1.3. WORK SEQUENCE
All thermochemical measurements have two stages: firstly, heat capacity of calorimeter is determined and then the studied process heat is measured.
Determination of Calorimeter Heat Capacity
Heat capacity (thermal constant) of calorimeter Сk is a heat amount required to be added to calorimetric system to heat it by one degree. Heat capacity of a calorimetric setup is a sum of heat capacities of liquid and calorimeter parts that contact it and participate in heat exchange: a cup, a mixer, and a thermometer. One of experimental determination methods involves the reaction carried out in calorimeter with known thermal effect and measurement of corresponding temperature change. In this practicum, thermal constant of calorimeter is determined using known heat of dissolution of potassium chloride in water. Experiment is carried out in the following sequence. After preliminary thorough grinding, 2 grams of salt is weighed on the technical weighs. At 25 ºC temperature 200 ml of distilled water is poured in the inner cup. Testing tube with salt is installed in a special hole in the lid. Beckman thermometer is installed in the second hole, it should be preliminary adjusted so that after its immersing in a calorimetric liquid, mercury level should be in the middle of thermometer scale (such thermometer position makes it possible to study both endothermic and exothermic processes). After checking by hand that the mixer does not touch both the thermometer and the testing tube, the mixer driving electrical motor is switched on, then wait 10 minutes until the temperatures of all parts of the calorimetric system are equalized. Calorimetric system is considered to be prepared for an experiment when equal time periods correspond to equal and small temperature changes (at most 0.02 deg/min). It is also possible, when a thermochemical process temperature does not substantially change with time.
In case of differences between temperatures of a calorimeter and the surrounding air, the heat exchange will occur, that leads to distortion of temperature changes measured during the experiment. To provide corrections related to heat exchange with the surrounding environment, all calorimetric experiment is to be subdivided into three stages:
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1)Preliminary stage, at this stage before the experiment, regularities of temperature change with time is determined (10 readings every 30 seconds);
2)Main stage includes whole lead time of thermochemical process (its duration depends on the reaction rate);
3)Completion stage, at this stage temperature change with time is
recorded after completion of the studied process (10 readings every 30 seconds).
After preliminary preparation of calorimeter, temperature is registered with Beckman thermometer.
Every 30 seconds 10 temperature readings are recorded within the accuracy of 0,005°. This data characterize the preliminary stage. Then continuing the temperature records, testing tube with KCl is taken out and salt is poured with a funnel in the inner calorimetric cup, testing tube is installed in the previous place.
A moment when salt is poured in calorimeter is considered to be a start of the main stage of calorimetric experiment. Endothermic process of KCl dissolution in water is accompanied by abrupt temperature decrease. At the main stage, registration of temperature continues 10 readings every 30 seconds are recorded until temperature change again becomes regular. At the completion stage, 10 readings every 30 seconds are recorded and after this, the experiment is considered to be completed. The results are written in the Table 1.1:
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Experimental Data |
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Table 1.1 |
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Dissolution of КСl |
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Preliminary |
Main |
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Completion Stage |
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Stage |
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Period |
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# |
Т, °С |
# |
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Т, °С |
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Т, °С |
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1 |
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1 |
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1 |
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2 |
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2 |
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2 |
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… |
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… |
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… |
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Based on the obtained data, a temperature-time diagram is plotted on graph-paper (advised scale is 2 mm per 30 seconds, 2-4 mm per 0.01°).
In accurate experiment, temperature changes with time, relating to periods before and after reaction, correspond to straight lines, tilting of lines along the abscissa axis is determined by the ration between temperatures of the calorimeter and the environment.
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Fig. 1.3. Change of temperature during calorimetric experiment
Possible dependence of the temperature change on time is presented at Figure 1.3. Segment AB corresponds to preliminary stage, BC – main stage and CD - completion stage. Tangent lines to preliminary and completion stages are drown to determine the temperature change during the process. Perpendiculars to the abscissa axis are drawn from the terminal tangency points. The beginning of the main stage corresponds to the point B, end of the main stage is considered to be the first point on the line crossing all points of the completion stage. Time of the main stage В´С´ is divided in two periods: from point М´ perpendicular is to be erected until it intersects both extrapolated curves. Segment of perpendicular MN between these lines is the real temperature change ∆Т (including the heat exchange).
