Difficult Topics in the Chemistry Curriculum – Bulgarian Students‘ View

Chemistry is often considered a difficult subject, and the way students perceive learning affects their achievements. The research aims to identify: the topics in the secondary Chemistry curriculum perceived as difficult and as interesting by the Bulgarian students, the reasons for students’ difficulties and ways to overcome it. An inquiry was conducted with 321 students aged 16 – 17 years. According to them, the most difficult topics are Organic Chemistry and Chemical calculations (abstract and requiring specific skills). The most interesting topics are Organic Chemistry and Theory of Electrolytic Dissociation. Acids and bases. There is no significant correlation between levels of perceived difficulty and interest. Students relate difficulties to: information overload, emphasis on memorization, and lack of connections to everyday life. More lab activities and the practical application of the knowledge are recommended. Our research results can help improve Chemistry curricula and teaching practice.

Development of an Electrolytic Pilot Plant for the Production of Chlorine Gas “In Situ” in the Disinfecting Water Process

Technology of gaseous chlorine in water treatment has shifted to the dosage of sodium hypochlorite or calcium hypochlorite because of their greater biocide power; require less contact time with the microorganisms in the pretreated water, and the pH of slightly affect water. The generation of chlorine gas in place is based on the principles of electrolytic dissociation and laws of electrolysis by Faraday. The equipment corresponds to an electrolytic cell of three (3) compartments , a central one (anode) where chlorine gas emerges , and two side (cathodes) where sodium hydroxide is produced in the central compartment must be refilled chloride solution sodium consumed due to the electrochemical reaction , the chlorine evolved being of gaseous nature ascends the column of sodium chloride and is captured by the vacuum venture system , who to put in direct contact with target that has been previously subjected to the processes of uptake , coagulation , sedimentation and filtration. Compartments anode-cathode - anode are physically separated by porous diaphragm, whose purpose is to permit selective flow of sodium and chloride ions , avoiding side reactions recombination , as additional product is the formation of sodium hydroxide in the cathode chambers, which should be removed from the cell through side pipes. Statistical models were used to optimize the performance and operation of the prototype.

CONSTANTS OF ELECTROLYTIC DISSOCIATION OF THE LITHIUM, SODIUM AND POTASSIUM SULFATES IN AQUEOUS ISOPROPANOL SOLUTIONS

At introduction of isopropyl alcohol in saturated aqueous solutions of sulfates of lithium, sodium and potassium at 25 °C physico-chemical properties of the studied systems Li2SO4-H2O-C3H7OH, Na2SO4-H2O-C3H7OH and K2SO4-H2O-C3H7OH are changed. This reduces the density of solutions and salt content in aqueous isopropanol solutions due to a decrease in solubility of salts. It is shown that the variation of the volume content of alcohol from 0% to 90% results in the decrease of Li2SO4 solubility 1280 times, Na2SO4 – 548 times, K2SO4 – in 278 times. Alcohol additives also affect the degree of electrolytic dissociation of salt in aqueous isopropanol solutions. To study the electrochemical properties of salts we used conductometric method based on the measurement of the molar conductivity of solutions depending on salt concentration. In aqueous solutions, alkali metal sulfates exhibit the properties of strong electrolytes and almost completely dissociate into ions. When the volume content of isopropanol in the solution is more than 30%, alkali metal sulfates begin to show the properties of weak electrolytes, as evidenced by the correlation of the molar conductivity of the diluted solution with the salt concentration under the equation describing the state of weak electrolytes. From the transformations of experimental data in the coordinates of this equation, the values of the electrolytic dissociation constants of the studied salts were determined, which vary (8.30 ± 0.01)·10-5 to (4,35 ± 0,01)·10-8 (mol/l)2 when varying the alcohol content from 30 to 90% volume. It is shown that isopropanol additives reduce the constant (and hence the degree) of electrolytic dissociation of alkali metal sulfates: the higher the alcohol concentration in the solution, the weaker the salt becomes as an electrolyte. The value of the electrolytic dissociation constant depends on the nature of the salt: with an increase in the size of the sulfate cation, the electrolytic dissociation constant decreases.

CONSTANT OF ELECTROLYTIC DISSOCIATION OF LITHIUM, SODIUM AND POTASSIUM SULPHATES IN AQUEOUS ETHANOL SOLUTIONS

Constants of electrolytic dissociation of the lithium, sodium and potassium sulphates in aqueous ethanol solutions at 25 °C were determined by conductometric method. Li2SO4, Na2SO4, K2SO4 were shown to decrease their ability to dissociation. It is generally accepted that the degree of electrolytic dissociation of electrolytes in a solution depends on the dielectric constant of the solvent. Water and ethanol differ greatly in their dielectric constant. For this reason, it is possible to prepare the solvent with different dielectric permittivity by changing the alcohol content in a mixture with water, thus, influencing the equilibrium state of the salt in solution. Therefore, in water-ethanol solutions with an increase in the alcohol content, sulfates of lithium, sodium and potassium should exhibit the properties of a weak electrolyte. In this case, the dependence of the molar conductivity on the concentration of sulfates of lithium, sodium and potassium in water-ethanol solutions will have a different appearance. Indeed, with an increase in the alcohol content in aqueous ethanol solutions, the molar conductivity decreases due to a decrease in the degree of electrolytic dissociation. With a decrease in the salt concentration in the solution, the molar electrical conductivity increases, approaching λ0, for both water and water-alcohol solutions. As it follows from the obtained data, the dependence of λ on the total concentrations of Li2SO4, Na2SO4, K2SO4) for the water solution with a high correlation coefficient is transformed into a straight line in the coordinates of the Kolraush equation since the sulfates of lithium, sodium and potassium act as strong electrolytes in the aqueous medium. For water-ethanol solutions, a high correlation coefficient in the coordinates of the equation 4λ3· C0(M2SO4)2 = КDIS · λ03 - КDIS - λ02·λ is observed in the case of [C2H5OH]> 50% of the volume. This means that in these solutions lithium, sodium and potassium sulphates exhibit the properties of a weak electrolyte. When the alcohol content is from 10% to 40% by volume, sulfates of lithium, sodium and potassium are medium-strength electrolyte and, therefore, low correlation coefficients are characteristic of transformations in the coordinates of the equations Kohlrausch and 4λ3· C0(M2SO4)2 = КDIS · λ03 - КDIS - λ02·λ.