A student conducts an experiment to determine the enthalpy of neutralization for phosphoric acid
reacting with sodium hydroxide. The student combines 25.0 mL of equimolar solutions of acid and
base, both having an initial temperature of 22.5 °C, in an open polystyrene calorimeter. Each
solution has a density of 1.00 g/mL and a specific heat of 4.18
J
g • °C
. The student collects data until
a maximum temperature of 26.4 °C is reached.
(a) What assumption is the student allowed to make?
(b) Explain how the student knows data collection is complete.
(c) Calculate the qrxn for the neutralization reaction.
(d) Is the reaction exothermic or endothermic? Justify your answer.
(e) Calculate the ΔHneut
if the initial molarities of both solutions is equal to 0.80 M. Be sure to
report your answer with the correct sign and units.
See your post above.
(a) The assumption the student is allowed to make is that the heat capacities of the polystyrene calorimeter and the solution are negligible compared to the heat released or absorbed during the neutralization reaction.
(b) The student can determine that data collection is complete when the temperature stops increasing and reaches a stable maximum value. This indicates that the reaction has reached completion, and no more heat is being released or absorbed.
(c) To calculate the qrxn for the neutralization reaction, we can use the equation:
qrxn = m * C * ΔT
where qrxn is the heat released or absorbed by the reaction, m is the mass of the solution (which can be calculated from the volume and density), C is the specific heat of the solution, and ΔT is the change in temperature.
First, we need to calculate the mass of the solution:
mass = volume * density
mass = 25.0 mL * 1.00 g/mL
mass = 25.0 g
Next, we can calculate the change in temperature:
ΔT = final temperature - initial temperature
ΔT = 26.4 °C - 22.5 °C
ΔT = 3.9 °C
Now, we can calculate the qrxn:
qrxn = 25.0 g * 4.18 J/g • °C * 3.9 °C
qrxn ≈ 388 J
Therefore, the value of qrxn for the neutralization reaction is approximately 388 J.
(d) To determine whether the reaction is exothermic or endothermic, we need to analyze whether heat is being released or absorbed. In this case, since the temperature of the solution increased, heat is being released, which indicates that the reaction is exothermic.
(e) To calculate the ΔHneut, we need to use the equation:
ΔHneut = qrxn / moles of limiting reactant
Since both the acid and the base are equimolar and have an initial molarity of 0.80 M, we can assume that the limiting reactant is either the acid or the base. In this case, we can choose either one.
Let's assume the limiting reactant is the acid. This means that 25.0 mL (0.025 L) of the acid solution contains 0.80 M * 0.025 L = 0.02 moles of acid.
Therefore, ΔHneut = 388 J / 0.02 moles of acid = 19400 J/mol
Since enthalpy is usually reported in kilojoules per mole (kJ/mol), we can convert the answer to the desired units:
ΔHneut ≈ 19.4 kJ/mol (exothermic)