Calculation of Calorimeter Heat Capacity
Calorimeter heat capacity Ск is calculated using the equation of the heat balance:
∆H m KCI gKCI |
= −(C |
K |
+ m c ) ∆T , |
(1.29), |
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M KCl |
1 1 |
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where ∆Hm KCI is an integral heat of KCl dissolution (see Appendix);
gKCl, is the weight of a sample, g;
МKCl is the molecular weight of KCl, g/mole;
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m1 is the weight of solution in the calorimetric cup (water weight + salt weight), g;
с1 is the specific heat capacity of this solution (see Appendix), J/g∙deg; Т is the temperature change during the experiment (determined on
the base of the diagram), °С.
The left part of the equation corresponds to the heat of potassium dissolution in water, the right part of the equation is the multiplied total heat capacity of the calorimetric system (including heat capacity of the calorimeter and a solution in it) and the temperature change ∆Т caused by this heat.
Calorimeter heat capacity is calculated by the following equation:
C |
k |
= − |
∆Hm KCl gKCl |
−m c |
(1.30). |
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M KCl ∆T |
1 1 |
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LABORATORY EXERCISE 1.
DETERMINATION OF THE HEAT OF NEUTRALIZATION
OF A STRONG ACID BY A STRONG BASE
Work objective: to determine heat of neutralization of strong acid (HCl) by strong base (KOH).
The reaction between a strong acid and a strong base is used to determine the neutralization heat. Their molecules should completely dissociate into ions in moderately concentrated solutions.
When the calorimeter heat capacity is determined, the setup is prepared to study this thermochemical process. For this purpose, 5 ml of 2 М НС1 solution is measured with a pipette and poured into the inner cup containing calorimetric liquid (water solution of KCl remained after determination of calorimeter heat capacity). 5 ml of 2 М KOH solution is poured into another testing tube that is fixed in the lid of a calorimeter. Wait 10-15 minutes until temperature change becomes regular, then temperature is registered every 30 seconds. When 10 records were made, the alkali solution is poured into the inner cup of a calorimeter, where neutralization reaction runs, then temperature is registered with the Beckman thermometer during 5-6 minutes (10-12 readings), a completion stage includes 10 readings every 30 seconds.
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A “Temperature – time” diagram is plotted on the basis of obtained data, temperature change ∆Т is determined for neutralization process. To determine true value of temperature change corrected to heat exchange, the diagram is processed as stated in the Section 1.
Calculation of Neutralization Teat
Heat of neutralization ∆Hneutral. is calculated by the following equation:
∆Hneutral. = −(CK + m2 с1 ) ∆Tneutral. |
(1.31), |
where Ск is a heat capacity of calorimeter, determined in the previous experiment, J/deg;
m2 is a total weight of solution in calorimeter (weight of KCl solution + weight of acid + weight of base), g;
с1 is a specific heat capacity of solution (see in Appendix), J/deg;
Тneutral. is a temperature change during the experiment (determined from the diagram), °С.
Thermal effect, observed during base pouring into an acid solution, comprises the neutralization heat and the heat of acid dissolution, but the dissolution heat is sufficiently small, because of small volumes of acid and base, compared to the volume of water, so the dissolution heat can be neglected.
Neutralization enthalpy change ∆Hexper. per 1 gram-equivalent of acid is determined by the following equation:
∆Hexper. = |
∆Hneutral. 1,000 |
(1.32) |
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V C |
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where V is an acid volume, ml, С is an acid concentration, mole/l. Finally, the relative error of the determined value is calculated by the
following equation:
σ = |
∆Htheor. −∆Hexper. |
100% |
(1.33). |
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∆Htheor. |
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Make a conclusion.
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LABORATORY EXERCISE 2.
CALCULATION OF HEAT OF DISSOCIATION OF A WEAK ACID
OR A WEAK BASE
Work objective: to determine heat of dissociation of weak acid (СН3СООН).
