Balance the equations for the reactions occurring in the solution at the anode

and cathode:

Assume that no other reactions occur at the anode and cathode.

Write down and balance the overall equation for the electrolysis of an aqueous solution of $\rm{CuSO_4}$.

Reactions in the solution at the anode and cathode are respectievely

\[ 2\mathrm{H_2O} - 4e^- \to 4\mathrm{H^+ + O_2} \uparrow, \]

\[ \mathrm{Cu^{2+}} + 2e^- \to \mathrm{Cu^0}.\]

The overall equation for the electrolysis is

\[ \rm 2Cu SO_4 + 2H_2O \to 2H_2SO_4 + 2Cu + O_2 \uparrow. \]

Prepare $V_0=150$ mL of copper sulfate with a molar concentration of $c_0=0.400~\mathrm{M}$. You are given copper sulfate powder ($\rm CuSO_4 \cdot 5 H_2 O$). How many grams $m_{bs}$ of powder are needed to prepare the specified solution? It can be assumed that the volume of the resultant copper sulfate solution is the same as the volume of the added water.

We will call the resulting solution “solution A2.” Pour $5$ mL of solution A2 into Answer tube A2.

The amount of copper sulfate that needs to be added is determined as $\nu_{\rm bs}=c_0V_0$.

Therefore, the mass is equal to

$$m_{\rm bs}=c_0V_0⋅M({\rm CuSO_4⋅5H_2O}).$$

In accordance with instruction G2, perform electrolysis of $120$ mL of solution A2 for $t_0=1$ h at a current of $I=1$ A.

Record the dependence of the volume of oxygen released $V_{\rm O_2}$ on time $t$. Take at least 10 measurements. Plot the resulting dependence and draw an approximation curve.

Ответ:

After electrolysis, stir the solution remaining in the electrolyzer. In accordance with the G2 instructions, filter approximately 20-25 mL of the stirred solution after electrolysis.

We will refer to the filtered solution as “solution A4.” Pour 5 ml of solution A4 into Answer tube A4.

The jury will perform spectroscopy on this solution.

Substituting the known values $I=1$ A and $t=3600$ s, we calculate the charge $Q=I\cdot t$

We can determine the number of oxygen molecules:
$$N_{O_2}=\frac{N_Ap_0V_{O_2}}{RT_0}.$$Four electrons are needed to reduce one oxygen molecule, therefore:
$$Q_{O_2}=4e⋅N_{O_2}=\frac{4eN_Ap_0V_{O_2}}{RT_0}=2870~\rm C.$$

Note that the initial solution A2 has very strong absorption, so in this task you calculate its dilution by a factor of 10 or more.

 The total volume of the resulting solution is

$$V_0=V_{\bf A2}+V_{H_2O}.$$

The amount of copper ions does not change after dilution with water:

$$[\rm Cu^{2+}] \cdot V_0 = c_0 V_\textbf{A2}.$$

Hence

$$ \begin{cases} V_\textbf{A2} =  V_0\dfrac{[\rm Cu^{2+}]}{c_0}, \\ V_{\rm H_2O} = V_0\dfrac{c_0 - [\rm Cu^{2+}]}{c_0}. \end{cases}$$

Using the calculations made in the previous task, prepare five solutions in optical cuvettes. In accordance with instruction G1, measure the absorption spectrum of each of the five solutions.

Save the measured spectra in the “Results/B2” folder on your desktop under the names “B2.{cuvette number}.txt” (for example, “B2.3.txt”).

Ответ:

Save the measured spectrum in the “Results/B5” folder on your desktop under the name “B5.txt”.

Ответ:

Determine the concentration of copper ions $[\rm Cu^{2+}]_\textbf{A4}$ in solution A4.

Absorption at a wavelength of $\lambda_n = 760 \text{ nm}$ is $A(\lambda_n) = 0.6564 \text{ a.u.}$

$$[{\rm Cu}^{2+}]_{\bf A4}=10 \cdot  A(\lambda_n) / s$$

According to the graph, we determine $A_{iso}=0.34$ a. u. We record the $A_{peak}$ value for each $\rm pH$ value in the table.

