Heat of Formation of Nitronium Perchlorate

Abstract

Calorimetric measurements of the heat of solution of nitronium perchlorate (NO₂ClO₄) and of a mixture of potassium nitrate and potassium perchlorate in aqueous potassium hydroxide have been made. These are combined to give:

$$\begin{array}{c}{\text{NO}}_2{\text{ClO}}_4\left(c\right)+4\text{KOH}\left(\text{in}5300{\mathrm{H}}_2\mathrm{O}\right)\to {\text{KClO}}_4\left(c\right)+{\text{KNO}}_3\left(c\right)+2\text{KOH}\left(\text{in}5300{\mathrm{H}}_2\mathrm{O}\right)+{\mathrm{H}}_2\mathrm{O}\left(\text{liq}\right)\\ \mathrm{\Delta }H=-285.80\pm 0.38\text{kj}/\text{mole}\\ =-68.31\pm 0.09\text{kcal}/\text{mole}\end{array}$$

from which the standard heat of formation of nitronium perchlorate is calculated as

$$\begin{array}{c}\mathrm{\Delta }Hf°\left(25°\mathrm{C}\right){\text{NO}}_2{\text{ClO}}_4\left(c\right)=37.19\pm 1.0\text{kj}/\text{mole}\\ =8.89\pm 0.25\text{kcal}/\text{mole,}\end{array}$$

in which the uncertainty represents twice the estimated overall standard deviation of the result.

1. Introduction

This investigation was carried out in the Thermochemistry Section of the National Bureau of Standards as part of a program, currently in progress, on the determination of the thermodynamic properties of the “light element” compounds. Nitronium perchlorate is a white crystalline material stable at temperatures up to 120 °C. It is extremely hygroscopic, reacting rapidly with water to form nitric and perchloric acids. The heat of formation of NO₂ClO₄ can be determined by measuring this heat of decomposition. In this investigation, aqueous solutions of potassium hydroxide were used, as the potassium salts of the acids provided more suitable reference substances.

2. Materials

The potassium nitrate and potassium perchlorate were reagent grade materials, dried at 120 °C and stored in a desiccator over anhydrous magnesium perchlorate. The potassium hydroxide was reagent grade material; the calorimetric solutions were prepared with CO₂-free distilled water, and standardized against potassium acid phthalate. The NO₂ClO₄ was obtained from the Callery Chemical Company as a white crystalline powder; an analysis furnished by them showed:

Nitrogen	6.75−6.74	theoretical	6.874 milliatoms/g
Chlorine	6.90−6.82	theoretical	6.874 milliatoms/g
Total acid	13.61–13.71	theoretical	13.75 milliequiv/g
NO+	0.06–0.1	theoretical	0.00 milliequiv/g

Analyses for total acid by titrations with standard base, and for NO⁺ by oxidation with standard ceric sulfate solution and back titration with standard ferrous ammonium sulfate gave:

Total acid	13.74	±0.01 milliequiv/g
NO+	0.06	±0.01 milliequiv/g

The addition of silver nitrate to the acid solution gave a negative test for chloride ion.

From the ratio of total acid measured to total acid theoretical, the purity of the sample was estimated to be 99.93 mole percent or 99.65 weight percent. Since NO₂ClO₄ is extremely hygroscopic the major impurity is presumed to be water.

3. Units of Energy, Molecular Weights, and Conversion Factors

The joule was taken as the unit of energy. All instruments were calibrated in terms of standards maintained at the National Bureau of Standards. For conversion to the conventional thermochemical calorie, one calorie is taken as 4.1840 joules. All weights are corrected to vacuum.

All atomic weights were taken from the 1957 International Table of Atomic Weights [1].1 The heat capacities were taken, where possible, from the literature [2]. For NO₂ClO₄, an estimated value of 36 cal/deg mole was used.

