We take sodium thiosulfate and three acids (sulfuric, hydrochloric and orthophosphoric):
Na2S2O3 + H2SO4 = Na2SO4 + SO2 + S + H2O
Na2S2O3 + 2 HCl = 2 NaCl + SO2 + S + H2O
3 Na2S2O3 +2 H3PO4 = 2 Na3PO4 + 3 SO2 + 3 S + 3 H2O
Pour into three test tubes 8 ml of sodium thiosulfate solution. Pour 8 ml of sulfuric acid into the first test tube with a solution of sodium thiosulfate, mix quickly and note the time in seconds from the start of the reaction to the cloudiness of the solution. To better notice the end of the reaction, glue a strip of black paper on the opposite side of the test tube wall. We finish the time report at the moment when this strip is not visible through the cloudy solution.
Similarly, we carry out experiments with other acids. The results are entered in the table (Appendix 1, Table 1). The reaction rate is defined as a value inversely proportional to time: υ = 1/ t. Based on the table, we build a graph of the dependence of the reaction rate on the nature of the reactants (Appendix 2, graph 1).
Conclusion: thus, the nature of acids affects the rate of a chemical reaction. And, since the strength of acids is determined by the concentration of hydrogen ions, the reaction rate also depends on the concentration of the reactants.
B. Consider the reaction of the interaction of various metals with hydrochloric acid. The reaction rate will be determined by the volume of released hydrogen, which is collected by the method of water displacement (Appendix 3, Figure 1).
In four test tubes we place 0.05 g of metals: magnesium, zinc, iron and copper. In turn, pour equal volumes of hydrochloric acid (1:2) into each test tube (a). Hydrogen, which will be rapidly depleted, will enter the test tube (b). Note the time it takes for the tube to fill with hydrogen. Based on the results (Appendix 4, Table 2), we build a graph depending on the nature of the reactants (Appendix 4, Chart 2).
Conclusion: not all metals can interact with acids by removing hydrogen. Metals that displace hydrogen from acid solutions are located in the series N.N. Beketov to hydrogen, and metals that do not displace hydrogen - after hydrogen (in our case, this is copper). But the first group of metals also differ in the degree of activity: magnesium-zinc-iron, therefore, the intensity of hydrogen evolution is different.
Thus, the rate of a chemical reaction depends on the nature of the reactants.
2. Dependence of the rate of a chemical reaction on the concentration of interacting substances.
Target. Establish a graphical dependence of the effect of concentration on the reaction rate.
For the experiment, we use the same solutions of sodium thiosulfate and sulfuric acid that were used in the first experiment (A).
Pour the indicated amounts of milliliters of sodium thiosulfate solution and water into numbered test tubes. Pour 8 ml of sulfuric acid solution into the first test tube, mix quickly and note the time from the beginning of the reaction to the cloudiness of the solution (see experiment 1 A). We carry out similar experiments with the rest of the test tubes. We enter the results in a table (Appendix 6, Table 3), on the basis of which we build a graph of the dependence of the rate of a chemical reaction on the concentration of reactants (Appendix 7, Chart 3). We obtained a similar result by keeping the concentration of sodium thiosulfate constant, but changing the concentration of sulfuric acid.
Conclusion: thus, the rate of a chemical reaction depends on the concentration of the reacting substances: the higher the concentration, the greater the reaction rate.
3. Dependence of the rate of a chemical reaction on temperature.
Purpose: To test whether the rate of a chemical reaction depends on temperature.
We carry out the experiment with solutions of sodium thiosulfate and sulfuric acid (see experiment 1), additionally we prepare a beaker, a thermometer.
Pour 8 ml of sodium thiosulfate solution into four test tubes, 8 ml of sulfuric acid solution into 4 others. We place all test tubes in a glass of water and measure the temperature of the water. After 5 minutes, we take out two test tubes with solutions of sodium thiosulfate and sulfuric acid, drain them, mix and note the time until the solution becomes cloudy. We heat a glass with water and test tubes by 10 ° C and repeat the experiment with the next two test tubes. We carry out the same experiments with the rest of the test tubes, each time increasing the water temperature by 10°C. The results obtained are recorded in a table (Appendix 8, Table 4) and we plot the dependence of the reaction rate on temperature (Appendix 9, Chart 4).