Ответ:

This step uses a thin glass cuvette with an adapter. Following the G1 instructions for thin cuvettes, obtain the absorption spectrum of undiluted solution A4 without indicator. Save the measured spectrum to the “Results/C2” folder on your desktop under the name “C2.txt”.

Ответ:

This step uses a thin glass cuvette with an adapter. Following the G1 instructions for thin cuvettes, obtain the absorption spectrum of the undiluted A4 solution with the indicator. Save the measured spectrum to the “Results/C3” folder on your desktop under the name “C3.txt”.

Ответ:

Considering the dependence $A_{\rm peak}/A_{\rm iso} ({\rm pH})$ to be linear, we determine the value from the graph from C1:

$$
A'_{\rm peak}/A'_{\rm iso} = 2.92 \\
{\rm pH_{fin}} = 0.85$$

Perform the experiment described above, adding the specified amount $\Delta V$ of acid with concentration $C_{\rm HCl}$ at each step. Measure and save the absorption spectrum at each step according to instruction G1. Save the measured spectra in the folder on your desktop named “Results/C7” under the names “C7.{step number}.txt” (for example, “C7.2.txt”). You should end up with 8 spectra. Pour the remaining solution after obtaining all spectra into Answer tube C7.

Ответ:

This step uses a thin glass cuvette with an adapter. Following the G1 instructions for thin cuvettes, obtain the absorption spectrum of the undiluted A2 solution without indicator. Save the measured spectrum to the “Results/C10” folder on your desktop under the name “C10.txt”.

This step uses a thin glass cuvette with an adapter. Following the G1 instructions for thin cuvettes, obtain the absorption spectrum of the undiluted A2 solution with indicator. Save the measured spectrum to the “Results/C11” folder on your desktop under the name “C11.txt”.

Based on the measurements in questions C10-C11, calculate the absorption $A'_{peak}$ at wavelength $\lambda^{BB}_{peak}$ caused only by the absorption of the indicator. What is the absorption $A'_{iso}$ at a wavelength of $\lambda^{BB}_{iso}$ nm caused only by the absorption of the indicator?

$$A'_{peak}/A'_{iso} = 2.52\\
\text{pH}_{\text{ini}} = 3.92$$

The initial and final concentrations of hydrogen ions are $10^{-\operatorname{pH}_{\mathrm{ini}}}\,\mathrm{M}$ and $10^{-\operatorname{pH}_{\mathrm{fin}}}\,\mathrm{M}$.

The change in the concentration of hydrogen ions in the solution is equal to $\frac{Q_{\operatorname{pH}}}{eN_A V_i}$. Thus, $$Q_{\operatorname{pH}}=eN_AV_i\cdot\left(10^{-\operatorname{pH}_{\mathrm{fin}}}-10^{-\operatorname{pH}_{\mathrm{ini}}}\right) \,\mathrm{M}\simeq eN_AV_i\cdot10^{-\operatorname{pH}_{\mathrm{fin}}}\, \mathrm{M}.$$

1. Correct. Indeed, some of the oxygen formed dissolves in the liquid and can then escape through the surface of the solution.
2. Incorrect. Reduction occurs at the cathode, while the formation of $\rm O_2$ is an oxidation process. Therefore, molecular oxygen cannot be formed at the cathode.
3. Correct. It is this effect that leads to underestimated values of the measured oxygen volume during electrolysis and the calculated value of the charge $Q_{\text{O}_2}$ based on it.
4. Correct. A small amount of hydrogen can be released at the cathode, and carbon dioxide at the anode (as a result of the interaction of oxygen with the electrode).
5. Incorrect. $\rm CuSO_4$ dissociates very well.
6. Incorrect. Copper will still be reduced from the solution at the cathode. The change in $\operatorname{pH}$ will be the same.
7. Correct. If the anode is replaced with a copper one, the copper oxidation reaction will occur on it first (instead of water oxidation). Therefore, neither $[\rm Cu^{2+}]$ nor $\operatorname{pH}$ will change in the solution. This means that it is impossible to determine the charge based on the change in the corresponding values.