4. Apparatus and Procedure

The glass calorimeter, thermometric system, apparatus for measurement of electrical energy, and general calorimetric procedure have been described [3, 4, 5]. The calorimeter contained 454.6 g of a 0.04126 N KOH solution. The sealed glass ampoule contained about 0.005 mole NO₂ClO₄. The calorimeter assembly was placed in a thermostatically controlled water-bath maintained at 25.0 ±0.002 °C. After the run, the calorimeter solution was titrated with standard acid, using a Beckman pH meter, as a check on the mass of sample and completeness of reaction. No calculations were based on the titrated values of NO₂ClO₄, since the weighed mass of sample was more accurate.

The heat of solution of an equimolar mixture of crystalline KNO₃ and KClO₄ was measured in the same apparatus. The calorimeter contained 454.6 g of a 0.02077 N KOH solution. The sample size was adjusted to give the same final solution as in the NO₂ClO₄ hydrolysis experiment.

Two series of electrical energy calibrations were carried out, one with 454.6 g of 0.04126 N KOH solution and a bulb containing 0.005 mole NO₂ClO₄, the second with 454.6 g of 0.02077 N KOH solution and a bulb containing 0.005 moles each of KNO₃ and KClO₄.

5. Results and Calculations

The results of the calibration experiments on the calorimetric system used for the measurement of the heat of reaction of NO₂ClO₄ are given in table 1. ΔRc corresponds to the corrected temperature rise of the system [6]. The energy equivalent, Es, of the “standard” system was obtained as the ratio of the quantity of electrical energy, Ee, to ΔRc, the corresponding rise in temperature.

Table 1.

Electrical calibrations of the NO₂ClO₄ system

Expt.	ΔRc	Ee	Es
	Ohm	j	j/ohm
1	0.094266	1,990.86	21,119.6
2	.097337	2,053.87	21,100.6
3	.096767	2,042.58	21,108.2
4	.096023	2,026.13	21,100.5
5	.095990	2,025.34	21,105.6
Mean			21,106.9
Standard deviation of the mean			±3.5

The results of the experiments on the hydrolysis and neutralization of NO₂ClO₄ in solution I are given in table 3. Here, Δe is the change in the energy equivalent from the “standard” system, due to deviations in the mass of the glass bulb and sample from that of the reference bulb and sample. The term q(dil) represents the correction for the dilution of the individual final solutions to a uniform concentration. This was calculated from the data in [2], assuming that only the KOH(aq) contributed any heat effect. The total energy, q, is shown by the equation q=ΔRc(Es+Δe) +q(dil).

Table 3.

Results of the experiments on the hydrolysis of NO₂ClO₄

Expt.	Δe	ΔRc	q(dil)	q	Weighed NO2ClO4	Titrated NO2ClO4	$\frac{{\text{NO}}_2{\text{ClO}}_4\left(t\right)}{{\text{NO}}_2{\text{ClO}}_4\left(w\right)}$	−ΔH (25 °C)
	j	Ohm	j	j	Mole	Mole	%	kj/mole
1a	−13.6	0.023575	0.84	498.11	0.0024629	0.00238b	96.7	202.24
2	−10.1	.042890	.15	904.99	.0045364	.004546	100.2	199.50
3a	−10.2	.041598	.15	877.73	.0044650	.004434	99.4	196.58
4	−9.7	.049401	.00	1042.22	.0052450	.005253	100.2	198.71
5	−7.0	.063000	−.53	1328.76	.0066728	.006603	99.0	199.13
6	−7.7	.059705	−.38	1259.35	.0063211	.006246	98.8	199.23
7	−9.9	.046153	.04	973.73	.0049043	.004901	100.0	198.55
Mean								199.02
Standard deviation of the mean								±0.17

^aNot included in mean value.

^bMethyl orange end point.

The results of the electrical calibration experiments on the KNO₃/KClO₄ system, and heat of solution experiments with the equimolar mixture of KNO₃ and KClO₄, are given in tables 2 and 4, respectively.

Table 2.