Conclusion: this experiment led to the conclusion that the rate of a chemical reaction increases with an increase in temperature for every 10°C by 2–4 times, i.e. proved the validity of Van't Hoff's law.
4. Effect of a catalyst on the rate of a chemical reaction.
Purpose: to check whether the rate of a chemical reaction depends on the catalyst, and whether the catalysts have specificity.
A. To test the specificity of the catalyst, we used the decomposition reaction of hydrogen peroxide: 2H2O2 = 2H2O + H2. They took a 3% solution, the decomposition of hydrogen peroxide is very weak, even a smoldering splinter dropped into a test tube does not flare up. We used silicon dioxide SiO2, manganese dioxide MnO2, potassium permanganate KMnO4, and sodium chloride NaCl as catalysts. Only when manganese (IV) oxide powder was added did a rapid evolution of oxygen occur, a smoldering splinter, lowered into a test tube, flared up brightly.
Thus, catalysts are substances that speed up a chemical reaction, and, most often, a particular reaction requires its own catalyst.
5. Kinetics of the catalytic decomposition of hydrogen peroxide.
Purpose: to find out the dependence of the reaction rate on the concentration of substances, temperature and catalyst.
The decomposition of a very weak solution of hydrogen peroxide begins under the influence of a catalyst. With the course of the reaction, the concentration of hydrogen peroxide decreases, as can be judged by the amount of oxygen released per unit time. We carry out the experiment in the device (Appendix 10, Figure 2): put 0.1 g of manganese dioxide powder into a test tube, attach it to a rubber tube, pour 40 ml of a 3% hydrogen peroxide solution into the flask, connect it with a test tube using a rubber tube. We fill the cylinder (burette) with water, lower it into the crystallizer, fix it vertically in the clamp of the tripod, and bring the gas outlet tube from the Wurtz flask under it. Without a catalyst, oxygen evolution is not observed. After adding manganese dioxide, every minute for 10 minutes we note and write down in the table the volume of released oxygen (Appendix 11, Table 5). Based on the data, we build a graph of the dependence of the volumes of released oxygen on time (Appendix 12, graph 5)
6. Influence of the contact surface of reactants on the rate of a chemical reaction.
Target. Find out whether the contact surface of the reactants affects the rate of a heterogeneous chemical reaction.
The same amount (0.5 g) of chalk (CaCO3) in the form of a piece and powder was weighed on a balance, the weighed portions were placed in two test tubes, into which the same amount of hydrochloric acid (1:2) was poured. We observe the release of carbon dioxide, and in the first test tube (chalk in the form of a piece) the reaction is less vigorous than in the second (chalk in the form of powder) (Appendix 13, photos 1.2): CaCO3 + 2 HCl = CaCl2 + CO2 + H2O
Sodium thiosulfate is a synthetic compound known in chemistry as sodium sulphate, and in the food industry as additive E539, approved for use in food production.
Sodium thiosulfate acts as an acidity regulator (antioxidant), anti-caking agent or preservative. The use of thiosulfate as a food additive allows you to increase the shelf life and product quality, prevent rotting, souring, fermentation. In its pure form, this substance is involved in the technological processes for the manufacture of food iodized salt as an iodine stabilizer and is used to process bakery flour, which is prone to caking and clumping.
The use of food additive E539 is limited exclusively to the industrial sector, the substance is not available for retail sale. For medical purposes, sodium thiosulfate is used as an antidote for severe poisoning and as an external anti-inflammatory agent.
general information
Thiosulfate (hyposulfite) is an inorganic compound that is the sodium salt of thiosulfuric acid. The substance is a colorless, odorless powder, which on closer examination turns out to be transparent monoclinic crystals.
Hyposulfite is an unstable compound that does not occur naturally. The substance forms a crystalline hydrate, which, when heated above 40 ° C, melts in its own crystalline water and dissolves. Molten sodium thiosulfate is prone to supercooling, and at a temperature of about 220 ° C, the compound is completely destroyed.
Sodium thiosulfate: synthesis
Sodium sulphate was first obtained artificially in the laboratory by the Leblanc method. This compound is a by-product of soda production that results from the oxidation of calcium sulfide. Interacting with oxygen, calcium sulfide is partially oxidized to thiosulfate, from which Na 2 S 2 O 3 is obtained using sodium sulfate.
