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

Addition of a solvent to corresponding solutions with final concentration is also accompanied by thermal effects (dilution heat), because of changes in interparticle interactions. Lower dilution heats are observed for more dilute solutions.

There are the following types of dilution heats:

Integral dilution heat ( Hm0 ) is a heat effect occurs while solution

contained a mole of dissolved substance dilutes from concentration m to an infinitely small concentration m 0 .

Intermediate integral dilution heat ( H m1 ) is a heat effect occurs

m2

while solution contained one mole of dissolved substance dilutes from concentration m2 to an infinitely small concentration m1

Ratios between integral dilution heat and integral dissolution heat are represented by the following equations:

Differential (partial) molar heats of dissolution and dilution are also used in calculations. Differential molar heats are calculated from the integral heats. Differential molar heat of dilution ( H1,m )

describes an addition of a mole of solvent to infinite large amount of solution of concentration m. Differential molar heat of dissociation

( H2,m ) describes an addition of a mole of dissolved substance to

infinite large amount of solution of concentration m.

Integral heats of dissolution are determined experimentally, differential heats are determined by calculations.

Along with heats of dissolution heats of neutralization, dissociation, and crystallohydrate formation are used in thermochemistry.

Neutralization heat (Δ H neutr) is a heat effect of neutralization reaction between one mole of any strong mono acid ( HCl, HNO3 , etc.) and strong base ( NaOH , KOH , etc). Neutralization reaction is as follows:

11

CALORIMETERS AND THEIR APPLICATION

Calorimeters (or calorimetric systems) are used to measure heat effects. Calorimetric system is a reactor placed into a container. A container works in one of two possible regimes: it prevents heat exchange between reactor and environment (an isolated system) or it makes the control of heat exchange easier (a closed system).

In general, calorimeters are subdivided into calorimeters with constant or varying temperature. In the first case, a container includes melting solid substances (such calorimeters are called ice calorimeters) or a vaporizing liquid. During an experiment in such a calorimeter, the temperature is constant, because all obtained heat is consumed for a phase transition. Heat is evaluated using the amount of melted of evaporated substance.

In case of calorimeters with varying temperature, two following measurement methods are possible:

Adiabatic method is a method when a container temperature is varied during the experiment in such a way, that it equals to a reactor temperature at any time; in this case heat exchange does not proceed, so reactor is an isolated system; these calorimeters are applied for measuring small thermal effects or thermal effects of slow processes;

Diathermic method when the heat exchange occurs between a reactor and an isothermal container (a reactor is the closed system); container has the almost constant temperature, because it contains a significant amount of water that has a relatively high heat capacity.

These calorimeters are usually applied for measuring heat capacity, heat of dissolution, dilution, neutralization, state change, and combustion. In the latter case, the reaction runs in a calorimeter bomb with a constant volume. At this practicum, a calorimeter with an air isothermal container is used; its schematic configuration is presented at the Figure 1.

Cup 1 is an external container of calorimeter. Inside isothermal container, there is calorimeter cup 2, where thermochemical process runs. The cup is covered with the thermal insulating lid 3. Air layer between cups serves for protective layer that decrease heat exchange between calorimeter and environment.

13

There are holes in the calorimeter lid: for mixer 4, for thermometer 5 and the testing tube 6 with studied reagent. Electric motor driven mixer 4 provides rapid temperature equalization and active mixing of studied substances. During the thermochemical experiment the temperature change is measured with Beckman thermometer.

Beckman thermometer (Figure 2) serves to measure small temperature changes and consists of main 1 and additional 2 mercury reservoirs connected

with capillary

3. Additional reservoir makes it possible to adjust the thermometer by mercury transferring from one reservoir to another at the temperature from - 20 to 15 ºC. Beckman thermometer used at this practicum can measure the temperature changes in the range from 5 to 0.005 ºC.

WORK SEQUENCE

All thermochemical measurements have two stages: firstly, heat capacity of

calorimeter is determined and then the studied process heat is measured.

14

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:

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°).

16

Calculation of Calorimeter Heat Capacity

Calorimeter heat capacity Ск is calculated using the equation of the heat balance:

Appendix);

gKCl, is the weight of a sample, g;

МKCl is the molecular weight of KCl, g/mole;

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:

2. 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).

18

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.

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:

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;

19

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,

mole/l.

Finally, the relative error of the determined value is calculated by the following equation:

3. 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.

20