The reaction between a weak acid and a strong base or a strong acid and a weak base is a complex process comprising of neutralization, dissociation and inter-dilution of solutions. Weak electrolytes, in particular, weak acids and bases, incompletely dissociate in ions in solution. For this reason, while mixing weak acid and strong base or weak base and strong acid, water is formed from Н+ and ОН- ions, weak acid or weak base dissociation process also occurs.
When mixing it is necessary to take into account dilution of acids and bases, so in this case mixing heat ∆Hmixing. equals to the sum of the following thermal effects:
—heat of water formation from H+ and OH– ions – ∆Hneutral. ;
—heat of dissociation of weak acids or bases - ∆Hdiss. ;
—heat of dilution of acid by base – ∆Hdillut.1 ;
—heats of dilution of base by acid – ∆Hdillut.2 ;
∆Hmixing = ∆Hneutral. + ∆Hdillut.1 + ∆Hdillut.2 + ∆Hdiss (1.34).
Work Sequence
The constant of a calorimeter is determined by the procedure described in the Section 1.
The reaction heat ( ∆Hmixing. )is determined for the solution of
СН3СООН and NaOH. 150 ml of 0.1 M alkali solution is poured into the clean calorimetric cup installed in calorimeter. 5 ml of 5 М acid solution is poured with pipette into the special testing tube and then installed into the calorimeter lid. When calorimetric setup is assembled, it is necessary to wait 10 minutes for equilibration in calorimeter. Then temperature is registered every 30 seconds during the preliminary stage. When 10 – 15 records were made, acid solution is poured through a
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funnel in the inner cup of calorimeter to carry out a reaction, continuing registration of temperature change (20 records every 30 seconds).
Graphical method is applied to determine time of reaction between
weak acid and strong base and corresponding value ΔTmixing.
Heat of water formation (neutralization of HCl by NaOH) can be from
the experiment |
or taken |
from a table |
of |
reference values |
∆Htheor. = −55.9kJ / mole ; |
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Then heat of |
dilution of |
СН3СООН acid |
by |
alkali ∆Hdillut.1 is |
determined. For this 200 ml of distilled water is poured in a clean calorimetric cup. 5 ml of acetic acid solution (С= 5 mole/l) is poured into testing tube installed in the calorimeter lid. An experiment is carried out similarly to determination of heat of mixing reaction. Time of dilution
reaction and corresponding temperature change ΔTdillut.1 are determined by the graphical method.
Since alkali volume is high and does not change while acid adding, value can be regretted: ∆Hdillut.2 ≈ 0.
Calculation of Dissociation Heat
∆Hmixing is calculated by the following equation (1.35):
∆Hmixing = −[mc +Ck ]∆Tmixing |
(1.35), |
where Ck is a calorimeter constant, J/deg;
m is a total weight of solution in calorimeter (weight of acid solution + weight of alkali solution), g;
с is a water heat capacity – 4.18 J/g∙deg,
∆Tmixing is a temperature change during a mixing reaction °С. Heat of dilution of acid by alkali is calculated by the equation (36).
∆Hdillut.1 = −[mc + Ck ]∆Tdillut.1 |
(36), |
where m is a total weight of solution in calorimeter (water weight + acid weight),
с is a water heat capacity – 4.18 J/g∙deg,
∆Tdillut.1 is a temperature change of dilution reaction, °С,
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Ck is a calorimeter constant, J/deg.
In this case, the heat of dissociation reaction is calculated by the
equation 35. Heat of dilution of alkali by acid ∆Hdillut.2 can be neglected, because alkali volume is considerably higher than the acid volume.
∆Hdiss = ∆Hmixing − ∆Hneutral. − ∆Hdillut.1 |
(1.35). |
Make a conclusion.
LABORATORY EXERCISE 3.
CALCULATION OF INTEGRAL HEAT OF SALT DISSOLUTION
Work objective: to determine dissolution heats of salts: NaCl, Na2SO4, NiSO4, KJ, NH4Cl, LiCl, K2SO4
Work Sequence
Firstly, the heat capacity of a calorimeter is determined (see Section Determination of Calorimeter Heat Capacity).