Modern chemistry offers several ways to synthesize sodium sulphate:
- oxidation of sodium sulfides;
- boiling sulfur with sodium sulfite;
- interaction of hydrogen sulfide and sulfur oxide with sodium hydroxide;
- boiling sulfur with sodium hydroxide.
The above methods make it possible to obtain sodium thiosulfate as a by-product of the reaction or as an aqueous solution from which the liquid must be evaporated. You can get an alkaline solution of sodium sulphate by dissolving its sulfide in oxygenated water.
The pure anhydrous compound of thiosulfate is the result of the reaction of the sodium salt of nitrous acid with sulfur in a substance known as formamide. The synthesis reaction proceeds at a temperature of 80 ° C and lasts about half an hour, its products are thiosulfate and its oxide.
In all chemical reactions, hyposulfite manifests itself as a strong reducing agent. In interaction reactions with strong oxidizing agents, Na 2 S 2 O 3 is oxidized to sulfate or sulfuric acid, with weak oxidants to a tetrathione salt. The oxidation reaction of thiosulfate is the basis of the iodometric method for determining substances.
Special attention deserves the interaction of sodium thiosulfate with free chlorine, which is a strong oxidizing agent and a toxic substance. Hyposulfite is easily oxidized by chlorine and converts it into harmless water-soluble compounds. Thus, this compound prevents the destructive and toxic effects of chlorine.
Under industrial conditions, thiosulfate is extracted from gas production waste. The most common raw material is lighting gas, which is released during coal coking and contains impurities of hydrogen sulfide. Calcium sulfide is synthesized from it, which is subjected to hydrolysis and oxidation, after which it is combined with sodium sulfate to obtain thiosulfate. Despite the multi-stage nature, this method is considered the most cost-effective and environmentally friendly method for extracting hyposulfite.
| Systematic name | Sodium thiosulfate (Sodium thiosulfate) |
|---|---|
| Traditional Appellations | Sodium sulfate, hyposulfite (sodium) soda, antichlor |
| International marking | E539 |
| Chemical formula | Na 2 S 2 O 3 |
| Group | Inorganic thiosulfates (salts) |
| State of aggregation | Colorless monoclinic crystals (powder) |
| Solubility | Soluble in, insoluble in |
| Melting temperature | 50 °C |
| Critical temperature | 220 °С |
| Properties | Reducing (antioxidant), complexing |
| Dietary Supplement Category | Acidity regulators, anti-caking agents (anti-caking agents) |
| Origin | Synthetic |
| Toxicity | Not tested, the substance is conditionally safe |
| Areas of use | Food, textile, leather industry, photography, pharmaceuticals, analytical chemistry |
Sodium thiosulfate: application
Sodium sulphate has been used for a variety of purposes long before its inclusion in food supplements and medicines. Antichlor was impregnated with gauze bandages and filters of gas masks to protect the respiratory organs from poisonous chlorine during the First World War.
Modern areas of application of hyposulfite in industry:
- film processing and fixing images on photographic paper;
- dechlorination and bacteriological analysis of drinking water;
- removal of chlorine stains when bleaching fabrics;
- leaching of gold ore;
- production of copper alloys and patina;
- skin tanning.
Sodium sulphate is used as a reagent in analytical and organic chemistry, it neutralizes strong acids, neutralizes heavy metals and their toxic compounds. The interaction reactions of thiosulfate with various substances are the basis of iodometry and bromometry.
Food supplement E539
Sodium thiosulfate is not a widely used food additive and is not freely available due to the instability of the compound and the toxicity of its degradation products. Hyposulfite is involved in technological processes for the production of food iodized salt and bakery products as an acidity regulator and anti-caking agent (anti-caking agent).
Additive E539 performs the functions of an antioxidant and preservative in the manufacture of canned vegetables and fish, desserts and alcoholic beverages. This substance is also part of the chemicals that treat the surface of fresh, dried and frozen vegetables and fruits.
The preservative and antioxidant E539 is used to improve the quality and increase the shelf life of such products:
- fresh and frozen vegetables, fruits, seafood;
- , nuts, seeds;
- vegetables, mushrooms and seaweed preserved in or oil;
- jams, jellies, candied fruits, fruit purees and fillings;
- fresh, frozen, smoked and dried fish, seafood, canned food;
- flour, starches, sauces, seasonings, vinegar, ;
- white and cane, sweeteners (dextrose and), sugar syrups;
- fruit and vegetable juices, soft drinks, soft drinks, grape juices.
