
2HCl(aq)
+
Mg(s)
MgCl2(aq)
+
H2(g)
QUESTION: What is the enthalpy change (∆H) for the reaction between hydrochloric acid
magnesium metal?
SAFETY PRECAUTIONS:


Aprons and safety glasses must be worn
If you get hydrochloric acid on your skin or in your eyes, flush your skin or eyes
with cold water for 15 minutes for eyes or 5 minutes for skin.
MATERIALS:




1.00 mol/L HCl (aq) – corrosive
and poisonous
magnesium strip
styrofoam cups (2)
thermometer





stir rod
100mL graduated cylinder
250mL beaker
stop watch or timing device
weigh scale
PROCEDURE:
1. Build a calorimeter out of the three Styrofoam cups as demonstrated in class.
You will need two holes in the top to put your stir rod and thermometer. The
holes should be as small as possible to minimize thermal energy exchange with
the surroundings.
2. Measure 50.0mL of HCl(aq) with the graduate cylinder and add the HCl(aq) to the
calorimeter.
3. Record the initial temperature of the HCl(aq).
4. Measure and record the mass of the magnesium strip. Quickly and carefully add
the magnesium strip to the 50.0mL of HCl(aq) in the calorimeter.
5. Place the top of the calorimeter in place and record the temperature every 30s.
Stir gently and continuously. Ensure that the thermometer is not touching the
bottom of the calorimeter.
6. When the temperature levels off, record the final temperature, tf. The
magnesium strip should be completely dissolved. If the magnesium metal is not
completely dissolved, wash off the remaining metal strip, allow it to dry and weigh
the remaining metal strip.
7. Repeat steps 4-7 two more times for a total of three trials.
OBSERVATIONS:
Trail #1
Initial Temperature of HCl(aq):___________
Time
(min. : sec.)
0:30
1:00
1:30
2:00
2:30
3:00
3:30
4:00
4:30
2:30
3:00
3:30
4:00
4:30
2:30
3:00
3:30
4:00
4:30
Temperature
(oC)
tf = _________________________
Trail #2
Initial Temperature of HCl(aq):___________
Time
(min. : sec.)
0:30
1:00
1:30
2:00
Temperature
(oC)
tf = _________________________
Trail #3
Initial Temperature of HCl(aq):___________
Time
(min. : sec.)
0:30
1:00
1:30
Temperature
(oC)
tf = _________________________
2:00
ANALYSIS:
1. Is the reaction of dissolving magnesium metal endothermic or exothermic?
Explain.
2. Determine the molar enthalpy change (∆rH) for the magnesium metal that
dissolved for each trail. Show only one sample calculation for trail 1and then
simply record your calculated values for trial 2 and 3.
3. Calculate the average molar enthalpy change for the reaction of magnesium
metal.
4. Write a thermochemical equation for the reaction. (Show the actual calculation
for how you go from a molar enthalpy change in question 3 to what I am asking in
this question.
5. Based off of your thermochemical equation, what is the molar enthalpy change
for HCl?
6. If the magnesium metal didn’t completely dissolve, why is it necessary to weigh
the remaining amount of magnesium metal left? If a student forgot to weigh the
remaining amount of magnesium and just used the total mass of the magnesium
in the calculations, how would the measured enthalpy and molar enthalpy
change differ from the correct values (ie. increase, decease, remain the same)?
7. Suppose you added the same amount of magnesium metal, but instead the
magnesium was granular (cut up into small pieces) instead of one strip. Would
you have obtained a different enthalpy change? Explain.