2 grams of examined salt should be poured into a dry testing tube installed in the calorimeter lid hole. Experiment is carried out similarly to the determination of the calorimeter constant without pouring the liquid out of the calorimetric cup.
A “Temperature - time” diagram is plotted based on obtained results,
then temperature change ∆Тdissol., resulted from the heat of studied thermochemical process, is determined from the diagram.
Calculation of Heat of Salt Dissolution
Heat ∆Hdissol (J/mole) of salt sample dissolution is calculated by the following equation:
∆Hdissol. = − |
(CK +m |
с1 ) ∆Tdissol. M s |
(1.38), |
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where m is a weight of solution in calorimeter (water weight+ KCl weight + salt weight), g;
с1 is a specific heat capacity of this solution (without big error, it is assumed to be equal to specific heat capacity of water at this temperature) 4.18 J/g∙deg;
gs is a weight of salt, g;
Ms is a molar weight of salt.
Equation 39 is used to estimate the accuracy of the calculated value of an integral heat of dissolution to the reference value (see Appendix).
s = |
∆Htheor. −∆Hdissol. 100% |
(1.39). |
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∆Htheor. |
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Make a conclusion.
LABORATORY EXERCISE 4.
CALCULATION OF HEAT OF CRYSTALLOHYDRATE
FORMATION (HYDRATION HEAT)
Work objective: to determine heats of formation of СuSO4·5H2O from CuSO4 and H2O.
Crystallohydrate formation heat is called a heat of formation of a mole of solid crystallohydrate from a solid anhydrous salt and corresponding amount of water.
Dissolution of anhydrous cuprum sulphate runs according to the following equation:
CuSO4(cryst.) + nH2O(liq) = CuSO4(solution) |
∆Hdissol.anhydr.salt |
CuSO4(cryst.) + 5H2O(liq.) = CuSO4 5H2O(cryst.) |
∆Hhydration |
CuSO4 5H2O(cryst.) + (n −5)H2O(liq) = CuSO4(solution) ∆Hdissol.hydr.salt. |
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According to the Hess’s law: |
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∆Hdissol.anhydr.salt = ∆Hhydration + ∆Нdissol.hydr.salt. |
(1.40). |
Heat of crystallohydrate formation ∆Hhydration |
cannot be directly |
measured in a calorimeter, because of the small rate of crystallohydrate
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formation. This value is calculated from the difference between integral dissolution heats of an anhydrous salt ∆Hdissol.anhydr.salt and crystallohydrate
∆Hdissol.hydr.salt. .
Work Sequence
Samples of an anhydrous salt and a crystallohydrate should have the weights providing the same concentration for prepared solutions. Two samples of preliminary powdered CuSO4 5H2O crystallohydrate are used to achieve it. The weight of each sample is 2 grams. One sample is heated in a drying oven at the temperature of 240-250 °C until sample obtains the constant weight (m = 1.28 g) and has its color changed from blue to white that corresponds to the anhydrous salt CuSO4. The anhydrous salt CuSO4 is cooled down and stored in desiccators of testing tube plugged with a rubber stopper.
The heat capacity of a calorimeter is determined by the method described in the Section 1. Calorimetric setup is discharged and prepared for the next experiment. To determine hydration heat, it is necessary to carefully follow the instruction described in the Section 4: determine dissolution heats of an anhydrous salt and the same salt containing crystallization water. The values Т1 и Т2 are determined by the graphical method.
Calculation of Hydration Heat
Integral dissolution heat of an anhydrous salt is calculated by the following equation:
∆Hdissol.anhydr.salt = − |
(CK + m1 с1 ) ∆T1 M anhydr.salt |
(1.41). |
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Integral dissolution heat of a crystallohydrate is calculated by the following equation:
∆Hdissol.hydr.salt = − |
(CK + m2 с1 ) ∆T2 Mhydr.salt |
(1.42). |
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According to the Hess’s law heat of hydrates formation equals to the difference between molar heats of dissolution of an anhydrous salt and a crystallohydrate:
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