In the manufacture of table iodized salt, the food additive E539 is used to stabilize iodine, which can significantly extend the shelf life of the product and preserve its nutritional value. The maximum allowable concentration of E539 in table salt is 250 mg per 1 kg.
In the baking business, sodium thiosulfate is actively used as part of various additives to improve product quality. Bread improvers are oxidative and reductive. Anti-caking agent E539 refers to improvers of restorative action that allow you to change the properties.
Dough made from dense flour with short-tearing gluten is difficult to process, cakes, does not reach the required volume and cracks during baking. The anti-caking agent E539 destroys disulfide bonds and structures gluten proteins, as a result of which the dough rises well, the crumb becomes loose and elastic, and the crust does not crack when baking.
At enterprises, an anti-caking agent is added to flour along with yeast immediately before dough is kneaded. The content of thiosulfate in flour is 0.001-0.002% of its mass, depending on the technology of manufacturing a bakery product. Sanitary and hygienic standards for the additive E539 are 50 mg per 1 kg of wheat flour.
Anti-caking agent E539 is used in technological processes in a strict dosage, so there is no risk of thiosulfate poisoning when using flour products. Flour intended for retail sale is not processed before sale. Within the normal range, the supplement is safe and does not have a toxic effect on the body.
Use in medicine and its effect on the body
Soda hyposulfite is included in the list of essential medicines of the World Health Organization as one of the most effective and safe medicines. It is injected under the skin, intramuscularly and intravenously as an injection or used as an external agent.
In the early twentieth century, sodium thiosulfate was first used as an antidote for hydrocyanic acid poisoning. In combination with sodium nitrite, thiosulfate is recommended for particularly severe cases of cyanide poisoning and is administered intravenously to convert cyanide to non-toxic thiocyanates that can then be safely excreted from the body.
Medical use of sodium sulphate:
The effect of hyposulfite on the human body when taken orally has not been studied, therefore it is impossible to judge the benefits and harms of the substance in its pure form or as part of food. There have been no cases of poisoning with the E539 additive, so it is considered to be non-toxic.
Sodium thiosulfate and legislation
Sodium thiosulfate is included in the list of food additives approved for use in the manufacture of food products in Russia and Ukraine. Anti-caking agent and acidity regulator E539 are used in accordance with established sanitary and hygienic standards exclusively for industrial purposes.
Due to the fact that the effect of the chemical on the human body when administered orally has not yet been studied, the E539 supplement is not approved for use in the EU and the USA.
Sulfuric acid esters include dialkyl sulfates (RO 2)SO 2 . These are high-boiling liquids; the lower ones are soluble in water; in the presence of alkalis, they form alcohol and salts of sulfuric acid. Lower dialkyl sulfates are alkylating agents.
diethyl sulfate(C 2 H 5) 2 SO 4 . Melting point -26°C, boiling point 210°C, soluble in alcohols, insoluble in water. Obtained by the interaction of sulfuric acid with ethanol. It is an ethylating agent in organic synthesis. Penetrates through the skin.
dimethyl sulfate(CH 3) 2 SO 4 . Melting point -26.8°C, boiling point 188.5°C. Let's dissolve in alcohols, it is bad - in water. Reacts with ammonia in the absence of a solvent (explosively); sulfonates some aromatic compounds, such as phenol esters. Obtained by interaction of 60% oleum with methanol at 150°C. It is a methylating agent in organic synthesis. Carcinogen, affects the eyes, skin, respiratory organs.
Sodium thiosulfate Na 2 S 2 O 3
Salt of thiosulfuric acid, in which two sulfur atoms have different oxidation states: +6 and -2. Crystalline substance, highly soluble in water. It is produced in the form of Na 2 S 2 O 3 5H 2 O crystalline hydrate, commonly called hyposulfite. Obtained by the interaction of sodium sulfite with sulfur during boiling:
Na 2 SO 3 + S \u003d Na 2 S 2 O 3
Like thiosulfuric acid, it is a strong reducing agent. It is easily oxidized by chlorine to sulfuric acid:
Na 2 S 2 O 3 + 4Cl 2 + 5H 2 O \u003d 2H 2 SO 4 + 2NaCl + 6HCl
The use of sodium thiosulfate to absorb chlorine (in the first gas masks) was based on this reaction.