Electrical calibration on KNO₃/KClO₄ system

Expt.	ΔRc	Ee	Es
	Ohm	j	j/ohm
1	0.092808	1962.35	21,144.1
2	.085947	1817.33	21,144.8
3	.095268	2013.70	21,137.2
4	.093441	1975.56	21.142.3
Mean			21,142.1
Standard deviation of the mean			±1.7

Table 4.

Results of the experiments on the solution of KNO₃ and KClO₄

Expt.	Δe	−ΔRc	KNO3	KClO4	qKNO3	q	ΔH (25 °C)
	j	Ohm	Mole	Mole	j	j	kj/mole
1	−0.47	0.020236	0.0048956	0.0049288	−1.16	428.98	87.03
2	−1.43	.020453	.0049324	.0049920	−2.09	434.48	87.04
3	−1.54	.020401	.0049127	.0049985	−3.60	434.29	86.88
4	−1.32	.020485	.0049677	.0049943	−0.93	434.00	86.90
5	−1.36	.020450	.0049381	.0049863	−1.69	434.02	87.04
Mean							86.98
Standard deviation of the mean							±0.04

The term q(KNO₃) in table 4 is the product of the difference between moles of KClO₄ and moles of KNO₃ and the heat of solution of KNO₃. It is applied to make the molar quantities of KNO₃ and KClO₄ exactly equal.

For the reaction

$$\begin{array}{l}{\text{NO}}_2{\text{ClO}}_4\left(c\right)+\left[4\text{KOH}+5300{\mathrm{H}}_2\mathrm{O}\right]\left(\text{soln}\right)\to \\ \left[{\text{KNO}}_3+{\text{KClO}}_4+2\text{KOH}+5301{\mathrm{H}}_2\mathrm{O}\right]\left(\text{soln}\right)\\ \mathrm{\Delta }H=-199.02\pm 0.35\text{kj}/145.465\mathrm{g}\text{sample}\\ =-47.57\pm 0.08\text{kcal}/145.465\mathrm{g}\text{sample}\end{array}$$ (1)

based upon mass of sample.

This value must be corrected for the presence of 0.35 weight percent moisture. If it is present as the hydrolysis products HNO₃ and HClO₄, it corresponds to 0.0283 mole of each acid.

Therefore eq (1) actually corresponds to

$$\begin{array}{c}0.9682{\text{NO}}_2{\text{ClO}}_4\left(c\right)+0.0283{\text{HNO}}_3\left(\text{liq}\right)+0.0283\\ {\text{HClO}}_4\left(\text{liq}\right)+\left[4\text{KOH}+5300{\mathrm{H}}_2\mathrm{O}\right]\left(\text{soln}\right)\to \\ \left[0.9965{\text{KNO}}_3+0.9965{\text{KClO}}_4+2.0070\text{KOH}+5301{\mathrm{H}}_2\mathrm{O}\right]\left(\text{soln}\right)\\ \mathrm{\Delta }H=-199.02\pm 0.35\text{kj}\\ =-47.57\pm 0.08\text{kcal}.\end{array}$$

For 1 mole of NO₂ClO₄ this becomes

$$\begin{array}{l}{\text{NO}}_2{\text{ClO}}_4\left(c\right)+0.0292{\text{HNO}}_3\left(\text{liq}\right)+0.0292\\ {\text{HClO}}_4\left(\text{liq}\right)+1.0328\left[4\text{KOH}+5300{\mathrm{H}}_2\mathrm{O}\right]\to \\ \left[1.0292{\text{KNO}}_3+1.0292{\text{KClO}}_4+2.0728\text{KOH}+5475{\mathrm{H}}_2\mathrm{O}\right]+1.0584{\mathrm{H}}_2\mathrm{O}\left(\text{liq}\right)\\ \mathrm{\Delta }H=-205.56\pm 0.36\text{kj}/\text{mole}\\ =-49.13\pm 0.09\text{kcal}/\text{mole}.\end{array}$$ (1a)

