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22 14. What is the chemical mechanism of the toxic effect of arsenic for living organisms? Write the scheme of action. 15. What is the mechanism of the toxic effect of Be  2+ ions on the body? Explain this with the help of reaction equations. 16. A 3% aqueous solution of hydrogen peroxide is stable for a long time. However, when the open wound is treated with this solution, hydrogen peroxide decomposes very rapidly. Why is this happening? Laboratory work Characteristic and qualitative reactions of biogenic elements. Experiment 1. Catalytic decomposition of hydrogen peroxide (performed in a fume hood!). Add 6–7 drops of a saturated solution of potassium dichromate to the test tube. The tube is slightly heated on the flame of the burner, add 2 drops of 30% hydrogen peroxide solution. Observe the blackening of the solution. Intermediates of the type К 2 [Cr 2 O 7 ×H 2 O 2 ] are formed. There is a rapid decomposition of these compounds with the release of oxygen. After 1–2 minutes after the reaction, the orange color of potassium dichromate appears again. Write the equation for the decomposition reaction of hydrogen peroxide. What type of oxidation-reduction reactions is this reaction? Experiment 2. Oxidizing properties of hydrogen peroxide. Add 3–4 drops of potassium iodide solution, 1 drop of 1 M sulfuric acid solution and 1 drop of 3% hydrogen peroxide solution to the test tube. Observe the discoloration of the solution. What substance has a characteristic emerging color? Write the equation of the reaction, select the coefficients by the electronic balance method. Experiment 3. Preparation and properties of chromium hydroxide (III). Add a solution of chromium (III) into two tubes and add a dilute solution of ammonia dropwise to form a precipitate of chromium hydroxide. In one of the tubes add a solution of sulfuric acid to dissolve the precipitate, and in the other – a solution of sodium hydroxide until the precipitate dissolves. What is the color of the resulting solutions? Add a solution of hydrochloric acid to the second test tube until a turbidity or precipitate appears. Explain the observed phenomenon and write the equations of all reactions. Experiment 4. Reducing properties of chromium (III) compounds. Add a solution of alkali to a solution of chromium (III) salt. Aprecipitate forms. Add an alkali solution, the precipitate must be dissolved, then add hydrogen peroxideto this tube. Explain the change in color of the solution. Write the equation of the reaction, arrange the coefficients by the electronic balance method.