Sodium thiosulfate is oxidized somewhat differently by weak oxidizing agents. In this case, salts of tetrathionic acid are formed, for example:
2Na 2 S 2 O 3 + I 2 \u003d Na 2 S 4 O 6 + 2NaI
Sodium thiosulfate is a by-product in the production of NaHSO 3 , sulfur dyes, in the purification of industrial gases from sulfur. It is used to remove traces of chlorine after bleaching fabrics, to extract silver from ores; is a fixer in photography, a reagent in iodometry, an antidote for poisoning with arsenic, mercury compounds, an anti-inflammatory agent.
Lesson motto:
“Just knowing is not everything, knowledge must be used.”
Lesson Objectives:
Educational:
- expand students' understanding of the speed of chemical reactions;
- to understand the essence of the law of mass action (LMA);
- introduce students to new concepts (homogeneous and heterogeneous reactions);
- experimentally investigate the dependence of the rate of a chemical reaction on the concentration of reactants.
Developing:
- continue the formation of experimental skills of students;
- develop the ability to work in groups and individually;
- continue the formation of chemical thinking, the development of speech, memory, cognitive interest in the subject, independence, the ability to draw conclusions.
Educational:
- to cultivate the ability to work in pairs, communication skills.
Equipment:
- For the teacher : porcelain bowl, porcelain pestle, computer, video projector.
- On the student's desktop : four test tubes, test tube stand, clock with a second hand, black paper.
Reagents: Sodium thiosulfate, sulfuric acid, water, aluminum, iodine.
During the classes
1. Introductory part: the message of the topic of the lesson, the mood of the students for the lesson.
Teacher. Kinetics is a branch of chemistry that includes the study of such topics as the reversibility of chemical reactions, the thermal effect of reactions, the rate of chemical reactions, and chemical equilibrium. We start with a topic whose name you need to guess (the topic on the board is closed; I show an experiment demonstrating the dependence of the reaction rate of the interaction of aluminum and crystalline iodine on a catalyst).
Question to the class. Why do we start the study of chemical kinetics with this topic?
The topic of the rate of chemical reactions is relevant, since various processes are constantly taking place around us and their speed is different. These processes are important and occur in all corners of nature, human life. (Picture 1). Discussion among the guys - comparing the rates of the proposed reactions. The class comes to conclusion: All processes run at different speeds.
Questions to the class:
1. What is the reaction rate? Which of the following formulas corresponds to the speed chemical reactions?
2. In what units is the rate of chemical reactions measured?
It is important not only to know the rate of a chemical reaction, but also to learn how to control it. For what? To speed up the desired reaction and slow down the unwanted. As Goethe said: "Just knowing is not everything, knowledge must be used." Let's look at the screen: the figure shows the dependence of the reaction rate on certain external factors (Figure 2).
3. What factors affect the rate of chemical reactions?
The guys name the temperature, the catalyst, the nature of the substances, the area of contact of the reacting substances, give examples in which the influence of these factors is observed.
2. The main part.
Teacher. And what factor is not here, but affecting the rate of chemical reactions?
This is the concentration of reactants, it increases the rate of reactions in a liquid and gaseous medium. Therefore, in this lesson, we experimentally study the effect of the concentration of substances on the rate of chemical processes. In the 9th grade it was the experience of the interaction of zinc with dilute and concentrated hydrochloric acid, and in the 10th grade we use the reaction of the interaction of sodium thiosulfate with sulfuric acid.
A little about sodium thiosulfate: the chemical formula is Na 2 S 2 O 3, it is widely used in medicine. In photography, it is known as fixing salt. With its help, undecomposed silver bromide is removed from plates, paper or film. This process is based on the ability of sodium thiosulfate to form a water-soluble compound with silver bromide. Films treated with it and thoroughly washed with water become insensitive to the further action of light.
The meaning of the chemical reaction underlying the experiment: when sodium thiosulfate interacts with sulfuric acid, turbidity is observed - the appearance of pure sulfur (a sign of a chemical reaction). This reaction proceeds in two stages.
