Chemistry Quiz

Photosynthesis is an

• exothermic redox reaction.

• endothermic redox reaction.

• exothermic condensation reaction.

• endothermic condensation reaction.

Answer explanation

Photosynthesis is an endothermic process because it absorbs energy from sunlight to convert carbon dioxide and water into glucose and oxygen. It is also a redox reaction, as it involves the transfer of electrons.

Which one of the following statements is correct?

• The oxidation of glucose is reversible in the body.

• The combustion of glucose provides less energy per gram than the combustion of hydrogen.

• The oxidation of glucose is given by the equation 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.

• The combustion of oxygen occurs during cellular respiration.

Answer explanation

The correct statement describes the oxidation of glucose, which is represented by the equation 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. This reflects the process of photosynthesis, where glucose is synthesized, not oxidized.

0.50 mol of ethane, C₂H₆, and 100.0 g of air that is 22.0% m/m oxygen, O₂, are injected into a combustion chamber. The equation of the combustion of ethane is C₂H₆(g) + 3.5O₂(g) → 2CO₂(g) + 3H₂O(l). If complete combustion takes place, which reactant is in excess and by how much?

• C₂H₆, 0.25 mol

• C₂H₆, 0.30 mol

• O₂, 12 g

• O₂, 97 g

Answer explanation

To find the limiting reactant, calculate moles of O₂ in 100 g of air (22% O₂): 100 g * 0.22 = 22 g O₂, which is 1 mol. Ethane needs 0.50 mol * 3.5 = 1.75 mol O₂. Thus, O₂ is in excess by 97 g.

Consider the following reaction. \[ \text{Cr}_2\text{O}_7^{2-} (\text{aq}) + 14\text{H}^+ (\text{aq}) + 6\text{I}^- (\text{aq}) \rightarrow 2\text{Cr}^{3+} (\text{aq}) + 3\text{I}_2 (\text{aq}) + 7\text{H}_2\text{O} (\text{l}) \] Which one of the following correctly identifies the oxidising agent and a conjugate redox pair (in the form oxidising agent/reducing agent)?

• Cr₂O₇²⁻ / I₂/I⁻

• Cr₂O₇²⁻ / I⁻/I₂

• I⁻ / Cr₂O₇²⁻/Cr³⁺

• I⁻ / Cr³⁺/Cr₂O₇²⁻

Answer explanation

In the reaction, Cr₂O₇²⁻ is reduced to Cr³⁺, making it the oxidising agent. I⁻ is oxidised to I₂, forming the conjugate redox pair I₂/I⁻. Thus, the correct choice is Cr₂O₇²⁻ / I₂/I⁻.

When the galvanic cell is operating

• total chemical energy is increasing and electrons flow to half-cell A.

• total chemical energy is increasing and electrons flow to half-cell B.

• total chemical energy is decreasing and electrons flow to half-cell A.

• total chemical energy is decreasing and electrons flow to half-cell B.

Answer explanation

In a galvanic cell, chemical energy is converted to electrical energy, leading to a decrease in total chemical energy. Electrons flow from the anode (half-cell A) to the cathode (half-cell B), confirming the correct choice.

Consider the following half-equation. \[ \text{IO}_3^- (aq) + 6\text{H}^+ (aq) + 5e^- \rightarrow \frac{1}{2}\text{I}_2(aq) + 3\text{H}_2\text{O}(l) \quad E^0 = +1.20 \, \text{V} \] A galvanic cell is set up using inert electrodes and the following chemicals: - half-cell 1: 1.0 M acidified solution of potassium iodate/iodine solution, KIO\(_3\)(aq)/I\(_2\)(aq) - half-cell 2: oxygen gas, O\(_2\)(g), in an alkaline solution. Which one of the following statements is correct when the galvanic cell is operating?

• A. Hydrogen gas is produced at the cathode.

• B. The oxidising agent is IO\(_3^-\) and the reducing agent is O\(_2\).

• C. The concentration of OH\(^-\) ions decreases at the negative electrode.

• D. The mass of the electrode in half-cell 1 decreases.

Answer explanation

In the galvanic cell, IO3- is reduced to I2, making it the oxidizing agent. O2 is oxidized, acting as the reducing agent. Thus, statement B is correct.

In a galvanic cell, copper(II) ions, Cu\(^{2+}\), are converted to copper metal, Cu. What mass of Cu is deposited for each 0.50 mol of electrons transferred in the cell?

• A. 1.6 × 10 g

• B. 3.2 × 10 g

• C. 6.4 × 10 g

• D. 1.3 × 10\(^2\) g

Answer explanation

To find the mass of Cu deposited, use the equation: mass = moles × molar mass. For Cu, molar mass = 63.5 g/mol. For 0.50 mol of electrons, Cu deposits 0.25 mol (1:2 ratio). Thus, mass = 0.25 mol × 63.5 g/mol = 15.875 g, or 6.4 × 10 g.

Consider the following statements about hydrogen fuel cells. I. The use of porous electrodes reduces cell efficiency. II. As the ratio of the amount of hydrogen supplied to the amount of electricity produced increases, the efficiency increases. Which of the statements about hydrogen fuel cells is/are correct?

• A. I only

• B. II only

• C. both I and II

• D. neither I nor II

Answer explanation

Statement I is incorrect; porous electrodes can enhance efficiency by improving reaction rates. Statement II is correct; a higher hydrogen-to-electricity ratio indicates better efficiency. Thus, the correct answer is B.

The overall reaction in an alkaline–ethanol fuel cell is shown below. C₂H₅OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(l) What amount of hydroxide ions, OH⁻, reacts with 1 mol of ethanol, C₂H₅OH, at the negative electrode of the fuel cell?

• 4 mol

• 6 mol

• 8 mol

• 12 mol

Answer explanation

In the reaction, 1 mol of C₂H₅OH reacts with 3 mol of O₂, producing 2 mol of CO₂ and 3 mol of H₂O. Each O₂ requires 2 OH⁻ ions, totaling 6 OH⁻ for O₂ and 2 OH⁻ for ethanol, resulting in 8 mol of OH⁻.

Consider the following statements regarding fuels. I. Bioethanol fuel is sustainable because it results in no net release of carbon dioxide into the atmosphere. II. Hydrogen peroxide, H₂O₂, can be used as an oxidising agent in fuel cells. Which of the statements regarding fuels is/are correct?

• I only

• II only

• both I and II

• neither I nor II

Answer explanation

Statement I is incorrect; bioethanol can still release CO2, but it's considered renewable. Statement II is correct; hydrogen peroxide can act as an oxidizing agent in fuel cells. Thus, the correct answer is II only.