Сalorimetric measurements of thermal effects of chemical reactions and physicochemical processes. Laboratory training guidance

When mixing it is necessary to take into account dilution of acids and bases, so in this case mixing heat H 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 (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 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 valuesHtheor. 55.9kJ / mole ;

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

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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 (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).

where m is a total weight of solution in calorimeter (water weight

+acid weight),

сis a water heat capacity – 4.18 J/g∙deg,

Tdillut.1 is a temperature change of dilution reaction, °С, Ck is a calorimeter constant, J/deg.

In this case, the heat of dissociation reaction is calculated by the equation 37. Heat of dilution of alkali by acid Hdillut.2 can be neglected, because alkali volume is considerably higher than the acid volume.

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4. 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 1).

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:

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).

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5. 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:

Heat of crystallohydrate formation Hhydration cannot be directly measured in a calorimeter, because of the small rate of crystallohydrate formation. This value is calculated from the difference between integral dissolution heats of an anhydrous saltHdissol.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

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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:

Integral dissolution heat of a crystallohydrate is calculated by the

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:

Hhydrat. formation. Hdissol.anhydr.salt. Нdissol.hydr.salt. (43).

Estimation of Measurement Error

The relative error of the measured thermal constant of a calorimeter Ск is calculated by the equation 44:

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Different measurements give absolute errors Δ: temperature registration with the naked eye using Beckman thermometer - ∆(∆Т) = ± 0.005°, weighing on a technical weight ∆g = ± 0,02 g. Error of water weight measurement can be regretted because of its small value.

The relative error of a process heat ∆Н is determined by the equation

where ∆Т is the temperature change during the studied reaction, g is the reactant weight.

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APPENDIX

where H1 , H2 are reference values of integral heats of dissolution of salt in water; m1 , m2 are corresponding molarities of solutions.

Table 2

Integral Heats of Dissolution of Studied Salts

(for the molarity of the solution obtained by dissolution of 2 g of salt in 200 ml of Н2О)

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Table 3

Special heat capacity of KCl solutions

(for the molarity of the solution obtained by dissolution of 2 g of salt in 200 ml of Н2О)

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Test Questions

1.What is a thermodynamic system?

2.What is a state parameter?

3.What is a state equation?

4.Determine the first law of thermodynamics.

5.Write the mathematical expression for the first law of thermodynamics.

6.What quantities are bonded by the first law of thermodynamics?

7.What is thermochemistry?

8.What does Kirchhoff's law allow to calculate?

9.What is an essence of Hess’s law?

10.Write the mathematical expression for Kirchhoff's law in differential form.

11.Write the mathematical expression for Kirchhoff's law in integral form with consideration of dependence of reaction heat

capacity change on temperature СР =f(T).

12.Write the mathematical expression for Kirchhoff's law in integral form for case, when heat capacity change does not depend on temperature ( Ср =const).

13.Write the mathematical expression for Kirchhoff's law in integral form for case, when heat capacity does not change during reaction ( Ср =0).

14.Write the mathematical expression for Hess’s law.

15.What is a molar heat capacity?

16.What is a molar specific capacity? What is the dimension of a molar specific capacity?

17.What is a mean heat capacity?

18.Write the mathematical expression for true molar heat capacity? What is the dimension of a true molar heat capacity?

19.Write the mathematical expression for a mean heat capacity. What is the dimension of a mean heat capacity?

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20.Give a definition for the heat of a chemical reaction.

21.What is a standard heat of formation ( Н°f,298 )?

22.What substances are elementary substances and complex substances?

23.Give the definition for a standard heat of combustion.

24.Describe reactions and processes which occur during the dissolution of solid salts.

25.Explain why the same heat amount releases under neutralization of any strong acid by any strong base.

26.What is an integral molar heat of dissolution?

27.Which factors influence the value of an integral heat of dissolution?

28.Explain the difference between the first, intermediate and last integral heats of dissolution.

29.What is the difference between an integral dissolution heat and an integral dilution heat?

30.What is a calorimetric measurements method?

31.What calorimeters do you know?

32.What is the heat capacity (constant) of a calorimeter?

33.How is constant of calorimeter determined?

34.What is the method of a graphical determination of the reaction time and the respective temperature change?

35.What is the method for the determination of the neutralization heat of a strong acid by a strong base?

36.Describe all stages of the experiment for the determination of a salt dissolution heat.

37.What is a sequence to determine the heat of dissociation of weak acids and alkali?

38.Describe all the heats which can be calculated when we determine dissociation heats of weak acids and alkali.

39.How is a heat of crystallohydrate dissolution determined?

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