Stage I: Na 2 S 2 O 3 + H 2 SO 4 = Na 2 SO 4 + H2S2O3(thiosulfuric acid)
Stage II: H 2 S 2 O 3 \u003d H 2 SO 3 + S v
Sulfur is insoluble in water, which is why it precipitates. Before starting the experiment, let's look at the table that is on your tables - the instructions for conducting the experiment (Figure 3). It indicates the concentration of sodium thiosulfate in drops (conditional concentration). We will change it with water. The concentration of sulfuric acid remains unchanged - 1 drop. In the next column, write down the reaction time with a pencil. What is considered the start time of the reaction? The moment of draining solutions of sodium thiosulfate, water and sulfuric acid is considered zero, then you count the time until cloudiness appears. To better see the formation of sulfur in the reaction, use black paper.
Let's do a preliminary experience of the interaction of sodium thiosulfate with sulfuric acid and note the reaction time (second hand).
After the experiment, we plot the dependence of the reaction time on the concentration of sodium thiosulfate (Figure 4). We build a chart on half a page. We set aside the concentration in drops, the time in seconds. You have 10 minutes to work. Get started.
Let's look at the results of the experiment. On the board, the student enters his data in a pre-prepared table. I compare with my data (experiment spend the day before). I note which of the pairs conducted the experiment more accurately. The student then draws a graph of reaction time versus sodium thiosulfate concentration. class does conclusion:
The rate of a chemical reaction depends on the concentration. The larger it is, the faster the reaction rate.
Questions to the class:
1. Why does the rate of a chemical reaction increase, because with an increase in concentration, the reaction time decreases? (the answer is an inverse relationship between speed and time - see the formula).
2. What does a graph of reaction rate versus time look like? The guys build a graph (Figure 5). Why?
The dependence of the rate of a chemical reaction on the concentration of substances is expressed by the law of mass action (LMA), discovered in the 19th century. For example, for a conditional response
the rate of a chemical reaction is equal to the product of the rate constant of the chemical reaction k on the molar concentrations of the reactants raised to the power of their stoichiometric coefficients, if necessary: ? = k C A C B 2
Where S A And M B– molar concentration of substances A and B, mol/l.
physical sense k : when C A \u003d C B \u003d 1 mol / l, then k=v.
But here it is important to take into account in what medium the reaction proceeds: in homogeneous or heterogeneous. According to ZDM, the concentrations of substances in the dissolved and gaseous state are recorded in the expression for the reaction rate. If the substance is in the solid state, then its concentration is neglected (two students go to the blackboard to write down the expression for the reaction rate in a homogeneous and heterogeneous medium):
| 2SO 2 + O 2 \u003d 2SO 3 | C + O 2 = CO 2 |
| v= k With O2 With 2 SO2 | v= k With O2 |
That is, ZDM is valid for homogeneous reactions. And what does the expression for the rate of a chemical reaction look like for a homogeneous and heterogeneous reaction?
For a homogeneous reaction:
For a heterogeneous reaction:
Control. To consolidate the topic, students answer the test questions (Figure 6).
Then, students check all the answers with the screen where the answers are projected for verification (Figure 7).
The result of the lesson: deepened knowledge on the topic of the rate of chemical reactions, experimentally investigated the effect of the concentration of substances on the reaction rate. I think that you have acquired new knowledge, skills that will be useful to you in the future. And, finally, a small wish in chemical language.
IV. Reflection.
I wish you not with loud words,
So that they do not explode like hydrogen, in case of failures
What's behind you
And were not inert, like neon, on the way,
What you haven't seen yet.You be patient like fate
Don't oxidize like a group of alkali metals
hardworking always
For long, long years.Let there be fewer inhibitors
Like a burden, slowing down the path at times.
Let there be more individuals
Talented and creative of you.Be active in our mad life,
Like a free radical.
Catalysts on your way are promised
Love, patience and kindness.
1. Influence of concentration on the rate of reaction of sodium thiosulfate with sulfuric acid . Pour 0.1 N into three test tubes. sodium thiosulfate solution: in the first - 5 ml, in the second - 10 ml and in the third - 15 ml. Then add 10 ml of distilled water to the first test tube and 5 ml of distilled water to the second one. Then, in three other test tubes, pour 5 ml of 0.1 N. sulfuric acid solution. Pour off the prepared solutions in pairs, resulting in a reaction
Na 2 S 2 O 3 + H 2 SO 4 \u003d Na 2 SO 4 + SO 2 + H 2 O + S
Using a stopwatch, note how long it takes for sulfur to appear in each tube. Record the results in the following table:
Table 9.1
What conclusion can be drawn from the obtained data?