From data in the literature [2], eq (1b) may be evaluated:

$$\begin{array}{l}{\text{HClO}}_4\left(\text{liq}\right)+{\text{HNO}}_3\left(\text{liq}\right)+\left[4\text{KOH}+5300{\mathrm{H}}_2\mathrm{O}\right]\to \\ \left[{\text{KClO}}_4+{\text{KNO}}_3+2\text{KOH}+5300{\mathrm{H}}_2\mathrm{O}\right]+2{\mathrm{H}}_2\mathrm{O}\left(\text{liq}\right)\\ \mathrm{\Delta }H=-230.9\text{kj}/\text{mole}\\ =-55.2\text{kcal}/\text{mole}.\end{array}$$ (1b)

Appropriate combination of eqs (1a) and (1b) gives

$$\begin{array}{l}{\text{NO}}_2{\text{ClO}}_4\left(c\right)+1.0036\left[4\text{KOH}+5300{\mathrm{H}}_2\mathrm{O}\right]\to \\ \left[{\text{KClO}}_4+{\text{KNO}}_3+2.0144\text{KOH}+5320{\mathrm{H}}_2\mathrm{O}\right]+{\mathrm{H}}_2\mathrm{O}\left(\text{liq}\right)\\ \mathrm{\Delta }H=-198.82\text{kj/mole}\\ =-47.52\text{kcal/mole}.\end{array}$$ (2)

From table 4 we have

$$\begin{array}{l}{\text{KNO}}_3\left(c\right)+{\text{KClO}}_4\left(c\right)+\left[2\text{KOH}+5300{\mathrm{H}}_2\mathrm{O}\right]\left(\text{soln}\right)\to \\ \left[{\text{KNO}}_3+{\text{KClO}}_4+2\text{KOH}+5300{\mathrm{H}}_2\mathrm{O}\right]\left(\text{soln}\right)\\ \mathrm{\Delta }H=86.98\text{kj/mole}\\ =20.79\pm 0.02\text{kcal/mole}.\end{array}$$ (3)

Combining eqs (2) and (3) and neglecting small dilution effects, we obtain:

$$\begin{array}{c}{\text{NO}}_2{\text{ClO}}_4\left(c\right)+4\text{KOH}\left(\text{in}5300{\mathrm{H}}_2\mathrm{O}\right)\to \\ {\text{KClO}}_4\left(c\right)+{\text{KNO}}_3\left(c\right)+2\text{KOH}\left(\text{in}5300{\mathrm{H}}_2\mathrm{O}\right)+{\mathrm{H}}_2\mathrm{O}\left(\text{liq}\right)\\ \mathrm{\Delta }H=-285.80\text{kj/mole}\\ =-68.31\text{kcal/mole}.\end{array}$$ (4)

If we take the following values for the heats of formation of the other substances in the reaction [7]

Substance	ΔHf°298.15 (kcal/mole)
KClO4(c)	−103.45
KNO3(c)	−118.12
H2O(liq)	−68.314
KOH(2650 H2O)	−115.23

we compute for NO₂ClO₄(c)

$$\begin{array}{l}\mathrm{\Delta }Hf{°}_{298.15}=37.19\pm 1.0\text{kj/mole}\\ =8.89\pm 0.25\text{kcal/mole}.\end{array}$$

The uncertainty interval for the measured heats has been taken as twice the overall standard deviation of the mean based on the sum of the variances from the calibration and reaction experiments and reasonable variances assigned to the auxiliary data and the analysis for total acid.

6. Discussion

Cordes and Fetter [8] measured the heat of reaction of NO₂ClO₄ with water, they report a value for the heat of formation of NO₂ClO₄(c) of 8.0 ±0.4 kcal/mole. Recalculating their data with the more recent auxiliary heats of formation used for this work, their value becomes 8.7±0.4 kcal/mole, in excellent agreement with the results obtained here.