Lab 9: Reaction Stoichiometry from Heat of Reaction

Jeffrey Wang05 November 2013

Abstract

Samples of potassium thiocyanate (KSCN) and of hydrogen peroxide (H2O2) with a combined

volume of 100 mL were mixed in a Styrofoam cup, causing an exothermic reaction. The change in

temperature was measured, and the volume for which the highest temperature change would

theoretically be observed was found. This volume was approximately 69 mL, yielding an H2O2-to-KSCN

ratio of 2.22 to 1. These findings suggest that the coefficient of hydrogen peroxide in the unbalanced

chemical equation KSCN + H2O2 products + q is 2.22.

Introduction

The purpose of this experiment was to determine the stoichiometry (i.e. the ratio of reactants)

of a reaction from the amount of heat evolved by the reaction. The reaction examined involved

potassium thiocyanate (KSCN) and hydrogen peroxide (H2O2). Stoichiometric coefficients may be

determined by noting how the mixture’s properties (in this case, heat) change depending on the ratios

in which the reactants are mixed, since the change should be at its maximum when the reactants are

combined in proportions equal to that of their coefficients in the chemical equation (Parr). Knowledge

of coefficients is crucial in understanding chemical interactions, and this particular method of calculating

them—via calorimetry—is a versatile system with applications in, for example, enzymology (Sturtevant).

Experimental

This experiment required the following equipment: two Styrofoam cups, two 50-mL graduated

cylinders (100-mL graduated cylinders, which the lab manual called for, were unavailable), two 400-mL

beakers, one glass stirring rod, and the PASCO system with temperature probe. Chemicals required

were as follows: 0.50 M KI, 0.50 M H2O2 in 1.0 M HCl, 0.50 M Na2SO3, 0.50 M Na2S2O3, and 0.50 M KSCN

in 0.1 M NaOH. Only the KSCN and the H2O2 were used in this particular experiment.

Standard lab safety equipment (lab coat, gloves, goggles) was worn. Contact with chemicals was

scrupulously avoided.

To measure temperature changes, samples of KSCN and H2O2 measuring approximately 300 mL

each were obtained in separate beakers (the lab manual called for 600-mL samples, but the limitation of

beaker size meant that two 300-mL samples of each substance had to be obtained over the course of

the experiment instead). Using the graduated cylinders, samples of KSCN and H2O2 were measured out

in volumes that were multiples of 10 and totalled 100 mL, starting with 90 mL of KSCN and 10 mL of

H2O2, for a total of nine trials. For each trial, the temperature of the KSCN was taken in the graduated

cylinder; the H2O2 was poured into the Styrofoam cup (two were used, one inside the other, for extra

insulation) and stirred while its temperature was taken. The temperature probe was held in place by a

clamp attached to a ring stand and lowered into the Styrofoam cup such that its tip was approximately

an inch from the bottom of the cup. The KSCN was then poured into the cup and the mixture was

stirred consistently. The maximum temperature reached was recorded, and the temperature change

was calculated, using the average of the KSCN and H2O2 temperatures prior to mixing as the initial

temperature.

The change in temperature (ΔT) was then plotted against the volume of H2O2 used, with ΔT on

the y-axis and H2O2 volume on the x-axis. Best-fit lines were drawn for the two distinct regions, and the

x-value at the intersection was taken as the stoichiometrically correct volume of H2O2 solution required.

Results

Table 1: Measuring Temperature ChangesKSCN:H2O2 90:10 80:20 70:30 60:40 50:50 40:60 30:70 20:80 10:90

KSCN temp. (°C)

23.5°C 23.3°C 23.7°C 23.5°C 23.3°C 24.0°C 23.6°C 23.8°C 23.5°C

H2O2 temp. (°C) 21.6°C 21.8°C 22.1°C 22.3°C 22.4°C 22.4°C 22.6°C 22.7°C 22.2°C

Tinitial (°C) 22.55°C 22.55°C 22.9°C 22.9°C22.85°

C23.2°C 23.1°C 23.25°C 22.85°C

Tfinal (°C) 24.6°C 29.8°C 34.7°C 38.4°C 42.2°C 45.0°C 46.1°C 41.5°C 32.5°C

ΔT (°C) 2.05°C 7.25°C 11.8°C 15.5°C19.35°

C21.8°C 23.0°C 18.25°C 9.65°C

Figure 1: ΔT (°C) vs. Volume of H2O2 (mL)

0 10 20 30 40 50 60 70 80 90 1000

5

10

15

20

25

30

Volume of H2O2 (mL)

T (°C) Δ

The mixture was found to rise in temperature until H2O2 volume had reached 70 mL and

decreased after that point (Table 1). The perceived stoichiometrically correct volume of H2O2—that is,

the point of intersection of the two best-fit lines—appears to be slightly less than 70 mL, perhaps at

approximately 69 mL. From this the ratio can be calculated to be 0.0345 moles of H2O2 to 0.0155 moles

of KSCN, or about 2.22, implying that the (unbalanced) chemical equation is as follows:

KSCN + 2.22H2O2 aKH + bCO2 + cSO2 + dNH3 + q,

where a, b, c, and d are the unknown coefficients of the products.

However, this is not necessarily accurate; as is evident in Figure 1, the data point at 70 mL H2O2

is a clear outlier, fitting neither with the left or right side’s best-fit lines yet influencing both. This may

be due to experimental error (e.g. recording the temperature too soon, or mixing inaccurate amounts of

each solution), but a more likely explanation is that the insulation was insufficient. As the temperature

increased, it deviated further and further from room temperature, reaching a maximum recorded

temperature of 46.1°C (Table 1). Hot air rises, and the cup was uncovered; furthermore, the act of

stirring would have constantly exposed new molecules to the air above the cup, thus facilitating the

cooling process and leading to a slight reduction in the observed temperature change. Indeed, the value

of ΔT at 60 mL H2O2 had already begun to deviate from the best-fit line, and the three values of ΔT from

50 to 70 mL H2O2 form an unmistakable curve, indicating a downward trend in the rate of ΔT’s increase.

65 70 75 80 85 90 950

5

10

15

20

25

30

Volume of H2O2 (mL)

T (°C) Δ

Moreover, error may also arise from the lack of data; only nine points of data were acquired,

and the right-hand best-fit line is based off of only three points. The accuracy of this experiment would

have benefited from additional trials focused on the H2O2 volumes between 60 and 80 mL.

Discussion

The calculated ratio of H2O2 to KSCN was approximately 2.22 to 1, although a number of flaws in

the experimental procedure may have produced some error in this result.

When determining the stoichiometric ratio, it was important to maintain the total volume of the

reacting solutions at a constant value because the temperature change is a result of the amount of

reactants consumed in the reaction. Varying the total volume of reacting solutions would have

introduced inconsistencies and disparities in other aspects of the reaction, such as reaction speed (since

amount of reactants affects reaction rate); keeping the total volume the same removes these variables

from consideration and ensures that the temperature change is, for the most part, a direct result of the

volume of reactants consumed.

If the reaction occurred with the absorption of heat, the graph of ΔT vs. volume of H2O2 would

be shaped like the letter ‘V’, like so:

ΔT would be a negative quantity, since the reaction would cause the mixture’s temperature to drop.

The mixture must have a sufficiently low freezing point, however, because observation would become

significantly more difficult if it cooled down to the point that it froze over; moreover, results past this

point would have a larger degree of error because the energy required to freeze would have to be

accounted for.

References

Parr. Advanced General Chemistry 115a Lab Manual, Fall 2013.

Sturtevant, J. M. “Calorimetry”. Methods Enzymol. 1972, 26, 227.