2. Temperature dependence of the reaction rate . Influence of temperature on the reaction rate of interaction of sodium thiosulfate with sulfuric acid. Prepare six identical glasses. In three glasses, pour 15 ml of 0.1 N. sodium thiosulfate solution, and in the other three glasses - 15 ml of 0.1 n. sulfuric acid solution. Heat one pair of glasses with solutions of sodium thiosulfate and sulfuric acid in a water bath to a temperature 10 ° C higher, and another pair of glasses 20 ° C higher than room temperature for 15–20 minutes, controlling the water temperature with a thermometer. While the solutions are heating, drain off the remaining sodium thiosulfate and sulfuric acid solutions at room temperature. Note the time the sulfur appears in the glasses. Do the same with heated solutions. Record the data obtained in the table:
Table 9.2
What conclusions can be drawn regarding the effect of temperature on the reaction rate from the results obtained?
3. Studying the Reaction Rate of Hydrogen Peroxide Decomposition . Hydrogen peroxide spontaneously slowly decomposes in accordance with the equation: H 2 O 2 =H 2 O+1/2O 2 . The rate of this process can be increased by the introduction of a catalyst and the amount of oxygen released over a certain period of time can be estimated. The experiment is carried out in the apparatus shown in Fig. 2. Pour water through the funnel into the burette until approximately zero division, tightly close the burette opening with a stopper with a glass tube. Using a funnel, pour 1 ml of a solution of ferric chloride III into one leg of the Landolt vessel - a catalyst. Using a funnel, pour hydrogen peroxide into the other knee at a concentration specified by the teacher. Then connect the Landolt vessel to the burette using a stopper with a gas outlet tube. Check the tightness of the device. Place the Landolt vessel in a thermostat with a given temperature and hold for 10–15 minutes. Set the equal water level in the equalizing funnel and buret, record the level. By tilting the Landolt vessel, bring the hydrogen peroxide into contact with the catalyst. Every 1–2 min for 30 min, measure the volume of released oxygen V τ . Record the measurement results in table. 9.3.
Table 9.3
After complete decomposition of hydrogen peroxide, cool the Landolt vessel to the initial temperature of the thermostat, and again measure the volume of completely released oxygen V ∞ . According to Table. 9.3 and according to the formula
calculate the reaction rate constant. Build a dependency graph:
Determine the rate constant of the reaction by the tangent of the slope of the straight line to the abscissa axis and compare with the arithmetic mean value (9.17). It is advisable to carry out experiments at two temperatures: 15–25°C and 30–40°C.
According to the values of the reaction rate constant for two temperatures according to the formula:
where R=8.314 J/mol∙K, calculate the activation energy of the hydrogen peroxide decomposition reaction.
4.The influence of the concentration of reagents on chemical equilibrium . When a solution of iron (III) chloride reacts with potassium thiocyanate, soluble substances are formed and the color of the solutions changes. The reaction is reversible:
FeCl 3 +3KCNS Fe(CNS) 3 +3KCl
Record in the table the colors of the solutions of all substances of the system:
Table 9.4.
Mix in a test tube 5 ml of solutions of iron (III) chloride and potassium thiocyanate. Note the color of the resulting solution. Indicate the substance that imparted color to the system. Pour the resulting solution into four test tubes, if possible in equal parts. Add a little concentrated solution of ferric chloride to the first test tube, a solution of potassium thiocyanate to the second, and a little crystalline potassium chloride to the third. Leave the fourth tube for comparison. Compare the color of the solutions in the test tubes and indicate in which direction the equilibrium shifted when adding FeCl 3 , KSCN and KCl. Write an equation for the equilibrium constant of the studied reaction.
5. Effect of temperature change on chemical equilibrium . Under the action of iodine on starch, an unstable compound of complex composition is formed, colored blue. The equilibrium of the system can be conditionally represented by the following equation:
Starch + iodine starch iodine complex
Pour 2-3 ml of starch solution into a test tube and add a few drops of iodine water until a blue color of the solution appears. Heat the tube until the solution becomes clear and then cool until the blue color returns. Determine which reaction (direct or reverse) is exothermic, which is endothermic. Explain the change in color when heated and cooled.