THE HEAT OF REACTION
In this experiment, you will measure and compare the quantity of heat
involved in three chemical reactions.
The reactions are:
Reaction 1: Solid sodium hydroxide dissolves in water to form an
aqueous solution of ions:
NaOH(s)
--> [Na]+(aq) +
[OH]-(aq) + x1J /\H = -x1J
Reaction 2: Solid sodium hydroxide reacts with an aqueous
solution of hydrogen chloride to form water and an aqueous solution of sodium
chloride:
NaOH(s)
+ [H]+(aq) +
[Cl]-(aq) --> H2O +
[Na]+(aq) + [Cl]-(aq) + x2J /\H= -x2J
Reaction 3: An aqueous solution of sodium hydroxide reacts with an
aqueous solution of hydrogen chloride to form water and an aqueous solution of
sodium chloride:
[Na]+(aq) + [OH]-(aq) +
[H]+(aq) + [Cl]- --> H2O + [Na]+(aq) + [Cl]-(aq) + x3J /\H= -x3J
Note that the energy term is shown both as part of the equation and
separately with the "/\H" notation. You will calculate the amount of energy after measuring the
temperature change of a known quantity of water. An expanded polystyrene cup will serve as the calorimeter in this
experiment. Assume that the heat of
reaction will be used to change the temperature of the aqueous solution
only. Neglect small losses to the
surroundings. You do not need to find
the mass of the water used since 1.0mL of water has a mass of 1.0g and you will
measure the volume of water to the nearest millilitre.
Procedure
Part 1 - DETERMINATION OF THE HEAT OF REACTION 1
Caution: Sodium hydroxide is extremely corrosive to the skin and causes
blindness.
1. Put 150mL of cool tap water into an expanded polystyrene
cup. Stir carefully with a thermometer
until a constant temperature is reached (about room temperature). Measure this temperature as precisely as
possible and record it.
2. Weigh out about 4g of solid sodium hydroxide, NaOH(s),
as precisely as possible. Since sodium
hydroxide becomes moist as it is being weighed in the open air, your teacher
will demonstrate the rapid weighing of a prescribed number of solid pellets
which will approximate 4g (between 3.9 and 4.1).
3. Pour the weighed NaOH(s) into the water in the expanded
polystyrene cup. Place the thermometer
into the solution and stir gently but continuously until the sodium
hydroxide is dissolved. Record the
extreme temperature reached. Before
proceeding to reaction 2, discard the solution and rinse the cup thoroughly
with water.
Part 2 - DETERMINATION OF THE HEAT OF REACTION 2
1. Repeat steps 1, 2
and 3 of Part 1, but substitute 150mL of 0.50M HCl for the tap water in step 1.
2. Discard the solution
and rinse the cup before proceeding to reaction 3.
Part 3 - DETERMINATION OF THE HEAT OF REACTION 3
1. Measure 75mL of 1.0M HCl into the expanded polystyrene
cup and 75mL of 1.0M NaOH into a 250mL beaker.
Both of these solutions should be at, or slightly below, room
temperature. Check this with the
thermometer (rinse and dry the thermometer before changing from one solution to
the other). Record the temperatures.
2. Add the sodium hydroxide solution to the hydrochloric
acid solution in the expanded polystyrene cup.
Mix quickly and record the extreme temperature reached.
Calculations and Results
1. For each reaction,
calculate:
a) the change in temperature
b) the amount of heat absorbed by the solution
c) the number of moles of NaOH used
d) the amount of heat evolved per mole
of NaOH
2. Express the above
results as heats of reaction: /\H1, /\H2, /\H3.
3. Write the net ionic
equation for reactions 2 and 3.
4. In reaction 1, /\H1 represents the
heat evolved as solid NaOH dissolves.
Look at the net ionic equations for reactions 2 and 3 and make similar
statements as to what /\H2 and /\H3
represent.
5a. Compare /\H2
with (/\H1 + /\H3) and explain in terms of
your answer to question 4.
b. Calculate the percentage difference between /\H2
with (/\H1 + /\H3), assuming /\H2
to be the correct value.
[/\H2 - (/\H1 + /\H3)]//\H2
* 100%
6. Suppose you had used
8g of NaOH(s) in reaction 1.
a) How would this have affected the change in
temperature?
b) What would have been the number of joules
evolved in your experiment?
c) What effect would this have on your
calculations of /\H1, the heat evolved per mole?
Data Table format:
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Experiment # |
mNaOH |
VNaOH |
CNaOH |
VHCl |
CHCl |
VH2O |
nNaOH |
nHCl |
m heat absorber |
T1 |
T2 |
/\T |
Q |
/\H |
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