Explanation on question #4:

The reduction potential of $\rm Cu^{2+}$ ions is greater than that of $\rm H^{+}$ ions. However, at non-zero current, the electrode is not in equilibrium with the solution, and therefore the ratio of the rates of these reactions cannot be determined from the reduction potentials alone. During galvanostatic electrolysis, a constant and non-zero current flows in the solution due to the ions dissolved in it. Ion movement in the near-electrode region of the solution near the cathode occurs due to diffusion, since the electric field from the electrode side is shielded by the electrolyte, and convection is hindered by viscosity. The intensity of diffusion, and therefore the difference between the copper ion concentrations near the cathode and in the bulk of the solution, is controlled by the current. Hydrogen gas evolution at the cathode begins at a current at which the copper ion concentration near the cathode is sufficiently low (under the experimental conditions, negligible compared to the copper ion concentration in the bulk of the solution).

The values of charges $Q_{\mathrm{O}_2}$ and $Q_{\operatorname{pH}}$ may differ from reality due to the reaction between the carbon electrode and oxygen. As a result, less oxygen is released and carbon dioxide dissolves, acidifying the medium.
This effect does not affect the value of $Q_{\mathrm{Cu}}$, and in the experiment, the corresponding value is closest to the value calculated from the current $Q = I \cdot t$.

For each LED battery, measure the voltage across the 3 LEDs connected in series and calculate the voltage across single LED when the power source is turned on. Fill in the table in the answer sheets.

For this task, use microorganism $A$. Prepare the setup for measurements according to the G3 instructions. Turn on the light source and start timing.

If no oxygen release is observed 30 minutes after the start of the experiment, record zero values for $V_{O_2}$ in the table in the answer sheets.

If oxygen release is observed 30 minutes after the start of the experiment, continue the experiment for another 1.5 hours. Record the volume of oxygen $V_{O_2}$ released when illuminated by different colors of light in the table on the answer sheet.

The displacements of the water pistons after 2 hours are
\begin{align}
\Delta x_{\rm red} = 14.1 \ \text{cm},\\
\Delta x_{\rm green} = 6.0 \ \text{cm}, \\
\Delta x_{\rm blue} = 10.5 \ \text{cm}.
\end{align}

According to instruction G4, use Goryaev's chamber to count the number of cells in the four small squares $n_A$ and $n_B$ of microorganisms $A$ and $B$.

The edge of the large square of the Goryaev chamber is 0.2 mm, the depth of the chamber is 0.1 mm, and the large square consists of 16 small squares. Count the total number of cells $N_A$ and $N_B$ of microorganisms $A$ and $B$ inside a 20 ml syringe. Write down the calculation formula showing how $n_A$ and $N_A$ are related.

$$n_A = 25, \; n_B = 17.$$$$N = n \cdot 20 \text{ mL} \cdot \dfrac{16}{0.2 \times 0.2 \times 0.1 \text{ mm}^3 \cdot 4}.$$

Using the data obtained in questions E3 and E7, fill in the table in the answer sheets.

According to instruction G5, perform thin-layer chromatography of extracts of microorganisms $A$ and $B$.

Immediately after completing the chromatography and drying the plate, analyze the table and carefully mark the spots corresponding to chlorophylls with an “X” and the spots corresponding to carotenoids with an “O” on the plate with a pencil.

Raise the HELP sign so that an assistant can come to you and photograph the plate.

Place the marked plate in Answer tube F1.

The jury will examine this plate.

In accordance with instruction G1, obtain the absorption spectrum of extracts from microorganisms $A$ and $B$.

Save the measured spectra in the folder on the desktop “Results/F2” under the names “F2.A.txt” and “F2.B.txt” for microorganisms $A$ and $B$, respectively.

Pour 3 mL of the microorganism extract solutions you measured into Answer tube F2.A and Answer tube F2.B.

The F2 author's spectrum

The jury will perform spectroscopy on this solution.

Select the correct statements about microorganisms.

The figure shows a diagram of a small pond with poor water circulation. Identify the zones of the pond (A-D) where the following microorganisms will live.

1. cyanobacteria and green algae
2. anaerobic decomposers of organic matter
3. green bacteria
4. purple bacteria

Write the numbers of the organisms in the table on the answer sheet.